How to Avoid Sink Marks in Injection Molding?


1. Introduction

Injection molding is an ideal manufacturing process for high-volume production of plastic components. However, it is not uncommon for these components to suffer from surface defects that can negatively impact their structural integrity and aesthetics. One of the most common defects in injection molding is sink marks. In this article, we will explore the causes of sink marks and provide practical tips on how to avoid them, ensuring high-quality components.

2. Understanding the Cause of Sink Marks

In injection molding, the causes of sink mark defects may include the following:

Material shrinkage: Thermoplastic materials shrink during the cooling process, and if the shrinkage is uneven, it can result in sink marks.

Mold design issues: Improper mold design, such as improper gate placement or poor venting, can lead to uneven material filling and subsequent sink marks.

Improper injection speed and time control: Injection speed that is too fast or too slow, or insufficient holding pressure time, can cause material flow issues within the mold or inadequate compensation for material shrinkage, resulting in sink marks.

Inadequate mold temperature control: Mold temperature that is too high or too low can affect material flowability and shrinkage rate, leading to the formation of sink marks.

Insufficient supply of material: If the nozzle hole of the injection molding machine is too small or there is local blockage at the nozzle, it can cause significant loss of injection pressure, resulting in sink marks and depressions.

3. How to Avoid Sink Marks in Injection Molding

A. Optimize Mold Design

Gate location: Setting the gate at the thickest part of the product will help the material fill the mold evenly.

Air venting system: Set up a good air venting system to avoid air bubbles in the mold that can cause sink marks.

Cooling system: Design an effective cooling system to ensure uniform mold temperature and reduce material shrinkage.

B. Adjustment of injection speed and time

Proper injection speed and holding time are crucial to prevent dents. They need to be adjusted according to factors such as the thickness, shape and material of the product. Generally speaking, slower injection speeds and sufficient holding time help to minimize the production of dents.

C. Controlling Mold Temperature

Mold temperature has a significant effect on material flow and shrinkage. Keeping the mold temperature stable and close to the recommended processing temperature range for the material will help reduce dents. Mold temperature can be more precisely controlled by using a mold temperature control system.

D. Using the right material

Selecting a material that matches the requirements of the product is also an important step in preventing sink marks. Considering the material's shrinkage, fluidity, and other characteristics, selecting a more suitable material can help minimize denting problems.

E. Adding support structures

Adding support structures to thick-walled or suspended sections of the product helps reduce sink marks caused by material shrinkage. These support structures can be removed during the post-processing stage.

F. Quality Inspection and Control

Establish a comprehensive quality inspection and control system and conduct regular inspections of the products to detect and deal with dent problems in a timely manner. Utilize tools such as Statistical Process Control (SPC) to analyze data, identify the root causes of problems and take appropriate measures.

4. Conclusion

Sink marks can be a significant concern in injection molding, impacting the quality and appearance of the final product. By considering design factors, material selection, process optimization, gate design, cooling system design, and mold maintenance, manufacturers can effectively prevent sink marks and achieve high-quality injection-molded parts. By implementing these strategies, businesses can enhance their production efficiency, reduce scrap rates, and deliver visually appealing products to meet customer expectations.