Sink Mark: A defect in Injection Molding

In the process of injection molding, a thermoplastic polymer is heated beyond its melting point, turning it from a solid to a fluid that has a relatively low in viscosity. This melt is mechanically injected or pressed into mold that mimics the ultimate shape of the product that will be produced. This what we called injection molding.

So what does an injection molding sink marks means? Why does this happen and how can a manufacturer stop it?

The variation in wall thickness on the surface of injection molded parts, such defect known as sink mark.

Sink marks can appear when the part thickness is too thick for the resin being injected into the mold. They are commonly found in thicker areas as a result of varying cooling rates throughout the part or insufficient cooling while the part is in the mold. Low pressure in the mold cavity or an excessive temperature at the gate through which material is injected into the mold can also cause sink marks.

Sink Mark Causes & Solutions:

1. Inadequate control on molding condition:

• If the depression and shrinkage mark appear near the gate, increase the holding time. When the plastic part has a depression at the wall thickness, the cooling time in the mold should be extended.
• By raising the insert temperature, depression and shrinkage mark can be stopped.
• If the injection molding machine’s nozzle hole is too small or the nozzle is locally blocked, the local loss of injection pressure will result in depression and shrinkage marks. As a result, the nozzle must be replaced or cleaned.
• Surface depression can be fixed by the increased feeding amount

2. Mold deficiencies:

• Sink marks that appear far away from the gate are usually caused by poor melt flow in a specific part of the mold structure, which prevents pressure transmission. In this regard, the structural size of the mold gating system should be appropriately expanded, particularly for the “bottleneck” that impedes melt flow, the sprue section should be increased, and the sprue should be extended to the recessed part.
• Wing gates should be used for thick-walled plastic injection molded parts. In this manner, for plastic injection molding parts that are not suitable for directly setting the gate on the plastic part and are likely to cause residual deformation at the gate after molding, a wing-shaped body can be attached to the plastic part, and the gate can then be set on the small wing. The winglets’ gates can be inverted and point gates, transferring the plastic part’s concave defects to the winglets, and the winglets are cut off after the plastic part is formed.

3. Insufficient raw material for molding:

• The surface of the plastic injection molding part will show depressions and shrinkage marks if:
• the shrinkage rate of the molding material is too high,
• the flow performance is too bad,
• the lubricant in the raw material is insufficient,
• or the raw material is damp.

In order to the greatest extent possible, low shrinkage resin grade should be chosen for plastic parts with high surface requirements.

4. The plastic injection molded part’s structure design is absurd.

• Because of insufficient pressure during molding, thick-walled parts are prone to dents and sink marks if the wall thicknesses of the plastic parts vary greatly. As a result, the wall thickness should be as consistent as possible when designing the shape and structure of injection molding parts. If the wall thickness of the plastic part varies, special cases can be solved by adjusting the structural parameters of the gating system.

There are numerous methods for preventing sink marks in injection molded parts, and understanding how and why they occur is the first step. CADMOULD is one of the effective and potential software for sink marks.