How to Improve Sink Marks in Injection Molding

Plastic injection molding is a standard process for producing plastic parts. This process allows us to create parts in a variety of shapes and textures. During this process, plastic material is melted in a hot runner and poured into a mold. This production process requires three main components: a molding machine, a mold, and the plastic feedstock. The plastic injection molding process requires focus and expertise. Failure to exercise due care can lead to defects that can compromise the quality of the molded part.

What are sink marks in plastic injection molding

In injection molding, sink marks (also known as dents or depressions) are localized depressions or unevenness on the surface of a plastic part. They typically appear as tiny depressions in smooth areas or shrinkage marks at locations with varying wall thicknesses, such as the bases of ribs, bosses, and bosses. They are a common cosmetic defect in injection molding. They are essentially caused by uneven localized volumetric shrinkage during the cooling and solidification of the plastic melt, resulting in the surface layer being pulled down by internal contraction forces.

Causes of sink marks in injection molding

1.Reasons related to raw material characteristics

• The plastic shrinkage rate is too high: Different plastics have different shrinkage rates (e.g., PE shrinkage rate is 1.5%-3.5%, PC is about 0.5%-0.7%). Materials with high shrinkage rates change more significantly in volume during cooling. If the mold or process is not matched, shrinkage marks are likely to occur.

• Poor fluidity of raw materials: When plastics with poor fluidity (such as PVC and PC) are filling the mold, the pressure at the front end of the melt decays quickly, making it difficult to fully compact the plastic part, especially the part far away from the gate, which is prone to forming sink marks after shrinkage.

2.Mold design related reasons

• Uneven wall thickness of plastic parts: Parts of the plastic part are too thick (such as the connection between ribs, bosses and the main body). The thick wall cools slowly and the melt continues to shrink after solidification, while the thin wall has solidified and cannot replenish the volume, resulting in sink marks in the thick wall area.

• The gate position is unreasonable: the gate is far away from the thick wall area, and the pressure of the melt is insufficient when it reaches the thick wall, which cannot fully fill and maintain pressure, resulting in sink marks.

• Poor exhaust: When the mold cavity is not well vented, gas is trapped inside or on the surface of the plastic part, hindering the filling of the melt, resulting in insufficient local pressure and forming sink marks after cooling and shrinkage.

3.Process parameter related reasons

• Insufficient injection pressure will lead to insufficient melt filling, low density of plastic parts, and large cooling shrinkage.

• The melt temperature is too high: the plastic cooling time is prolonged, the shrinkage time is increased, and the decrease in melt viscosity may lead to large pressure loss in the holding stage and the shrinkage cannot be compensated.

• The mold temperature is too high: the plastic part cools slowly, the surface solidifies prematurely, and the surface cannot follow the deformation when the inner layer shrinks, forming sink marks (similar to "the outer shell is tight and the inside is shrinking").

• Injection speed is too slow: the melt filling time is long, the front end melt cools down prematurely, and the subsequent pressure is difficult to transmit to the end of the cavity, resulting in shrinkage at the end and sink marks; injection speed is too fast: it may cause melt turbulence, entrainment of gas, indirectly affecting pressure transmission and causing sink marks.

4.Equipment and operation-related reasons

• Excessive equipment pressure loss: Wear of the nozzle and barrel of the injection molding machine, or poor sealing of the check ring, will lead to pressure leakage during the injection and pressure holding stages. The actual pressure transmitted to the cavity is insufficient and cannot suppress shrinkage.

• Inaccurate metering: The plastic in the barrel is not measured enough, the melt volume cannot completely fill the cavity, and the shrinkage after cooling exceeds the compensation capacity of the remaining material, resulting in sink marks.

• Insufficient back pressure: Too low back pressure will cause uneven melting of the plastic, containing bubbles or insufficiently plasticized particles. These defects will aggravate local shrinkage marks during cooling and shrinkage.

How to Improve Sink Marks in Injection Molding

1、Optimize plastic part and mold design

• Improve part structure to avoid wall thickness variations

• Optimize gate and runner design

• Enhance mold venting and cooling

• Adjust injection molding process parameters

2、Increase holding pressure and extend holding time

• Optimize temperature parameters

• Adjust injection speed and time

3、Optimize raw materials and equipment status

• Select low-shrinkage or modified raw materials

• Ensure equipment pressure transmission efficiency

Summary

Improving sink marks in injection molding requires addressing two key areas: compensating for shrinkage and reducing shrinkage differences. This involves systematic optimization, combining raw material characteristics, mold design, process parameters, and equipment status. The core logic behind improving sink marks is to ensure that areas with significant shrinkage (such as thick walls) receive sufficient material replenishment during the cooling process. In practice, it is recommended to prioritize addressing the problem at its source through mold and structural optimization (for long-term effectiveness), then combining this with process parameter adjustments (for quick results), and flexibly matching solutions based on material properties (such as shrinkage and fluidity). During mold trials, the optimal parameters can be determined by gradually changing a single variable (such as holding pressure) and observing the changes in sink marks.