What are the common shortcomings of engineering plastic extrusion?

Engineering plastic extrusion production line is a processing solution widely used in the production of pipes, plates, films, profiles and other products. Since the extrusion process involves multiple links such as high temperature, high pressure, melt flow and cooling and solidification, various problems are prone to occur in actual production, affecting the appearance, dimensional accuracy and mechanical properties of the product. Understanding the causes of these defects and taking corresponding improvement measures can effectively improve product quality and production efficiency.

1. Appearance defects

(1). Surface roughness (shark skin, melt fracture)

Phenomenon: Irregular ripples, roughness or cracks appear on the surface of the product. In severe cases, "shark skin" or "melt fracture" may even appear.
Causes:
Excessive shear stress of the melt at the die
Excessive extrusion speed leads to unstable melt flow
Melt temperature is too low, resulting in poor fluidity
Irrational die design and poor flow channel
Solutions:
Increase melt temperature to improve fluidity
Reduce extrusion speed to reduce shear stress
Optimize die flow channel design to reduce flow resistance

(2). Bubbles and pores

Phenomenon: Bubbles, pores or voids appear on the surface or inside the product. Causes:
The raw materials are not fully dried, leaving residual moisture or volatiles
The extrusion temperature is too high, causing the material to decompose and produce gas
Poor exhaust, gas cannot be discharged
Solutions:
Strictly control the drying conditions of the raw materials (such as drying at 80~120℃ for 4~6 hours)
Optimize the extrusion temperature to avoid local overheating
Use an exhaust extruder or add an exhaust device

(3). Surface stripes or scratches

Phenomenon: Longitudinal or transverse stripes, scratches or uneven gloss appear on the surface of the product.
Causes:
Contamination, wear or scratches on the die or shaping die
Uneven melt temperature, resulting in unstable flow
Impurities mixed in (such as degraded materials, metal particles, etc.)
Solutions:
Regularly clean and maintain the die and shaping die
Ensure uniform melt temperature to avoid cold material from entering
Strengthen raw material screening to prevent impurities from mixing in

2. Dimensional stability issues

(1). Dimensional fluctuation (uneven wall thickness, diameter variation)

Phenomenon: Product dimensions are unstable, such as uneven wall thickness of pipes, fluctuating thickness of plates, etc.
Causes:
Unstable feeding leads to fluctuations in extrusion volume
Inconsistent pulling speed and extrusion speed
Uneven cooling and inconsistent shrinkage
Solutions:
Use a metering feeding device to ensure uniform feeding
Adjust the pulling speed to synchronize it with the extrusion speed
Optimize the cooling system to ensure uniform cooling

(2). Warping

Phenomenon: The product bends, twists, or shrinks unevenly after cooling.
Causes:
Cooling rate is too fast or uneven
Material shrinkage is large, internal stress distribution is uneven
Irrational mold design leads to flow imbalance
Solutions:
Use segmented cooling to avoid sudden cooling
Select low shrinkage materials or add fillers
Optimize mold design to improve melt flow balance

(3). Shrinkage depression

Phenomenon: Depression or shrinkage marks appear on the surface of the product.
Causes: 
Insufficient cooling, internal shrinkage leading to surface collapse Large wall thickness difference, more obvious shrinkage in thick wall parts
Solution: 
Extend cooling time or reduce cooling temperature Optimize product structure to avoid sudden change in wall thickness

3. Mechanical property defects

(1). Insufficient strength

Phenomenon: The mechanical properties of the product (such as tensile strength and impact strength) are lower than expected.
Causes:
Excessive orientation of molecular chains (such as excessive stretching leading to decreased strength)
Low weld strength (such as poor fusion during multi-channel extrusion)
Material degradation or uneven distribution of additives
Solutions:
Adjust extrusion and pulling speeds to avoid excessive stretching
Optimize mold design to reduce the impact of weld marks
Ensure that the material is fully plasticized to avoid degradation

(2). Decreased toughness (increased brittleness)

Phenomenon: The product becomes brittle and prone to breakage or cracking.
Causes:
Processing temperature is too low, leading to poor plasticization
Cooling rate is too fast, material crystallinity changes
Material aging or additive failure
Solutions:
Increase extrusion temperature to ensure sufficient plasticization
Use appropriate cooling rate to avoid sudden cooling
Check raw material quality and avoid using aged materials

4. Process-related defects

(1). Extrusion fluctuations (unstable pressure, output fluctuations)

Phenomenon: Unstable extrusion pressure leads to output fluctuations or uneven product quality.
Causes:
Unstable screw speed
Poor feeding (such as "bridging" phenomenon)
Changes in melt viscosity (such as temperature fluctuations)
Solutions:
Use servo motor to control screw speed
Optimize hopper design to prevent bridging
Strictly control temperature to ensure stable melt viscosity

(2). Degradation and discoloration

Phenomenon: Material discoloration (such as yellowing, blackening) or mechanical properties deteriorate.
Causes:
Excessively high extrusion temperature leads to material decomposition
Excessively long retention time leads to material overheating and degradation
Screw or barrel wear leads to local overheating
Solutions:
Optimize temperature setting to avoid overheating
Reduce downtime to prevent material retention
Regularly check equipment wear