1、 Overview of surface defects in extrusion molding

Dalian extrusion molding, as an efficient and economical plastic processing method, is widely used in the production of products such as pipes, sheets, and profiles. However, in the actual production process, various defects often appear on the surface of the product, such as stripes, bubbles, shark skin, melt rupture, etc. These defects not only affect the appearance of the product, but may also reduce its mechanical performance and service life.

The occurrence of surface defects is often the result of multiple factors working together, including raw material characteristics, extrusion process parameters, mold design, cooling conditions, etc. To effectively address these issues, it is necessary to start from a systemic perspective, comprehensively analyze the causes of defects, and take targeted measures.

2、 Common types and causes of surface defects

1. Shark skin phenomenon

Shark skin is characterized by a rough texture resembling shark skin on the surface of the product, which is mainly caused by:

The melt undergoes elastic recovery at the exit of the mold

Excessive extrusion speed leads to melt fracture

The temperature of the melt does not match the temperature of the mold

2. Melt fracture

Melt fracture is characterized by irregular cracks or wavy patterns on the surface of the product, mainly due to:

The shear rate exceeds the critical value

Excessive elasticity of the melt

Unreasonable design of mold entrance angle

3. Surface stripes

Surface stripes are common linear defects in extruded products, which may be caused by the following factors:

Accumulation or damage inside the mold

Uneven melt temperature

Extruder screw wear

4. Bubbles and pores

Bubble defects seriously affect the appearance and performance of products, and the main causes include:

The moisture content of the raw material is too high

Excessive melt temperature leads to decomposition

Incomplete exhaust system

3、 System solution for surface defects

1. Raw material control

The quality of raw materials is the fundamental factor affecting the surface quality of extruded products

Strict selection of raw materials: choose resins with suitable fluidity and good thermal stability

Adequate drying treatment: Pre drying of hygroscopic materials such as PA, PET, etc

Reasonable formula design: adding appropriate lubricants, stabilizers and other additives

2. Optimization of process parameters

Process parameters have a direct impact on surface quality and need to be carefully controlled

Temperature control: Establish a reasonable temperature gradient to avoid local overheating

Screw speed: Adjust according to material characteristics to avoid excessively high shear rates

Traction speed: matches the extrusion speed to prevent product deformation

3. Mold design and maintenance

Mold is a key component that determines the surface quality of a product:

Channel design: Adopting a reasonable channel structure to ensure uniform flow of the melt

Surface treatment: Polishing or coating treatment on the working surface of the mold

Regular maintenance: timely cleaning of accumulated dirt and repairing damaged areas

4. Improvement of cooling system

The cooling process has a significant impact on the surface quality of the product:

Cooling method selection: Adopt water cooling, air cooling, or combined cooling according to product characteristics

Cooling temperature control: avoid sudden cooling causing internal stress

Cooling uniformity: Ensure consistent cooling rates for all parts of the product

5. Online detection and feedback control

Establishing a quality monitoring system can promptly detect and correct defects:

Visual inspection system: Real time monitoring of product surface condition

Process parameter recording: Establish a database linking parameters to quality

Automatic feedback adjustment: Automatically adjust key parameters based on detection results

4、 Specific solutions for specific defects

1. Solution to Shark Skin Phenomenon

Reduce the extrusion speed to keep the shear rate below the critical value

Raise the melt temperature and reduce the melt elasticity

Increase the length of the shaping section at the mold outlet

Add processing aids such as fluoropolymers

2. Improvement methods for melt fracture

Optimize the entrance angle of the mold (usually using 30 ° -60 °)

Adopting a screw design with graded compression

Raise processing temperature and reduce melt viscosity

Choose raw materials with narrow molecular weight distribution

3. Measures to eliminate surface stripes

Regularly clean the mold to remove dirt and debris

Check and repair mold damage

Optimize the filtration system to prevent impurities from entering

Adjust screw combination to improve plasticization uniformity

4. Solution to Bubble Problem

Pre drying treatment of raw materials (usually 105-120 ℃, 2-4 hours)

Optimize exhaust system design

Reduce the melt temperature to prevent thermal decomposition

Reduce the screw speed appropriately

5、 Preventive Maintenance and Management Strategy

1. Establish standardized operating procedures

Develop detailed process operation procedures

Record key parameters and quality data

Implement the handover information transmission system

2. Equipment maintenance plan

Regularly check the wear of screws and barrels

Establish a mold maintenance cycle

Calibrate temperature, pressure and other sensors

3. Personnel training and skill enhancement

Training on Process Knowledge for Operators

Cultivation of Quality Problem Analysis Ability

Emergency response capability training

The solution to surface defects in extruded products requires a systematic and comprehensive approach. From raw material control, process optimization, mold improvement to quality management, every link is crucial. By scientifically analyzing the causes of defects, taking targeted solutions, and establishing a preventive maintenance system, the surface quality of products can be significantly improved, the scrap rate can be reduced, and production efficiency can be enhanced.

In actual production, it is recommended that enterprises establish a comprehensive quality tracking system to record defect types, occurrence conditions, and solutions, form a knowledge base, and provide reference for subsequent production. At the same time, with the development of new materials and processes, we should continue to pay attention to industry technological progress, continuously optimize production plans, and maintain competitive advantages.