Internal and External Lubrication Balance in PVC Processing
Introduction
Lubricants play a critical role in PVC processing, especially during extrusion, calendering, and injection molding. Lubricants are generally classified into two types:
- Internal lubricants – reduce friction between PVC molecular chains and improve melt flow.
- External lubricants – reduce friction between the PVC melt and processing equipment.
A proper balance between internal and external lubrication is essential to ensure stable processing, good product appearance, and optimal mechanical properties.
Effects of Lubricant Imbalance
1. Insufficient Internal and External Lubrication
When both internal and external lubricants are insufficient, several processing problems may occur:
- High melt viscosity
- High plasticizing torque
- Severe melt adhesion to processing equipment
- Rough or uneven inner surface of products
- Premature thermal decomposition
- Poor mechanical properties
In proper conditions, when internal and external lubrication are balanced, the melted material flows smoothly without sticking to the rotor or equipment surfaces.
2. Excessive Internal and External Lubrication
When both lubricants are added excessively:
- Plasticizing torque becomes very low
- Additive precipitation may occur
- Products may appear smooth but have poor mechanical strength
In severe cases, the product surface may feel oily when touched, and the material may become brittle and prone to breakage.
3. Insufficient Internal Lubricant but Excessive External Lubricant
If internal lubrication is insufficient while external lubrication is excessive:
- Plasticizing time becomes longer
- Internal friction of PVC resin increases
- Risk of thermal degradation increases
- Plasticizing torque rises
Sometimes the product surface may feel slightly oily, while the product becomes brittle and mechanically weak.
4. Excessive Internal Lubricant but Insufficient External Lubricant
If internal lubrication is excessive but external lubrication is insufficient:
- Plasticization occurs too quickly
- Melt viscosity becomes high during processing
- Strong adhesion to metal surfaces occurs
- Equipment cleaning becomes difficult
This condition may also reduce thermal stability time, resulting in yellowing or decomposition lines in the final product.
Summary of Lubrication Effects
In general:
When External Lubricant is Insufficient
- Material adhesion to processing equipment
- Short plasticization time
- Poor surface smoothness
When External Lubricant is Excessive
- Longer plasticization time
- Reduced mechanical properties
- Possible additive precipitation
- Oily surface feeling
When Internal Lubricant is Insufficient
- Higher plasticizing torque
- Longer plasticizing time
- Increased internal friction
When Internal Lubricant is Excessive
- Short plasticizing time
- Reduced plasticizing torque
- Shorter thermal stability time
- Increased risk of thermal degradation
Relationship Between Plasticization and Mechanical Properties
The mechanical properties of PVC products are closely related to the degree of plasticization:
- Tensile strength is highest when plasticization is about 60%.
- Impact strength reaches its maximum around 65%.
- Elongation at break is highest when plasticization reaches approximately 70%.
Maintaining the proper lubrication balance helps achieve the optimal plasticization level and overall product performance.
Conclusion
Proper control of internal and external lubrication is essential in PVC processing. An imbalance in lubricant levels can lead to processing problems, poor surface quality, and reduced mechanical performance.
By carefully adjusting lubricant types and dosages, manufacturers can achieve stable processing conditions, improved product quality, and optimized mechanical properties in PVC products.
