Best Ways to Prevent and Fix Sink Marks in Injection Molding

Best Ways to Prevent and Fix Sink Marks in Injection Molding

Challenges from sink marks are significant in the sphere of injection molding. These common flaws can jeopardize the final excellence of fabricated plastic elements.

To achieve superior results in the injection molding industry, it’s crucial to fully comprehend the root triggers and feasible solutions to these sink marks.

This article aims to delve into these causal factors and mitigations and illustrate how Design for Manufacturability (DFM) approaches can assist in averting sink marks, thus guaranteeing superior production output.

Continue reading to broaden your understanding and gain crucial insights to combat the challenges of sink marks in injection molding.

What’s Sink Mark in Injection Molding?

During plastic manufacturing via injection molding, unavoidable defects, such as sink marks, often manifest. They tend to materialize as indentations or hollows on the exterior of a crafted plastic component.

Generally, these imperfections derive from non-uniform thermal management while the substance is transitioning to a solid state.

In the injection molding procedure, the plastic granules undergo a metamorphosis from firm to liquefied and are then vigorously propelled into an empty mold.

This fluid plastic begins to cool down, shifting again into its firm phase, during which it shrinks and diminishes in its dimensions. Such shrinkage introduces internal tension within the substance.

However, any irregularities or deficiencies in the cooling phase might not grant the plastic sufficient duration for complete solidification, leading to depressions referred to as sink marks.

Sink marks exhibit varying degrees of depth and visual significance, ranging from subtle surface imperfections to noticeable depressions.

They can substantially affect both the final product’s aesthetic appeal and structural robustness, making it essential to comprehend their origins and implement suitable preventative strategies.

Implementing Design for Manufacturability (DFM) practices can also help prevent sink marks. During the design stage, designers can proactively address sink mark issues by considering material selection, part thickness, wall thickness transitions, and gate placement.

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Causes and Remedies of Sink Marks in Injection Molding

Causes and Remedies of Sink Marks in Injection Molding

Sink Marks Cause 1: Incorrect Melt Temperature

An inadequate temperature during the melting phase significantly contributes to the occurrence of sink marks in the injection molding procedure. When the melt temperature is excessively high, it may lead to an extended cooling period and insufficient hardening, producing sink marks.

On the other hand, if the melt temperature is overly low, the material might not circulate effectively, causing an uneven spread and subsequent sink marks.

Remedy: The remedy for incorrect melt temperature is finding the optimal temperature range for the specific material used. Conducting melt temperature experiments and adjusting the settings accordingly can help achieve the desired balance.

It is crucial to consult material data sheets, perform trials, and work closely with material suppliers to determine the ideal melt temperature for the given resin.

Sink Marks Cause 2: Low Pack and Hold Time

Inadequate duration of pack and hold stages can actively foster the development of sink marks. If the pack and hold time falls short, it hinders the material’s ability to settle properly and conform within the mold cavity.

Under such circumstances, the material might not have sufficient time to properly occupy the mold cavity and densely pack, forming sink marks as the substance cools and contracts.

Remedy: By lengthening the packing and holding phase period, the material gets ample time to occupy the cavity and pack compactly.

However, it is essential to strike a balance as an excessive pack and hold time can lead to other issues, such as over-packing and part distortion.

Sink Marks Cause 3: Improper Pack or Hold Pressure

During the injection molding process, if the pressure for packing or holding is inadequate, it can result in sink marks. Insufficient pressure may result in incomplete mold cavity filling or poor packing, leading to sink marks as the material solidifies and shrinks unevenly.

Remedy: Adjustments to the pack and hold pressure settings can help address this concern. Increasing the pack or hold pressure can facilitate better compaction of the substance inside the mold cavity.

Sink Marks Cause 4: High Mold Temperature

Sink Marks Cause 4- High Mold Temperature

In the context of injection molding, increased thermal conditions could prompt the occurrence of sink marks. If the mold temperature soars too high, it may cause the material to cool swiftly, leading to irregular contraction and, consequently, the development of sink marks.

Remedy: Ensuring a suitable temperature for the mold promotes an even cooling effect, thus decreasing the chances of developing indentations.

Vigilant observation and adaptation of the mold temperature parameters, based on the substance and component requirements, are vital for realizing the best outcomes.

Sink Marks Cause 5: Improper Part Geometry

Sink Marks Cause 5- Improper Part Geometry

Improper part geometry is a significant contributor to sink marks. Complex or thick sections in the part design can create challenges regarding proper material flow and cooling, leading to sink marks.

Remedy: Design optimization is crucial to address sink marks caused by improper part geometry. Considerations such as reducing wall thickness, utilizing ribs or gussets for reinforcement, and incorporating proper gating and venting can help improve material flow and cooling, minimizing sink marks.

DFM (Design for Manufacturability) Helps Prevent Sink Marks in Plastic Injection Molding

Design for Manufacturability (DFM) averts sink marks in plastic injection molding. Engineers can pre-emptively tackle problems that might result in sink marks by contemplating specific factors during the design phase.

Here are essential methods by which DFM aids in thwarting sink marks:

1. Material Selection:

Material Selection

Certain materials are particularly prone to developing sink marks as a result of their natural tendency to shrink. To reduce the likelihood of these marks, one should favor materials that have fewer shrinkage properties or use specialized additives that can limit shrinkage.

2. Part Thickness:

Part Thickness

Appropriate part thickness is pivotal in avoiding sink marks. Enhanced thickness in certain areas tends to impede the cooling process, thereby augmenting the probability of sink marks.

Nevertheless, adopting a design that ensures unwavering and precisely balanced wall thickness across the entire part diminishes the occurrence of sink marks. It facilitates a harmonized dissipation of heat throughout the part.

3. Wall Thickness Transitions:

Wall Thickness Transitions

Smooth transitions between different wall thicknesses in part are essential to avoid abrupt changes in cooling rates. Sudden transitions can cause differential cooling, leading to sink marks. Utilizing gradual transitions or incorporating appropriate ribbing can help mitigate this issue.

4. Gate Placement:

The gate placement, where molten material enters the mold cavity, is critical in preventing sink marks. Improper gate placement can result in uneven material flow, inadequate packing, and sink marks.

DFM principles guide engineers in selecting optimal gate locations to ensure proper material distribution and minimize sink marks.

5. Venting:

Proper venting in the mold is essential for preventing sink marks. Trapped air or gas in the mold cavity can hinder material flow and packing, leading to sink marks. Well-designed vents allow for efficient air escape, ensuring complete filling and minimizing sink marks.

6. Mold Cooling:

Mold Cooling

Efficient mold cooling is vital in preventing sink marks. Ensuring the presence of well-designed cooling channels and effective control over the cooling rate plays a crucial role in achieving uniform cooling across the entire part.

This approach significantly diminishes the potential for sink marks by facilitating consistent temperature distribution throughout the material during the cooling process. DFM considerations guide the placement and design of cooling channels to optimize cooling efficiency.

When incorporated at the initial stages of the design process, DFM approaches allow manufacturers to detect and mitigate potential sink mark problems before they reach production.

Manufacturers can achieve a remarkable reduction in the occurrence of sink marks through meticulous refinement of material selection, optimization of part thickness, strategic placement of gating, effective venting techniques, and meticulous control of mold cooling.

Check Out: It All Starts with Design: A Detailed Guideline on Insert Molding Design Guide

Kemal – Your Best Injection Molding Partner

Kemal – Your Best Injection Molding Partner

Regarding injection molding solutions, Kemal stands out as a trusted partner in the industry. With years of experience and expertise, Kemal offers a comprehensive range of injection molding services to meet your needs.

Whether you require assistance designing for manufacturability, optimizing production processes, or ensuring the highest quality standards, Kemal has you covered.

We leverage advanced technologies and state-of-the-art equipment to deliver exceptional results. With a solid commitment to quality, Kemal adheres to strict manufacturing standards and utilizes rigorous quality control measures.

This ensures our products meet the highest industry standards, free from sink marks and other defects.

To learn more about Kemal and how we can assist you with your injection molding needs, visit our website. Contact us directly for personalized support and expert guidance throughout your injection molding journey.

Partner with Kemal and experience the difference between working with a dedicated and reliable injection molding partner. Let us help you achieve your goals of producing high-quality, sink-mark-free plastic parts efficiently and cost-effectively.


Sink marks in injection molding can be caused by several factors, including:

  • Incorrect melt temperature leads to inadequate solidification and uneven cooling.
  • Low pack and hold time, resulting in insufficient packing and material shrinkage.
  • Improper pack or hold pressure leads to incomplete filling and uneven material distribution.
  • High mold temperature, causing rapid cooling and uneven shrinkage.
  • Improper part geometry, such as thick sections or abrupt transitions, hinders proper material flow and cooling.

To prevent sink marks, consider the following measures:

  • Optimize melt temperature based on material specifications and conduct trials to find the optimal range.
  • Increase pack and hold time to ensure complete filling and proper packing of the mold cavity.
  • Adjust pack or hold pressure to achieve adequate material compression and distribution.
  • Optimize part geometry, reducing wall thickness and incorporating smooth transitions to facilitate proper material flow and cooling.

DFM prevents sink marks by considering material selection, part thickness, gate placement, venting, and mold cooling.

By designing parts with uniform thickness, proper gate locations, and optimized cooling channels, DFM ensures efficient material flow, adequate packing, and uniform cooling, thereby minimizing sink marks.

Materials with high shrinkage rates, such as certain thermoplastics like ABS or polypropylene, are more prone to sink marks than materials with lower shrinkage rates, like polyethylene or polycarbonate.

It is important to consider material properties and select the most suitable material for minimizing sink marks.

Although cooling time is an important aspect of the injection molding process, solely modifying the cooling time may not completely eradicate sink marks.

These marks often arise due to insufficient material packing, improper material flow, or incorrect melt temperature.

To achieve an optimal reduction of sink marks, a comprehensive approach encompassing the harmonization of various process parameters alongside the appropriate manipulation of cooling time is indispensable.

Yes, mold design plays a significant role in sink mark prevention. Factors such as the location and size of cooling channels, gate design, venting, and mold temperature control can significantly influence the occurrence of sink marks.

Yes. Visual inspection and surface analysis techniques like 3D scanning, profilometry, or optical inspection systems can help detect and quantify sink marks.

These methods allow manufacturers to assess the severity of sink marks and take corrective measures if necessary.

Employing design optimization techniques such as adding ribs, reducing wall thickness, or modifying gating locations can mitigate sink marks while maintaining the desired part aesthetics and functionality.


Ultimately, ensuring the absence and rectification of sink marks in injection molding is paramount in attaining superior plastic components.

Manufacturers can significantly minimize sink marks by implementing proper process parameters, leveraging design optimization strategies, and partnering with experts like Kemal.

What steps will you take to enhance your injection molding process and create flawless, sink-mark-free parts? Take action today and explore the possibilities with Kemal.

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