How Vacuum Casting Works

 The vacuum casting process has three steps: master pattern, molding, and casting.

Step 1. Master Pattern Making

The quality of the casting urethane parts is determined by the quality of the master pattern, which is best machined by CNC when you need to hold tight tolerance, glossy finish, or even optically clear. In addition, we also will consider making the master by SLA for those parts with more complex shapes and structures.

We can paint parts surface for texture or satin effect to simulate the final mold texture of the product. The silicone molds will copy the details and textures from the original master, so the cast urethane parts will come out the same as the master model without any extra finish on the surface.

Step 2. Silicone Mold Making​

Casting molds are made from liquid silicone resin, also known as RTV molds. The chemical stability, self-releasing properties and flexibility of silicone rubber are excellent in that it gives minimal shrinkage and carries fine details effectively from master to mold. The mold making process starts as follows:

Paste tape in a flat position around the master, so that it is easy to open the mold later, which will also become the parting surface of the final mold.

The master model is suspended in a casting box and placed with plastic rods to set gates and vents.

Liquid silicone is poured around the master and removal all air, creating a vacuum chamber to ensure the master mold is free of air bubbles. Then is cured liquid material in an oven, at a temperature of 40℃. It generally takes 8-16 hours depending on the volume of the mold.

Once the silicone rubber is cured, cut open the casting box, remove the master model from the silicone to form an empty cavity, and the silicone mold is made.

Step 3. Casting Process

Standardized operational procedures assure accurate part replication and consistent mechanical properties. Our vacuum casting technicians to control all operational parameters: mixing, de-gassing, stirring, preheating, casting, and de-molding, allowing for near-perfect copies of the master pattern to be created. The manufacturing processes of vacuum casting parts are as follows:

First, put the silicone mold into the oven and preheat it to 60-70°C.

It is very important to choose a suitable release agent before closing the mold and use it correctly to avoid stickiness and surface defects.

Prepare the polyurethane resins to preheat them to about 40°C before use. Mix two-component casting resins in exact proportions, and then fully stir and degas for 50-60 seconds under vacuum to avoid air pockets or bubbles

The resin is poured into the mold inside the vacuum chamber under computer control and cured again in the oven. Average cure time: 1-3 hours for small parts and 3-6 hours for big parts.

Remove the casting part from the silicone mold after curing. Prepare to repeat this vacuum casting work cycle.