One of the defects that can rear its ugly head while molding, are sink marks. A sink mark can be defined as a depression in the plastic part; it can even resemble a dimple or a groove. It is caused by excessive localized heat with resultant contraction (sinking) of the resin/composite as the finished part cools. While sink marks can occur during start-up, more problematic is when the defects appears during a run. The causes can run the gamut of part design, mold, machine, and process.
Primarily, work with your mold designer and molder (even better if they are one in the same) as up front as possible. Potential for sink marks comes from concentrated heat. Part features like excessive thickness where mating walls meet and non-uniform and excessively thick wall sections can be recognized and eliminated.
Next, with collaboration on the design you draw attention to mold design issues that can eliminate variables. Even when mating walls have been optimized, they are a localized heat source. If the mold inadequately removes heat from the part, (cooling line placement or access) it will eject hotter than desired and sinks will form in due time. This defect can show up well into the run as the mold builds up heat. So auto-running processes could yield acceptable parts and then produce sink marks later increasing scrap and labor. In addition, discussion can bring light onto the size of the gates and runners in the mold. While mold designers want to be conservative and steel safe, when too small, gates and runners will constrict flow of the resin over the shot time. When not fully packed out, the part has room to contract and sinks form. Lastly, the gate location, or more accurately the features opposite the gate, has the potential to cause early cooling and improper mold filling, also resulting in sink marks.
Of course, processing is a significant contributor to sink marks. Suffice it to say that time, temperature and pressures all must stay consistent during the run. Systems creep in hydraulics, wear on the non-return (check ring) valve, the flow rate of the resin, even a consistent cycle by the press and or operator can be a cause.
So next time your discussion turns to sink marks, preparation and maintenance can help prevent that sinking feeling as you look at your parts.
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