All thermoplastic materials shrink to some extent as they cool from their melt temperature. Crystalline materials, such as Arnite, tend to shrink much more than amorphous materials. The addition of fillers and reinforcements to crystalline materials has a dramatic effect on mould shrinkage. For example, the addition of glass fibres will reduce the total shrinkage, but will also lead to anisotropic shrinkage. Arnite, like all crystalline materials, produces mould shrinkage, which is somewhat anisotropic. This will result in part warpage if not taken into account when laying out the mould.
The amount of shrinkage and warpage are affected by product, process, and material-dependent factors. Process-dependent factors include wall thickness, flow path length, and gating system.
Increases in wall thickness produce fairly steep increases in shrinkage while narrower gates and larger flow paths also make for higher shrinkage. The gate location is of importance as well: injection of the material at the thickest section is most effective in controlling shrinkage.
Therefore wall thickness and mould temperature will have an effect on the amount of shrinkage.
Approximate shrinkage figures (%) for wall thickness to 2 mm
* parallel and perpendicular to flow direction, respectively
Additional process dependent factors include gate dimensions, melt and mould temperature, holding pressure, holding time, and injection rate. Mould temperature, in particular, has a large influence on the shrinkage of crystalline Arnite grades: the shrinkage decreases as mould temperatures fall.
For best results, the maximum allowable moisture contents for molding should be 0.03% for Arnite T and 0.02% for Arnite A, moisture content. At these moisture levels, no significant deterioration of properties will occur during processing.