Indirect Molding Method
The direct molding method can use different materials in different parts according to requirements. However, the direct molding method is limited by the process itself, and the manufactured mold cannot meet the requirements of high-precision metal molds in terms of surface and dimensional accuracy, size, and shape freedom. The competitive rapid molding method is mainly an indirect molding method that combines rapid prototyping with traditional molding processes such as casting, spraying, electroplating, and powder molding. There are several methods for indirect molding.
1.Epoxy resin mold
The production process is similar to the silicone mold production process, except that the silica gel is replaced with an epoxy resin doped with aluminum powder. The entire mold needs to be cast in two parts because the epoxy resin cannot be cut and cut like a silicone. The epoxy resin is accompanied by a small amount of shrinkage during the curing process, so the master mold is often damaged. The thermal conductivity of epoxy resin is extremely poor, and the heat generated by injection molding using a mold made of pure epoxy resin is difficult to dissipate. The solution is to make the surface of the mold of epoxy resin, and the back is filled with a material with good thermal conductivity. The mold thus produced has a good compressive strength and can be used for pressure forming like injection molding, and the abrasive material can also be injection molded, and the service life is several thousand pieces.
2. Silicone mold
Silicone molds are used in a wide range of applications. First, the “positive” master mold of the same piece to be molded is produced by the RP method, and then the silicone rubber is filled around the master mold. After curing, the silicone rubber was cut along the desired parting surface, and the mother mold was taken out to prepare a silicone mold.
In order to ensure the quality of the silicone mold, the surface of the master mold is polished, because all the features of the surface of the master mold are almost copied to the surface of the mold and then copied to the parts. Silicone molds are extremely fast and can be used to cast a variety of thermoset plastics. The molded parts have the advantages of good precision and very low price, but the silicone mold cannot produce parts with high precision and has a short life span, and usually can only be cast 25 to 30 times.
3. Surface spray metal mold
A layer of atomized molten metal is sprayed on the outer mold made by RP technology, and the metal surface is formed after the liquid metal is solidified. The spraying equipment and the master mold are limited by temperature. Generally, the metal material used is a low melting point metal such as lead-tin alloy, zinc alloy and nickel. The common spraying method is arc spraying. If the master can withstand high temperatures, it can also be sprayed with high melting point metals such as stainless steel.
The life of such a mold for injection molding can reach about 2,500 pieces, and it is impossible to use a larger clamping force than a pure metal mold. The research results show that if nickel is sprayed on the surface of the master mold, the service life of the mold can be further improved. Because of its high surface hardness and wear resistance, the corrosion resistance is good, and the injection molded parts are easy to take out after the mold is opened. Such a mold can be completed in less than two days. It is worth noting that the temperature of the use of such molds cannot exceed 300 °C.
4. Nickel and ceramic mixture mold manufacturing technology
This method uses plastic RP and model as the mother type, and a nickel metal is plated on the mother plate by electroplating to produce a thin nickel metal shell. The surface of the thin shell and the female mold completely reflects the injection molding. The surface shape and dimensional characteristics of the part.
Due to the low strength of the thin shell, the non-molded surface of the thin shell is filled with high-strength ceramic powder, and the ceramic material is required to have a small shrinkage coefficient and suitable thermophysical properties. This composite mold is very suitable for manufacturing larger parts (greater than 250mm × 250mm × 250mm). If the mother type is made by the stereoscopic light modeling (SL) method, the dimensional accuracy of this method will not be lower than the accuracy of the SL part. It has a life of at least 5,000 pieces during plastic injection molding.
5. 3D Keltool mould
Keltool is currently considered to be the most promising method for the rapid manufacture of metal molds. It first uses the RP master model to produce a silica mold with higher precision, and then injects fine powdered A6 tool steel (or stainless steel) into the silica gel mold, and a mixture of finer tungsten carbide particles (note that the size of the powder particles is controlled). A very important factor in the final quality of the mold). An epoxy resin binder is then added to the mixture to form a green powder in the silica gel mold.
After the mold is opened, the green member is heated in the furnace to volatilize the binder, and at the same time, the metal powder mixture is sintered. At this time, there is still 30% of the voids in the mold, so the final copper infiltration is required, and finally it is available. For mass production, up to a million pieces of hard mold. Its dimensional accuracy is ±0.04mm per 250mm error. The downside is that the largest mold size that can be made by this method is currently only 127mm. Generally, this method can complete the mold cavity or core production in more than ten days, saving 25% to 40% of the cost compared with the traditional CNC machine tool processing.
6. Using the RP prototype to make electrodes for EDM
The EDM method is widely used in the field of mold making. It can be used to machine extremely complex shapes and cores. It can process extremely hard materials that cannot be machined by CNC machines. It can also process heat-treated materials to avoid Thermal deformation caused by heat treatment after processing. The quality of the EDM electrode is a key factor affecting the quality of the workpiece. The cost of the electrode itself accounts for 50% to 80% of the cost of the EDM process. At present, graphite or copper electrodes are often used, and the service life is extremely short. Sometimes, it is necessary to replace a plurality of electrodes for processing one cavity, which seriously affects the processing efficiency of EDM. The RP method can be used to quickly manufacture EDM electrodes of any shape, which can make up for this deficiency.
(1)Graphite electrode forming method
The RP prototype was used to reproduce the graphite electrode lap, and the graphite electrode was fabricated by the translational grinding method using the lap, and then the metal mold was processed by electric discharge.
(2) Electroformed copper electrode method
It is sprayed with a layer of conductive medium on the surface of the RP, and then electroformed to deposit a layer of copper on the surface of the RP and the prototype to obtain an electroplated copper electrode, and then use an electrode EDM mold. The advantage of the electrode forming method is that as the complexity of the mold cavity increases, the batch size increases, and the superiority thereof can be reflected.