Sink Marks

Views: 0 Author: Site Editor Publish Time: 2020-07-26 Origin: Site

Sink marks on a plastic part appear as a depression or dip on the part surface. These are created when the shrinkage of the molten core of a part wall is greater that that of the skin. This larger shrinkage of the molten core"sucks"in the surface, causing a depression to occur.The list causes and solutions for sink marks.

Causes:

1. High melt temperatures cause sink marks because they prolong the cooling time of the part. This slows down the cooling of the molten core

2. If the pack pressure or holding pressure is too low, not enough material will be pushed against the cavity wall. This lack of pressure will not supply enough resistance to the molten core shrinking away from the cooler part wall

3. Lower injection forward time can create sink marks since both pressure and time are needed to assure that the part is cooled and packed sufficiently

4. Short cooling times will prevent the molten core of the part from solidifying creating sink marks

5. Obstructions in the nozzle restrict flow, underpack the part, and also cause sink marks

6. If the gate freezes off too soon, the part will not pack out completely, allowing sink marks to form

7. Smaller cross-section runners, such as half rounds, will cool and solidify faster and as a result restrict flow to the cavity

8. Mold temperatures that are too cold will set up the skin of the part much faster than the molten core

Solutions:

1. Reducing the melt temperature will reduce the cooling time of the part and reduce the sink marks

2. Higher packing pressure or hold pressure will provide more pressure on the cavity wall to cool the skin and core to prevent si nk marks

3. High injection speeds increase melt temperature due to high shear, and lowering injection speed will reduce melt temperature and sink marks

4. Workpieces need enough cooling time in the cavity to solidify the skin and core

5. Enough shot size needs to be provided to assure that enough material enters the cavity and fills it completely

6. Gate seal time needs to be examined to determine if the gate is freezing off before the cavity is full

7. Moving to larger-diameter runners, such as full round trapezoidal, and modified trapezoidal runners, will keep the runner more molten and hotter, allowing the material to flow more easily

8. Increasing mold temperature will slow down the as dead spots into the cavity areas where the melt can stagnate