Moisture Streaks
U-shaped silvery streaks caused by moisture in the granulate or on the mold surface. The core of the solution is correct pre-drying.
Moisture streaks appear when the water in the raw material evaporates at injection temperature and forms bubbles in the melt. They leave silvery lines running in the flow direction on the part surface. In hygroscopic plastics (PA, PC, PET, ABS, PBT) the moisture has penetrated into the granule, so surface drying is not enough; the real issue is the type of dryer and the residence time.
How to Recognize It
- Silvery streaks opening in a U shape on the surface; the area around the streak is often rough and porous.
- If the mold surface is damp, the streaks take the form of large, matte and layered (lamellar) areas.
- If the melt gives off steam or shows bubbles during slow injection "into the open", the granulate is damp.
- With hygroscopic materials (PA, ABS, PC, PBT, PMMA, SAN) the first suspect is moisture.
Physical Causes
- The granulate has absorbed moisture; the water vapor bubbles forming in the melt are carried to the flow front, burst and freeze on the mold wall.
- Insufficient/incorrect pre-drying or a dryer fault (regeneration not working).
- Unsuitable storage, open bags, taking up moisture again by waiting a long time in the hopper.
- The mold cooling circuit is leaking, or there is condensation on a cold mold surface.
- The feed (throat) temperature is too low.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Check the mold surface first: if it is damp, look for a leak in the cooling circuit and raise the mold wall temperature.
- Measure the residual moisture of the granulate; dry again according to the manufacturer's instructions (e.g. ABS 80°C/2-3 hours, PA6 80°C/3-5 hours, PC 120°C/2-3 hours).
- Reduce the residence time in the hopper; feed from the dryer to the machine through a closed system.
- Raise the throat temperature to the manufacturer's range.
Mold, Design and Material Solutions
- If the condensation problem is persistent, consider a dry-air-curtain system — a short cycle can be maintained with a low mold temperature.
- Review the packaging and storage conditions (an opened bag must be kept closed).
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Burn Marks | A burn mark is brownish and persists even when the raw material is dried. A moisture streak is silvery and disappears completely after correct drying. |
| Air Streaks | An air streak usually follows a particular path from the gate and is related to air drawn in during metering. A moisture streak spreads more widely over the part surface. |
| Stress Whitening | Stress whitening forms in the area strained during ejection and is a lightening of color that can be felt to the touch; a moisture streak is a glossy line on a flat surface. |
Frequently asked questions
Why are there still moisture streaks in material coming out of the dryer?
Look at three points: is the dryer type right (hygroscopic material needs dry air), is the residence time sufficient, and is the dried material picking up moisture on its way to the machine? An open hopper or an uninsulated line re-moistens dried granulate within minutes.
Does regrind increase moisture streaks?
Yes. Because the surface area of regrind is larger than that of granulate, it takes up moisture faster, and it has also been through thermal processing once already. If the regrind ratio is high, review the drying time and the ratio together.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice