Why Do PC Parts Have Silver Streaks?
Silver streaks are common surface defects in PC injection molding.
They usually look like silver-white lines or streaks extending from the gate or along the flow direction.
Moisture is a common cause, but drying the PC material does not always solve the problem.
If the streaks remain after proper drying, you need to check the molding process and mold design.
1. The Material May Still Contain Moisture
PC is highly sensitive to moisture.
Even after drying, the material can absorb moisture again during storage or transportation.
Check:
- Drying temperature
- Drying time
- Dryer performance
- Material storage
- Time between drying and molding
The material should remain properly sealed after drying.
2. Excessive Shear Can Create Silver Streaks
If the PC passes through a very small gate or runner at high speed, the material experiences high shear.
This can generate excessive heat and damage the material.
The result can be silver streaks around the gate and along the flow direction.
This is why simply increasing injection speed is not always a good solution.
A suitable gate and runner design can reduce unnecessary shear.
Our injection mold manufacturing service focuses on runner, gate, and cooling design before mold construction.
3. The Gate May Be Too Small
A small gate creates high resistance.
The material must pass through the gate at high speed, which increases shear stress.
This is especially important for PC because PC generally requires higher processing temperatures and pressure than materials such as PP or ABS.
If silver streaks always start around the gate, check the gate size first.
4. Material Degradation Can Also Cause Streaks
PC can degrade when exposed to excessive heat or remains in the barrel for too long.
This can happen when:
- Barrel temperature is too high
- Residence time is too long
- The machine has too much capacity for the shot size
- The material stays in the barrel during long stops
Degraded material may produce surface streaks and a noticeable change in appearance.
5. Poor Venting Can Trap Gas
Not all silver streaks come from moisture.
Air or gas can also become trapped during filling.
Poor venting makes it difficult for the gas to escape.
This can create streaks, burns, or other surface defects.
For complex parts, proper venting should be considered together with the gate and flow direction.
6. Recycled Material Needs Attention
Excessive use of recycled PC can also increase the risk of appearance problems.
Repeated thermal processing can reduce material performance.
For high-quality or appearance-critical PC parts, control the percentage and quality of regrind carefully.
How to Find the Real Cause
If drying does not solve the problem, do not immediately increase the drying temperature.
Instead, check the problem step by step:
- Confirm the material is actually dry.
- Check the gate and runner size.
- Check injection speed and shear.
- Check barrel temperature.
- Check material residence time.
- Check mold venting.
- Check the amount of regrind.
- Inspect the streak location and direction.
The location of the silver streak often provides useful clues.
If it starts at the gate, investigate shear and gate design first.
If it appears near the end of fill, check venting and trapped gas.
Final Thoughts
Drying is important for PC, but it is not the only solution for silver streaks.
When the material is already dry, the next things to check are shear, gate design, material degradation, venting, and processing conditions.
For difficult PC products, solving the problem at the mold design stage is usually more effective than trying to correct everything through machine settings.
Our injection molding production service supports PC and other engineering plastics.
For more practical injection molding topics, visit the Fentormold blog.