 | | Production and measurement of shrinkage sheets
All plastics have a more or less pronounced shrinkage behavior. This means that the molded thermoplastic part has smaller dimensions than the mold in which it is produced.
The total shrinkage of a molding is the molding shrinkage added to the after-shrinkage.
As it cools, the part begins to shrink in the injection mold. This process also continues after demolding. The dimensional changes until thermodynamic equilibrium is reached after 16 to 24h at 23 ºC ambient temperature are referred to as molding shrinkage.
Fully automated shrinkage test facility |  | The injection-molded part continues to shrink after this time, for example after post-crystallization in the case of semi-crystalline thermoplastics. This process can be speeded up by storing the moldings in conditioning ovens and is termed after-shrinkage.
Since different processing parameters have a huge impact on the overall shrinkage of a molding, it is very important to run the production process for the molding in accordance with a strictly defined formulation strategy as defined for example in ISO standard 294-4. |
In addition, with reinforced plastics in particular, processing causes the filters to become oriented in different ways, longitudinal and transverse to the direction of flow. For this reason, in particular with moldings made from reinforced plastics, there are clear differences in the longitudinal and transverse shrinkage.
In order to determine the shrinkage properties, standardized sheets of dimensions 60 mm x 60 mm x 2 mm (ISO 294-4) or 150 mm x 105 mm x d (in-house method) are usually produced by injection molding.
Through the combination of standard sheet production with complete process data compilation, temperature-controlled storage and subsequent quality-controlled measurement, it is possible to obtain a meaningful longitudinal and transverse shrinkage value for both the molding and after-shrinkage.
Taking into account the boundary conditions relating to the application, this shrinkage data makes it possible to carry out precise dimensional correction when designing new injection molding tools, there by adhering to the desired dimensional tolerances for the finished injection-molded parts.
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