Overview of Vacuum Casting

What exactly is vacuum casting?

Vacuum casting is a casting process that uses a vacuum to draw a liquid material into a silicone mold to create complex components. Elastomers such as plastic resins and liquid silicone rubber are used as liquid materials in vacuum casting.

Because polyurethane resin, a type of elastomer, is used as the casting material, vacuum casting is also known as polyurethane casting or urethane casting. Because the production cost of vacuum casting is lower, it is a more cost-effective alternative to injection molding for making plastic components.

Vacuum casting is an excellent manufacturing method for batch production and other low-volume jobs. Vacuum casting is widely used in the production of consumer goods as well as industrial electronics.

What materials are used in vacuum casting?

Casting polymers are available in a variety of hardness and surface texture combinations. Depending on your application, you can also create parts that are fully opaque, translucent, or completely transparent. 

Polyurethane is the most commonly used casting material in this process. Polyurethanes have a variety of properties, including structural rigidity, elasticity, flexibility, shock and temperature resistance, and so on. As a result, different polyurethanes can be used depending on the type of product being manufactured. 

The following are some of the most common materials we use:

Polypropylene (PP) like (80D)

Polycarbonate (PC) like (75d)

Acrylonitrile butadiene styrene (ABS) like (83D)

Pe-like (65D)

Santoprene-like

Thermoplastic polyolefins (TPO or TPE-O) like

Talc-filled polypropylene like

Glass-filled nylon-like

Thermoplastic polyurethane (TPU) like

Fire-retardant (FR) PU like (78D, 80D, 85D)

High impact (85D, 88D, 80D – 90D)

Polyamide (PA) like (80D)

Acrylonitrile butadiene styrene (ABS) + high density polyethylene (HDPE) like (70D)

Elastomer like (60A, 65A, 70A, 90A, 95A)

Polyurethane (pur or pu) like (25A, 30A 40A, 50A, 60A, 70A, 80A, 84D, 85A, 90A)

What is the Process of Vacuum Casting?

Vacuum casting is similar to injection molding in that the cast material is injected into molds to produce products. The types of molds used distinguish vacuum casting from injection molding.

Molds for injection molding are made of aluminum, steel, and other materials. Silicone molds, on the other hand, are used in vacuum casting. Because silicone is less expensive and has a longer lifespan, this significantly reduces the cost of the mold. Below are the typical steps of vacuum casting.

#1 Design of the component

The first step in making any vacuum cast component is to create a 3D model of the component in question. The model is created using the same design guidelines as injection molding molds.

Before creating the 3D model, proper calculations must be performed. Deformities in the model can have an impact on the finished product after casting. As a result, it is critical to adhere to design guidelines and make necessary adjustments to calculations.

#2 Creating a mold master pattern

The mold’s master pattern is created using 3D rendering. The master pattern is the primary mold used to create the silicone molds that will be used for casting. It is a prototype for silicone molds made of high-durability materials such as metals or other plastics.

Previously, master patterns were created using CNC or hand tools, but today, processes such as 3D printing, stereolithography, and other such processes are used to create master patterns that are more cost-effective and precise.

Because the silicone molds are copies of the master pattern, any deformation on the master pattern will cause the silicone molds to deform. As a result, a high level of precision is required when creating the master pattern.

#3 Producing silicone molds

The next step is to create silicone molds after creating the master pattern. The master pattern and a casting box are used to create silicone molds directly.

Inside the casting box, the master pattern is suspended. Cores and inserts are inserted into the master pattern. Along with the core, the casing box contains casting gates and risers for pouring molten silicone and removing the final mold.

Around the master pattern, hot, molten silicone is poured into the casting box. It flows inside the master pattern, filling all of the prototype mold’s crevices. For about 8-16 hours, the molten silicone is allowed to cure inside the casting box, which is kept at 40°C.

When the silicone has solidified and the curing process has been completed, the risers are removed and the silicone mold is removed from the casting box. To keep casting deformations to a minimum, the entire process is carried out under a vacuum.

#4 Polyurethane resin preparation

The polyurethane (PU) resin mixture is prepared before cast by mixing the resin with other casting components such as pigments and other agents and heating the resin. Depending on the type of product to be manufactured, various polyurethanes with varying material and structural qualities are available.

To make the resin mixture, combine the two parts of the resin in a container and heat the mixture to around 40°C. At this stage, color pigments are also mixed. It should be noted that the resin and pigments have completely mixed into a homogeneous solution before pouring the resin mixture into the molds.

When the solution is finished, it is poured into the silicone molds using a funnel to ensure uniform distribution and to prevent air bubbles from forming. When the solution has been completely poured into the mold, it is sealed and ready for casting.

#5 Curing and removing the mold

The mold is placed in the vacuum casting machine for casting after the solution has been completely set in the mold and properly sealed. The temperature and time for casting may vary depending on the type of polyurethane used, but the process remains the same. When the casting is finished, the mold is left in the casting machine to cure. Curing aids in the retention of structural rigidity after the cast is removed from the mold.

To properly cure the cast, the casting machine is kept at around 70°C. After curing, the mold is removed from the machine and its two halves are separated to properly remove the cast. It is critical to properly de-mold the cast and ensure that it is not damaged while being removed from the mold. Excess materials, such as gates and runners, are removed from the mold as the cast is removed.

As a result, the resin casting is complete, and the product is ready for use.

The benefits of vacuum casting

• Multiple components from the master pattern can be used within 24 hours, reducing the product design prototyping stage and saving time and money on new product development.
• Vacuum casting can be used to produce small batches of high-quality injection molding parts for prototypes or low-volume end-use parts for low-stress, relatively benign environments.
• A variety of vacuum casting resins are available to suit a variety of applications, including clear, rubbery, flame retardant, food-grade, and colored resins.
• To test various material types for a given application or part, the resin type can be easily changed without the need to retool.
• Multi-segment molds and cores can accommodate complex shapes and features.
• In-mold inserts made of aluminum or brass can also be used.

The drawbacks of vacuum casting 

• Because the mold is made of soft silicone, it wears out quickly and only lasts about 30 – 50 parts. Tool wear is determined by surface textures, features, and size. Mold life would be reduced by larger parts, rough surface textures, and a large number of proud features.
• Shrinkage can cause irregular, overly thin, and thick features to deviate from normal tolerance, just like injection molding.
• Thermal expansion of the liquid and the flexible mold causes a 15% – 0.25 percent shrinkage rate. In general, the manufacturer would allow and add shrinkage tolerance, but it is worth speaking with them before placing an order.
• Because the surface finish is dependent on the part’s post-processing, it is sometimes limited to only the outside because it is difficult to access some internal features of the pattern Sharp corners and texts will occasionally have rounded edges
• Some features will also necessitate the use of plugs and cores.

In the end

Vacuum casting is a very cost-effective and efficient manufacturing process. When compared to traditional methods such as plastic injection molding and pressure die casting, vacuum casting has a very low manufacturing cost.

Because of the cost-effectiveness of manufacturing, vacuum casting is ideal for batch production and rapid prototyping jobs that require a small number of products.

Because only a limited number of products can be cast at a single time, vacuum casting is not as efficient for mass production as other traditional manufacturing processes.

Thus, vacuum casting is an excellent manufacturing process for small batch production and rapid prototyping, with efficient manufacturing and low production costs.

It is a highly adaptable and versatile manufacturing process. As a result, it is the preferred manufacturing process for custom and low-volume manufacturing jobs.