By David

July 19, 2022 at 8:02 am

If you’d just like to see the results of the vacuum casting process, below is a video from a recent client who we helped prototype a rubber sanding block with this manufacturing method:

Otherwise, keep reading to learn more about the process.

Overview of the Vacuum Casting Process

Industrial manufacturing processes are getting increasingly challenging owing to the complex parts required and high production costs. Meeting these needs cost-effectively has birthed the vacuum casting process.

Vacuum casting, otherwise called polyurethane casting or urethane casting, is a manufacturing process that makes a cast of an object by drawing a liquid material into a silicone mold using a vacuum. This differs from a closely related process, injection molding (IM), in which the liquid materials are pushed into the mold by force.

The vacuum chamber offers the advantage of removing air bubbles that may interfere with the fine details of the final product. Hence, this process is a great method for rapid prototyping and manufacturing products with undercuts or fine details.

Production quality parts can be made using the vacuum casting process, and this type of manufacturing is often used to create visually enticing prototypes as well as non-mechanical consumer products.

The material used in this casting process is polyurethane and several types of poly are available depending on the temperature, flexibility, and durability required in the final product. These include rubber, PP, ABS, PC, etc. These materials offer varying properties for your rubber components or other polyurethane parts.

Vacuum casting or Urethane casting is a cost-effective process compared to IM batch production and other manufacturing processes for low volumes production.

Vacuum Casting vs Injection Molding

Vacuum casting machine

A vacuum casting machine. Image by Lukadent, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>

The vacuum casting process and injection molding share a lot of similarities. However, one of their biggest differences lies in the type of mold used.

In the latter, aluminum, steel, etc. are used in making molds. Vacuum casting on the other hand uses silicone molds. The cheap cost of silicone means that many manufacturers choose vacuum casting as a cost-effective manufacturing process for shorter production runs.

Vacuum casting allows for the manufacturing of multiple components of products simultaneously, hence increasing the efficiency and reducing the cost of the manufacturing process. Also, since the quality of the products is comparable to those from IM, vacuum-cast models can be used for fit and function testing and other marketing purposes. These molds have a limited lifespan however, so for large volume production runs an aluminum mold and manufacturing via an injection process is more appropriate.

Steps in the Vacuum Casting Process

This casting process involves the following steps;

1. Making a 3D Model of the Master Pattern

The vacuum casting process begins with making the vacuum cast component via the creation of a master model using rapid prototyping 3D Computer Aided Design (CAD).

The model specifications should be precisely calculated while creating the 3D model. Adhere to design guidelines and make adjustments where necessary.

2. Creating the Master Pattern For the Mold.

3D printing

After the design process the next step is 3D printing. The raw material for 3D printing can range from Polyurethane resin to metals and carbon fibers.

The master pattern is the object that will be used in making the silicone mold. The mold’s complexity is determined by the design of this primary model. Any deformities on the master pattern will be transferred to the silicone molds so it’s important to get the prototype right.

Other ways of making the primary pattern

Aside from using a 3d printer, there are several other ways of creating the master pattern. These include CNC machining, Stereolithography, and other economical methods that offer high precision and surface finish that creates high-quality prototypes.

3. Creation of the Silicone Mold

Making the mold requires the master pattern and a casting box.

The process starts with hanging the master pattern in the empty cavity of the casting box. Cores and inserts are also attached to the inside of the master model and in addition to these, casting gates and risers are also incorporated for pouring liquid silicone around the model suspended inside the casting box.

When the liquid silicone is poured into the casting box, it flows inside the design filling all crevices. It is allowed to cure inside the box at a temperature of 40°c for about 8-16 hours.

After the curing is completed, the mold is taken out of the box and cut along a preferred parting plane and the master model is removed leaving behind a mold. The entire process is done under vacuum pressure to keep casting deformations to a minimum.

It is important to note that silicone molds are subject to shrinkage owing to the thermal expansion of the silicone in the heating chamber. This shrinkage ranges from 1% to 4% and necessitates using high-quality silicone when using vacuum casting processes.

4. Preparing the Casting Resins

Before polyurethane casting, the poly resin mixture needs to be prepared. This is done by mixing the resin and other agents such as color pigment and heating the resin. The structural qualities of the product will determine the type of polyurethane to be used.

The resin mixture is made at 40°C in the heating chamber. The color pigment is also mixed at this stage. The mixture should be made homogeneous before pouring into the silicone molds.

When the solution is made homogeneous, the two halves of the mold are assembled and the mixture is poured into the mold using a funnel to aid uniform distribution and prevent air pockets from forming. The solution is then ready for the casting process.

5. Curing and Demolding

After the casting materials have been set in the mold and sealed, the mold is placed in the casting machine for casting under vacuum pressure.

The temperature and time of casting depend on the type of polyurethane used. When casting is complete, the mold is removed from the vacuum machine and its two halves are separated to remove the cast. The demolding stage of the vacuum casting process should be carefully done to avoid damage to the cast.

While removing the cast, excess material from gates and runners is trimmed off and any other additional finishing is done while making sure to maintain surface textures.

After these tasks are finished, the vacuum casting process is complete and the products are ready to be used.

 

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Technical Specifications of the Vacuum Casting Process

Maximum Part Dimensions:

The size of the mold for the vacuum casting process is limited by the dimensions of the vacuum chamber which is typically 1900 x 900 x 750mm. The volume of the product is also a limiting factor with a maximum volume of 10 liters.

Minimum Wall Thickness:

To ensure that the mold is filled properly a wall thickness of at least 0.75mm is required. However, we recommend a wall thickness of at least 1.5mm.

Typical Quantities:

Depending on the mold’s complexity and casting material, each mold can be used to produce 20 to 25 copies of the prototype. Vacuum casting can be used to make small batches of quality parts using silicone molds. If you require larger numbers of the finished product consider aluminum molds instead.

Accuracy:

The vacuum casting process has an accuracy of ±0.3% with a lower limit of ±0.3mm on dimensions smaller than 100mm.

Typical Lead Time:

The vacuum casting process has a lead time of up to 20 parts in 15 days or less depending on the specifications and volumes of parts required.

Starting a short run or vacuum casting project

If you have an upcoming need for a short production run of items, look no further than the team here at JCAD!

We have decades of experience in all facets of manufacturing, including CAD design, prototyping, making master patterns and creating molds, either in aluminum for more durability, or silicone to aid in cost-efficiency.

Get in touch today via our quote form, talking to us through live chat or calling 1.888.202.2052. We look forward to speaking with you!

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