Vacuum casting

The process

Vacuum casting is a primary forming process in which liquid casting resin is drawn through an applied vacuum into a previously created mold. This avoids air pockets in the resulting part and produces a highly reproducible, high part quality. Vacuum casting is particularly useful for producing parts with very smooth surfaces that must meet high tightness requirements, as well as parts with very fine details and complex geometries. The molds used in vacuum casting are typically taken from a 3D-printed master model, or additively manufactured directly in the stereolithography (SLA) process. This allows the flexibility and detail quality of the SLA process to be utilized while maintaining good mechanical properties of the casting resin used. Unlike plastic injection molding, the molds used are comparatively inexpensive to produce, so vacuum casting is already suitable for small quantities, small batches and medium-sized series.

Production of the mold

To produce a mold, the master pattern must first be machined so that it has the desired surface quality. Then the mold concept must be defined with parting lines, sliders, risers where residual air can escape and sprue. The master pattern with sprue and risers is then hung in a box and cast with a 2-component silicone. After a curing time of approx. 12 hours, the mold can be cut open by hand with a scalpel and the master pattern removed.

Advantages of vacuum casting

• High precision: Very precise and complex shaped parts can be produced. The master model of the mold can be produced using the SLA process, thereby taking advantage of the flexibility and surface quality of additive manufacturing. 
• Smooth surfaces: By using the vacuum molds, the components obtain a very high surface quality, comparable to that of injection molding.
• Good mechanical properties: The casting resins used have high strength, so the main disadvantage of the SLA process is circumvented.
• Cost efficiency: The process does not require high initial costs in mold making and enables the production of larger quantities in a short time.
• Material selection: There are many casting resins that can be used for different applications. Possible properties include, good electrical conductivity (ESD), high heat resistance, good transparency, strength, rubber elastic behavior, etc.
• Flexibility: The process is flexible and suitable for the production of both prototypes and series parts.

Comparison with other manufacturing processes

Vacuum casting is suitable for the production of small quantities up to medium-sized series. This places the manufacturing process between 3D printing (single part production and small batches) and plastic injection molding (large batches) when considering the suitable number of production pieces. Due to the required mold, the process loses some flexibility and speed compared to 3D printing. In order to enable removal from the mold, so-called demolding, the components must not have large undercuts, so that the producible complexity of the components is lower compared to additive manufacturing.

On the other hand, vacuum casting impresses with very good surface qualities in combination with a wider range of materials and good strength values, where additive manufacturing reaches its limits. Depending on the casting resin used, the components that can be produced can exhibit high strength and stiffness or rubber-elastic ductility and thus in many cases meet the requirements for a series application.

In comparison with the fully automated, highly efficient injection molding process, the significantly lower initial costs are particularly striking. The tools used in injection molding, elaborately milled steel molds, account for a large proportion of the resulting component price and can cost several hundred thousand euros. In order to pass on the high tooling costs to the component price, these components are designed for quantities from 10,000 up to several million.

The molds for vacuum casting are made by hand from silicone or directly additively manufactured. Unlike the stiff steel molds used in injection molding, the silicone molds allow the production of smaller undercuts and highly pronounced surface textures in all component directions. At the same time, the softer plastic molds last less time before they have to be replaced, but they are also significantly cheaper than steel molds. This results in significantly lower initial costs, which have to be passed on to the component price, making vacuum casting a viable alternative to uneconomical injection molding.