Vacuum Casting | A Step-By-Step Guide to Creating High-Quality Plastic Prototypes
- Zaid McAllister
- Jan 13
- 9 min read
Updated: 2 days ago
Although 3D printing is everywhere now, vacuum casting still produces prototypes that are in many ways superior, but nobody talks about it.
This is a low-volume manufacturing process using thermoset resins to create parts with isotropic mechanical properties and an excellent surface finish.
Isotropic = "Having a physical property which has the same value when measured in different directions."
Anisotropic = "Having a physical property which has a different value when measured in different directions. An example is wood, which is stronger along the grain than across it.
An Overview of Vacuum Casting
What is Vacuum Casting?
Vacuum casting involves replicating a "master pattern" by casting a silicone mould tool around an existing part.
Once the master is removed, a two-part thermosetting plastic resin is mixed and injected under a vacuum until the resin forms a solid part.
The now solidified part is a near-perfect replica of the original master model used to create the silicone mould. Vacuum casting is famous for its ability to accurately reproduce the texture applied to the master model.
The master model can be an existing part or one produced by CNC machining or 3D printing. Parts with minor undercuts & zero draft are still compatible with this process.
Materials
The poor thermal conductivity of silicone combined with its flexibility makes it unsuitable for injecting thermoplastics at the high pressures associated with injection moulding.
Instead, thermoset polyurethanes are used with varying chemical compositions and additives that simulate the properties of common thermoplastics, such as nylon, ABS and polycarbonate.
Two-part polyurethane casting resins now come with elastomeric varieties that simulate the properties of silicone rubber for prototyping. Silicone is not recommended for casting the parts themselves as it would bond to the silicone mould.
Vacuum Casting vs Other Prototyping Methods
The rapid prototyping space is a competitive one when it comes to technologies. The most popular methods are:
- Injection Moulding
- Vacuum Casting
- CNC Machining
- 3D Printing
Vacuum casting is best utilized when you want production-quality models with isotropic mechanical properties but without using the end-use material. It is also suitable for prototyping complex parts that may not be suitable for other processes.
Cost
To better understand the cost of each process we sent an RFQ to my go-to Chinese supplier HordRt. The quote used a basic part and specified a nylon material (or nylon-like for vacuum casting).
These charts show that vacuum casting is priced above 3D printing and CNC. From 0-100 parts, injection moulding is the most expensive. This is until the low part cost starts to outweigh the initial tooling investment.
Keep in mind that the above part was not optimized for CNC machining so it would be priced at around 2/3 of the above cost with modifications.
Lead Time
We used the same RFQ to create a lead time graph. Lead times are difficult to measure as they vary across suppliers & part geometries. This is still, however, a useful chart for comparison purposes.
A Step-By-Step Guide To Vacuum Casting
Step 1: Creating The Master Pattern
The master pattern is the part that is going to be replicated with vacuum casting. This means that the master model needs to be as close as possible in both geometry & surface finish to the final cast parts.
To create a master pattern the first step is to model the pattern in CAD software such as Solidworks or Fusion 360. This can then be reproduced in-house or by a manufacturer using STEP files and one of the below processes:
CNC Machining - It is possible to CNC machine a master model however, one limitation of CNC machining is the inability to machine sharp internal corners. This means it's unlikely that the parts produced will be able to replicate all of the features on the CAD model.
The main advantage that CNC machining has over 3D printing when vacuum casting is the tight tolerances that can be achieved on critical features.
3D Printing - This is the most popular method of creating master models for vacuum casting. SLA resin 3D printing is used most due to its reasonable tolerances and smooth surface finish.
Other processes such as SLS & FDM can be used but will require more post-processing to achieve a smooth or glossy surface finish.
Step 2: Preparing The Master Pattern
Something that catches many off-guard when they first try out vacuum casting is that many chemicals will actually inhibit the curing of silicone. This means that when you remove the master pattern from the mould there will be a layer of un-cured liquid silicone.
This is why master pattern preparation is such an important part of the vacuum casting process. The surface finish applied to the master pattern will be replicated during moulding.
Three-step finishing process:
Rough finishing: All features that are not intended to be replicated such as supports or inserts are removed. The surface is sanded with high-grit sandpaper for a uniform rough finish.
Priming: A primer is applied to the part and sanded with fine-grit sandpaper. This is repeated until the surface is smooth and uniform. For major imperfections filler material can be used before the primer.
Painting: The clearcoat used should match the desired surface finish for the part. A glossy clearcoat is used for glossy parts and a matt clearcoat for matt parts. The clearcoat must be free of sulphur and other cure inhibitors.
Step 3: Adding A Split Line
With vacuum casting, split lines, gating, and venting must still be considered. This should be considered carefully, and when possible, match the features of the production mould tooling.
To create the two-part silicone mould, once the silicone mould material is cast around the part, a knife is used to split the mould using a wave pattern to ensure alignment during part moulding.
In order to achieve a consistent and controlled flash pattern during this process, coloured tape is applied to the part along the intended parting line. The tape should be as thin as possible to minimize flash.
Step 4: Adding The Sprue
The sprue is the channel that will later be used to inject the resin into the silicone tool during vacuum casting. In order to prevent trapped bubbles the "gate low vent high" philosophy is adhered to by positioning the gate below the vents.
To create the sprue an acrylic dowl can be used. The dowl should be cylindrical with the end narrowing to a small point forming a cone shape. This point should be slightly narrower than the wall thickness of the part.
A great way to create this geometry is to place an acrylic dowl into an electric drill chuck and use it as a lathe. Sandpaper or a file can be held against the flat edge to produce the cone needed for moulding.
The sprue gate is then adhered to the master pattern. A good tip is to use a super glue with a quick-dry catalyst. The catalyst is applied to the master pattern first and then a small drop is placed on the acrylic dowl before joining.
There are other options for gating such as creating a tab gate. This would involve an acrylic cylinder and a flat piece of plastic adhered to the edge of the part.
For large parts, multiple gates can be used. This should be done with strategic venting to prevent trapped air and unfilled sections.
Step 5: Adding Vents
If air gets trapped during moulding it can leave vacuum-cast parts with serious defects. To avoid this multiple vents should be positioned around the part. Intuition is used to position the gates in a way that would minimize trapped air.
The technique for making and connecting the vents to the part is identical to that of the sprue. The only difference is that the vents come to a much sharper point and use a narrower dowl with the same diameter of a drinking straw.
Step 6: Building the Mould Box
There are lots of ways to build a mould box. The mould box is the container that will be filled with silicone when casting the vacuum mould.
This part does not make a huge difference to the quality of the final part. A smooth and sealed material like MDF or plastic sheet is used and edges are sealed with something non-permeant like hot glue.
The next step is to suspend the part in the box using the sprues and vents. It is suggested that there is at least 40MM of space around the parting tape to ensure a good alignment. The sprues & vent pins can be attached to the mould box with hot glue.
Step 7: Casting the Silicone
The first step to casting the silicone is selecting the material. The two types of silicone used are:
Platinum Cure (addition cure): Virtually no shrinkage during curing making it suitable for tight tolerance parts. Highly sensitive to curing issues in the presence of sulphur.
Tin Cure (condensation cure): Slight mould shrinkage can occur. It is the most commonly used silicone for vacuum casting thanks to its low cost and ease of use.
The silicone will come in two parts that need mixing to a specific ratio. The trick during this step is to minimize bubbles during mixing as much as possible. Using a vacuum chamber, each of the two resin parts are degassed individually first.
Two are then thoroughly mixed and degassed a second time. Pour this mixture into the mould box slowly to ensure no air is trapped during pouring. If a large enough vacuum chamber is available then the entire mould base is degassed once poured.
Vacuum chambers can be picked up on Amazon for less than £100 or made at home using a pot and a vacuum pump.
Step 8: Silicone Mould Prep
After following the above steps, what's left is a large silicone cube with some acrylic pieces protruding from the top. The next step is to split the mould into two halves.
A very sharp sculpting knife is recommended to ensure a clean cut. For the medical engineers, a scalpel will work just fine. Once the coloured tape is located, an incision is made with the cutting tool creating a cut from the mould edge to the tape edge.
The edge of the tape is followed by the blade while the hand is moving in a wave motion. This is continued until the mould is fully split and ready for separation.
Remove the master pattern from the silicone mould and clean the mould to remove any debris like paint or plastic chips. The tool is now ready for vacuum casting.
A thin plastic sheet is used to create a funnel that is inserted into the sprue hole. Plastic straws are placed into the vent holes to form vents.
Now a wooden block, slightly larger than the mould, is placed below the mould. This is done to minimize mould warpage due to uneven clamping pressure.
Step 9: Part Casting
The parting line is wrapped in duct tape to form a seal, followed by a wrap of tape around the entire mould tool assembly to provide a clamping force.
Following the same guidance as the silicone, the polyurethane casting resin is mixed and degassed. The resin is poured into the plastic sprue funnel until the venting straws are filled with resin. Now the mould is placed into the vacuum chamber for further degassing.
Step 10: Part Removal
Arguably the most exciting, yet nerve-wracking, is removing the part from the mould tool. Any errors in any of the above steps will have resulted in a failed part, which requires the process to be repeated.
To remove the part first remove all of the tape surrounding the mould. Then pull the two halves apart to reveal the part. Remove the part, sprues and vents as a single moulded piece.
Post-processing is required to remove the sprues and vents from the part. If these had to be positioned on an aesthetic surface then the surface can be sanded down and a clearcoat added for an even surface finish.
Special features
Often, when working with processes such as injection moulding, you may want to add additional non-plastic features to your parts. Vacuum casting allows for many of these special features to be added during the moulding process.
Adding Threads
As vacuum cast parts are made from thermoset plastics, not thermoplastics, it is not possible to use heat staking to install threaded inserts. The main methods used for adding threads to thermoset parts are:
Press Fitting: Press-fit threaded inserts can be installed after moulding. These provide a durable thread but also generate large amounts of stress.
Insert Moulding: Metal threaded inserts can be added to the mould prior to casting creating a very durable thread
Self-tapping: This speciality screw requires an undersized hole to function and provides great vibration resistance. They are not suitable for applications that require repeated assembly and disassembly.
Overmoulding
Overmolding is a process where one material is moulded over another in a secondary operation. This is often done with either two colours or two different material grades. The most common use of this process is to cover a rigid part with an elastomer.
When vacuum casting, two different grades of polyurethane can be used to produce both the rigid and elastomeric aspects of an overmolded part. In order to achieve this, two moulds will be used.
Mould 1: This mould will be used to cast the rigid component. To do this the same steps that were laid out above are followed.
Mould 2: This mould will be used to produce the overmould geometry. A master pattern is created by combining the rigid and overmould geometry in CAD to produce a single pattern. A silicone mould is created using the pattern and the above method.
When the mould is ready for casting the rigid part is inserted into the mould cavity and the mould is closed holding the rigid part. When the resin fills the mould it will bond to the rigid part and form the desired overmould geometry.
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