Table of Contents

1. Introduction

2. What are Silver Streaks Splay Marks

3. Causes of Silver Streaks

4. Types of Silver Streaks，Causes, and Targeted solutions

5. Case Study – Eliminating Silver Streaks in a PBT Housing

6. FAQ

7. Conclusion

1. What are splay marks?

In the injection molding industry, silver streaks in injection molding—also called splay marks in injection molding or silver marks—are among the most common and frustrating injection molding surface defects.

These fine, silver-colored lines or streaks appear on molded parts, making them look scratched or damaged.

While silver streaks are often considered cosmetic, they can also indicate deeper problems with materials, process settings, or mold conditions.

At Alpinemold, with over 23 years of mold making and injection molding experience, we have helped numerous customers eliminate this defect, ensuring products meet both functional and aesthetic requirements.

2. What are Silver Streaks Splay Marks

Silver streaks injection molding are thin, silver-colored lines or streaks on the surface of an injection molded part. They can be straight, curved, or radiating from a point, and they disrupt the smooth appearance of the part.

They occur when moisture, trapped air, or degraded material escapes during injection, creating a visible pattern on the cooled surface.

Other names in the industry:

• Silver marks

• Silver lines injection molding

• Splay marks

3. Causes of Silver Streaks

Material-related causes

• Moisture in resin: Hygroscopic materials like PA, PET, PBT, and PC absorb moisture easily. If not dried properly, the steam escapes during injection, leaving plastic splay  defects such as silver streaks.

• Material contamination: Mixed pellets, dust, or regrind contamination can trap air.

• Material degradation: Overheating during processing can break down polymer chains, releasing gas.

Process-related causes

• High injection speed or excessive back pressure introducing air into the melt.

• Inadequate melt temperature control leading to uneven flow.

• Screw design or wear allowing air entrapment.

Mold-related causes

• Poor venting or blocked vents.

• Damaged mold surfaces.

• Mismatch in parting lines causing leakage and air traps.

4. Types of Silver Streaks, Causes, and Targeted Solutions

silver streaks in injection molding (also called silver marks or splay marks) mainly occur in three types. Each type has different causes and corresponding solutions

Type 1: Moisture-Induced Splay

Main Cause

Moisture-absorbing materials (such as PA, PET, PBT, PC) are not dried sufficiently, causing water to evaporate at high temperatures and burst out on the surface.

Raw material storage environment has high humidity, or the material has been exposed to air for a long time.

Common Detection Indicators

lMoisture content exceeds the manufacturer's recommended value

lInsufficient drying temperature or drying time.

Solutions

You can Strictly follow manufacturer’s drying parameters, Proper drying helps prevent plastic splay defects and improves surface quality in injection molding surface defects caused by moisture, for example:

Table 1. Recommended Drying Parameters for Common Plastics

Material	Recommended    Drying Temperature	Recommended    Drying Time	Moisture    Content Requirement
PBT	120°C	4 hours	≤ 0.02%
PC	120°C	4–6 hours	≤ 0.02%
PA6	80°C	4 hours	≤ 0.1%
PA66	80°C	4–6 hours	≤ 0.1%
PET	150°C	4–6 hours	≤ 0.02%
ABS	80°C	2–4 hours	≤ 0.1%
PMMA	80–90°C	2–4 hours	≤ 0.05%
TPU	90–110°C	2–3 hours	≤ 0.05%
POM	100–120°C	2–3 hours	≤ 0.2%
PSU	150°C	3–4 hours	≤ 0.02%
PEI	150°C	3–4 hours	≤ 0.02%
PPS	120–130°C	3–4 hours	≤ 0.02%

Table 2. How to Determine if the Material is Properly Dried

Method	Description
Visual observation	Molded parts should have a smooth surface   without silver streaks, bubbles, or haze. Defects indicate possible   insufficient drying.
Moisture meter testing	Use a portable moisture meter before   molding to ensure moisture content meets the limits in Table 1.
Weight loss test	Weigh a small sample before and after   drying; if the weight loss matches expected moisture removal, drying is   effective.
Processing performance	Properly dried material ensures   consistent melt flow, stable injection pressure, and uniform part color.
Material handling	Always transfer dried resin directly to   the hopper without exposing it to ambient air to avoid reabsorption of   moisture.

Use sealed hopper dryers and install moisture-proof devices at the hopper outlet (to prevent re-absorption after drying).

Recommend adding online moisture detection (portable moisture meters).

Type 2: Air-Trap Splay

Main cause

Insufficient mold venting or clogged venting grooves trap air inside the cavity, which gets compressed by the hot melt and bursts out on the surface, causing splay marks in injection molding.

Injection speed too fast causing turbulence, entraining air into the cavity.

Common Detection Indicators

lDefects concentrated at the cavity end or melt front convergence areas.

lMold vent groove depth insufficient

(generally recommended 0.02–0.04 mm depth, 3–5 mm width).

Solutions

Add or optimize mold venting locations (especially at flow ends and thin-walled sections).

Regularly clean vent grooves to avoid residue or carbon buildup blocking them。

Adjust injection speed profile:

lModerate speed at the front stage (reduce turbulence)

lSlightly increased speed near the end (to fill cavity completely)

For long flow paths or multi-gate designs, consider vacuum-assisted venting systems.

Type 3: Degradation Splay

Main Cause

Plastic degrades inside the screw due to excessively high temperature or shear, generating gases, which leads to silver streaks in injection molding often accompanied by discoloration or burning.

Long downtime without purging causes residual material to carbonize under high temperature.

Common Detection Indicators

Defect location varies, often accompanied by burning or discoloration.

Melt temperature above the recommended range for the material.

Solutions

• Adjust temperature settings:

Control melt temperature in the middle to lower end of the recommended range (e.g., PBT recommended 240–260°C).

lReduce screw speed (to reduce shear heat) and control back pressure at 5–15 bar.

lPurge machine if downtime exceeds 30 minutes

(use cleaning pellets or low-degradation materials like PE).

• Check screw and barrel wear to prevent melt retention.

Summary Comparison Table

Type	Main Cause	Key Detection Point	Targeted Solutions
Moisture-Induced Splay	High material moisture	Moisture content > manufacturer spec	Strict drying, sealed storage, online   moisture detection
Air-Trap Splay	Poor mold venting / high injection speed	Defects at cavity end / shallow or   clogged vents	Optimize venting, clean grooves, adjust   speed profile
Degradation Splay	Overheating / overshear / degraded   residuals	Burning, discoloration, temperature above   spec	Lower temperature, control back pressure,   purge machine, inspect screw

5. Case Study – Eliminating Silver Streaks in a PBT Housing

Background:

A customer approached Alpinemold with a PBT electrical housing showing severe silver streaks in injection molding, a typical injection molding surface defect that affected their brand image and increasing scrap rate.

Solution steps:

1. Dried material at 120°C for 4 hours before molding.

2. Reduced injection speed by 15% to lower turbulence.

3. Cleaned mold vents and improved venting depth from 0.02 mm to      0.03 mm.

Result:

By optimizing drying, reducing injection speed, and improving mold venting, the splay marks in injection molding were drastically reduced, and silver lines in injection molding nearly eliminated, lowering the defect rate from 18% to less than 1%.

6. FAQ

Q1: Are silver streaks in injection molding only cosmetic?

Not always. In some cases, they can indicate internal material degradation that affects mechanical properties.

Q2: Which materials are most prone to plastic splay marks?

Hygroscopic materials such as nylon (PA), PET, PBT, and PC are more prone due to moisture absorption.

Q3: Can silver streaks be fixed after molding?

In most cases, no. The best approach is to prevent them during the molding process.

7. Conclusion

Preventing silver streaks in injection molding requires proper material preparation, optimized processing parameters, and well-maintained molds.

At Alpinemold, we apply strict quality control and technical expertise to ensure defect-free production and resolve injection molding surface defects like plastic splay defects and splay marks in injection molding.

If you are struggling with silver streaks in injection molding or other defects, contact Alpinemold today for expert troubleshooting and manufacturing support.