Vacuum casting compare with injection molding, which is suitable for your plastic part project?

Vacuum casting compare with injection molding, which is suitable for your plastic part project?

There are more than ten kinds of plastic parts molding processes, but the more common processes in various new project development and parts mass production are vacuum casting and injection molding. The common point of both processes is to inject raw materials into molds to manufacture the parts. Both processes have unique advantages and disadvantages, but how to choose the most suitable process which depends on your part needs and various other factors.

What is vacuum casting?

Vacuum casting is called polyurethane casting sometimes. The polyurethane resin is degassed, stirred, preheated, injected under vacuum conditions, and then cured for 2-3 hours in a thermostat at 60°C-80°C. Vacuum casting is suitable for structural and functional prototypes in the early stage of product development, design & functional verification, new product testing, early market research and small batch production. Vacuum casting greatly saves the cost of mold building and shortens the product development cycle, and is also widely used in various fields, such as consumer electronics, medical, automobile manufacturing, aerospace and so on.

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Workflow of vacuum casting:

1: Use CNC machining or 3D printing to manufacture a 1:1 master part, then fix the master part in a sealed box, pour liquid silicone material, and then bake and cure. After the silicone is solidified, cut it in half and take out the master part. The remaining cavity is the silicone mold, and the next step is to copy the parts.

2: Preheat the silicone mold. Preheat the silicone mold to 60-70°C. If the mold temperature is too low, the curing will be incomplete, resulting in poor physical properties. The mold temperature has a direct relationship to the dimensional accuracy of the copy parts.

3: Polyurethane resin pre-degassed. The two liquids A and B were respectively subjected to vacuum defoaming treatment in a vacuum box for more than 30 minutes.

4: Injection. Place the containers which containing liquid A and liquid B in the work room respectively. After the working room is evacuated, stir liquid B from time to time to make it defoamed for 5-10 minutes, pour liquid A into liquid B, stir for 30-40 minutes, and quickly pour it into the silicone rubber mold. And the operation of restoring atmospheric pressure is carried out within 1 minute to 1 minute and 30 seconds from the start of mixing.

5: Cured. After curing for 30-60 minutes in an incubator at 60-70°C, it can be demolded. If necessary, carry out secondary curing for 2-3 hours in an incubator at 70-80°C.

vacuum casting

The advantage of vacuum casting

  • Low cost of molds, greatly shortened production cycle, is one of thecost-effective production processes for small batches.

  • Can copylarge parts in small quantities, as well as parts with more complex structures and uniform wall thicknesses.

  • Stable dimensional accuracy.

vacuum casting process-1

The disadvantage of vacuum casting

  • Parts with unequal wall thicknesses and very complex structures may deviate from normal tolerances due to shrinkage.

  • The service life of silicone moldsis short, typically only 15-25 copies can be made, but this also depends on the complexity of the part.

  • Manufacturing parts in high volume is not cost effective

What is injection molding?

Injection molding is a process of producing parts made of thermoplastics or thermosets. Injection molding is to heat and melt plastic in the barrel of an injection molding machine, when it is in a flowing state. Under the pressure of the plunger or screw, the molten plastic is compressed and moved forward, and then injected into the closed mold with low temperature at a very fast speed through the nozzle at the front end of the barrel. After a certain period of cooling and setting, the mold is opened to obtain the part. Injection molding is suitable for mass production of plastic parts with complex shapes. It is one of the important mass processing methods, and is also widely used in various industries and fields.

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The advantage of injection molding

  • The production speed is fast, the efficiency is high, the cost is low, and the operation can be automated.

  • There are many options of materials and colors, the shape can be from simple to complex, and the size can be from small to

  • Part withaccurate dimensions, good surface effect, part with complex shape and structure can be manufactured in large quantities.

The disadvantage of injection molding

  • High initial investment cost for the mold.

  • The mold buildingtime is long, generally up to 4-8 weeks.

  • Higher mold maintenance costs.

Vacuum casting compare with injection molding

  • Mold cost. The cost of building a set of injection molding molds (steel or aluminum molds) will be hundreds or thousands of times that of a vacuum injection molding mold (silicone). The material used in the mold, the structure and production method of the mold, and the service life of the mold will affect the cost of the mold  The silicone mold used in vacuum casting is quite simple to build and is only suitable for small quantity production. However, the steel mold used for injection molding requires various precision machining. In addition, the mold structure is also quite complex, and the service life of the mold is relatively long. Generally, it can produce more than 1000K+ parts. For the aluminum mold used for injection molding, the cost of the mold will be much lower than that of the rigid mold, but the service life will be much shorter than that of the steel mold. Usually it can produce 10K-20K parts. In addition, the mold building time will be shorter than the steel mold. Aluminum mold is commonly known as rapid tooling.

  • Quantity.If your project is in the early stages of development and only needs a few plastic parts for design & functional verification, then vacuum injection molding is definitely the most cost-effective option. After the project implement tomass production, if the number of parts you need is between 200-20K, it will be better to choose a fast aluminum mold. But when you need more than 20K parts, it will be more cost-effective to choose a steel mold.

  • Material.The material of vacuum injection molded parts, which can only simulate the material of the final product,but cannot be completely replaced. For some parts with critically required characteristics, injection molding manufacturing can use the same process and materials for manufacturing.

  • Delivery time.The manufacturing of steel molds and aluminum molds usually requires more complex and precise machining, and it usually takes several weeks to prepare, so the delivery time of injection molding is divided into mold building time and injection molding time, and the lead time may up to 2 months. On the other hand, the silicone molds used in vacuum casting can be made in a very short time, usually only in one week to complete the manufacturing of silicone molds and parts. 

  • Individual part cost.Generally speaking, the part cost of vacuum casting will be much higher than injection molding, because the number of parts that can be produced is quite less and the manual operation is more. However, the overall cost of vacuum casting is generally much lower than injection molding due to lower tooling and material costs.

Choose the Best Manufacturing Process for Your Plastic Parts

Both manufacturing methods have their advantages and disadvantages. There is no direct way to tell which manufacturing technique is best for your design; it ultimately depends on the part or prototype you are designing. In order to choose a better solution, following points need take into consideration.

  • What stageof your project, R&D/trial production/mass production?

  • How many quantity you need for the part?

  • What are the specific design parameters?

It doesn’t matter if you can’t make the final decision to use which manufacturing technique. Just contact with DDPrototype (A prototype manufacturer located in China) for get more technical support. We can produce the plastic parts which you need by using a variety of other molding processing.