How to Avoid Sink Marks in Injection Molded Plastic Parts / XuFeng Tool Tech Group Limited

How to Avoid Sink Marks in Injection Molded Plastic Parts

Introduction

Sink marks are one of the most common defects in injection molded plastic parts. They appear as small surface depressions, usually opposite thick ribs, bosses, screw posts, mounting pads or other areas with local material buildup.

The core cause is uneven volumetric shrinkage. When a thick section cools and shrinks after the outer surface has already solidified, the inner material may not receive enough packing compensation. The surface is then pulled inward, creating a visible sink mark.

For overseas buyers and engineers, sink marks are not only cosmetic. They may indicate risks in wall thickness design, rib and boss structure, gate location, packing pressure, cooling layout or material selection. If these risks are not reviewed before tooling, the mold may require repeated trials, design changes or tooling modification.

At Xu Feng, we review plastic part drawings and 3D models before tooling to check wall thickness, rib design, boss structure, material shrinkage, gate location, cooling risk and cosmetic surface requirements. The goal is to identify sink mark risks before mold manufacturing starts.

Problem

Sink marks usually happen when one area of the plastic part is much thicker than the surrounding wall. The thicker area takes longer to cool, so it continues shrinking after the surface skin has already formed. If packing pressure cannot effectively feed extra material into that area, a depression may appear on the opposite surface.

Common drawing features that may cause sink marks include:

Thick ribs connected to visible outer walls
Solid bosses, screw posts or mounting columns
Thick mounting pads or reinforcement blocks
Abrupt transitions from thin walls to thick sections
Local material buildup near corners, intersections or ribs
Internal structures placed behind cosmetic surfaces
Uneven wall thickness around functional or assembly areas

In many projects, the outside of the part may look simple, but the internal structure can create local thick sections. These areas are often the real source of sink marks.

Manufacturing Risk

If sink mark risks are not reviewed before tooling, the project may face several production problems.

Visible surfaces may fail cosmetic requirements, especially for housings, covers, panels and consumer-facing plastic parts.
Sink marks near screw posts, clips, ribs or assembly surfaces may affect customer acceptance or local part strength.
Thick sections may require longer cooling time, increasing cycle time and part cost.
Repeated mold trials may be needed to adjust packing pressure, holding time, melt temperature, mold temperature, gate size or cooling conditions.
If the root cause comes from part geometry, process adjustment alone may not solve the defect, and design or tooling modification may be required.

DFM Review Points

1. Uniform Wall Thickness

Uniform wall thickness is the first design rule for reducing sink marks. Large thickness changes cause uneven cooling and uneven shrinkage. During DFM review, thick local areas, sudden thickness transitions and unnecessary material buildup should be identified early.

If a thick section is required for function, coring can be used to remove bulk material and keep the wall thickness more consistent. Protolabs also recommends preventing variations in wall thickness and coring out thicker areas to reduce sink risk.^[1]

2. Rib Thickness

Ribs improve stiffness without increasing the whole wall thickness, but ribs that are too thick can create sink marks on the opposite surface.

A common DFM guideline is to keep rib thickness around 40% to 60% of the adjoining wall thickness, depending on material, cosmetic requirements and part structure. For high-gloss or highly visible surfaces, a lower ratio is usually safer. Protolabs notes that bosses should follow a 40-60% surrounding-wall guideline to avoid sink, while Xometry states that rib thickness should generally be no more than 60% of nominal wall thickness. ^[2][3]

3. Boss and Screw Post Design

Bosses and screw posts are common sink mark sources because they often create thick circular sections at the base. During DFM review, bosses should be checked for excessive thickness, location behind cosmetic surfaces, weak draft, poor coring or too much material at the root.

A better structure is usually a cored boss supported by thinner ribs or gussets, instead of a solid block. Hubs also recommends hollowing thick sections and using ribs to improve stiffness, while warning that excessive rib thickness can result in sink marks. ^[4]

4. Smooth Thickness Transitions

Abrupt transitions from thin to thick sections increase shrinkage difference and stress concentration. Smooth transitions, radii and fillets help material flow more consistently and reduce local cooling differences.

For practical DFM, thick-to-thin transitions should be gradual. The exact transition angle depends on part function, material, mold design and appearance requirements, but the key principle is to avoid sudden material buildup.

5. Cosmetic Surface Location

If ribs, bosses or mounting pads are directly behind a visible surface, the risk of visible sink marks is much higher. Cosmetic surfaces should be marked clearly on the 2D drawing before mold design.

This helps the supplier decide whether rib thickness, boss position, gate location, texture, cooling or process control needs special attention.

6. Gate Location and Packing Path

Gate location affects how melt fills the cavity and how packing pressure reaches thick sections. If the gate is too far from a thick area or freezes too early, packing pressure may not compensate shrinkage effectively.

During DFM and mold design, thick areas should be reviewed together with gate position, gate size, flow length, pressure loss and expected gate freeze time.

7. Cooling Design

Uneven cooling can make sink marks more difficult to control. Thick sections may require better cooling support, such as optimized cooling channels, local inserts with higher thermal conductivity or improved mold temperature balance.

Autodesk notes that Moldflow sink mark results can help identify potential design or processing concerns opposite features like ribs, and can estimate sink mark probability and depth. ^[5]

8. Material Shrinkage Behavior

Different plastics shrink differently. PP, PE, PA, POM, ABS, PC, PMMA and glass-filled grades may behave very differently in molding. Fiber-filled materials may reduce overall shrinkage but may also create directional shrinkage depending on fiber orientation.

Material selection should be reviewed together with wall thickness, part size, rib design, gate location and cosmetic requirements.

Practical Suggestions

For real projects, sink mark troubleshooting should follow a practical order: process adjustment first, tooling optimization second, and material or design change when necessary. This sequence helps reduce unnecessary mold modification while still addressing the root cause.

Troubleshooting Level	Typical Actions	When to Use	Notes
1. Process Adjustment	Increase packing pressure/time, adjust melt and mold temperature, optimize injection speed, extend cooling time	Fastest trial method when sink marks are mild or process-related	Validate through part weight, appearance and dimensional stability
2. Tooling Optimization	Adjust gate size/location, improve local cooling, modify thick areas, add coring or optimize inserts	When process adjustment cannot solve sink marks	More effective for root-cause issues but requires mold work
3. Material or Design Change	Use lower-shrinkage resin, glass-filled grade, wall thickness optimization or structural redesign	When shrinkage risk is linked to material or product geometry	Should be evaluated before production tooling when possible

1. Optimize Packing Pressure and Holding Time

Because sink marks often form during the packing and cooling stage, packing pressure and holding time are usually the first process parameters to review. Autodesk Moldflow help states that the most effective way to reduce or eliminate sink marks is to control packing pressure correctly.^[6]

In mold trial, packing pressure may be increased gradually within a safe processing window. Holding time should be long enough for the gate to freeze and for the part weight to stabilize. A practical way to confirm holding time is the part-weight method: increase holding time step by step until the molded part weight no longer increases significantly.

Some factories may start trials with packing pressure around 80-90% of filling pressure, but this should not be treated as a universal rule. The final setting depends on resin type, part thickness, gate design, machine capacity, mold structure and flash risk.

2. Adjust Melt Temperature and Mold Temperature Carefully

Higher melt temperature can reduce viscosity and improve material flow into thick areas, which may help packing compensation. However, too high a melt temperature can increase cycle time, degradation risk or shrinkage problems.

Mold temperature also affects surface solidification and the packing window. For some thick-wall areas, slightly higher local mold temperature may delay surface freeze-off and allow better packing. But this must be balanced with cooling time, cycle time and dimensional stability.

3. Optimize Injection Speed

Injection speed affects filling balance, molecular orientation, local density and pressure transmission. For thick sections, a smoother filling strategy may reduce density variation and help later packing pressure transfer. However, injection speed must also avoid short shots, flow marks, jetting or excessive shear.

4. Extend Cooling Time When Necessary

If the part is ejected before thick sections are sufficiently solidified, post-ejection shrinkage may continue and sink marks may become more visible. Extending cooling time can improve dimensional stability, especially for parts with thick bosses, ribs or mounting pads. The disadvantage is longer cycle time, so cooling changes should be evaluated with cost impact.

5. Improve Gate Location and Gate Size

If a thick section cannot receive enough packing pressure, the gate location or gate size may need review. In general, thick areas should be placed closer to effective packing paths where possible. Larger or better-positioned gates can reduce pressure loss and delay gate freeze, allowing more compensation for volumetric shrinkage.

For new molds, gate strategy should be reviewed before tooling. For existing molds, gate modification may help, but it can affect appearance, weld lines, flow balance and trimming requirements.

6. Strengthen Local Cooling

For severe sink marks caused by local heat concentration, cooling design is often more important than simple parameter adjustment. Possible tooling improvements include adding cooling near thick sections, using high-conductivity inserts such as beryllium copper where appropriate, or improving the cooling channel layout.

For complex parts, conformal cooling or optimized cooling circuits may help reduce hot spots and improve shrinkage uniformity. The exact cooling distance and channel design must be evaluated by mold structure, material, mold steel, insert strength and maintenance requirements.

7. Reduce Local Material Buildup in Tooling or Part Design

If the thick plastic section is the root cause, the most effective solution is to reduce material buildup. This may include coring out thick sections, reducing rib thickness, thinning boss roots, adding hollow structures or changing reinforcement layout.

For ribs, a 40-60% wall-thickness guideline is commonly used, but high-gloss cosmetic surfaces may require thinner ribs. For bosses, a hollow boss supported by thin ribs is usually safer than a solid cylindrical post.

8. Consider Low-Shrinkage or Filled Materials

If the selected plastic has high shrinkage, material change may be considered. Glass-filled materials often shrink less overall than unfilled resin, but they may introduce anisotropic shrinkage and affect strength, appearance, weld lines, tool wear and surface finish.

Material change should be reviewed together with product performance requirements, dimensional tolerance, appearance, certification requirements and cost.

9. Use Mold Flow Analysis for High-Risk Parts

For parts with thick sections, high cosmetic requirements, complex ribs or expensive tooling, mold flow analysis can help identify high-risk shrinkage areas before mold manufacturing. Autodesk explains that sink mark results can predict potential sink behavior opposite features such as ribs. ^[5]

Mold flow should not replace engineering judgment, but it is useful for reviewing gate location, flow balance, packing pressure, cooling layout, volumetric shrinkage and possible hot spots before cutting steel.

When to Ask Supplier for Review

You should ask your supplier for DFM review before tooling if your plastic part has:

Visible cosmetic surfaces
Ribs behind outer walls
Bosses, screw posts or insert features
Uneven wall thickness or local thick sections
Thick mounting pads or reinforcement blocks
Large flat surfaces
Tight assembly requirements
High-gloss or textured appearance requirements
Prototype mold or production mold planning

Early DFM review helps identify whether sink mark risk is mainly caused by part design, molding parameters, mold structure, cooling design or material selection. It is usually easier and more cost-effective to prevent sink marks before mold manufacturing than to correct them after mold trial.

Conclusion

Sink marks in injection molded plastic parts are mainly caused by uneven volumetric shrinkage and insufficient packing compensation in thick sections. The risk increases when a part has uneven wall thickness, thick ribs, solid bosses, local material buildup, poor gate location, insufficient packing or uneven cooling.

The best prevention method is to review the design before tooling: keep wall thickness as uniform as possible, core out thick sections, design ribs and bosses properly, use smooth transitions, mark cosmetic surfaces clearly, and review material shrinkage behavior.

During mold trial, process adjustment should be checked first, especially packing pressure, holding time, melt temperature, mold temperature, injection speed and cooling time. If process tuning cannot solve the problem, tooling optimization, cooling improvement, gate modification, material change or part redesign may be required.

CTA

Have a plastic part design with ribs, bosses, screw posts or cosmetic surface requirements?

Upload your 2D drawing and 3D CAD file for DFM analysis. Our team can help review sink mark risks, wall thickness, mold feasibility, gate and cooling risks, and possible design improvements before quotation.