How to Remove Mold Sink Marks
Release Date: 2021.11.12  Category: Corporate News
I. Causes of Sink Marks
1-1 Different thicknesses in various parts of the product.
1-2 Insufficient internal pressure in the mold.
1-3 Inadequate mold cooling.
1-4 Deformation caused by insufficient cooling time.
II. Related Knowledge
2-1 During the production process, sink marks are the most frequent defect. Plastic injected into the mold undergoes volumetric shrinkage during cooling. The early-cooled part, the surface, hardens first, creating bubbles internally. Sink marks occur when the slow-cooling part creates noticeable concave surfaces in the direction of the bubble shrinkage.
2-2 Materials with high shrinkage are prone to sink marks. To eliminate sink marks by changing molding conditions, settings should be adjusted toward low shrinkage. Specifically, reduce mold and barrel temperatures and increase injection pressure, though awareness of potential residual internal stress is necessary.
2-3 Since it is best for sink marks to be inconspicuous, a "corrosion" pattern (such as textured or granulated finishes) is sometimes intentionally processed on the mold if it doesn't affect appearance. Additionally, if the material is HIPS, lowering the mold temperature to reduce the finish luster can be effective. However, once sink marks occur, repairing and polishing the product becomes difficult.
III. Solutions
3-1 Immediate: Increase injection pressure, extend injection holding time, lower barrel and mold temperatures, and implement forced cooling at the site of the sink marks.
3-2 Short-term: Patch the flow edge at the site of the sink mark (Figure A). If there is a narrow passage near the sink mark area, thicken that part of the edge (Figure B).
3-3 Long-term: Completely avoid design differences in product thickness. Ribs that are prone to sink marks and narrow/long shapes should be kept as short as possible.
IV. Material Differences
4-1 Materials with high molding shrinkage produce larger sink marks. For example, PE and PP can produce sink marks even with slight reinforcement ribs.
Reference Matters:
5-1 When the temperature is lowered to a point where sink marks do not form, if there is still pressure on the material inside the cavity, sink marks should not occur. The internal pressure (static pressure) of the material within the mold is not uniform everywhere. Pressure is high near the gate; if the flow passage is wide, pressure transfer to all corners is efficient. If the pressure difference between areas near and far from the gate is small compared to the overall pressure, sink marks will not form, and products without residual internal stress can be obtained. When material has difficulty flowing into certain areas, high pressure remains there while pressure in other areas drops, causing sink marks. This residual high pressure also increases internal stress. Ideally, with an increase in material and mold temperatures, fluidity improves, and the injection static pressure state becomes more uniform.
5-2 When changing molding conditions, a table of temperature, pressure, and time combinations should be prepared in advance. By proceeding sequentially, results can be known early. By first setting a very long time, it becomes easier to detect the effects of minor pressure changes. Note: To obtain results for temperature changes, they should be observed after the temperature has dropped following injection.