---
title: "Sink Marks Guide: Root Causes, Fixes, and Prevention for Injection Molded Parts - JATERSON"
description: "Injection molded part sink marks cause costly production delays and rework for global procurement and engineering teams. Learn root causes, step-by-step diagnosis, and actionable prevention measures from a Zhejiang manufacturing expert to cut defect rates and protect project timelines."
url: "https://www.jaterson.com/manufacturing/sink-marks-guide-root-causes-fixes-prevention-injection-molded-parts.html"
language: "en"
type: "Article"
category: "Injection Molding Guide"
datePublished: "2026-08-30"
dateModified: "2026-08-30"
brand: "JATERSON"
manufacturer: "JATERSON"
image: "https://static.jaterson.com/uploads/industry/plasticparts/lfcOMwGB35lbU.webp"
---

# Sink Marks Guide: Root Causes, Fixes, and Prevention for Injection Molded Parts

If you’ve worked with injection molded plastic parts,you’ve likely heard the common claim that sink marks are nothing more than a minor cosmetic flaw,easily fixed by cranking up injection pressure on the press.This is one of the most costly and widespread misconceptions in plastic manufacturing.

In reality,sink marks signal uneven material shrinkage during the cooling phase,and their root cause often traces back to part design,mold construction,or material selection — not just machine settings.Attempting to fix them with pressure alone can lead to flash,excessive internal stress,or even mold damage,creating more problems than it solves.This guide walks through a systematic,manufacturing-focused approach to diagnosing,correcting,and preventing sink marks,with actionable steps for procurement,engineering,and quality teams.

![Sink Marks Guide: Root Causes, Fixes, and Prevention for Injection Molded Parts](https://static.jaterson.com/uploads/industry/plasticparts/lfcOMwGB35lbU.webp)

## What Are Sink Marks,and Why Do They Form?

Sink marks are shallow,localized depressions on the surface of injection molded plastic parts,typically appearing on thicker wall sections,or directly above ribs,bosses,gussets,or other internal features with increased material volume.They form because thermoplastic resins shrink as they cool from molten to solid state — the rate of shrinkage varies by resin type,but all resins experience some level of volumetric shrinkage during cooling.

The failure mechanism is straightforward: when a part has uneven wall thickness,thinner sections cool and solidify first,while thicker,adjacent sections retain heat longer.As the molten material in the thick section cools and shrinks,it pulls material inward from the already solidified outer skin.If the outer skin is not rigid enough to resist the pull,a depression (sink mark) forms on the part surface.If the skin is rigid enough,the material pulls inward from the center instead,creating internal voids or bubbles.

This is why the “just increase pressure” myth is so dangerous: if the temperature differential between thick and thin sections is large enough,no amount of hold pressure will fully compensate for the uneven shrinkage,and excessive pressure will only create new defects.

## Step 1: Systematic Sink Mark Diagnosis

Before making any adjustments to process settings or mold design,you must first confirm the defect is a sink mark (not another surface flaw) and identify its root cause category.Skipping this step leads to wasted time and unnecessary costs.

### 1.1 Defect Validation Checklist

Use these checks to confirm you are dealing with a sink mark,not an ejector pin mark,short shot,or air trap:

- Run a clean fingernail across the defect: if it catches on a shallow depression that follows the contour of an internal thick feature (rib,boss,screw boss),it is almost certainly a sink mark.Ejector pin marks will be uniformly round and aligned with known ejector pin locations on the mold.
- Measure local wall thickness: sink marks almost exclusively occur where local [wall thickness](https://www.jaterson.com/knowledge/wall-thickness/) is **20% or more greater** than the adjacent nominal wall thickness.If the wall is uniform,the defect is likely caused by something else.
- Map the defect relative to the gate: sink marks far from the gate may be related to pressure drop during fill,while those on thick sections near the gate are almost always driven by uneven cooling and shrinkage.
- Cut a cross-section of the affected area: if internal voids are present under the surface,the root cause is the same uneven shrinkage,but voids carry higher structural risk than surface sink marks.

![Step-by-Step Sink Marks Diagnosis & Correction Guide for Molded Components](https://static.jaterson.com/uploads/industry/default/bl2ZTlye4Xuix.webp)

### 1.2 Root Cause Triage

Once you confirm the defect is a sink mark,narrow down the root cause category using these key indicators:

- **Design-related:** Sink marks are consistent across all production runs,all cavities,and all machine settings,located directly on thick features.This is the most common root cause for new parts.
- **Mold-related:** Sink marks appear only on specific cavities,or worsen as the production run progresses and mold temperature rises.This usually points to uneven cooling or blocked cooling channels.
- **Process-related:** Sink marks appear randomly across runs,or start after a machine changeover,material batch change,or process parameter adjustment.These are the easiest fixes if the root cause is truly process-related.
- **Material-related:** Sink marks appear only when using a specific resin grade or batch,even with identical process settings.High-shrinkage resins (like semi-crystalline PP or PE) are far more prone to sink marks than low-shrinkage amorphous resins (like ABS or PC).

## Step 2: Targeted Corrective Actions by Root Cause

Once you have identified the root cause category,apply targeted fixes in order of lowest cost to highest cost.Always make one change at a time,run 10-20 test shots,and measure results before moving to the next adjustment — this is the only way to build a stable,repeatable process.

### Design-Related Sink Mark Fixes

Design-related sink marks are the most expensive to fix after a mold is built,so these changes are best applied during the DFM phase.For existing molds,use workaround solutions where possible:

- Adjust rib and boss thickness: Follow the standard design rule that rib thickness should be **40-60% of nominal wall thickness** for amorphous resins,and 50-70% for semi-crystalline resins (which have higher shrinkage rates).Reducing rib thickness eliminates the temperature differential that causes sink marks.
- Add a sink mark pocket (counterbore): For existing molds where design changes to the boss or rib are not feasible,machine a shallow recess on the outer surface directly above the thick feature.The [sink mark](https://www.jaterson.com/knowledge/injection-molding-sink-marks/) will form inside the recess,making it invisible or within acceptable limits for cosmetic surfaces.This is a low-cost workaround that can resolve 60-70% of cosmetic sink mark issues on existing tools.
- Relocate thick features closer to the gate: Moving high-volume features closer to the gate ensures they receive full hold pressure before the gate freezes,reducing sink mark severity.

### Mold-Related Sink Mark Fixes

If the root cause is uneven cooling or poor mold design,these adjustments will address the issue at the source:

- Improve cooling near thick sections: Add beryllium copper (BeCu) inserts in high-heat areas to conduct heat away faster,or add conformal cooling channels for more uniform temperature control.This reduces the cooling rate differential between thick and thin sections.
- Increase gate size or relocate gates: Larger gates stay molten longer,allowing more material to pack into the cavity during the hold phase.Moving gates closer to thick sections has a similar effect,as hold pressure reaches the thick area before the gate freezes.
- Clean and maintain cooling channels: Mineral buildup,rust,or debris can block cooling channels over time,leading to uneven mold temperature.A regular cooling system flush and maintenance schedule prevents this issue and extends mold life.
- Check venting: Insufficient venting can trap air in the cavity,preventing full packing and creating surface depressions that mimic sink marks.Ensure vent depth is 0.02-0.03mm for most engineering resins,and vents are located at the end of fill paths.

### Process-Related Sink Mark Fixes

Process adjustments are the first line of defense for existing production runs,but they only work if the root cause is not a fundamental design or mold issue:

- Increase hold pressure: Higher hold pressure pushes more material into the cavity to compensate for shrinkage,but only while the gate remains molten.Never exceed the mold or machine’s maximum pressure rating,as this can cause flash,mold deflection,or tool damage.
- Optimize hold time with a gate freeze study: Hold pressure has no effect after the gate freezes.To find the optimal hold time,increase hold time in 0.5-second increments until part weight stops increasing — this is the minimum effective hold time,and any additional time only wastes cycle time.
- Reduce melt temperature: Lower melt temperature reduces total volumetric shrinkage,which can reduce sink mark severity.Adjust in 5°C increments and monitor for short shots or other fill defects.
- Adjust injection speed: Faster injection fills the cavity before the outer skin layer solidifies too much,allowing better packing of thick sections.Avoid excessive speed,which can cause jetting,shear heat,or mold erosion.

### Material-Related Sink Mark Fixes

If the sink mark severity is tied to resin selection,these adjustments can help without changing part or mold design:

- Switch to a lower-shrinkage resin grade: Adding glass fiber,[mineral filler](https://www.jaterson.com/knowledge/mineral-filler/),or impact modifiers can reduce resin shrinkage rate by 30-60%,which often eliminates or reduces sink marks.Always consult your engineering team before changing resin,as filler content affects mechanical properties,cost,and processing requirements.
- Verify material drying conditions: Moisture in hygroscopic resins (nylon,PET,PC) can cause surface defects and internal voids that look identical to sink marks.Follow the resin manufacturer’s drying temperature,time,and dew point specifications exactly.
- Use shrinkage-reducing additives: Some resin suppliers offer flow modifiers or shrinkage control additives that reduce uneven shrinkage,which can help with moderate sink mark issues on existing parts.

## Step 3: Long-Term Sink Mark Prevention for New Projects

The most cost-effective way to handle sink marks is to prevent them before mold cutting,when design changes cost a fraction of mold rework.Use these steps for all new injection molding projects:

- Conduct a full DFM review before tooling: Work with your manufacturing partner to review wall thickness ratios,rib design,boss placement,and gate location to identify sink mark risks early.A 1-hour DFM review can prevent 80% of common sink mark issues.
- Run mold flow analysis: For parts with complex geometry,strict cosmetic requirements,or high production volumes,mold flow simulation predicts where sink marks are likely to occur,allowing design or mold adjustments before physical tooling is built.
- Build prototype tooling for high-risk parts: Soft prototype molds (aluminum or P20 steel) allow you to test design iterations and process settings at low cost,before investing in hard production tooling.
- Define clear acceptance criteria upfront: Agree on sink mark limits for cosmetic and functional surfaces with your manufacturing partner before production starts,using a golden sample for reference.This eliminates costly disputes later.

## Sink Mark Acceptance Criteria for Procurement & Quality Teams

Sink mark acceptability depends entirely on the part’s application,surface type,and functional requirements.There is no one-size-fits-all standard,but these general guidelines provide a starting point for discussions with your manufacturing partner:

| Part Surface Type | Acceptable Sink Mark Depth | Acceptable Size & Quantity | Validation Method |
| --- | --- | --- | --- |
| Class A Cosmetic (high-visibility,high-gloss,painted/plated) | < 0.02mm | No visible sink marks under 500 lux lighting at 30cm viewing distance | Visual inspection per customer-approved standard; depth micrometer for verification |
| Class B Cosmetic (visible,matte finish,low-visibility area) | < 0.05mm | Diameter < 3mm,no more than 2 marks per 100cm² area | Visual inspection + fingernail test (no catching) |
| Functional / Non-Visible Surface | < 0.1mm,or < 2% of local wall thickness | No size restriction as long as structural performance is unimpaired | Cross-section measurement; load testing for load-bearing features |
| Sealing / Precision Fit Surface | 0mm (no sink marks allowed) | N/A | [Coordinate measuring machine (CMM)](https://www.jaterson.com/knowledge/coordinate-measuring-machine-cmm/) or height gauge measurement |

Always align these criteria with your end product’s performance and cosmetic requirements before production begins.For high-volume parts,a signed-off golden sample is the most reliable way to avoid misalignment between customer and manufacturer.

## Common Sink Mark Fix Mistakes to Avoid

In our two decades of injection molding production in Zhejiang,we’ve seen teams waste weeks of production time and thousands of dollars on ineffective sink mark fixes.These are the most common mistakes to avoid:

- **Over-reliance on injection pressure:** As noted earlier,many teams jump straight to increasing hold pressure when they see sink marks.If the root cause is a 2x wall thickness variation,even maximum pressure will not eliminate the sink,and will likely cause flash,increased internal stress,or mold damage.We once worked with a customer who tried to fix a sink mark on a mounting boss by increasing hold pressure by 30% — the result was a 12% flash rate and a 3-week delay while we repaired the mold’s worn parting line.
- **Making multiple changes at once:** If you adjust melt temperature,hold pressure,and injection speed all in one shot,you will never know which change fixed the problem (or made it worse).Change one parameter at a time,run enough shots to stabilize the process,and measure results before moving on.
- **Ignoring structural risks:** Sink marks on load-bearing ribs,bosses,or brackets can reduce cross-sectional area by 10-20%,leading to premature failure under load.Never assume sink marks are only cosmetic — always conduct functional testing if they appear on structural features,even if they are on non-visible surfaces.
- **Skipping root cause diagnosis:** It’s tempting to apply quick fixes,but taking 30 minutes to diagnose the root cause will save hours of trial and error.For example,if the sink mark is caused by a blocked cooling channel,adjusting process settings will only be a temporary fix,and the problem will return on the next production run.

For procurement and engineering teams working with injection molding suppliers in China,the key to managing sink marks is early collaboration.Bringing your manufacturing partner into the design phase — before mold cutting — eliminates most sink mark risks at a fraction of the cost of fixing them later.When evaluating suppliers,ask to see their DFM review process and mold flow analysis capabilities,as these are strong indicators of their ability to prevent and resolve sink mark issues efficiently.

At JATERSON,we include a full DFM review and sink mark risk assessment as part of every new project kickoff,and our engineering team works directly with customers to balance design requirements,cost,and manufacturability.Whether you’re launching a new part design or troubleshooting an existing production issue,applying the systematic steps in this guide will help you resolve sink mark issues faster and with lower total cost.

## Related Resources

- [Injection Molding Guide](https://www.jaterson.com/manufacturing/injection-molding/)
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