Definition: A shallow depression or dimple in the surface of an injection molded article, caused by local internal shrinkage, usually in area of thicker sections.
Sink marks in a plastic part can be attributed to a not enough material in the part. That is a simplistic way to describe the issue. Typically in processing of plastic parts we fill, and then pack till sinks are gone to start our process. The key point here is that it is with uniform walls, no thick section and no issues with ribs, cores, bosses etc. that may protrude from the wall.
A sink is the depression of the plastic surface, such that it does not have the contour of the mold and is visible by the eye. In most cases these can be resolved by packing more material into the mold, or possibly using a slower or faster fill speed into the mold this last fact is based on part design and flow pattern. As should be noted in all parts, the thicker the wall the slower the fill.
The issue is compounded when in fact we have thick cross section areas such as where ribs, bosses, or projection attach themselves to the wall. If one were to take a section of these areas one would notice that the nominal wall thickness increases. Thus a few things happen in these areas. 1st it is thicker and cools slower, secondly the surrounding walls cool first, thus preventing pressure to reach these areas thus they do not get packed out. As the heat from this mass tries to cool it radiates out to the wall softening the plastic at this point and then as the mass cools it shrinks inward creating the sink.
The solution is uniform walls, but that is not always an option, or even possible. Other ways to attack the issue if for only cosmetic reasons may be to run the steel temperature cooler, thus building up the thickness of the frozen wall so that the latent heat in the mass cannot soften and pull it in as it shrinks. The downside is that it than can create a void, which than becomes a stress raiser, but from a cosmetic standpoint the issue is gone, until of course the parts break. Using a heavier texture on the surface of the part may hide this defect and or make it acceptable, also placing design features at these points can aid in making them acceptable.
Design features can include, offsets, bandings, and different textures, features which might thin out the area or bring attention to the area.
Gate location can play a big part in sinks, especially if the flow of material is from thin to thick, then the thin section acts a pressure regulator to limit pressure to the thicker section thus creating sinks. The nominal wall thickness of the part is also critical, as this allows for pressure to be transmitted, and the thinner the wall the fastest the freeze, the greater the pressure drop. Using process tools such as pressures drops though the system can aid in determining the loss within the mold and feed system. Also gate freeze studies can aid in determining the time it takes to freeze off the gate.
Processing with injection speed works in two ways, a faster fill shall use more uniform pressure within the mold, but also at the same time it lowers the viscosity of the material so it may not pack out as good. While a slower fill has a higher pressure drop across the tool, the material well tend to pack out better when filling, due to a higher viscosity, though one has to ensure you can still fill the entire part at the injection speed selected.
The best case is to avoid thicker section from the beginning in the design phase if one can.
SL SILVEY
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