Vacuum Casting vs. Injection Moulding - What's the Difference?

Casting and injection moulding are two common manufacturing processes that are used to fabricate many of the products we encounter every day. Polyurethane, a thermoplastic material, is commonly used to make parts such as gaskets in automotive settings, casters and rollers for medical carts, and various hardware such as pulleys, bushings, shock absorbers, and scraper blades.

To determine whether vacuum casting vs. injection moulding is better for your particular application, consider whether the part is for prototyping or mass production and identify how many parts you intend to create.

What is vacuum casting?

Vacuum casting is the process of creating parts by pouring liquid plastic into a mould under a vacuum. The mould is typically made from a soft material such as silicone. Moulds filled with liquid plastic are then placed in an oven to cure. However, not all plastics require heating to cure. Some can cure at room temperature over time. Once cured, the mould is opened and the casted part can be removed.

acuum casting process animation

The vacuum casting process is often used to create finished plastic parts for small production runs or for the rapid prototyping of a product that will eventually be injection moulded.

The soft moulds for vacuum casting are created from a “master pattern.” A master model is a 3D-printed or CNC-machined replica of the part to be cast. The master pattern is then dipped in liquid silicone which then cures and sets. Once set, the master pattern is cut in half and the 3D-printed or CNC-machined portion is removed. This process leaves behind two mould halves whose internal cavities, taken together, are identical to the part to be cast.

What is the lead time of vacuum casting for production?

Lead times of production-level vacuum casting usually run from 1-2 weeks. While the master pattern and mould for vacuum casting can be completed between three to seven days, satisfying production volumes can take longer. This is because the plastic must cure before it can be removed from the mould, and curing time can’t be abbreviated. 

The production output for a single vacuum casting mould is generally about 1-10 parts per day. The output largely depends on factors such as the type of plastic and part geometry.

Vacuum cast part next to its mould

How is the volume produced by vacuum casting?

The volume of products produced by vacuum casting is small compared to other manufacturing methods. This is primarily because vacuum casting is a labour-intensive, manual process. It contrasts sharply with injection moulding or CNC machining processes which are heavily automated and require little manual intervention. Casting plastics must be prepared properly before casting. Some plastics may be purchased already in their liquid state. Others are purchased solid and must be melted in an oven. 

Once liquified, plastics must be mixed at the appropriate ratios with different chemicals to facilitate proper curing and to obtain desirable characteristics such as a particular colour. Additionally, these ratios can be adjusted to achieve properties such as abrasion resistance or increased compressive strength. 

While some fast-setting plastics can cure within 30 minutes, others may take several hours to cure. All these aspects make the process suitable for prototyping and small-scale production runs, but not large manufacturing operations.

What is the tools and parts cost for vacuum casting?

The tools and parts required for vacuum casting are significantly cheaper than those associated with other manufacturing methods. Silicone for the moulds is inexpensive and it’s much easier to create these flexible moulds than to make die sets for injection moulding. 

Additionally, suitable plastic resins and curing agents are affordable. The main barrier tends to be the master pattern since SLA printers or CNC machines are very costly. Many manufacturers outsource their master-pattern production to third parties. This, in many cases, is still significantly more cost-effective than other manufacturing technologies like injection moulding.

What is the tolerance of vacuum casting?

The tolerance of vacuum casting is usually 0.3% from the nominal size but can depend on several factors. The type of plastic, part size, and part complexity can all influence the tolerance of a vacuum-cast part. Tolerances can vary on a case-by-case basis.

Is vacuum casting expensive?

No, vacuum casting is not expensive. It is cost-effective for both prototyping and manufacturing (depending on the production volumes required).

What is injection moulding?

Injection moulding is the process of forming highly precise products by forcing molten plastic material into the cavities of a mould at very high pressure. This is unlike the casting process where gravity helps the plastic resin fill out the mould cavity. In injection moulding, the pressurised filling of cavities and rapid cooling of plastic parts makes it a significantly faster and repeatable process than casting. As such, it’s an ideal process for large-scale manufacturing operations.

Animation of the injection moulding process

Moulds for plastic injection moulding are fabricated through methods like CNC machining and electric discharge machining (EDM). These two processes can form cavities to tolerances as tight as ±0.1 mm on average and, in some cases, may get as low as ±0.025 mm depending on the application. 

Because of the high pressures and temperatures involved, moulds for injection moulding are typically made from hardened steel or aluminium. 

What is the lead time for injection moulding for production?

Lead times for the actual injection moulding tooling are long. The precision necessary in the metal moulds requires time to perfect. Once the moulds are completed, however, injection moulding production needs less lead time than casting. Large quantities of plastic parts can be completed at a much faster pace.

The high pressure and rapid cooling make for a much faster cycle time. While some die sets for injection moulding have only one or two cavities, others may have well over 100. This means injection moulding can produce hundreds, if not thousands, of parts per day to fulfil production orders.

How is the volume produced by injection moulding?

The volume of parts possible through injection moulding is significantly higher than that produced by casting. Injection moulds can have multiple cavities for identical plastic components and overall cycle times are short.

Injection moulded parts

What is the tools and parts cost for injection moulding?

The tools and parts required for injection moulding are much costlier than those needed for vacuum casting. Die sets for injection moulding are typically fabricated from metals such as hardened steel or aluminium and are made to precise tolerances. Several machining processes and post-processes are required to create a viable mould. 

Die sets are typically composed of many different parts such as ejection pins, springs, and limit switches. The cost of manufacturing and assembling die sets for injection moulding quickly runs into the thousands, or in some cases, hundreds of thousands of euros.

What is the tolerance of injection moulding?

Proper dimensions and tight tolerances in injection moulding are the most important things with respect to design and manufacturing. This is why we recommend referring to the industry standards DIN 16742:2013 where general tolerances for different linear dimension ranges are presented.

How to choose the right manufacturing process

When deciding whether vacuum casting or injection moulding is the right manufacturing technique, consider the items listed below:

1. Production Volume: Vacuum casting is better for prototyping or small production runs, while injection moulding is better for medium-to-large production runs. Casting can produce 1-10 parts per day while injection moulding can produce hundreds, if not thousands, depending on the number of cavities in your mould.
2. Tooling Cost: The tooling for vacuum casting is much cheaper than for injection moulding. Plastic resin, curing agents, and silicone moulds for casting are far less expensive than the hardened steel or aluminium die sets necessary for injection moulding.
3. Part Cost: Vacuum cast parts are more expensive than injection moulded parts because of the manual labour and low volumes produced by casting.
4. Material: Only a limited set of curable plastics can be used for casting while an extensive variety of thermoplastics and thermosets can be used with injection moulding.
5. Lifespan: Silicone moulds for vacuum casting are not as durable as hardened steel or aluminium injection-ready moulds that can last decades. Consider how long you expect to need your particular mould. 

What are the differences between vacuum casting and injection moulding?

The table below shows the differences, advantages, and disadvantages of vacuum casting and injection moulding:

Summary

This article presented vacuum casting and injection moulding, explained what they are, and discussed when to use each process best.

Xometry provides a wide range of manufacturing capabilities, including vacuum casting, injection moulding, die casting, compression moulding and other value-added services for all of your prototyping and production needs.