Troubleshooting Blow Molding Defect: Solutions for Common Issues

2026-01-24

In the high-speed world of plastic manufacturing, downtime and high scrap rates are the enemies of profitability. Even with the best materials, variables in temperature, pressure, and mechanical alignment can lead to product defects.

Troubleshooting Blow Molding Defect: Solutions for Common Issues-Kingplas Machinery Co., Ltd

As a leader in extrusion blow molding machine technology, KingPlas has compiled this troubleshooting guide to help you identify, diagnose, and fix the most common issues in bottle and container production.

1. Defect: Uneven Wall Thickness (Thin Spots)

Possible Causes:

Incorrect parison programming.
The parison is swinging or “curling” during extrusion.
The mold is not centered with the die head.

Solution:

Adjust your 100-point parison programmer to increase thickness at the specific points where the bottle stretches most.
Check for uneven heating in the die head.
Ensure the mold clamping unit and the die head are perfectly aligned

2. Defect: Surface Streaks or “Silver Lines”

Streaks ruin the aesthetic of cosmetic and food-grade packaging.

Possible Causes:

Moisture: The resin (especially PETG or PC) is damp.
Degradation: The melt temperature is too high, burning the plastic.
Contamination: Remnants of a previous color or a different resin type in the screw.

Solution:

Check your hopper dryer settings; ensure the resin is dried to the manufacturer’s specification.
Lower the heater band temperatures gradually.
Perform a thorough purge of the screw and barrel using a dedicated cleaning compound.

3. Defect: “Orange Peel” Surface (Rough Texture)

The bottle surface appears grainy and lacks gloss.

Possible Causes:

Melt temperature is too low (the plastic isn’t fluid enough).
Blow pressure is insufficient to push the plastic against the mold walls.
The mold surface is too cold.

Solution:

Slightly increase the melt temperature.
Increase the initial blow pressure.
Adjust the mold temperature controller (Chiller) to a slightly higher setting to allow the plastic to “flow” into the mold’s texture before solidifying.

4. Defect: Parison Sagging (Draw-down)

The parison stretches too much under its own weight before the mold closes, leading to thin tops.

Possible Causes:

Melt temperature is too high (viscosity is too low).
The extrusion speed is too slow.

Solution:

Lower the temperature of the die head and lower barrel zones.
Increase the screw speed or the accumulator push speed to minimize the “hanging time” of the parison.

FAQ:

Q1: Why is the bottom of my blow-molded bottle too thin or leaking?

A: This is usually caused by insufficient Parison Programming or incorrect blow timing.

The Fix: Adjust your parison programmer to increase the material thickness at the bottom of the parison. Also, ensure the “delay blow” time is not too long, which can cause the material to cool and fail to weld properly at the pinch-off area.

Q2: What causes “Black Spots” or charred particles in finished products?

A: Black spots are typically signs of thermal degradation (carbonization).

The Fix: Check if the resin is sitting in the barrel for too long at high temperatures during downtime. Inspect the heater bands for “hot spots” and ensure the screw is properly purged when switching between different resin types (e.g., switching from PP to HDPE).

Q3: Why does the parison swing or curl to one side during extrusion?

A: This indicates an imbalance in the melt flow or temperature.

The Fix: Clean the die and mandrel to remove any burnt material. Check if all heater bands on the die head are functioning equally. Even a small temperature difference on one side of the die head can cause the parison to “curl” toward the colder side.

Q4: My bottles have a rough surface (“Orange Peel”). How do I fix this?

A: This occurs when the plastic doesn’t press firmly enough against the polished mold surface.

The Fix: Increase the blowing pressure and slightly raise the melt temperature. This reduces the viscosity of the plastic, allowing it to capture the mold’s surface detail more accurately.

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.