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This is a casting process for elastomers which uses a vacuum to draw any liquid material into the mold. Vacuum casting is used when air entrapment is a problem with the mold. Additionally, the process can be used when there are intricate details and undercuts on the mold. Also, it is applied if the material used to make the mold is fiber or a reinforced wire. The process is sometimes called thermoforming because the manufacturing process involves rapid prototyping where the plastic sheets are preheated. The materials are preheated in an automated vacuum casting machine until they are soft and pliable.
Plastic films and sand are sculpted over a pattern using heat, vibration and vacuum to form a mould in the shape of the component to be cast. Suction by vacuum is used to hold the mould in place until the molten metal has solidified.
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Filling The Resin
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Copyright: Open University
Copyright: Open University
1.An alternative to “conventional” sand casting processes, with the advantage of a cleaner working environment. Suitable for either “jobbing”, or fully automated production, although ideally the foundry should be specifically designed for the process.
2.The foundry needs to be specially designed around this process, with special moulding boxes with integral cavities and vacuum lines to accompany moulds. The process is patented and licensed from Japan. Relatively few non-Japanese companies have adopted the process.
3.Uses dry, unbonded silica sand which requires little compaction and is almost fully reclaimable; a major economic advantage.
4.The ethylene vinyl acetate film is 50–100 µm thick and is heated to 100˚C; the temperature is judged by the loss in opacity of the film. After thermal softening the pattern must be uniformly coated, without wrinkles or tearing.
5.Patterns of cast aluminium are cheaper than for “bonded” sand moulding and last longer, since the sand does not come into contact with the pattern.
6.Gating systems should allow the cavity to be filled rapidly. Vents and risers must be located in such a way that the unfilled portion of the casting cavity is at atmospheric pressure, to allow the gases formed in the initial stages to escape to atmosphere.
7.Knock-out is easy, since removal of the vacuum allows the sand to flow out of the box.
8.When casting cast iron, or other similar medium to high melting point materials, the surge of hot air and mould gases released at knock-out produces a cloud of fine silica dust, which is a serious environmental health hazard.
9.Products that have been produced include manhole covers and tractor sprockets.
|Standard lead time||Up to 10 copies within 15 working days|
|Standard accuracy||±0.3% (with a lower limit on ± 0.3 mm on dimensions smaller than 100 mm)|
|Minimum wall thickness||To ensure that the mold is filled properly, a wall thickness of at least 0.75 mm is necessary. For best results, we recommend a wall thickness of at least 1.5 mm|
|Maximum part dimensions||The size of the mold is limited by the dimensions of the vacuum chamber (1900 x 900 x 750 mm) and by the volume of the product (maximum volume: 10 liters)|
|Typical quantities||Up to 25 copies per mold (depending on the mold’s complexity and the casting materials)|
|Surface structure||High-quality surface finish comparable to injection molding|
The table below depicts the general tolerances for industrial-grade vacuum casting services. Stresses during the build, support strategy and other geometry considerations may cause deviation in tolerances and flatness. Improved tolerances may be possible with a manual quote review, after successful completion of a prototype build, and must be approved on a case-by-case basis.