Injection molding defects rarely come from one variable alone. A sink mark may involve local wall thickness, material shrinkage, packing, or cooling. Warpage may be influenced by part geometry, uneven shrinkage, fiber orientation, mold conditions, or the molding process.
The visible defect is therefore only the starting point.
A useful troubleshooting process asks three questions first: What does the defect look like? Where does it appear? Is the pattern consistent? From there, the part design, material, mold, and processing conditions can be reviewed together.
Below are seven common injection molding defects and the areas worth checking before deciding what needs to change.
Why Injection Molding Defects Need Systematic Troubleshooting
When a molded part fails inspection, changing pressure, temperature, or cycle time may seem like the fastest response. But a process adjustment cannot correct every design, material, or tooling issue.
Four areas usually need to be considered:
- Part Design: Wall thickness, ribs, bosses, transitions, and flow restrictions influence filling, shrinkage, and cooling.
- Material: Resin grade, flow behavior, shrinkage, moisture sensitivity, and fillers can affect the molded result.
- Mold: Gate location, venting, cooling, parting surfaces, and tooling condition influence how the cavity fills and the part cools.
- Process: Melt and mold temperatures, filling, packing, and cooling conditions can contribute to defect formation.
Our Injection Molding DFM Guide covers the design side of these relationships, while the Injection Molding Materials Guide explains how polymer families, specific resin grades, moisture, shrinkage, and reinforcement can influence molding behavior.
The objective is not to blame one variable immediately. It is to narrow the possible causes using the defect pattern and the available production information.
1. Sink Marks
Sink marks are shallow depressions that often appear near locally thicker sections of a molded part.
Common locations include:
- Rib intersections
- Bosses
- Gussets
- Thick wall transitions
As a thicker section cools and contracts, the outer surface may be pulled inward.
Possible Contributors
- Localized thick sections
- Rib or boss geometry
- Material shrinkage
- Packing conditions
- Uneven cooling
What to Review
Compare the visible sink with the geometry behind the surface.
If the mark repeatedly appears opposite a rib or boss, review whether the feature contains more material than necessary for its function. Wall transitions, packing, and cooling should also be considered before assuming the mold itself needs modification.
2. Warpage
Warpage occurs when a molded component twists, bows, or moves away from its intended geometry after molding.
Possible contributors include:
- Uneven wall thickness
- Differential shrinkage
- Non-uniform cooling
- Gate location
- Material orientation
- Part geometry
- Processing conditions
Glass-filled materials require additional attention because fibers tend to orient with the melt flow. Depending on the resin, geometry, gate position, and process, this can produce directional differences in shrinkage and mechanical behavior.
What to Review
Compare the direction and location of the distortion with:
- Wall-thickness distribution
- Gate position
- Material flow direction
- Cooling conditions
- Resin grade
A large PP cover and a glass-filled nylon bracket can show similar visible distortion while having very different contributing factors.
3. Weld Lines
A weld line, sometimes called a knit line, can form where separate melt-flow fronts meet after moving around a hole, core, or another cavity feature.
They are commonly seen:
- Around holes
- Behind internal features
- Where the melt divides and rejoins
- In complex housing geometry
A visible weld line is not automatically a failed part. Its importance depends on its location, appearance requirements, material, and the functional loads on the component.
For fiber-reinforced materials, local fiber orientation around the meeting flow fronts may also affect mechanical behavior.
What to Review
Consider:
- Gate position
- Flow path
- Weld-line location
- Resin grade
- Mold and melt conditions
- Functional load near the affected area
For a cosmetic enclosure, visibility may be the main concern. For a structural component, the location relative to a loaded feature may matter more.
4. Short Shots
A short shot occurs when the cavity does not fill completely.
The result may be an incomplete:
- Corner
- Rib
- Wall
- Boss
- End-of-flow feature
Possible contributors include restricted flow, thin sections, long flow paths, venting, gate design, material behavior, and processing conditions.
What to Review
Start with the location of the incomplete feature.
If the same thin section repeatedly fails to fill, review the geometry and flow path. If the problem appears near the end of fill, venting and processing conditions may also deserve attention.
A material with different flow characteristics may change the result, but resin substitution should not be used as a shortcut for identifying the actual restriction.
5. Flash
Flash is excess plastic outside the intended part boundary. It commonly appears along:
- Parting lines
- Inserts
- Ejector areas
- Slides
- Shut-off features
Possible contributors include tooling fit or wear, clamping conditions, material behavior, and processing conditions.
What to Review
Look first at where the flash appears and whether it repeats in the same location.
Flash following one section of the parting line may point toward a different issue than flash concentrated around an insert or moving mold feature.
This makes defect location particularly useful when separating tooling-related factors from processing-related factors.
6. Burn Marks
Burn marks usually appear as dark or discolored areas on the molded surface.
One possible mechanism is trapped air or gas being compressed and heated as the melt advances. Material condition and processing conditions can also contribute.
What to Review
Check:
- Defect location
- End-of-flow areas
- Mold venting
- Material condition
- Filling behavior
- Processing conditions
Repeated discoloration near the final fill location may make venting an important review point, but the appearance of a burn mark alone does not establish the root cause.
7. Voids
Voids are internal empty spaces that may occur inside thicker molded sections.
Possible contributors include:
- Heavy local sections
- Internal shrinkage
- Packing behavior
- Gate freeze
- Cooling
- Part geometry
What to Review
Start with the section itself.
If the thick geometry is not functionally necessary, reducing or coring out the section may be worth evaluating. If the geometry must remain, packing, cooling, gate strategy, and material behavior should be reviewed together.
Avoid treating additional packing or longer cycle time as a universal solution.
Quick Injection Molding Defect Troubleshooting Reference
| Defect | Possible Contributors | First Review / Diagnostic Direction |
|---|---|---|
| Sink Marks | Thick sections, shrinkage, packing, cooling | Rib/boss geometry and wall transitions |
| Warpage | Differential shrinkage, cooling, material orientation | Wall balance, resin, flow direction and cooling |
| Weld Lines | Meeting flow fronts, gate location, material behavior | Gate position and weld-line location |
| Short Shots | Flow restriction, venting, thin sections | End-of-fill geometry, flow path and venting |
| Flash | Tooling fit, parting surfaces, clamping or process | Defect location and mold interface |
| Burn Marks | Trapped gas, material or processing conditions | Venting and end-of-flow areas |
| Voids | Thick sections, shrinkage, packing and cooling | Section thickness and local geometry |
This table is a troubleshooting starting point, not a fixed diagnosis. The same visible defect can have different causes on different parts.
A Practical Troubleshooting Workflow
When a molding problem appears, avoid changing several variables at the same time. A structured review makes it easier to understand what actually affects the result.
1. Identify the Defect
Record where it appears, how often it occurs, and whether the issue is cosmetic, dimensional, or functional.
2. Review the Part Geometry
Look for thick sections, abrupt transitions, restricted flow paths, thin end-of-fill features, ribs, bosses, and other DFM concerns.
3. Confirm the Material
Verify the actual resin manufacturer and grade where possible—not only the polymer family. Moisture condition, fillers, shrinkage, and flow behavior may be relevant.
4. Review the Mold
Consider gate location, venting, cooling, parting surfaces, and tooling condition.
5. Review Processing Conditions
Evaluate the process in the context of the part, resin, and mold rather than treating machine settings as an isolated variable.
This does not guarantee that every defect has a single root cause, but it provides a more useful way to narrow the investigation.
What to Send When Requesting an Injection Molding Review
For an existing molding problem, useful information includes:
- 3D CAD model
- 2D engineering drawing
- Resin manufacturer and grade
- Defect photographs and location
- Critical dimensions
- Surface requirements
- Production quantity
- Assembly information
- Existing mold information where available
For a new project, DFM can help identify some potential risks before tooling begins. For suitable projects, mold-flow simulation may also help evaluate filling behavior, potential air traps, weld-line locations, and shrinkage or warpage tendencies. These methods support engineering review but do not replace mold trials and production validation.
These requirements can be considered together with Mold Manufacturing and FSD Precision’s Injection Molding Service during quotation and tooling planning.
Frequently Asked Questions
What are the most common injection molding defects?
Common injection molding defects include sink marks, warpage, weld lines, short shots, flash, burn marks, voids, flow marks, and other dimensional or cosmetic variations.
What causes sink marks in injection molded parts?
Sink marks are often associated with locally thick sections, ribs or bosses, material shrinkage, packing, and cooling. The actual cause should be evaluated from the specific geometry, material, mold, and process.
Why do injection molded parts warp?
Warpage can involve differential shrinkage, wall-thickness variation, cooling, material orientation, gate location, geometry, and processing conditions. Several factors may contribute at the same time.
Are weld lines always a defect?
No. Their importance depends on location, appearance requirements, material, and the functional loads on the component. For reinforced materials, local flow and fiber orientation may also deserve review.
Can process adjustments fix all molding defects?
No. Processing changes may improve some issues, but defects driven primarily by geometry or tooling may require DFM or mold review. The likely root cause should be investigated before selecting a corrective action.
Can injection molding defects be identified before tooling?
Some risks can be evaluated before tooling through DFM and, where appropriate, mold-flow simulation. These methods may help identify potential filling, air-trap, weld-line, shrinkage, or warpage concerns, but they do not replace actual mold trials and production validation.
Have an Injection Molded Part Ready for Review?
If you are troubleshooting an existing molding issue or preparing a new component for tooling, send FSD Precision your CAD model, engineering drawing, resin specification, quantity, and any relevant defect photographs or inspection information.
Our engineering team can review the manufacturing requirements and discuss practical DFM, tooling, material, and injection molding considerations as part of the quotation process.
Need help sourcing custom OEM parts?
Send drawings, samples, part numbers, material requirements, quantity, and application details for engineering review.