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An introduction to Room Temperature Vacuum Casting

This process of molding is applied with the utilization of a vacuum chamber to extract the liquefied material into the mold. The method of vacuum casting is applicable when the mold that is desired has complex details or undercuts. It is also utilized when there is a concern of air entrapment or if the material has fibrous properties or wire reinforces.

A two piece silicone mold is placed in a vacuum chamber while the raw liquefied material is mixed and degassed; this is done when dissolved gasses are removed from liquid materials called degasification applied to prevent gas bubbles formation that is undesirable when a smooth finish is required. After the extraction of gasses is completed, the liquid material is then poured into the mold. The vacuum is then released and the mold is taken out of the vacuum chamber.

After this procedure, the vacuum casting is cured in an oven. Curing is a process of toughening or hardening of the liquefied material. This can also be applied to polyurethane and other plastics like acrylic. The silicone mold can be reused to up to 20 times maximum where it will start to deteriorate and can no longer be useful. 

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In the oven, it takes to up to hours for the liquid material to be cured depending on its size. After an exothermic reaction of the plastics, the cavity is opened and the molded plastic is removed. The silicone cavity is the closed and the procedure done again.

The disadvantage of Vacuum Casting is that it is expensive and is not preferred for mass production. However a substitute is available instead of this method known as Reaction Injection Molding. This process does not require expensive vacuum chambers and mixing facilities. The process utilizes simple plastic injection system with a couple of pressurized chambers.

So in Short

Vacuum casting is a prototyping technology used when you need a short run of your 3d cad model made out of a PolyUrethane with properties close to real injected plastics such as ABS, POM ,PA or TPE’s TPR rubbers

Usually series of 5-100 pieces will be run using this casting technology.

WHAT ARE THE ADVANTAGES OF Vacuum casting?

The main advantages of this process are amuch lower tooling costs and a quicker turn-around than traditional steel mould making. the lead-time to create casted models is around 7-10 days.

The products have an exceptional detail and accuracy which comes close to the final injected plastic parts.

Flexible silicone moulds allow significant undercuts, simplifying mould-making and containing costs.

The process:

The first step of vacuum casting process involves encapsulating/enclosing a master model in a liquid Silicone rubber. Than a vacuum is applied to the mould. The mould is then cured in an oven at around 70°C

Once cured the mould can be split up in 2 or more parts, at which point the master model is removed leaving a mould cavity perfectly replicating the master model.

what is possible with vacuum casting?;

We can make double or triple shot products, we can cast rubber or TPE or TPR like materials. (with 20-80 shore A hardness)

We can vacuum cast optical clear plastics ( same like PMMA) and we can use Inserts in the products.

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Vacuum casting

Vacuum Casting is widely used for low volume 3d real models or prototypes. Typically with one soft tool 20-25 duplicates can be made out of polyurethane material.

The Vacuum casting process is a 5 step process that starts with the creation of an accurate and highly finished master model which can be created using various RP techniques such as Stereo lithography, FDM, SLS, CNC or an existing part.

The second step; this mother model is placed in a mould box and liquid silicon rubber is poured over the master.

Once the silicon is cured, the mould is cut as per split line established and master model is removed, leaving an accurately formed cavity.

Step 4:  the mould cavity is used to cast large variety of polyurethane or epoxy materials available. This allows fast production of high quality parts in a short lead time

In the next chapter the advantaged and disadvantages will be elaborated.

The costs to create a silicone mould are much lower than steel moulds, the manufacturing of these moulds can be done in a short lead time. A lead time from 4-10 days is possible, but this depends on complexity and how the master model will be made.

There is a large variety of urethane materials available, which properties are very close to the injection molded materials ranging from soft rubber grades to hard plastics. Some polyurethane can be colored directly, It is also possible to sandblast or paint the finished parts. Because of the flexibility of the silicone moulds undercuts in the product can be made without sliders or cores. Also inserts can be placed in the moulds and even over-moulding 2 or more components is possible.

Disadvantages:

The tool life time of the silicone mould is limited; one cavity can only make 20-25 parts, than a new mould has to be created

Vacuum Cast parts are not identical to injection molded parts, because the conditions and method of manufacture are different.
Very thin wall sections are not recommended and chances of cracking or warping.

Recommendations:

The Minimum wall thicknesses are recommended not to be smaller than 0.8mm. Recommended size is above 1mm

Generally 20 pcs can be made from single mould so multiple of 20 can give you better pricing to cover up the silicon mould cost. E.G. Qty 20 or 40 has better pricing per piece than 30 or 50 pieces.

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Every project is different and there is a casting material to suit each of them. Jewellery creators for example may want to look at the various casting resins for interesting effects. If you are after a fun and creative party activity for children then you may want to investigate the casting of plaster ornaments and plaques for them to paint. Simple casting plaster is the most ideal casting material for this purpose. Every project is different and requires a specific casting solution and we will investigate this further in this article.

PLASTERS

Plaster is a versatile, cheap and safe casting material to use for many different types of applications. This material can be cast into almost all types of moulds and doesn’t require expensive silicone moulds to be made.

There are many types of plaster and each are suited to a specific task. Plaster of Paris is probably the least suitable for most casting applications as the surface hardness is low and detail is not defined.

Casting Plaster types are best suited for general purpose plaster casting such as for children’s plaster painting ornaments. This type of plaster has a harder surface to help prevent chipping.

Stone plasters are also available for those projects that require more strength and a harder surface. These plasters have superior strength but are more expensive.

RESINS

There are many types of resin. They generally made up of either polyester, epoxy or polyurethane and can come in either clear or opaque finishes. We will look at the various types of clear and opaque resins and their applications.

Opaque Resins: Polyurethane resin is the most common type used today. There are various products available with different setting times and hardnesses. The time it takes to set hard when mixed in most cases is very short, usually 3 – 5 minutes. When the resin is setting the temperature can be very high and the use of silicone rubber molding materials are recommended.

This type of resin can be coloured but the base colour which can range from white to creamy beige needs to be factored in to any colour mixing. Special polyurethane pigments can be mixed into part A before mixing in part B.

These types of resins are not as brittle as clear polyester resin and are therefore suitable for making model parts and small detailed items. The finished castings can be sanded, drilled and painted.

Transparent resins: The cheapest clear casting material is polyester resin. This type of resin requires the addition of MEKP to allow it to harden and this chemical is quite toxic. Gloves should be worn when using this casting material and a well ventilated are is essential.

These resins produce a hard and water clear casting which can be hand polished to produce glass like finishes. This product is excellent for embedding and due to the slow setting times it allows most bubbles time to rise to the surface.

Another type of transparent resin which is really starting to become popular is the epoxy type. These resins are much easier to mix at a ratio of usually 1 to 1 and have low odour. They are also a lot less toxic than polyester resins.

Epoxy resin has become quite popular with jewellery makers. One limitation of epoxy resin however is that larger castings can become problematic when setting. If the casting is larger than a fist then polyester is recommended.

METAL ALLOYS

Pewter casting is a popular hobby and people use low melt metals to create a wide range of model parts, toy soldiers badges and even jewellery. Special high temperature silicone rubber molding materials are needed when casting these metals.

A melting pot and a good set of gloves are recommended.

CONCRETE AND CEMENT

More and more people today are using moulds to cast concrete items for their gardens or for their own business. The most common types of moulds used with concrete are made from either durable ABS plastic or polyurethane rubbers.

Ready mix bags of concrete are ideal for the beginner to start casting and the more advanced caster can experiment with special finer concrete mixes which produce smoother and finer surfaces. The type of aggregate used is also important in getting a good surface finish. When the concrete has set in the mould and removed it needs at least a week to fully cure and during this time needs to be kept wet to prevent cracking.

CONCLUSION

These are just a few of the many casting material possibilities. We haven’t gone into additives which can enhance the materials mentioned above such as metal powders which can be added to transparent resins to make metal like cold casts. Before embarking on your casting project and mould making it is important to do you research and even talk to an expert. They can direct you onto the right path before you purchase any expensive mould making and casting material.