How to Improve Sink Marks in Injection Molding
Sink marks are a common defect in injection molding that can affect the surface quality and appearance of a product. They are primarily caused by the pressure and friction of the ejector pin on the plastic as it cools and solidifies.
There are several ways to improve sink marks:
1. Improvements in Mold Design
Ejector Pin Design: Ejector Pin Diameter: Increase the ejector pin diameter to reduce the pressure per unit area.
Ejector Pin Taper: Adjust the ejector pin taper to reduce the contact area between the ejector pin and the plastic part, decreasing friction.
Ejector Pin Material: Select a material with a smooth surface and high hardness, such as nitrided steel.
Venting System: Increase Venting Holes: Add more venting holes to the mold to promptly evacuate air from the cavity, reducing sink marks.
Optimize Venting Channels: Optimize the shape and location of venting channels to improve venting efficiency.
Draft Angle: Increase Draft Angle: Increase the draft angle between the plastic part and the mold cavity to reduce friction between the ejector pin and the plastic part.
2. Adjustments to Injection Molding Process Parameters
Injection Speed: Reduce Injection Speed: Reduce the injection speed to give the plastic sufficient time to fill the cavity, reducing the pressure of the ejector pin on the plastic.
Injection Pressure: Reduce Injection Pressure: Reduce the injection pressure to decrease the pressure on the ejector pin.
Dwell Time: Extend Dwell Time: Extend the dwell time to allow the plastic to fully cool and solidify, reducing the occurrence of sink marks.
Mold Temperature: Increase the mold temperature to reduce the plastic's viscosity, aid in its flow and filling, and reduce sink marks.
3. Material Selection
Select a Material with Good Flow: Choose a material with good flow properties to fill the cavity more quickly, reducing the time the ejector pin exerts pressure on the plastic.
Select a Material with Low Shrinkage: Select a material with low shrinkage to reduce the deformation of the plastic part during the cooling process, mitigating sink marks.
4. Post-Processing
Sanding and Polishing: Sanding and polishing the product can improve the surface appearance of sink marks.
Other Factors to Consider
Ejector Pin Location: If possible, place the ejector pin in a non-critical area or a hidden location on the plastic part.
Number of Ejector Pins: Determine the appropriate number and location of ejector pins based on the shape and complexity of the plastic part.
Ejector Pin Stroke: Adjust the ejector pin stroke to ensure it fully retracts from the plastic part, preventing deep sink marks.
