Shrinkage

This method is used to investigate the shrinkage characteristics of testing sheets manufactured from the material to be tested in conformity with the standard at fully automated test facilities both before and during the production process.

All plastics shrink to a greater or lesser degree. As a result, a molded thermoplastic part has smaller dimensions than the mold in which it is produced.

The total shrinkage of a molding is the sum of the molding shrinkage and 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 24 h at 23 ºC ambient temperature are referred to as molding shrinkage.

All measurements are monitored

The injection-molded part continues to shrink after reaching this state, for example due to post-crystallization in the case of semi-crystalline thermoplastics. This process can be speeded up by storing the molding in a conditioning oven and is termed after-shrinkage. Since a wide range of processing parameters have a huge impact on the overall shrinkage of a molding, it is very important to run the molding production process in conformity with a strictly defined formulation strategy as described, for example, in ISO 294-4.

In addition, processing causes the fillers to become oriented in different ways, either parallel or perpendicular to the direction of flow, particularly with reinforced plastics. For this reason, significant variations are observed in longitudinal and transverse shrinkage with reinforced plastic moldings. In order to determine the shrinkage characteristics, standardized sheets either 60 mm x 60 mm x 2 mm (ISO 294-4) or 150 mm x 105 mm x d (in-house standard) in size are usually manufactured by injection molding.

This combination of standardized sheet production and continuous acquisition of the process data at UL’s fully automated shrinkage test facilities, together with temperature-controlled storage and subsequent quality controlled measurement, enables meaningful longitudinal and transverse values to be obtained for both molding shrinkage and after-shrinkage.

Providing account is taken of the application’s diverse conditions and constraints, this shrinkage data facilitates precise dimensional corrections when designing new injection molding tools, so that the finished injection molded parts are always within the desired dimensional tolerances.

Fully automated shrinkage test facility Fully automated shrinkage test facility Fully automated shrinkage test facility