Dimensional stability

The following information on mold shrinkage, thermal expansion, and water absorption relates to the precision of a component both during molding and as secondary effects after molding.

Shrinkage or mold shrinkage is the difference between the mold cavity dimensions and the corresponding component dimensions. It is not possible to predict exact shrinkage values for a specific polymer grade. Therefore, the maximum and minimum values for the various DSM thermoplastics are provided in the figure below.

Shrinkage indication of DSM polymers in %.

During injection molding the polymer melt is injected into the mold. Once the mold is completely filled the dimensions of the molding are the same as the dimensions of the mold cavity at its service temperature (see "B" in figure below). While cooling down, the polymer starts to shrink (see "C" in figure below). During the holding stage of the injection molding cycle, shrinkage is compensated by post-filling/packing. Both the design of the part as well as the runner/gate should allow for sufficient filling and packing.

The process of shrinkage continues even after the part has been ejected. Shrinkage should be measured long enough after injection molding to take into account post-shrinkage (see "D" in figure below).

Dimensional stability through time.

If components are heated after molding for example during paint curing operation, this can cause temporary or even permanent dimensional changes. The operating environment will also have consequences for the dimensional stability of the component.

An important condition for the dimensions of a part is the use temperature. Thermoplastics show a relatively high thermal expansion (10-4/ °C) compared to metals (10-5/ °C). Thermal expansion cannot be ignored for large parts that are used at elevated temperatures (see "F" in figure above).

Akulon and Stanyl parts, like all polyamide moldings, show dimensional changes  increase) after molding due to moisture absorption (see figure above). Moisture absorption is a time dependent, reversible process that continues until equilibrium is reached. This equilibrium depends on temperature, relative humidity of the environment and the wall thickness of the molding.

A change in moisture content will result in different product dimensions. The designer should anticipate varying humidity conditions during use of the product (see E in figure above). The moisture absorption of reinforced grades differs from those of the unfilled grades.

The moisture content not only affects the dimensions but also various important properties. Yield stress, modulus of elasticity and hardness decrease with increasing moisture absorption, while toughness shows a considerable increase.

Although polyamide moldings are already comparatively tough in the dry state, the high toughness, which is characteristic of Akulon and Stanyl, is not reached until the material has absorbed 0.5-1 moisture. Unreinforced Stanyl already shows a dry as molded impact resistance twice as high as other polyamides, so conditioning is less critical.

Effect of time and humidity on moisture absorption.

Examples of the dimensional stability of unfilled and reinforced Akulon and Stanyl are shown in the figure below.  For polyamide grades in general, the swelling of the thickness is substantial, especially when compared to the swelling in the two other directions. This should be taken into account when designing parts with thick walls.

Dimensional deviations/tolerances. All factors discussed influence the final dimensions of the part. The maximum dimensional deviation of the part is the sum of the individual contributing factors (see dimensional stability figure above).

Dimensional stability of Akulon PA6/PA66  and Stanyl PA46.