During injection molding, if the gate is too small and the injection speed is too fast, the melt flow changes drastically and air is trapped in the melt. This results in noticeable shadows at the gate, bends, and steps of the plastic part. These shadows are called air streaks, as shown in Figures 3-63 and 3-64. ABS, PC, and PPO plastics are particularly prone to air streaks at the gate.
Figure 3-63 Airmarks on a plastic part (I)
Figure 3-64 Air bubbles on a plastic part (II)
The causes and solutions for air streaks are shown in the table below.
| # | Root Cause Analysis | Solutions |
|---|---|---|
| 1 | Material temperature too high or mold temperature too low | Lower the material temperature (to prevent degradation) or increase mold temperature |
| 2 | Gate too small or improperly positioned | Increase gate size or change gate position |
| 3 | Air bubbles or sink marks caused by excessively fast injection speed | Use multi-stage injection, slow down the injection speed at corresponding areas |
| 4 | Runner too long or too thin (too much cold material) | Shorten or enlarge the runner diameter |
| 5 | No fillet/round transition at product steps or corners | Add fillet/round transitions at steps and corners |
| 6 | Poor venting (trapped air) | Improve mold venting |
| 7 | Cold slug well too small or insufficient | Enlarge or add more cold slug wells |
| 8 | Insufficient drying of raw material or overheating decomposition | Fully dry the raw material and prevent overheating decomposition |
| 9 | High viscosity and poor flowability of the plastic | Switch to a plastic with better flowability |
Black lines (black stripes) and their solutions
Black streaks are black lines that appear on the surface of plastic parts, also known as black stripes, as shown in the image below.
The main cause of black streaks is the thermal decomposition of the molding material, commonly seen in plastics with poor thermal stability (such as PVC and POM). Effective measures to prevent black streaks include preventing excessively high melt temperatures in the barrel and slowing down the injection speed. If the barrel or screw has scratches or gaps, the material adhering to that part will overheat, causing thermal decomposition. Furthermore, cracks in the check ring can also cause thermal decomposition due to melt retention; therefore, special care should be taken to prevent black streaks from forming in high-viscosity or easily decomposed plastics.
The causes and solutions for black streaks are shown in the table below.
| # | Root Cause Analysis | Solutions |
|---|---|---|
| 1 | Material temperature too high | Lower material temperature / barrel temperature |
| 2 | Screw rotation speed or back pressure too high | Lower screw rotation speed or back pressure |
| 3 | Screw and barrel clearance too large, causing shear heat | Repair or replace screw/barrel |
| 4 | Nozzle hole too small or temperature too high | Replace nozzle with larger hole or lower nozzle temperature |
| 5 | Colorant unstable or poor heat resistance | Change to heat-resistant colorant or add heat stabilizer |
| 6 | Masterbatch contains impurities or incompatible materials | Clean the mixing head and hopper thoroughly |
| 7 | Reverse cooling: raw material inside the hopper is heated and degrades | Check hopper cooling; stop reverse cooling if there is no yellowing |
| 8 | Using recycled material (containing impurities) or colorant (fading) | Inspect or change recycled material/mouth |
| 9 | Nozzle hole too large or contains foreign matter | Change to smaller nozzle or clean out foreign matter inside the nozzle |
| 10 | Dwell time too long (material remains in barrel too long) | Reduce dwell time or shorten total molding cycle |