Indirect Rapid Tooling Methods in Mold Manufacturing: Processes, Advantages, and Applications

Direct rapid tooling can use different materials in different areas of a mold according to actual requirements. However, direct molding methods are still limited by the process itself. In many cases, the finished mold cannot fully meet the requirements of high-precision metal molds in terms of surface finish, dimensional accuracy, size range, and shape freedom.

For this reason, the more competitive approach in rapid tooling is often the indirect molding method, which combines rapid prototyping with traditional manufacturing technologies such as casting, spraying, electroplating, and powder molding. Several common indirect rapid tooling methods are described below.

1. Epoxy resin mold
The manufacturing process of an epoxy resin mold is similar to that of a silicone mold, except that the silicone material is replaced with epoxy resin filled with aluminum powder. Because epoxy resin cannot be cut easily like silicone rubber, the mold usually needs to be cast in two separate halves. During curing, epoxy resin undergoes slight shrinkage, which may damage the master pattern in some cases.

Epoxy resin also has very poor thermal conductivity, so heat generated during injection molding is difficult to dissipate if the entire mold is made of pure epoxy. A common solution is to use epoxy resin only on the mold surface, while filling the back side with a material that has better thermal conductivity. Molds made in this way can achieve good compressive strength, can be used for pressure forming such as injection molding, and can also process abrasive materials. Their service life can reach several thousand cycles.

2. Silicone mold
Silicone molds are widely used in prototype and small-batch production. First, an RP master pattern is produced, and then silicone rubber is poured around the master. After curing, the silicone mold is cut along the planned parting surface and the master pattern is removed.

To ensure mold quality, the surface of the master pattern should be polished carefully, since almost all surface features of the master are reproduced on the silicone mold and then transferred to the molded parts. Silicone molds can be produced very quickly and are suitable for casting a variety of thermoset plastics. The molded parts usually have good dimensional quality and low production cost. However, silicone molds have limited life and are generally suitable for only about 25 to 30 castings.

3. Surface-sprayed metal mold
In this method, a layer of atomized molten metal is sprayed onto the outer mold made by rapid prototyping, and a metal surface is formed after solidification. Because both the spraying equipment and the RP master pattern are limited by temperature, low-melting-point metals such as lead-tin alloys, zinc alloys, and nickel are commonly used. Arc spraying is one of the common spraying methods. If the master pattern can tolerate higher temperatures, higher-melting-point metals such as stainless steel may also be sprayed.

The service life of this type of mold in injection molding can reach about 2,500 shots, although the allowable clamping force is lower than that of a fully metal mold. Research has shown that spraying nickel onto the surface can further improve mold life because nickel offers higher surface hardness, better wear resistance, better corrosion resistance, and easier part release after molding. These molds can often be completed in less than two days, but their working temperature generally should not exceed 300°C.

4. Nickel shell and ceramic composite mold
This method uses a plastic RP pattern as the master, and a thin nickel shell is formed on the master surface by electroplating. The nickel shell reproduces the surface shape and dimensions of the injection molded part very accurately.

Because the thin nickel shell itself has limited strength, the non-molding side is backed with high-strength ceramic material. The ceramic filler should have low shrinkage and suitable thermal and mechanical properties. This composite mold structure is especially suitable for producing larger parts, typically larger than 250 mm × 250 mm × 250 mm. If the master pattern is made by stereolithography, the dimensional accuracy of the final mold can be comparable to that of the original SL part. In plastic injection molding, this type of mold can often achieve a service life of at least 5,000 shots.

5. 3D Keltool mold
Keltool is widely considered one of the most promising methods for the rapid production of metal molds. The process begins by using an RP master model to create a high-precision silicone mold. Fine A6 tool steel powder or stainless steel powder is then combined with finer tungsten carbide particles. An epoxy binder is added to form a green compact inside the silicone mold.

After demolding, the green part is heated in a furnace to remove the binder and sinter the metal powder. Because the sintered structure still contains about 30 percent porosity, a final copper infiltration process is used to densify the mold. The finished mold can then be used for mass production, and in some cases the service life can reach up to one million parts.

The dimensional accuracy of the Keltool process can reach about ±0.04 mm per 250 mm. One limitation is that the largest mold size currently achievable by this method is about 127 mm. Even so, Keltool can often complete mold cavity or core production in a little over ten days and may reduce cost by approximately 25 to 40 percent compared with traditional CNC machining.

6. Using RP prototypes to produce EDM electrodes
Electrical discharge machining is widely used in mold manufacturing for machining very complex cavities and cores. It is especially useful for hard materials that are difficult to machine by conventional CNC methods and for heat-treated materials where thermal distortion must be avoided.

The quality of the EDM electrode has a major influence on the quality of the finished cavity. In many EDM processes, the electrode itself accounts for 50 to 80 percent of the total machining cost. Traditional graphite or copper electrodes often have short service life, and several electrodes may be required to machine a single cavity. Rapid prototyping can help solve this problem by producing EDM electrodes of almost any shape more quickly.

(1) Graphite electrode forming method
An RP prototype is used to produce a graphite electrode master, and the graphite electrode is then fabricated by a transfer grinding process before being used for EDM machining of the metal mold.

(2) Electroformed copper electrode method
A conductive layer is first sprayed onto the surface of the RP prototype. Copper is then electroformed onto the prototype surface to create a copper electrode shell. This copper electrode is subsequently used in EDM mold machining. As mold cavity complexity increases and batch size grows, the advantages of this method become more obvious.

Overall, indirect rapid tooling provides a practical bridge between rapid prototyping and traditional mold manufacturing. It offers flexible solutions for prototype tooling, small- and medium-batch production, and complex mold development where speed and cost reduction are critical.