It is impossible to mould thick-walled thermoplastic parts without sink marks when using conventional injection moulding. The use of chemical or physical propellants - so-called structural foam moulding - can help to significantly reduce shrinkage in thick-walled areas and thereby prevent sink marks. However, this goes along with a dramatic deterioration of the surface quality like porosity and silver streaks. Thus far, these defects have rendered conventional injection moulding in combination with chemical or physical propellants useless for decorative parts with high-gloss or structured surfaces. The defects that are caused by the propellants can be avoided by heavily increasing the mould wall temperature. The level of required ideal mould wall temperature results from the used type of thermoplastic resin. This, in turn, drastically increases the cycle times and thereby the cost of production. The solution to the problem is as follows: The mould wall temperature has to be elevated as fast as possible to the required level within an injection cycle, which is to be cooled down then to the base temperature immediately thereafter. While these so-called variothermal processes have long been deemed state-of-the-art, they often involve excessive heating and cooling times, regardless of the technological principle that is applied.
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