WHAT IS VACUUM CASTING
Vacuum Casting (also known as urethane casting) is a technic used to produce part by using a silicone mold to reproduce the master part. It typically use a negative mold in silicone to produce a positive part by receiving a resine in its negative print cavity.
WHY USE VACUUM CASTING FOR RAPID PROTOTYPING
Vacuum Casting is a widely used process in rapid prototyping, typically for building 5 to 50 pieces of functional prototypes. So, this is well adapted for small run where you would not open a steel plastic injection mold for such small quantities. In short you can make some small production with silicone mold where you inject resine in it without investing in an expensive tooling.
Silicone molds are perfect for short run production because they are affordable and allow to make until 15-20 pieces per mold.
Usually series of 5-100 pieces will be run using this casting technology. Each Silicone mold can make maximum 15 to 25 pieces depending on the complexity of the part.
PROCESS OF VACUUM CASTING DURING PROTOTYPING
In order to create the silicone mold which will be the negative of the part, we first need to have a reference part (the positive, also called master) which will be used to create the silicone mold (the shell).
So, if a master is not existing yet, then we first need to make an accurate master first using SLA, 3D printing or CNC process. If the part is in rubber, we need to make a solid strong part to keep it straight.
Next step of vacuum casting process involves encapsulating/enclosing this master model in a liquid silicone rubber to give to the silicone the right shape of the part.
Then a vacuum is applied to remove air bubble and create a silicone mold adhering perfectly to the master. The mould is then cured in an oven at around 70°C in order to make the silicone to solidify. Some hole/pipe are made so the resine can feel the space inside the mold.
Once cured the silicone 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.
Then, in order to produce the prototype, the silicone mold being empty of master shoudl receive resine via a hole made into the mold from the top. With gravity, the resine will flow from top to bottom to fill the cavity shape. The whole is placed in oven for a while and then for drying, after what the solidified resine with shape of the parts is extracted.
Quite often, the part is presenting some dents due to the unperfect join of the parts of the mold, so a polishing is necessary to create a smooth surface of the part.
ADVANTAGES OF VACUUM CASTING FOR PROTOTYPE MAKING
- Small batch production, typically 15 – 30 parts per tool
- Affordable cost
- No need to open an expensive steel mold to perform pilot run
- Faster than opening a steel tooling
- Complex shapes and undercuts are possible without the need of joining the parts
- Wide array of colour and surface finishes can be achieved
- Overmolding, double and triple shot possible by injecting different material
- Double or triple shot products possible, rubber or TPE or TPR can be used. (with 20-80 shore A hardness)
- Can use direct color to avoid painting
- Wide range of imported PU simulant material available – Axson (France) and Hei Cast (Japan)Over-molding of rigid clear lens, inserts and soft PU materials
- RoHS certified materials so perfect for special purpose machine short run production (e.g. medical equipment).
LIMITATIONS OF VACUUM CASTING PROTOTYPE
- Maximum size of 1200 X 1000 X 1000 mm
- Material is not exactly the same as during plastic injection
- Mostly used for plastic
- Vacuum casting is relatively labour intensive and typically requires between 2 and 5 days to produce the first casted product.
MATERIALS THAT CAN BE USED WITH VACUUM CASTING
- Transparent PU
- Soft plastic PU
- ABS PU
- PP/PE PU
- Polycarbonate PU.
Example of materials below/
- 8150 PU – ABS simulant. Colors: White and black (Hei Cast)
- 8400 – Rubber simulant PU. 20 to 90 Shore A durometer. Colors: White and black (Hei Cast)
- PX 100/215 – HIPS simulant. Color: Natural (Axson)
- UP 4280 – ABS simulant with a higher flexural modulus of 2100 MPa. Color: White, Black and off-white (Axson)
- PX 223 HT – ABS simulant. High temperature resistance – 120 C. Color: Black (Axson)
- UP 6160 – ABS simulant. High temperature resistance – >200 C. Color: Natural (Axson)
- PX 527 – PMMA/PC or filled ABS/POM simulant. Color: Clear (Axson)
- PX 521 – PMMA/PC simulant with UV resistance. Color: Clear (Axson)
- UP 5690 – PP/PE/HDPE simulant. Color: White and Black (Axson)
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