Dec 27, 2018
In areas with thick walls and across from ribs, a hollow at the surface might appear.
Sometimes a difference in the gloss to the surrounding surface might be noticed only.
Mass accumulation leads to a higher volumetric shrinkage, which is insufficiently reached by packing pressure.
The surface is pulled to the inside during cooling and a sink mark appears.
Small wall thickness and higher packing pressure in the critical area reduce the risk of sink marks.
Jul 16, 2019
Heat can be described physically as oscillation of the molecules.
An excessive oscillation can impact the molecules of plastic drastically.
The effect of it is called thermal impact.
In the worst case, the molecules can break: the molecules degrade.
This is a damage of the plastic, which increases the higher the prevailing temperature.
Furthermore, the duration of the temperature effect – in injection molding called residence time – plays an important role.
An example from the daily life is the sunburn.
Here again, the two factors temperature and residence time are responsible for the thermal impact.
Injection Molding Defect: The longer and the higher the
thermal impact of a body, the darker it will get (Example: Vegetable).
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.
Dec 27, 2018
In areas with thick walls and across from ribs, a “hole” in the inside might appear.
Mass accumulation leads to a higher volumetric shrinkage, which is not insufficiently adjusted by packing pressure.
The melt is pulled to the surface during cooling, that a hollow remains in the inside.
CT-Scan and cutting the part help to detect these errors.
Small wall thickness and higher packing pressure in the critical area reduce the risk of voids.
Dec 19, 2018
When at least two melt fronts meet, a weld line occurs.
Weld lines display on one hand a reduction of the mechanical strength, on the other hand a beauty error in the part.
The lower the melt front temperatures, the more difficult it is for the melt fronts to merge.
The smaller the meeting angle (melt fronts meet head-on), the more critical.
For fiber filled materials, a weld line changes the orientation of the fibers, that they will orient perpendicular to the flow direction.
To reduce weld lines it is possible to work with sequential injection or overflow cavities.
