---
title: "What are the most common causes of melting defects in injection molding?"
description: "Struggling with splay and burn marks on incoming plastic parts? This analysis breaks down the root causes—material, machine, mold, and process—and provides actionable steps for parameter adjustment, mold maintenance, and material handling to secure consistent quality."
url: "https://www.jaterson.com/qa/common-causes-melting-defects-injection-molding.html"
language: "en"
type: "Q&A"
category: "Injection Molding Q&A"
datePublished: "2026-08-29"
dateModified: "2026-08-29"
brand: "OK TOOL"
manufacturer: "OK TOOL"
answerCount: 8
---

# What are the most common causes of melting defects in injection molding?

## Question

 I'm the quality lead for a consumer electronics OEM, and we've just started receiving a high-volume shipment of a new ABS housing from a supplier. Our incoming inspection is flagging a consistent issue: what looks like silver streaks and small, dark discolorations on the surface, mostly near the gate areas. The supplier's initial report calls it "minor cosmetic variation," but our spec is for a Class A finish, and these parts fail visual inspection. We're under pressure to approve the batch to keep the assembly line moving, but I'm concerned this is a fundamental process issue, not just a cosmetic one. I need to go back to the supplier with a clear, technical argument. From a manufacturing standpoint, what's the most likely root cause of these "melting defects," and what specific corrective actions should I be demanding from them to prevent this in future production runs? I need concrete parameters to check, not just a request to "fix it." 

## Answers
                            
### Answer 1 — Best Answer

The defects you describe—silver streaks (splay) and dark discolorations (burn marks)—are classic symptoms of material degradation during the injection molding process. While they may appear cosmetic, they often indicate underlying issues that can compromise mechanical properties. The root cause typically lies in an imbalance between four core pillars: material preparation, machine settings, mold condition, and process control. A supplier attributing this to "minor variation" is likely not addressing the systemic problem.

The first and most critical differentiation is between splay and burns. Splay (silver streaks) is caused by trapped moisture or volatiles vaporizing during injection, leaving a trail in the melt. Burns are localized, dark spots resulting from trapped air that compresses and overheats (diesel effect) or from excessive shear heat generation. Your observation that defects are near the gate is a key clue, as this is the point of highest shear and pressure.

Applicable scenarios and corrective paths break down as follows. For **material-related causes**, the primary suspect is improper drying. Hygroscopic materials like ABS must be dried to a moisture content often below 0.02% before processing. Demand to see the dryer's log: temperature (typically 80-90°C for ABS), dew point (below -40°C is ideal), and residence time (usually 3-4 hours). Also, verify the material hasn't been contaminated with regrind of a different type or has exceeded its recommended heat history.

For **machine and process parameters**, the injection speed and back pressure are often mis-set. Too high an injection speed can cause shear heating and trap air, leading to burns. A gradual speed profile or a switch to velocity-to-pressure (V-P) transfer at 95-98% cavity fill can help. Back pressure that's too low fails to compress and melt the material uniformly in the barrel, potentially leaving unmelted pellets that cause splay. Conversely, excessive melt temperature is a direct culprit. You should request a process data report showing the actual barrel zone temperatures, nozzle temperature, and screw RPM. For ABS, melt temperatures exceeding 240-250°C significantly increase degradation risk.

The **mold condition** is equally vital. Inadequate venting is the leading cause of burns. Vents must be deep enough to allow air escape but shallow enough to prevent flash (typically 0.01-0.03mm for ABS). They must also be clean and located at the end of fill and near high-risk areas like gates. A worn or damaged gate can create excessive shear. Ask for a record of the last mold maintenance and a review of the venting design. A simple test is to perform a short shot series to visually confirm the melt flow front and identify where air gets trapped.

Your selection advice for engaging the supplier should be systematic. First, insist on a controlled Process Capability (Cp/Cpk) study with the specific defect as the measured characteristic. This moves the conversation from opinion to data. Second, demand a detailed Corrective Action Report that addresses each pillar: material (drying validation), machine (parameter optimization report), mold (vent inspection/modification record), and process (established monitoring points like melt temp checks every 4 hours). Finally, require a first-article inspection sample from the corrected process before approving further production. The goal is to shift their focus from hiding defects to controlling the process variables that cause them.

**status:** accepted
**Author:** Emily Chen
**Date:** 2026-08-29

### Answer 2

From a tooling perspective, the root cause often traces back to the mold itself. The gate design is critical; a too-small gate diameter will force the material through at extremely high shear rates, generating localized heat that degrades the polymer and causes both splay and burns.

The mold steel quality and hardness matter for vent integrity—softer steel will see vents degrade quickly, losing their specified depth. Cooling line layout is another factor; insufficient cooling near the gate can lead to a hot spot, keeping the material at a degrading temperature for too long.

You should ask the supplier for the gate dimension details and the mold flow analysis report for that area. A request for the mold steel grade and its hardness certification is also reasonable to assess long-term durability against wear that exacerbates these issues.

**status:** suggested
**Author:** Jason Zhou
**Date:** 2026-08-29

### Answer 3

As a quality function, the approach is to define, measure, and contain. First, you need a clear, agreed-upon defect classification standard with the supplier. What is the allowable length and density of splay? Are burns acceptable in non-cosmetic areas? This must be documented with photos or physical limit samples.

For incoming inspection, consider using a go/no-go gauge or a dedicated UV light to make splay more visible. Internally, you should implement Statistical Process Control (SPC) charts for key parameters like melt temperature and dryer dew point, requiring the supplier to provide this data with each batch. The corrective action should follow an 8D methodology, ensuring the root cause (e.g., a faulty dryer desiccant bed) is fixed, not just the symptom on a few parts.

**status:** suggested
**Author:** Michael Wu
**Date:** 2026-08-29

### Answer 4

The precision of the mold's machined components directly influences melt behavior. The surface finish of the sprue, runner, and gate channels is crucial. A rough surface (high Ra value) creates turbulent flow and increases friction, contributing to shear heating.

The concentricity and alignment of the nozzle tip to the mold sprue bushing must be perfect; a misalignment can cause material hang-up and degradation. Furthermore, the tolerances on ejector pins and slides can affect venting if they become loose over time.

You should inquire about the machining strategies used for these critical mold features and the specified surface finishes. A high-precision, polished finish in flow channels is a fundamental expectation to ensure smooth laminar flow and reduce the risk of material hang-up and subsequent defects.

**status:** suggested
**Author:** Rachel Huang
**Date:** 2026-08-29

### Answer 5

While melting defects seem like a molding issue, they have direct consequences downstream in assembly. Splay can create weak spots that crack during press-fit operations for inserts or snap-fits. Discoloration from burns might be hidden on the exterior but could indicate degraded material strength in a load-bearing rib.

The real test is in the assembly fixture consistency. If parts with defects cause variation in fit—requiring more force to assemble or creating gaps—that's a functional failure, not just cosmetic.

You should perform a designed experiment: take a sample of parts with varying degrees of the defect and run them through the full assembly process, measuring insertion forces and checking final dimensions. This data transforms a visual quality debate into a hard, functional reliability concern for the supplier.

**status:** suggested
**Author:** Linda Xu
**Date:** 2026-08-29

### Answer 6

The end-use environment dictates the severity of the defect. For your electronic housing, internal burns near a heat source (like a processor heat sink) could be a nucleation point for further thermal degradation, leading to premature failure in the field.

Splay on the surface might affect the adhesion of paints or coatings, causing delamination. You need to correlate the defect location with the final assembly's Bill of Materials and thermal map.

Request that the supplier perform functional validation tests on affected parts—such as a heat aging test or a coating adhesion test—to prove the defects do not impact performance. This shifts the discussion from an aesthetic standard to a proven risk to product reliability, which carries much more weight in demanding corrective action.

**status:** suggested
**Author:** David Zhang
**Date:** 2026-08-29

### Answer 7

Many melting defects are designed into the part. A common DFM oversight is a wall thickness transition that is too abrupt, causing differential flow and cooling that can trap air and overheat material. The gate location being on a thick section can lead to sink and shear issues.

Insufficient draft angle can cause drag marks and localized heating during ejection. Review the part design with the supplier: was a formal DFM conducted? The ideal scenario is to have a mold flow simulation that predicts air traps, shear stress, and cooling times.

If the simulation was skipped or the results ignored, the mold may be fundamentally prone to these defects. Your demand should be for a retrospective mold flow analysis of the current design to identify and correct these inherent risks, which may require a minor mold modification.

**status:** suggested
**Author:** Eric Zhao
**Date:** 2026-08-29

### Answer 8

On the production floor, consistency is king. Melting defects that appear intermittently often point to process drift or operator intervention. Is the machine operator manually overriding parameters to solve a different problem, like short shots? Is the cycle time being pushed too fast, reducing cooling time and allowing heat to build up in the mold? Automation can help; an automated vision system at the press can catch defects in real-time and trigger an alarm for process adjustment.

Furthermore, the material handling system—from dryer to hopper to machine throat—must be sealed to prevent reabsorption of moisture. You should ask about the standard work instructions for machine setup and the frequency of process audits. The solution is often locking down the proven process parameters and removing human variability from the equation.

**status:** suggested
**Author:** Kevin Liu
**Date:** 2026-08-29

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- [Injection Molding Q&A](https://www.jaterson.com/qa/injection-molding/)
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