Rapid prototyping (RPM) technology has a variety of rapid prototyping processes with different characteristics and uses. Different users will make a comprehensive trade-off based on their own needs, equipment prices, operating costs, etc. when introducing rapid prototyping equipment. From the current development of rapid prototyping technology, the prototypes processed by SLA equipment have the highest accuracy. Correspondingly, the molds quickly manufactured using SLA prototypes are also more accurate, but its equipment and operating costs are also very high, which ordinary users cannot afford. The LOM process occupies a considerable proportion of the current domestic market due to its fast speed and low operating costs.
LOM molded parts have good mechanical strength, stable processing, and small shrinkage deformation. They can be used as simple injection molds, wax molds for precision casting, or wax mold molding molds to achieve small-batch trial production. They can also replace wooden molds for sand casting and repeatedly make 50 to 100 sand molds. According to previous literature [58-65], LOM rapid prototyping is generally used for rapid casting of metal parts and molds, that is, LOM paper prototypes are used instead of wooden molds to make plaster molds, ceramic resin sand molds and other casting molds, and then metal parts (or molds) are quickly cast.
During this process, the metal material undergoes a phase change from solid melting to liquid and from liquid cooling to solid, causing the part size to shrink. When establishing the CAD model, dimensional compensation needs to be performed in advance, otherwise it is difficult to ensure dimensional accuracy and the compensation of the CAD model is difficult to achieve the ideal effect. Therefore, it is necessary to find a way to reduce the dimensional shrinkage during the rapid prototyping to metal part conversion process.
The purpose of this chapter is to explore a method for manufacturing EDM electrodes from LOM prototypes by a non-casting method that maintains dimensional accuracy during the transition from LOM prototype to electrode. It involves layered solid manufacturing as well as the electroless plating, electroforming, and arc spraying techniques that have been studied in detail in previous chapters.
Layered Object Manufacturing (LOM) Technology
1.Basic Principles of Layered Object Manufacturing
Layered Object Manufacturing (LOM) is a rapid prototyping technology widely used in China. Its prototyping system is mainly composed of CNC system, data processing software, feeding mechanism, hot pressing device, laser cutting system, lifting table, etc. The specific working principle is shown in Figure 5-1.
The 3D CAD model of the product is transferred to the computer on the rapid prototyping system, and the CAD model is cut into a series of “thin slices” with a certain thickness along the height direction using data processing software. The 2D contour of each “thin slice” is obtained and stored in the computer. The feeding mechanism delivers thin layers of materials such as paper or plastic with hot melt adhesive on the bottom to the top of the workbench. Under the control of the computer, the laser cutting system cuts the raw material according to the 2D contour of the “thin slice” and cuts the contour-free area of ​​the material into small grids.
After each layer of material is cut, the workbench drops a layer thickness, and then a new layer of material is bonded to the cut upper layer of material through a hot pressing rod. The hot pressing cutting is repeated in this way, and layers are piled up until all layers are processed, and the excess waste is removed to obtain the final 3D product required.
The LOM process uses laser to cut materials. The M-RPMS-I multifunctional rapid prototyping machine used in this project uses a domestically produced sealed 50W continuous output CO laser. A high-frequency electronic switch is used at the output end to quickly respond to the system’s switch command for the laser. This system uses the XY high-speed precision linear guide rail of Japan’s THK company, and a reflector is installed on the guide rail slider to transmit the laser beam.
Two of the reflectors are fixed to the machine wall, and the other two reflectors are fixed on the sliders of the X and Y guide rails respectively. Under the control of the CNC system, the X and Y direction lead screw axes move simultaneously, and the laser can cut the material according to the two-dimensional contour information of the part on the X-Y plane. The laser and optical path system in the molding machine is equivalent to the tool system in the CNC machine tool, which is used to transmit the laser beam and also performs one-dimensional scanning movement, similar to the working principle of a plane plotter [66,67].
2.Analysis of Forming Accuracy in Layered Solid Manufacturing
The molding accuracy of the prototype obtained by LOM process is affected not only by system error, CAD/CAM error (that is, the error caused by replacing the CAD model with the STL formatted model and the principle error caused by slicing), but also by the thermal and wet deformation of the molding material and the LOM process parameter settings.
(1)Influence of system error on forming accuracy
â‘ Errors in system mechanical devices and transmission systems
The motion positioning accuracy and repeat positioning accuracy of the cutting head, the verticality of the X-axis and Y-axis guide rails, the verticality of the axis and the worktable, the motion positioning accuracy and repeat positioning accuracy of the Z-axis, etc. will affect the forming accuracy.
â‘¡Height sensor measurement accuracy
The molding system uses a height sensor to measure the real-time height of the paper surface after hot pressing, and feeds the measurement data to the computer for calculation. The workbench is raised and lowered to make the layer to be cut exactly on the focal plane, and the layered cross-section data at the corresponding height is called according to this data to perform contour cutting on the current layer.
Therefore, the accuracy of the height sensor will directly affect the processing accuracy of the molded part. Since the molding system uses a single height sensor, it cannot measure the entire layer, and can only measure the height value near the middle position in the X direction. At the same time, the measurement accuracy is also affected by temperature and mechanical vibration, which will lead to size and shape errors of the molded part[68].
â‘¢Temperature distribution on hot plate surface
The surface temperature of the hot press plate cannot be evenly distributed along the X-axis, and the verticality error between the worktable and the Z-axis causes the height of the hot press surface to be inconsistent. These factors lead to inconsistent melting states of the hot melt adhesive applied to the material, resulting in uneven distribution of adhesive thickness, thus affecting the Z dimensional accuracy[68].
â‘£Effect of optical path system deviation on molding accuracy
The optical path system of the molding system within the scanning processing range inevitably has deviations. When the cutting head moves to different positions, the position of the laser beam emitted by the laser after transmission and focusing on the cutting head through the focusing lens does not overlap, and is not always located at the center of the focusing lens. This will inevitably cause dimensional errors in the contour of the cut material part.
The non-overlapping of the laser spot on the focusing lens will also cause the focus after focusing to be not on the same horizontal plane, that is, the focal plane formed is a curved surface shape. In this way, the diameter of the laser spot at the laser cutting point will change within the scanning processing range of the part, reducing the accuracy of the incision, and will also cause dimensional errors in the cutting of the material contour, thereby affecting the size and shape accuracy of the molded part.
(2)The influence of layered slicing on molding accuracy
The layered slicing method used in the LOM process is to decompose the 3D CAD model into a series of layers with a certain thickness and a two-dimensional profile along the molding direction. The characteristics of the laser make it only possible to perform plane processing on each layer in the direction perpendicular to the workbench. Since each layer has a certain thickness, the curved 3D entity of the part within the thickness range is actually simplified into a cylinder, so there is an error in the external dimensions. In addition, when using the STL formatted model instead of the CAD model, errors are inevitable.
If the layer thickness method is used for layering, that is, the STL format model is sliced at one time according to the selected layer thickness, the computer sequentially calls the data of each layer to the CNC card during processing, and controls the molding machine to complete the processing of the prototype.
This layering method is relatively simple, but if the paper thickness is used as the layer thickness, the cumulative error of the paper thickness will cause the Z-direction dimensional accuracy of the molded part to be uncontrollable. If the height direction is detected in real time and fed back to the control system, although the final Z-direction dimension of the molded part can be controlled, it cannot guarantee that the cross-sectional profile at each height in the slice data is processed and realized, because during the processing, in order to meet the height requirements, some layers will not be processed.
If real-time thickness measurement is used for real-time layering, the lifting table is controlled in a closed loop. According to the measured height of the current layer of the molded part, the STL model is layered in real time to obtain the data of the corresponding section. Not only can the cross-sectional profile of the model at the corresponding height be obtained more realistically, but the influence of the cumulative error of the paper thickness on the Z-direction dimensional accuracy of the part can also be eliminated [69].
(3)The influence of thermal and wet deformation of molding materials on molding accuracy
During the LOM molding process, the cooling warping and moisture absorption growth of the material (especially paper) will cause the molded parts to warp, twist, crack, etc. The thermal and wet deformation of the material is one of the most critical and difficult to control factors affecting the molding accuracy of the LOM process-[69].
When using paper coated with hot melt adhesive as the molding material in the LOM process, due to the large difference in thermal expansion coefficients between paper and hot melt adhesive, when the heat is transferred to the parts by hot pressing on the hot press plate and laser cutting, the hot melt adhesive melts and expands rapidly after being heated, while the deformation of the paper is relatively small. In addition, the uneven contraction of the paper and adhesive during cooling will cause thermal warping and distortion of the molded parts.
LOM molded parts are made of laminated composite materials, and their wet deformation follows the expansion law of the composite materials. Experimental studies have shown that when water accumulates on the lateral open surface of the laminated composite material, it will immediately diffuse through the adhesive layer interface at a higher rate and enter the molded part through the loose fiber structure, causing the molded part to swell, weakening the bonding strength between the layers, and causing the molded part to deform or even crack [69].
By improving the coating method of hot melt adhesive and the post-processing method of molded parts, as well as pre-correcting the CAD model according to the thermal deformation law of molded parts, the influence of thermal and wet deformation on molding accuracy can be reduced.
(4)Influence of process parameter settings on molding accuracy
The LOM molding process is completed automatically, but its process parameters have a great impact on the accuracy of LOM molded parts, and the system process parameters need to be accurately set [70].
The nominal accuracy of the LOM process of the M-RPMS-II multifunctional rapid prototyping machine used in this project is t0.1mm/100mm. However, due to the inherent characteristics of the LOM process, the prototype will have a 1~2% dimensional rebound in the Z direction within a few hours after molding [70. In order to control the dimensional accuracy of the molded parts, it needs to be compensated when setting the system parameters.
The LOM process uses a focused laser beam to cut thin layers of material. The laser spot has a certain diameter (00.1~0.3mm) and the surface wheel line generated after layering by the slicing software is a theoretical trajectory line. Like the tool in CNC machining, the laser spot needs to be compensated for the radius to avoid affecting the accuracy of the actual cutting contour.
Therefore, during the laser cutting process, the movement trajectory of the center of the laser spot cannot be the cross-sectional contour line obtained after layering, but should be offset by a spot radius to the inside of the inner boundary or the outside of the outer boundary according to the inside and outside of the contour boundary. This offset is the radius compensation for the laser spot.
The parameter setting of LOM process should also consider the real-time matching of cutting speed and laser output power. Each layer of LOM prototype is cut by laser beam on thin layer of material. If the energy of laser beam acting on thin layer of material is uneven, it will lead to uneven thickness of cutting contour line, some contour lines are not cut off, while others are “overburned”. The former makes it difficult to separate waste material from molded parts, and the latter will not only lead to large deviation in size at the “overburned” part of the contour, but also cut the previous layer of formed paper, thus affecting the dimensional accuracy and surface quality of the prototype.
The parameter setting of LOM process should also consider the real-time matching of cutting speed and laser output power. Each layer of LOM prototype is cut by laser beam on thin layer of material. If the energy of laser beam acting on thin layer of material is uneven, it will lead to uneven thickness of cutting contour line, some contour lines are not cut off, while others are “overburned”. The former makes it difficult to separate the material from the molded part, and the latter will not lead to large deviation in the size at the “overburned” part of the wheel, but will cut the previous layer of formed paper, thus affecting the dimensional accuracy and surface quality of the prototype.
Therefore, when setting the process parameters of LOM molding, only a good match between the cutting speed and the laser output power can ensure that the material is not “overburned” due to the laser output power being too high, or that the material cannot be cut through due to the laser output power being too low, thereby ensuring that the molded parts have good dimensional accuracy and surface quality.
Rapid manufacturing of EDM electrodes based on LOM prototype
1.Process route for rapid manufacturing of EDM electrodes using LOM prototype
In rapid mold manufacturing, LOM prototypes are generally used to replace wooden molds to make sand molds, gypsum molds, ceramic molds, resin sand molds and other casting molds, and then quickly cast metal molds. However, the metal material will inevitably shrink during the casting process, affecting the dimensional accuracy of the parts.
Electrospark machining is an effective method for machining mold cavities with complex structures made of high-melting-point and high-hardness materials. The accuracy of electrospark machining depends largely on the manufacturing accuracy of the electrode. Traditional electrode manufacturing methods include milling, turning, CNC machining, etc. For electrodes with complex surfaces, these methods have low processing efficiency, high cost, or even cannot be processed at all. The emergence of rapid prototyping (RP) technology can quickly convert the CAD model of the tool electrode into a practical tool electrode, providing an effective way for the rapid manufacturing of electrospark electrodes[71].
Therefore, this section conducts research on manufacturing EDM electrodes based on LOM prototypes, combined with the chemical plating, electroforming, arc spraying and other technologies mentioned in the previous chapters, through non-casting intermediate conversion. First, assume that there are some possible process routes:
â‘ LOM process prototype (part positive type – post-processing – arc metal coating – adding backing – separation of spray coating (negative type) and prototype – separation of electroforming layer and coating – electric spark electrode (positive type).
â‘¡LOM process prototype (parts reverse type) – post-processing – arc spraying copper – backing – separation of spray layer (F type) and prototype – spark electrode (positive type)
â‘¢LOM process prototype (parts reverse type) – post-processing – injection of silicone rubber mold (positive type) – casting epoxy resin parts in silicone mold (reverse type) – chemical copper plating – electroforming copper – sand roughening treatment – arc spraying metal backing – separation from epoxy resin reverse type – electric spark copper electrode (positive type)
â‘£LOM process processing prototype parts positive type) – post-processing – pouring silicone rubber or epoxy resin to get reverse type – silicone reverse type (epoxy resin reverse type) chemical copper plating – electroforming copper – sand roughening treatment – arc coating metal backing – separation from silicone reverse type (epoxy resin reverse type) – electric spark electrode (positive type).
⑤LOM process processing prototype (part reverse type) – post-processing – spray special coating sealing treatment – chemical plating – electroforming copper – sandblasting roughening treatment – arc spraying metal backing – separation from the prototype – electric spark copper electrode (positive type).
For process route â‘ , the commonly used metals for arc spraying are Zn and AlCu. If copper is electroformed after arc coating on the LOM prototype, as described in Chapter 4, copper has a high melting point and poor spraying processability, making it difficult to obtain a good coating, and it will burn the prototype or even cause the prototype to crack due to heat; if Zn and Al are sprayed and then copper is electroformed in the tank, although the spraying processability is good, experiments have found that it is very difficult to electroform on the surface of Zn or Al substrates, because Zn and Al are too active in chemical properties.
Even if pre-plating treatment is performed, the substrate will undergo a replacement reaction in the electroforming liquid during electroforming, and a well-bonded copper electroforming layer cannot be electroformed on its surface. The cast layer is in powder form and cannot be electroformed further. Therefore, the manufacture of spark copper electrodes using process route O is not suitable for practical applications.
For process route â‘¡, if copper is directly arc sprayed on the LOM prototype as an EDM electrode, it is difficult to obtain a good copper spray layer because the LOM prototype cannot withstand the temperature during copper spraying, as mentioned above. Even if copper is sprayed after other steps, it is found in practice that the EDM electrode obtained by arc spraying copper has too many pores and poor toughness. The electrode wears out very quickly during processing, and the surface quality is also poor [71]. Therefore, process route 2 is also difficult to apply in practice.
For process route â‘¢, because it includes an additional step of making a silicone rubber mold from the LOM prototype and then pouring epoxy resin into the silicone mold, it not only increases the cost and construction period, but also too many steps make it more likely that the workpiece dimensional accuracy will be lost due to error accumulation. Therefore, it is not suitable for the rapid manufacturing of EDM electrodes.
Process routes ④ and ⑤ combine the LOM prototype with the electroforming process, and use the electrochemical principle to deposit metal on the treated LOM prototype surface or the transition resin surface replicated by the LOM prototype, which can obtain a copper electroforming layer with high replication accuracy, which can be used to manufacture EDM electrode processing metal mold cavity. The process routes and are respectively called the process route of reproducing resin inversion and the process route of sealing special coating. This section conducts experimental research on these two process routes and analyzes and compares their superiority.
2.Experimental study on the process route of resin remodeling
The previous chapters have analyzed the electroforming, chemical plating and arc spraying processes in detail, so this chapter focuses on how to successfully combine them and solve the key technologies in the specific process flow. According to the characteristics of this process route, it can be divided into several major parts and analyzed one by one.
(1) Conductive treatment
The electroforming process requires that the surface of the core mold is conductive. As described in Chapter 3, the conductivity of the LOM prototype before electroforming should be achieved by chemical plating, which is uniform and suitable for complex prototypes. Since the LOM prototype is made of layers of paper, it is impossible to perform chemical plating directly on its surface. Therefore, this process route adopts the method of first pouring epoxy resin inversion with it, and then performing chemical plating on the epoxy resin inversion. It is divided into the following steps:
â‘ Grinding and polishing of LOM prototype: Due to the thickness of the molding material, the LOM prototype has a step effect. After the prototype and the waste are peeled off, a rough surface will be left, affecting the surface quality. Therefore, it must be ground and polished before chemical plating to obtain a smooth surface.
â‘¡Pouring epoxy resin reverse mold: Place the LOM prototype in the mold frame and pour the epoxy resin. After the epoxy resin is cured, remove the paper LOM prototype mechanically to obtain the epoxy resin reverse mold. The curing shrinkage of the epoxy resin mixture should be as low as possible, so that the shrinkage distortion of the epoxy resin mold can be strictly controlled to ensure its manufacturing accuracy. Low-viscosity epoxy resin has good fluidity, which can improve the replication accuracy of the resin mold, and is also conducive to the degassing of the resin mixture and improve the density of the resin material. Low-molecular-weight epoxy resin has a lower viscosity. Adding a diluent to the epoxy resin mixture can also reduce its viscosity [72]. After many tests, the specifications in Table 5-1 were selected for epoxy resin pouring.
â‘¢Degreasing: Use the following alkaline degreasing process specifications to remove various oil stains on the surface of the LOM prototype
â‘£Roughening: Use the following process to roughen the surface of the epoxy resin to make it microscopically rough.
⑤Reduction: Further remove the coarsened Cr6+ ions, the formula is as follows
â‘¥Activation: Use an ionic activation solution with good adhesion and stable solution, the formula is as follows
⑦Reduction: Use a hydrated solution with good reduction effect to reduce the specimen according to the following formula:
⑧Chemical copper plating: The optimized process parameters obtained through process tests in Chapter 3 were used for chemical copper plating. Ni2Cl was added to it, which can not only improve the brightness of the coating but also make the coating more firmly bonded to the substrate [33].
Through the above process, an extremely thin layer of smooth and uniform copper is plated on the epoxy resin reverse, achieving surface conductivity.
(2)Pulse electroforming copper
After chemical copper plating, the epoxy resin reverse surface is placed in an acidic copper sulfate solution for pulse electroforming. Pulse electroforming uses a PWG-D150 pulse power supply, whose output waveform is a rectangular wave. There is a short negative pulse after the positive pulse output, which plays a role in removing burrs and leveling the coating, replacing the main function of the double pulse power supply. Using the optimized process parameters obtained from the orthogonal experiment in Chapter 2 as pulse electrical parameters, a copper deposition layer with dense crystals and low internal stress can be obtained. The surface morphology of the epoxy resin reverse is well replicated.
The electroforming anode material uses phosphor copper balls with a phosphorus content of 0.04-0.065% and a diameter of 25mm produced by Nanchang Xinyuan Copper Co., Ltd. The phosphor copper balls are placed in a basket with a mesh and then packed in a nylon bag to prevent the anode mud from entering the electroforming solution.
In the early stage of electroforming, a low current density of 0.2A/dm² is used to avoid burning the extremely thin chemical layer. Then the current density is gradually increased in a gradient manner, and increased to a normal current density of 5A/dm within 30 minutes to obtain a higher deposition rate, which can not only obtain a good replication surface, but also improve production efficiency. The specific electroforming liquid formula and process specifications are:
After electroforming for a period of time, when the average thickness of the shell reaches about 1mm, stop electroforming, take out the electroformed part, and clean and remove the residual electroforming solution. Because the strength of the electroformed layer is not strong enough, it is necessary to arc spray a metal coating on the back to strengthen it. In order to ensure sufficient mechanical strength during spraying, the electroformed part cannot be separated from the prototype at this time.
(3)Arc spraying metal backing
â‘ Determination of arc spraying backing material
Common arc spray wire materials include Al, Zn, and Cu. During EDM, the tool electrode does not contact the workpiece material, and the macroscopic mechanical force between the two is extremely small. The main factor causing electrode deformation is the instantaneous heat generated during pulse discharge. Therefore, the requirements for the arc spraying crotch layer are: good electrical conductivity and thermal conductivity, and firm bonding with the electroformed shell [13]. Therefore, the arc spray wire material should be selected based on indicators such as the electrical conductivity, thermal conductivity, porosity, and bonding strength with the copper electroformed layer of the coating.
A.Electrical conductivity and thermal conductivity
The electrical conductivity and heat transfer performance of the sprayed material are mainly determined by the physical properties of the sprayed material, the porosity of the coating and the content of the coating oxide. Referring to the references 52-54), the performance parameters of the three types of sprayed metal materials can be obtained, as shown in Table 5-2.
As can be seen from the table above, the electrical conductivity and thermal conductivity of copper wire are higher than those of aluminum and zinc wire. Arc spraying these three metal wires on the copper substrate, using the direct measurement method according to GB5935-86, the porosity of these three coatings is measured as shown in Table 5-3：
The greater the porosity, the more severe the oxidation of metal particles during the spraying process, and the worse the electrical conductivity and thermal conductivity. As shown in Table 5-3, the porosity of the Cu coating is greater than that of the other two metals. Due to the high melting point, the coating also oxidizes, but compared with the physical properties of the wire itself, the impact of this porosity and oxide is much smaller. Actual qualitative measurements show that the electrical conductivity and thermal conductivity of the copper coating are much better than those of the aluminum and zinc coatings. Therefore, from the perspective of electrical conductivity and thermal conductivity, copper wire should be selected as the spraying material.
B.Bonding strength
Another requirement for the spray coating is the bonding strength with the copper electroforming layer. As the basis of the electroforming layer, generally only the tensile and shear strengths between the substrate and the coating need to be considered. The tensile strength of the coating is the limit of the coating’s ability to withstand normal tensile stress; the shear strength of the coating is the limit of the coating’s ability to withstand tangential shear stress. These two are important indicators for evaluating the bonding strength of the coating.
During arc spraying, molten metal droplets are sprayed onto the surface of the electroformed shell at high speed through compressed air and rapidly cooled and solidified in a very short time. During the cooling and solidification process, the metal coating tends to shrink, but the shrinkage of the coating is fixed by the surface of the electroformed shell, and the coating will be subjected to large tensile stress [5I]. Moreover, the higher the melting point of the sprayed material, the greater the internal stress generated by the thermal shrinkage of the coating during the deposition process.
The bonding strength of the three metal coatings and the electroformed shell was determined experimentally. The tensile strength of the three coatings and the copper cylindrical substrate was determined by tensile tests according to the national standard GB8642-88; the shear strength of the three coatings and the copper cylindrical substrate was determined by shear tests according to the national standard GB13222-91. The specific process parameters of arc spraying used in the experiment are shown in Table 5-4.
After the copper substrate is sprayed, it is bonded to the counter specimen. After curing, the tensile strength is tested on the CSS-2220 electronic universal testing machine produced by the Changchun Testing Machine Research Institute of the Machinery Industry. Three specimens are sprayed for each type of spray wire, and the average value of the calculated tensile strength is used as the tensile strength of the wire. The specific test results are shown in Table 5-5:
When testing according to the national standard GB13222-91, the test surface 36 corundum is treated with chemical treatment, and then arc sprayed on the cylindrical surface. The coating should be 1mm thicker than the specified thickness, leaving a margin for the final processing to the specified thickness. The specified width of the coating on the cylindrical surface is 10mm. Similarly, for three circular surfaces of each material, the breaking force is tested on the CSS-2220 electronic universal testing machine, and the average value of the shear strength is taken. The specific test results are shown in Table 5-6:
It can be seen from Table 5-5 and Table 5-6 that the strength of the electroplating layer and the substrate is slightly lower than the shear strength of the other two layers, and much higher than the zinc spraying layer. Therefore, considering comprehensively, the bonding strength between the copper spraying layer and the copper electroforming layer is the highest.
From the above analysis of electrical conductivity, thermal conductivity and bonding strength, it can be seen that the best effect can be achieved by using copper spray coating as the backing material of copper electroforming layer. In addition, the materials of copper spray coating and copper electroforming shell are both copper, and the thermal expansion coefficients of the two are closer, so the thermal expansion generated in spraying and EDM is basically the same. Therefore, this experiment uses arc spray copper as the backing method.
â‘¡Sandblasting and arc spraying copper
When arc spraying, the roughening effect of the substrate surface directly affects the quality of the spraying, especially the bonding strength between the sprayed layer and the substrate. The roughening methods used in actual production applications include sandblasting, arc roughening, N/A1 bottom spraying and grinding wheel grinding. According to the results of previous studies [74], it can be seen that the coating and the substrate are most firmly bonded by sandblasting.
After determining the spraying material, the sandblasting method is used to roughen the copper electroforming layer first, so that the surface to be sprayed has a certain roughness. The experiment uses a CMD-SHP spray-suction recycling sandblasting machine, and 36 corundum sand is selected as the sandblasting abrasive. The air pressure is 0.6~0.7MPa. The electroforming parts should be cleaned before and after sandblasting. So as not to affect the effect of sandblasting and arc spraying.
After the surface roughening treatment, the electroformed parts are immediately arc sprayed. The experiment uses CMD-AS 1620 electrospraying equipment. The gas from the air compressor is processed by a refrigerated dryer to ensure that the compressed air used for sandblasting or spraying is oil-free and water-free. The specific arc spraying copper process parameters are shown in Table 5-4. The thickness of the copper spray layer should depend on the size and shape of the specific parts.
It can be seen from the literature [73] that the increase in coating thickness will reduce the bonding strength between the coating and the substrate surface, and is not conducive to the heat dissipation of the electrode during electrospark machining; during electrospark machining, the electrode will not be affected by mechanical force and does not require too high mechanical strength. Considering the above two factors, the thickness of the copper coating used for mounting reinforcement is suitable to be 2~3mm.
After spraying to a predetermined thickness, the coating is stopped and machined and trimmed. Then, the copper shell with backing is heated in water to separate from the epoxy resin by reverse molding, thereby obtaining an EDM electrode that can be used to process the cavity of an injection mold.
(4) Surface morphology and metallographic analysis of EDM electrodes
In the specific experiment, a 3D CAD solid model with a “heart-shaped” surface was first designed using Pro/e modeling software. Two identical solids were combined into one part during LOM processing. Then, the parts were processed by pouring epoxy resin, chemical copper plating, electroforming copper, arc spraying copper, etc. Finally, the copper shell and epoxy resin were separated to obtain the EDM electrode. The stage products obtained in the entire process are shown in Figure 5-2.
Figure 5-3 shows the polished electric spark electrode. From the cross-section of its local area, the bonding between the electrode electroforming layer and the arc sprayed backing layer can be clearly observed.
The surface roughness R of the polished LOM prototype, epoxy resin reverse, electroless copper conductive layer, and unpolished electrode after electroforming were measured, and the measurement results were: 0.509um, 0.521pm, 0.704um, and 0.726um. The measurement results show that the electroless copper deposition layer increases the surface roughness of the epoxy resin reverse, while the surface roughness of the LOM prototype and epoxy resin reverse, the electroformed electrode and the electroless copper conductive layer does not change much, which shows that the electroforming process does have a high replication accuracy.
Therefore, the surface quality of the electroformed electrode mainly depends on the surface quality of the LOM prototype and the electroless copper conductive layer. In the experiment, the electroformed electrode was subjected to general polishing treatment, and it was found that its surface roughness R could be reduced to 0.138um.
The metallographic structures of the electroformed layer and the sprayed layer of the electrode were analyzed, and the scanning electron microscope photos are shown in Figure 5-4. As can be seen from the figure, the electroformed layer is composed of a large number of fine grains and a small number of clustered coarse grains, with obvious interfaces between the grains, a relatively dense organizational structure, and obvious characteristics of grain growth; while the arc sprayed layer is composed of flat spray particles piled up and superimposed, with a loose organizational structure and a relatively large porosity.
The chemical elements of the electroformed layer and arc sprayed layer of the obtained EDM electrode were further analyzed. The arc sprayed copper coating, electrode electroformed layer and profile copper were subjected to energy spectrum analysis (i.e., micro-area phase analysis) using an ISIS-300 energy spectrum dispersion instrument. The phase analysis results are shown in Figures 5-5 to 5-7, respectively.
From the results of Figures 5-5 to 5-7, it can be seen that the phase compositions of arc sprayed copper, electroformed copper, and profile copper used for processing conventional EDM electrodes are: arc sprayed copper, electroformed copper, and profile copper.
It can be seen that the copper deposit obtained by pulse electroforming has the same physical composition as the copper profile used to process the copper electrode, and the content of each element is also relatively close: From the scanning electron microscope photo shown in Figure 5-4, it can be seen that the copper electroforming layer is mainly composed of a large number of fine grains and has a relatively dense structure. Therefore, from the metallographic structure and chemical composition of the electroformed electrode, it can be used as an electrode for EDM.
As to whether the copper electrodes obtained by pulse electroforming can be used in production practice, it is necessary to further verify their electromachining performance through electrospark machining tests.
(5) Electroforming performance test of electroformed electrodes
In order to investigate the electroforming performance of the obtained electroformed electrodes, electrode processing tests were conducted on a Japanese Sodick A50R CNC pulse EDM forming machine, and Japanese imported mold steel NAK80 was selected as the workpiece material to be processed. During pulse EDM, the peak current and the pulse width (on) and pulse interval (f) used to control the EDM discharge time jointly affect the surface roughness of the mold cavity and the loss of the tool electrode.
Therefore, the test selected the three electrical parameters of pulse EDM, pulse width (on), pulse interval (o) and peak current (P), as the main process parameters. In this test, the value ranges of these three parameters and their codes on the EDM machine are shown in Table 5-7.
The test takes the surface roughness of the workpiece’s processed cavity as the evaluation index, analyzes the processing performance of the electroforming spark electrode, designs the test plan shown in Table 5-8, uses the Surfcorder Model SE-3H surface wheel tester to test the surface roughness, and fills the test results in Table 5-8.
The experiment uses negative polarity machining, and uses the same electrode to machine 12 cavities according to 12 electrical machining standards. The depth of the machined cavity is set to 0.8mm. The test is carried out according to the test number in Table 5-8, and the 12 cavities after machining are marked with sequence numbers, as shown in Figure 5-8.
According to the test results, the following analysis is made:
â‘ The thickness of the electroformed layer of the electrode used in the experiment is about 1mm, and there is no electrode damage during the EDM process. However, in previous experiments, when the electrode with an electroformed layer thickness of 0.45mm was used for processing, the electrode was damaged. This shows that the thickness of the electroformed layer should be more than 1mm to meet the requirements of EDM.
â‘¡When the cavity processing depth is constant, the processing time is related to the pulse width, pulse interval and peak current, and mainly depends on the peak current. The larger the peak current IP and the longer the pulse width on, the higher the processing efficiency and the shorter the processing time.
â‘¢Surface roughness Ra is related to pulse width, pulse interval and peak current. As the peak current increases, the surface roughness increases; when the peak current is constant, the surface roughness increases with the increase of on and of values. Therefore, the surface roughness of the cavity under low current short pulse is significantly less than that under high current long pulse. Since the electrical machining standard selected in this test is not the optimal parameter setting of the machine tool, but the rough machining standard corresponding to the high loss rate, the surface roughness of the processed cavity is generally high.
The electroformed electrode with a surface roughness R of 0.138 μm after grinding as shown in Figure 5-3 (a) is processed according to the optimal rough and fine electromachining standards of the machine tool. The processed electrode and the processed cavity are shown in Figure 5-9. The cavity depth is 10 mm, and the measured surface roughness Ra is 1.94 μm.
3.Experimental study on the process route of sealing special coatings
The second process route proposed above: LOM process prototype (part reverse) – post-processing – spray special coating sealing treatment – chemical plating – electroforming copper – sandblasting roughening treatment – arc spraying metal backing – separation from the prototype – electric spark copper electrode (positive type).
Most of the problems have been solved in the research on process route one. It can be seen that the key problem that needs to be solved is how to perform chemical plating directly on the LOM prototype; in addition, in the research on process route one, the change in surface roughness between the electrode and the LOM prototype was emphasized, and its dimensional accuracy was not specifically investigated. Therefore, this part mainly studies these two aspects.
(1) Conductive surface of LOM prototype
As mentioned above, chemical plating should be used as the conductive surface method of LOM prototype. LOM prototype is made of layers of paper glued together and superimposed. Its chemical plating has the following problems: Due to the thickness of the paper itself (about 0.1mm) and the principle of layered processing, the molded part will inevitably appear “step-shaped”, especially on the surface with a large curvature, which affects the accuracy of the part; LOM prototype made of thin paper stacking, in the process of chemical plating and electroforming,Long-term immersion in chemical liquids will cause water seepage and deformation or even debonding, thus destroying the prototype: There will be a certain temperature during ordinary chemical plating and its pre-treatment process, which may cause the LOM prototype to expand and deform due to heat.
In view of the above problems, when implementing chemical plating of LOM prototype, the LOM prototype should be polished first to obtain a smooth surface; secondly, a good sealing method should be found to obtain a thin film on the surface of LOM prototype that can prevent liquid from penetrating and can be plated with metal; finally, a process method for low-temperature chemical plating and its pretreatment should be found. After a lot of experiments and analysis, the following process flow was found to implement chemical plating on LOM prototype, which solved the problem of surface conductivity of LOM prototype.
â‘ Polishing
After grinding, the step effect of the prototype is eliminated and the surface quality of the prototype is improved. Due to the special properties of paper fibers, it is difficult to grind the LOM prototype to a smooth surface by conventional methods, especially the rough surface left when the prototype and waste are peeled off. Therefore, this test adopts a two-time grinding method: first, the prototype is polished with sandpaper to remove the part with obvious step effect: then a certain mass ratio of ABS and PVA particles are dissolved in a solution mixed with a certain volume ratio of dimethylformane, chloroform and acetone, and the solution is sprayed on the LOM prototype with a spray gun.
After the solvent evaporates, the prototype is polished with fine sandpaper. In this way, because this film can fill the gaps between paper fibers well, the rough surface that is difficult to polish during the first grinding (especially the flat part on the prototype) can be removed during grinding. After actual measurement, the surface roughness Ra of the polished paper prototype can reach 0509um.
â‘¡Closed processing
This process is the key to the entire process. Obtaining a film that is reliably sealed and of uniform thickness is the prerequisite for the smooth progress of subsequent processes. After the polishing of the previous process, the surface of the prototype becomes smooth, so a film can be coated on its surface to seal the prototype.
The sealing method used in the experiment is: spray the paint prepared in the previous process onto the polished LOM prototype through a spray gun, and a uniform ABS film can be obtained after the solvent evaporates. When preparing the solution, the concentration should be controlled not to be too high, so as to reduce the generation of bubbles and ensure that the prototype will not lose dimensional accuracy due to the coating being too thick. The thickness of the film can generally be controlled between 30 and 50 um.
â‘¢Degreasing
Before the coating is prepared in the first two processes, the ABS and PVA particles are chemically degreased, so the film itself is basically free of oil stains. After the sealing treatment, degreasing with organic solvents can reliably remove various oil stains on the surface of the film and avoid the high temperature of chemical degreasing. When using organic solvents for degreasing, be careful not to use solvents that can dissolve ABS film.
â‘£Roughening
Use the general chemical roughening method, but pay attention that the temperature should not exceed 40C, and the roughening time should not be too long, so as not to cause the expansion and damage of the workpiece surface film and the heat deformation of the workpiece itself. The specific parameters are as follows. During the roughening process, the workpiece should be turned over continuously to ensure uniform roughening. Rinse with tap water after treatment.
⑤Roughening followed by reduction, activation, activation followed by reduction
Because these steps are all carried out at room temperature, they can be carried out in sequence using the same process specifications as the previous resin inversion process route.
â‘¥ Chemical copper plating
The same chemical copper plating solution as the previous process route is used, but the plating temperature is changed to room temperature. Although the plating speed is reduced, the dimensional accuracy is affected because the conductive layer is too thick. Therefore, at this plating speed, a uniform conductive layer can still be obtained within 30 minutes.
⑦Examples
A prototype as shown in Figure 5-10(a) was processed using the LOM process. After the above treatment, a smooth copper plating layer as shown in Figure 5-10(b) was obtained on its surface.
(2)Electroforming, roughening and arc spraying copper backing
The specific process methods of electroforming, sandblasting and arc spraying copper used in this process route are exactly the same as those in the previous process route. The LOM prototype and the final electrode can be separated by soaking in water at room temperature, then peeling off the paper stacked in the LOM prototype layer by layer, and finally cleaning the electrode surface with an organic solvent. Therefore, the electroformed electrode will not produce any thermal deformation or mechanical deformation. After separating the backing copper shell from the LOM prototype, the EDM electrode for processing the injection mold cavity can be obtained.
(3)Examples of dimensional accuracy
In order to investigate the dimensional accuracy of EDM electrodes manufactured quickly using LOM prototypes, an “E”-shaped CAD model with three raised English letters “DUT” was designed, as shown in Figure 5-11(a). The prototype produced by LOM molding process is shown in Figure 5-11b). Several key dimensions of the LOM prototype and the resulting electrode are investigated and analyzed as shown in Figure 5-11(c).
The prototype shown in Figure 5-11(b) processed by the LOM process has a copper conductive layer evenly deposited on its surface after the pretreatment mentioned above, and then undergoes electroforming, sandblasting, arc spraying, and finally the copper shell is separated from the prototype to obtain the EDM electrode shown in Figure 5-12(a). This electrode is used to process the workpiece on the EDM forming machine, and the resulting processing cavity is shown in Figure 5-12(b):
The polished LOM prototype and electroformed spark electrode were measured according to the dimensions shown in Figure 5-11(c). The results are shown in Table 5-9.
By analyzing the data in Table 5-9, we can draw the following conclusions:
â‘ As can be seen from the table, for the LOM prototype, the size of the entity part (E) is smaller than the nominal size of the CAD model, while the size of the non-entity part (A, B, C, D) is larger than its nominal size. The reason is that the LOM prototype needs to be polished due to the “step effect”, so that its entity part is polished and the actual size is smaller than the nominal size of the CAD model. The size of the non-entity part is just the opposite, and it is increased by polishing.
â‘¡The measured dimensions (A, B, C, D) of the physical parts of the electroformed copper electrode are smaller than the nominal dimensions, and have not increased due to the increase in the dimensions of the corresponding parts of the LOM prototype; the measured dimensions (E) of the non-physical parts are smaller than the nominal dimensions, and have not decreased due to the decrease in the dimensions of the corresponding parts of the LOM prototype, and the measured dimensions of the copper
electrode are closer to the nominal dimensions of the CAD model.
There are two reasons for this: When the LOM prototype is sealed, a thin film needs to be applied to the surface of the prototype, and the thickness of this film is generally controlled within 30~50um, which offsets the effect of grinding and polishing on the size of the prototype to a certain extent: During the chemical plating and electroforming process, although the LOM prototype is sealed by a thin film, it may cause its micro-expansion due to being immersed in the solution for a long time, which makes the corresponding dimensions of the electroformed part slightly increase or decrease, which also offsets the effect of grinding and polishing on the size of the prototype to a certain extent.
â‘¢From the experimental data, it can be seen that the absolute deviation of each dimension of the electroformed copper electrode from the corresponding nominal dimension is no less than 0.06mm, and the deviation percentage is no more than 0.15%. This shows that the electrode maintains good dimensional accuracy compared with the CAD model.
④The surface roughness R of the polished prototype, the sealed prototype, the chemical copper conductive layer, and the unpolished electroformed copper electrode were measured, and the actual measured values ​​were: 2.67um, 0.512um, 1.72um, and 1.76um, respectively. The R of the sealed prototype is lower than that of the LOM paper prototype.
This is because the surface quality has been improved by spraying special coatings: the R value of the chemical copper conductive layer is larger than that of the sealed prototype. This is because the deposition of copper during chemical copper plating is related to the distribution of catalytic active centers adsorbed on the surface of the closed film after activation treatment. If the distribution of catalytic active centers is not uniform and dense enough, it will lead to uneven deposition of copper and increase the roughness: the electroformed copper electrode well replicates the surface quality of the chemical copper layer, which illustrates the high replication performance of the electroforming process.
4.Comparison of two process routes
The above research proves that the above two process routes can combine the rapidity of LOM process and the high reproducibility of electroforming process to achieve rapid manufacturing of EDM electrodes. In practice, it is found that the two process routes have their own advantages and disadvantages, specifically in the following aspects:
(1) Compared with the process of remaking resin mold, the process of sealing special coating eliminates the process of pouring resin mold, thus reducing working hours and costs;
(2) Both can quickly manufacture EDM electrodes to process mold cavities, but the process of sealing special coating is to electroform on the sealed LOM paper prototype, so generally only copper can be electroformed at room temperature to obtain electrodes; the remade resin mold can withstand much higher temperatures, so nickel and its alloys can be electroformed at a temperature above 45°C on the resin surface after chemical plating to directly obtain a high-strength mold cavity without the need to use electrodes to process the mold cavity.
(3) When remaking resin mold, bubbles may be generated in the resin mold due to poor casting process and shrinkage and distortion during solidification, which cannot reproduce the surface of the LOM prototype well. When sealing special coating, the LOM prototype size may change significantly due to poor control of the thickness of the sealing film. However, if the accuracy of the resin mold is guaranteed under vacuum casting conditions, the cost will also be increased. Relatively speaking, the process of sealing coating is more difficult to master, but the cost is lower.
Summary of this chapter
This chapter studies the process of combining LOM rapid prototyping with chemical plating, electroforming, arc spraying and other processes to achieve rapid manufacturing of EDM electrodes without casting process. According to the characteristics of LOM prototype, the possible process routes are compared and analyzed, and two process routes for rapid manufacturing of EDM electrodes are determined:
(1) Route for making resin reverse mold
LOM prototype (zero positive post-processing resin reverse mold-resin reverse mold chemical copper plating-electroforming copper-sandblasting roughening treatment-arc spraying metal backing-copper shell and resin reverse mold separation-electrodischarge copper electrode (positive mold)
For this process route, combined with the research in the previous chapters, the optimized process parameters are used for chemical copper plating and pulse electroforming copper to obtain a densely crystalline copper casting layer: arc spraying is performed with copper wire to obtain a backing with good electrical and thermal conductivity and high bonding strength with the copper electroforming shell.
Finally, through the example of rapid manufacturing of “heart-shaped” electrodes with complex surfaces, it is proved that the thickness of the electroforming shell is generally above 1mm to meet the wall thickness requirements of the EDM electrode, and the copper spray layer can meet the requirements of electrode mounting reinforcement when it reaches 2~3mm thick. After electromachining experiments, it is confirmed that the electrode manufactured using this technology can achieve the electromachining performance of conventional machined copper electrodes, thereby shortening the manufacturing time of EDM electrodes with complex surfaces and effectively reducing the manufacturing cost of molds.
(2) Process route for sealing special coating
LOM prototype (part negative type) post-processing spraying special coating sealing treatment – chemical plating – electroforming copper – sandblasting roughening treatment – arc spraying metal backing – copper shell and prototype separation – electric spark copper electrode (positive type)
For this process route, two key problems are solved: @ sealing the LOM paper prototype by spraying special coating; @ adopting the whole process of low-temperature chemical copper plating to make the LOM paper prototype conductive. Through the example of rapid manufacturing of “E”-shaped electromachining electrodes, it is verified that this process method has the characteristics of fewer processes, high dimensional accuracy and low cost, and is particularly suitable for the rapid manufacturing of molds for the production of medium and small parts.