A sink mark is a local depression on the surface of a plastic injection molded part, typically found in a thicker area of a component. Sink marks can originate from processing methods, tooling design, part geometry, and material selection.
There are varying standards regarding whether sink marks can be present on plastic injection molded components, depending on the product and its end use. For example, parts for consumer electronics must be nearly free of sink marks, while toys often have visible sink marks on their surfaces.
What Causes Sink Marks
In plastic injection molding, a plastic resin material is melted and then injected into a mold, where it cools and solidifies into the final part or parts before being ejected. Sink marks can occur when there is a part thickness that is too large for the type of resin being injected into the mold. They are typically found in thicker areas due to varying cooling rates across the part or insufficient cooling while the part is in the mold. Sink marks can also be caused by low pressure in the mold cavity or an excessive temperature at the gate through which material is injected into the mold.
5 Techniques for Resolving Injection Molding Sink Marks
Here are some of the most common changes that can be made to resolve plastic injection molding sink marks:
- Lower the mold temperature: By pulling heat away more quickly from a thick area where sink is likely to occur, the chance of differential cooling between thick and thin areas can be reduced. The coolant temperature can be altered from core to cavity side, moving the sink mark in one direction or another, however caution must be taken, for this can cause problems such as distortion.
- Increase the holding pressure: When sink marks are caused by low pressure in the cavity, this may be addressed by increasing the holding or packing pressure. The pressure of molten plastic being injected into the mold can have a strong effect on thermal and mechanical stresses. Modifying this pressure can be helpful, but changes in holding pressure must be carefully controlled.
- Increase the holding time: Increasing the holding time encourages more adequate and even cooling. The ability to control sink marks in this way depends on the location of the gates in relation to the thicker wall sections, and whether there is enough time to fill all areas of the cavity before the gate freezes off. To avoid molded-in stress, pressure, injection speed, and holding time must all be taken into account and balanced.
- Reducing wall thickness: The part design can be adjusted to reduce the thickness of thick wall sections. This will promote quicker cooling and reduce the probability of sink marks.
- Moving the gate location: If the gate is sealed too early, sink marks can result. Relocating gates during design can change the timing of gate sealing, eliminating the sink marks.
Sink Marks and DFM
Injection molding contract manufacturers (CM) use design for manufacturability to prevent sink marks during the design phase, which is the least expensive time to address them. The CM engineers use specialized mold flow simulation software to simulate the injection molding processes, creating 3D simulation of flow, heat flux and warpage for injection molding, including the complete mold with all details. During the simulation, they inspect the project for sink marks and other problems using a checklist for DFM in plastic injection molding. They make changes to the design and plans for the mold, materials, etc., rerunning the simulation and revising until sink marks and other problems are eliminated.