In the first step, the surface of the master model is prepared so that it corresponds to the subsequent production part. EDM texturing and graining can be applied, as well as high-gloss surfaces. The master model can be made of any material such as wood, metal or plastic. However, “absorbent” materials such as wood must be sealed beforehand.
Starting from this prepared master model, a silicone casting tool is then produced. Once the sprue and parting line have been defined, the master model is placed in a casting frame.
We typically use addition-curing RTV-2 silicone rubbers in a ratio of 1:10. The required silicone mass is well mixed and pre-evacuated in a vacuum. After carefully filling the mold in a vacuum, the silicone mass is poured around the model.
The mold is either cured in a kiln or at room temperature. When the silicone is completely cured, the model can be removed from the mold.
When demolding the model, the silicone block is cut open at the previously defined parting line with a knife. A wave cut is created in the outer area of the parting line, so that the upper and lower parts later center on each other.
Preparing the casting resins
Production of the vacuum castings first requires the weighing and evacuation of the individual resin and hardener components. Most PU casting-resin systems consist of two components, however in some cases there may be several. The individual components are weighed out in accordance with the mixing ratio and casting quantity, and placed in the casting system.
Mixing and casting
The PU casting-resin components are mixed and stirred under vacuum, then cast into the silicone mold underneath.
Curing of the castings
In order to accelerate the curing process of the plastics, the fully cast mold can be placed in a heating cabinet and cured at elevated temperatures. However, since the curing temperatures have an influence on the shrinkage of the part, they must be selected accordingly.
Demolding and finish
When curing is complete, the finished part can be removed from the mold and finished. The sprue is removed cleanly, the risers (vents) are neatly separated from the component and smoothed. Depending on the requirements, the component is re-ground, possibly polished or subjected to further surface treatment (painting, etc.). Parts that are resistant to higher temperatures are then additionally trimmed to the temperature properties of the material via a tempering process.