TPE plastics can be processed on standard injection molding machines for thermoplastics.
THERMOLAST® K compounds enable longer residence times if this is required.
Due to the structural viscosity of TPS, these compounds are among the materials with the greatest potential in relation to the range of the processing window.
What general requirements have to be met when using TPEs in injection molding?
In general, materials from KRAIBURG TPE are free-flowing granules. These can be processed on standard injection molding machines for thermoplastics. The three-zone screws should have a compression ratio of at least 2:1 and an L/D ratio of at least 20. Processing with open nozzles is possible; working with shut-off nozzles is advantageous.
THERMOLAST® K compounds enable longer residence times if this is required. Short residence times should be the general aim, especially in the hot runner system. Care needs to be taken to ensure that the volume of the hot runner system does not exceed one to two times the shot weight.
How high should the injection speed be set?
Injection pressure and injection speed are determined by factors such as the design of the item and gate as well as the structural viscosity of the material and may vary according to the compound. For structurally viscose plastics, the cavity should be filled at a high injection speed. This makes it possible to achieve long flow paths even for very thin-walled components.
How is the filling study carried out?
The filling study begins when the machine is started at a medium rate and without holding pressure. Depending on the article’s geometry, an injection profile may also be effective. The shot volume is increased in steps of ten percent, starting from half of the calculated volume. This ensures efficiency of the ejectors.
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The solidification point is determined using a combination of holding pressure time and holding pressure level. The holding pressure only serves to compensate for processing shrinkage and not to fill the item. Starting with a holding pressure of 40 to 60 percent of the injection pressure achieved at the changeover point, the holding pressure time is increased step by step without overpacking the mold. The weight is determined after each shot.
If no further change can be detected, the solidification point has been reached and the holding pressure phase has been completed. After assessing the item quality, this process can be repeated at higher holding pressures. It may also make sense to use holding pressure profiles.
Optimization of residual melt cushion
The residual melt cushion ensures neat pressure transmission during the holding pressure phase. It should be kept as small as possible. An optimum residual melt cushion is 10 to 15 percent of the screw diameter.
