Foam Injection Molding (TSG) Defects
Streaks, air hooks, folding, inflation, discoloration and sink marks seen in thermoplastic foam injection molding — and the separate solution logic for each.
How to Recognize It
- Silvery streaks: if fine and evenly distributed, even gas release at the front; coarse air hooks point to large gas bubbles.
- Layer-like surface / folding: wrinkles running across the flow direction.
- Inflation: the part blisters locally, especially at the base of ribs.
- Discoloration: browning/yellowing as production goes on.
- Sink marks in thin-walled areas; pimples on high-gloss parts after storage.
Physical Causes
- Streaks/air hooks: gas is released through the low (atmospheric) pressure at the melt front and the expansion of the gas; bubbles are carried to the front, burst and freeze on the wall. Early expansion in the space in front of the screw or in the hot runner produces coarse hooks.
- Folding: eruptions spurting from the front, or at high injection speed the fresh outer layer tearing and being pushed to the end of flow.
- Inflation: the outer layer has not solidified enough to withstand the internal gas pressure; with an exothermic agent, heat generation after ejection softens the layer. Hot spots in the mold increase the risk.
- Discoloration: long residence with an exothermic agent → temperature rise in the unit → thermal degradation. Endothermic agents release water in the reaction; this can degrade hygroscopic material.
- Sink marks: insufficient gas pressure cannot take the place of holding + rapid solidification in the thin area.
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.
- Streaks over the WHOLE surface: measure the residual moisture, check for early expansion in front of the screw (DISABLE decompression), increase the back pressure and injection speed, reduce the agent dosage, increase the mold wall temperature.
- Air hooks: increase the back pressure and injection speed, optimize the clamping force, reduce the venting.
- Folding: REDUCE the injection speed and the mold wall temperature.
- Inflated areas: check the temperature homogeneity in the cavity (hot spot!), increase the remaining cooling time, reduce the mold temperature and agent dosage, switch from an exothermic to an ENDOTHERMIC system.
- Discoloration: reduce the melt temperature and residence time, increase the metering delay, change the active agent.
- Sink marks (in a thin area): set a LOW holding pressure, increase the mold temperature. Note: holding pressure SUPPRESSES foaming — even a very short/low holding phase can make the sink mark more pronounced than with no holding at all.
Mold, Design and Material Solutions
- Arrange filling from thin to thick; create smooth wall thickness transitions; avoid excessive differences in thickness.
- Gates and runners must be SMALLER than for compact injection; in multi-cavity molds a hot runner is preferred.
- A correctly timed shut-off nozzle in the unit or hot runner is essential; screw position control is needed in all phases.
- Optimize the venting system (a variable design can be considered).
- For plate-out/corrosion: change the active agent, use a coating or corrosion-resistant steel.
- For parts to be coated, choose citric acid derivatives as the endothermic agent and plan a minimum storage time before coating.
Important Notes
- ⚠️ The main gains of foam injection molding: weight and material savings, shorter cycles, less warpage and compensation of shrinkage. Its most important disadvantage is STREAK formation — visible parts require additional measures.
- ⚠️ Weld line areas are MORE CRITICAL than with compact injection, both in terms of surface and mechanically.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice