Why Rapid Tooling Process is Great for Small Batch Production



In the past 10 years, the manufacturing market environment has undergone tremendous changes, and the rapid introduction of products to the market has become an important guarantee for manufacturers to seize market opportunities. Rapid Prototyping and Manufacturing (RP&M) technology is to shorten the development cycle of new products and introduce new manufacturing technologies that are marketable and multi-variety and small-volume products manufactured according to customers' intentions.
RP&M technology is one of the most significant advances in manufacturing technology in the second half of the 20th century. With the advent of the era of multi-variety small batch production, enterprises require molds to ensure that new products quickly occupy the market, and the development of fast and economical molds has received more and more attention. According to the statistics of the US automobile manufacturing industry, an average of 67% of planned investments are related to molds. Therefore, the rapid development of products will become the key to winning the 21st century manufacturing market.

At present, the rapid injection molding that are popular in application are:

1. Silicon rubber mold for vacuum injection molding.
2. Aluminum alloy for reaction injection molding, ABS mold or epoxy resin mold.
3. Aluminum mold or simple steel mold for injection molding
This article focuses on the silicon rubber molds that are best suited for small batch production in these processes.

Silicon mold features:
Due to the flexibility and elasticity of the silicon mold, it is possible to produce complex structures, fine pattern, no draft angles. Even the invert draft angle and the parts with the deep groove type (the resin piece).
Moreover, the material has good cutting performance, can be easily cut with a sheet and the cut surface is very smooth to each other. Therefore, when you use it to copy the mold, you needn't first divide the upper and lower molds. After the soft mold is integrally cast, it is cut along the predetermined parting surface, and the master mold is taken out to obtain the upper and lower soft molds. And basically does not affect the dimensional accuracy. It has the advantages of extremely short manufacturing cycle and high quality.
Since the prototype material produced by RP technology does not react with silicon rubber, there is no deformation and no wrinkles in the manufacturing process, and the same surface quality as precision plastic parts. The specific process of manufacturing silicon soft mold by using RP prototype is as follows:

Prepare the master → mold frame production → fixed the master → silicon stirring and vacuuming → mold vacuuming → mold curing → open the mold → completion.
1. Mandatory tools:
2mm plastic plate, hot melt glue gun and rubber strip, release agent, tape, steel ruler, utility knife, electronic scale, calculator, ABS rod (length > 50mm,diameter 5mm), material cup, special open mold clamp and scalpels.
2. Preparation steps for the master:
Usually the master is made by CNC or 3D printing technology.
(1) Use the blade to remove the burrs and supports of the master, taking care not to damage the surface and details of the sample.
(2) Surface treatment of the sample: If it is a machined part, sand the sample with sandpaper until the surface is not visible to the naked eye.
(3) According to the drawings, the master is trimmed to ensure the dimensional accuracy.
(4) Spray the release agent evenly on the surface of the master, and ensure that all surfaces have release agents, especially deep holes or narrow grooves.
(5) Weigh the sample weight with an electronic scale and record.
(6) Determine the parting line and mark it with a mark.
3. The production of the mold frame and the placement of the master
(1)According to the geometrical dimensions of the parts, the unilateral increase of 20 ~ 40mm principle is good.
(2)The mold frame material is ABS sheet (or other surface with a smooth surface thickness of more than 2 mm is appropriate).
(3) Paste the mold frame with a hot melt glue gun.
(4) Place the part in the middle of the mold frame and fix the prototype in the mold frame by means of sprue, exhaust support, and direct placement.
4. Silicon stirring and vacuuming
Place the silicon-filled cup and mold frame into the vacuum injection molding machine.

5. Mold vacuuming
Vacuum the cavity in a vacuum injection molding machine and wait for the silicon and curing agent to stir well and then automatically pour into the lower mold frame.

6. Mold curing
The mold frame is placed in a curing box for curing, and after the silica gel is completely cured, the mold frame is removed.

7. Open the mold
(1) Start the determined part parting line and find the parting surface of the mold.
(2) According to the corresponding parting surface of the mold, first use the marker to draw the wave parting line on the surface of the mold, and then use the scalpel to separate the mold along the direction of the marker to ensure that the part is free from external force or less stress. In the case of smooth demoulding.
The problems and solutions of silicon mold production:

1. In the process of making silicon molds, mold curing is often incomplete. The main reason is that the ratio of silica gel is not correct, and the curing time after casting is different.
Solution：silicon rubber material ratio and curing time for reference: The ratio of silicon rubber to curing agent is 1:0.1. The curing time after the casting is completed is 12 to 16 hours at 40 ° C and 4 hours at 70 ° C.
2.The outer surface of the silicon mold is uneven, and the silicon mold has pores inside.
Solution：Mainly because the time of vacuuming after the silica gel and the curing agent are stirred is not accurate. It should be stirred for 10 minutes and then poured into the mold frame. When there are many bubbles, the vacuum pump should be turned off, air should be added to eliminate the bubbles, and then vacuum is applied for about 1 minute.
3.The phenomenon of flashing often occurs when the silicon mold is finished.
Solution: This is caused by insufficient sealing of the cutting surface during mold opening. Therefore, when using the open mold tongs and the scalpel to open the mold, the row cutter should be a straight line with the tip of the knife, and the curve of the knife tail will make the cutting surface become a mountain-like curved surface to achieve sealing. Role, but be careful not to damage the fine structure on the mold.
In actual operation, the quality of silicon mold production is also closely related to the quality of the material, the environment and temperature of the production.
Conclusion:
Rapid tooling technology has been widely used in the automotive, home appliance, aviation, medical and other industries for more than ten years. Mastering vacuum casting technology is of great significance to prototype manufacturers.

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