Troubleshooting Spots, Bubbles, and Streaks in Blow Molding

2026-03-18

Technical Guide to Eliminating Blow Molding Defects:

In the competitive world of plastic packaging, “surface perfection” is a primary benchmark for quality. However, even with advanced machinery, blow molding machine manufacturers often face the frustration of surface defects (Surface Defects) such as black spots, bubbles, or vertical streaks. These imperfections can lead to rejected batches and increased production costs.

Troubleshooting Spots, Bubbles, and Streaks in Blow Molding-Kingplas Machinery Co., Ltd

As a specialist in blow molding machinery, KingPlas Machinery has analyzed the root causes of these common aesthetic issues to help operators maintain a zero-defect production line.

1. Bubbles and Voids: The Moisture Challenge

The most common cause of bubbles in the bottle wall is moisture. When manufacturers fail to dry resins like PETG, PC, or certain HDPE blends, moisture turns into steam inside the heated barrel. This process creates ‘silver streaks’ or trapped air bubbles in the final part.
Solution: Implement a high-efficiency hopper dryer and ensure the resin is pre-heated to the manufacturer’s recommended dew point.

2. Black Spots: The Carbonization Warning

Black spots typically signal ‘carbonized’ plastic—resin that burns after staying in the barrel or die head too long. Foreign contamination in the raw material can also cause these defects.
Solution: Perform regular purging with a specialized cleaning compound. If spots persist, it may be time to dismantle and clean the die head (Die Head) to remove stubborn carbon buildup on the mandrel.

3. Vertical Streaks: The Die Mouth Issue

If you see consistent vertical lines or “scratches” on every bottle, the problem is likely at the die opening. A tiny burr on the metal or a piece of burnt plastic stuck at the die lip will “score” the parison as it extrudes.
Solution: Carefully clean the die mouth using a copper tool (never steel, to avoid scratching the polished surface). Ensuring the die is perfectly polished is critical for high-gloss applications.

Engineer’s Suggestion

“Surface defects are often the machine’s way of telling you that the material or the temperature is out of balance,” says a senior engineer at KingPlas. “By focusing on material purity and precise heat control, most of these issues can be resolved in minutes.”

FAQ:

Q1: Why do bubbles suddenly appear in the middle of a production run?

A: Environmental humidity changes or new, undried resin batches often cause this issue. Check for ‘feed throat cooling’ problems. A throat that is too cold causes air moisture to condense and enter the barrel.

Q2: What causes “Ripples” or horizontal lines on the bottle?

A: Unstable extrusion speed or a ‘shaking’ parison usually cause horizontal ripples. Check for fluctuating extruder motor speeds or an external breeze—such as a cooling fan—disturbing the parison before the mold closes.

Q3: Will increasing the blow pressure help remove surface bubbles?

A: Not usually. Blowing pressure helps the plastic take the shape of the mold, but bubbles trapped inside the plastic wall are a material/melting issue. Increasing pressure may actually make a bubble “pop,” resulting in a hole or a “blow-out.”

Why Choose Kingplas Machinery?

Has 30 years blow molding experience. Our blow molding machine portfolio is designed to handle a wide range of thermoplastic materials, including HDPE, PP, PE, PVC, PC, and PETG, for containers ranging from 10ml to 1250L.

Automatic Extrusion Blow Molding Machines: High-speed continuous extrusion systems (Single/Double Station) for cosmetics, lubricants, and household chemicals.
Accumulator Head Blow Molding Machine: Robust solutions for large-scale industrial parts, such as 200L drums, IBC tanks, and automotive components.
Multi-Layer Co-Extrusion Technology: Advanced 2-to-6 layer systems that provide superior barrier properties for agrochemical and fuel tank applications.
Turnkey Solutions: Integrated production lines including precision mold design and auxiliary equipment.