Tiger Stripes
Regularly alternating matte and glossy bands on the surface. The cause lies in the material; the most effective intervention is the injection speed.
Tiger stripes (tiger lines) are glossy-matte band alternations repeating from the gate onward. They arise from the melt front alternately sticking and slipping as it advances instead of flowing smoothly. They are especially pronounced in TPE and some filled materials, and on parts with visible surfaces they are a direct reason for rejection.
How to Recognize It
- Regularly spaced alternating matte-glossy bands spreading from the gate (concentric rings).
- DISTINGUISHING FEATURE: where an area is glossy on one face of the part, the same area on the opposite face is MATTE.
- Only in certain materials: blends (PA/ABS, PC/PET) and multi-phase systems (PP + EPDM).
Physical Causes
- The combination of incipient partial crystallization of the outer layer + high shear stress leads to variable replication of surface detail.
- The elasticity of the melt also plays a part. The main cause lies IN THE MATERIAL; the influence of process parameters is limited.
- With cascade gating, when another nozzle opens the pressure conditions change; the melt front speed jumps and the stripes may appear only in some areas.
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.
- REDUCE the injection speed — it is the parameter with the greatest effect on the defect.
- Increase the melt and mold wall temperatures (indirect help: makes it possible to lower the speed).
- In some cases the exact opposite may be needed: INCREASING the injection speed considerably.
- Check for melt inhomogeneities (defects that look similar can also come from inhomogeneity).
Mold, Design and Material Solutions
- Increase the runner and/or manifold cross-section; increase the wall thickness.
- In a cascade system, use SPEED- and STROKE-dependent needle valve opening control — this prevents uncontrolled entry of the melt into the cavity and sudden changes in front speed.
- A pressure-controlled cascade system for each nozzle provides constant pressure as the gates open.
- Material change: the highest possible MFI, the narrowest molecular weight distribution, the highest soft phase content (e.g. for PP-EPDM).
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 |
|---|
| Record Groove Effect | The record groove effect forms regular, evenly spaced rings. Tiger stripes appear as irregularly spaced bands running in the flow direction. |
| Flow Lines | Flow and color streaks are a difference in shade. With tiger stripes the color is the same and the gloss changes alternately. |
Frequently asked questions
Is changing the injection speed enough?
Lowering the speed often improves it but may not solve it completely. The effective approach is a staged speed profile: slow at the gate, constant along the flow path. **Keeping the front speed constant** is the key point.
How much of it is down to the material?
A lot. With TPE and soft materials this defect appears more easily by nature. If the problem disappears with another supplier's material in the same process, the difference in flow behavior is decisive.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice