Sink Marks (Shrinkage Marks) on the surface of plastic extruded profiles are a very common and frustrating defect in extrusion production, especially for large hollow/complex cross-section profiles with thick walls and internal ribs (stiffeners/partitions) as shown in the figure.

In the image, you can clearly see very distinct longitudinal depressions/shrinkage lines on the large outer surface of the profile, corresponding exactly to the locations where the internal ribs (partitions) are connected.

I. Core Causes of Shrinkage Marks

Shrinkage marks are essentially caused by uneven localized volume shrinkage:

1. Heat Sink Effect / Uneven Wall Thickness

The "T-junction" or "cross-junction" where the internal ribs intersect with the outer wall is where the plastic accumulates the most thickness.

During cooling, the outer surface layer solidifies first, forming a hard shell, while the internal junction area, being thicker and cooling more slowly, remains in a high-temperature molten/softened state.

As the internal junction continues to cool and shrink, it generates an inward pulling force that "pulls" the not-yet-fully-hardened (or just hardened but still low-strength) outer wall surface inward, creating a concave groove.

2. Insufficient Vacuum Calibration and Cooling

Insufficient vacuum/weak adhesion: The vacuum suction force in the calibration die (calibrator/sizing sleeve) is not strong enough, preventing the molten outer surface from tightly adhering to the inner wall of the calibration die.

Uneven cooling water temperature/water circuit distribution: The cooling efficiency of the calibration die's water circuits in thick-wall or junction areas is too low, failing to rapidly freeze the surface layer.

3. Formulation and Melt State Issues

Excessively high melt temperature: While this improves flowability, it also increases the coefficient of thermal expansion, leading to greater overall shrinkage and slower cooling.

Insufficient melt strength/pressure: Insufficient die head pressure results in less dense extrudate, with internal voids or poor shrinkage compensation capacity.

4. Die Design (Die and Flow Channel)

Mismatched output speed between the rib and the main wall in the die design (if the rib outputs too fast, localized material buildup occurs; if too slow, it causes thinning/stretching).

The transition angle (fillet radius) between the rib and the outer wall is either too rigid or too large, increasing melt accumulation at the junction.

II. Repair and Troubleshooting Solutions

Solving shrinkage marks generally follows the troubleshooting logic of "adjust the process first, then modify the formulation, and finally modify the die."

1. Adjust the Extrusion Process (Fastest Results)

Optimize vacuum calibration (primary check):

Increase the vacuum level in the first/front sections of the calibration die to ensure the profile's outer skin is firmly suctioned against the calibration die's inner wall immediately after exiting the die head, counteracting the internal shrinkage force.

Check whether vacuum holes/slots are clogged with degraded material or scale.

Enhance cooling efficiency:

Lower the cooling water temperature in the calibration tank (especially in the first calibration die).

Increase the cooling water flow rate in the calibration die's internal circuits or water tank areas corresponding to the shrinkage marks.

Lower the melt temperature:

Appropriately reduce the temperatures of the die head and the front section of the calibration stage (without affecting plasticization and surface gloss), thereby reducing the overall volume shrinkage from the molten state to the solid state.

Control extrusion speed and back pressure:

Appropriately increase screw speed/feed rate to raise die head back pressure, making the extrudate denser; or finely adjust the haul-off speed to avoid over-stretching and thinning of the profile.

2. Die Modification and Optimization (If Process Adjustments Reach Their Limit)

If surface indentations remain after process optimization, targeted modifications to the die (die head and calibration die) are required:

Optimize vacuum slots and water circuits in the calibration die:

Add vacuum suction holes/slots at the outer wall positions corresponding to the shrinkage marks to enhance the pulling force in that area.

Add dedicated cooling channels inside the calibration die at the corresponding junction positions to implement localized intensified cooling.

Adjust die cross-section/output flow:

Appropriately reduce the wall thickness of the internal ribs. As a rule of thumb, the internal rib wall thickness should be controlled at 0.6 to 0.8 times the outer wall thickness. If the ribs are too thick, the heat sink effect at the T-junction is extremely difficult to eliminate.

Modify the fillet radius (R-angle) at the connection between the ribs and the outer wall to avoid excessive material buildup.

Fine-tune die output resistance (adjust choker bars/restrictor angles): Ensure uniform output speeds between the outer wall and the internal ribs.

3. Raw Material and Formulation Adjustments

Increase melt strength: Add high-molecular-weight processing aids (such as high-viscosity ACR) to improve melt coating strength and tensile resistance, enabling the surface layer to withstand internal pulling forces before complete cooling.

Increase inorganic fillers/modification: Add appropriate amounts of fillers such as calcium carbonate (within performance tolerance limits). Inorganic fillers have extremely low thermal shrinkage rates and can effectively reduce overall shrinkage.

Check raw material crystallization and shrinkage characteristics: If the material is PP, PE, or other crystalline plastics, which have high crystallization shrinkage, nucleating agents should be used to refine crystal grains and reduce shrinkage.