Vacuum casting

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The vacuum casting is a widely used molding method for fast and cost duplication of master models / prototypes of which are to be made of plastic-like materials mainly casts / small series. With this method, a master model previously produced using various processes (e.g. by 3D printing or stereolithography) is reproduced in a silicone rubber mold. The casting takes place in a vacuum casting machine in order to avoid air inclusions in the cast part.

Vacuum casting is part of a series of processes that are summarized under the umbrella term "Rapid Prototyping". It is carried out by specialized suppliers and specialist departments in vacuum casting systems (widespread) or (less often) using "rapid injector" systems.

The following materials are currently available for the casts to be made with the aid of vacuum casting :

  • Casting resins based on 2K polyurethane; (Plastics) based on 3K polyamide;
  • Meltable wax materials (for use as master models for investment casting);
  • Low-melting metal alloys.

process

The basis for production is the master model. After defining the shape division, sprue and raising, the master model is fixed in a frame. Then the frame is filled with liquid, transparent 2K silicone rubber, everything is degassed under vacuum for a short time (until air pockets are eliminated) and then cured under atmospheric pressure (depending on the application, also under the influence of heat in the oven). The heat only accelerates the hardening of the 2K silicone rubber; it is not absolutely necessary for hardening - usually a chemical reaction of the polyaddition type . It should be noted that curing silicone exposed to heat can be disadvantageous, since silicone cured under heat shrinks significantly - this affects the accuracy of the parts to be cast later due to dimensional deviations or distortions.

After the transparent mold has hardened, the master model embedded in it is exposed using sharp cutting tools such as scalpels, etc. The master model is removed and the shape obtained is heated in an oven and then closed again. The heated closed mold is placed in a vacuum casting machine and filled with liquid resin mixture under vacuum. After removing it from the vacuum, the filled mold is placed in an oven, in which the liquid resin in the mold hardens to a solid substance. The parts produced in this way are removed from the mold after a defined demolding time and finished (i.e. sanded and, if necessary, painted). The mold is then available for further casts.

advantages

  • inexpensive mold production
  • short term mold making
  • Undercuts can be produced (silicone rubber mold can be divided as often as required, elastic, inserts can be used in the mold)
  • easy demoulding
  • high reproduction accuracy
  • Various 2K casting resins available, which simulate series plastics such as B. ABS, PA, PS, PP / PE, transparent PC + PMMA (with optical characteristics), elastomeric materials and special types.
  • Use of mold inserts to improve the quality of the molded part is possible
  • Integration of standard and molded parts (e.g. bearing bushes, threaded bolts) into the plastic parts is possible during the casting process

disadvantage

  • faster wear (on average only 15 to 25 casts per mold possible, as the resins used have different chemically aggressive effects on the silicone)
  • The processing temperature of the material must be feasible through the silicone mold (40 ° C-70 ° C for PUR vacuum casting resins)
  • Problems with the production of shaped elements with a very large height in relation to the cross-section (for example slim bores), unless a vacuum casting machine with differential pressure equipment is used.
  • Disposal of the used molds made of polyaddition-curing silicone is actually unproblematic, since this material is a chemically non-reactive (inert) material.
  • Hollow bodies can only be produced with the help of special cores (using the melting process)

The effectiveness of the process can be increased by simultaneously molding several parts in one mold (multiple mold) or, if possible, by choosing a PUR casting resin with a less aggressive effect on the mold silicone or by using a wax-based mold release spray.

The application area for vacuum casting is the production of small series and prototype series within the process chain ( Rapid Product Development ).