Why Is The Extrusion Temperature Higher Than The Melting Point?

During the processing of plastic film, whether it is cast film or blown film, the plastic pellets will first be heated and melted in the extruder, and the heating temperature is often much higher than the melting point of the plastic. Why is this?

Part.1 What is the basis for setting the extrusion temperature?

As we all know, each plastic material has its specific melting point, which is the temperature at which it begins to melt. The extrusion temperature needs to be higher than the melting point of the material to ensure full melting. However, in the actual process, it is also necessary to consider the melt flow characteristics of the material, the characteristics of the processing equipment, the presence or absence of additives, etc.

(1) Melt flow characteristics

Different plastic materials have different melt flow behaviors. Some materials require higher temperatures to reduce melt viscosity and thus improve their fluidity.

(2) Characteristics of processing equipment

The design of the extruder, the screw configuration, and the distribution of the heating zone will affect the temperature setting. The temperature needs to be optimized according to the characteristics of the equipment to ensure that the material can pass through the equipment smoothly.

(3) Product quality requirements

In order to obtain high-quality products, it is necessary to set an appropriate temperature to control the dimensional accuracy, surface finish and physical properties of the product.

(4) Effect of additives and fillers

If stabilizers, plasticizers, fillers, etc. are added to the material, the thermal stability and melting behavior of these components also need to be taken into account, which may require adjustment of the extrusion temperature.

(5) Production speed

The speed of production will also affect the extrusion temperature. When the production speed is faster, a higher temperature may be required to ensure the melting and flow of the material.

(6) Thermal stability of materials

The stability of materials at high temperatures is also an important factor. If the thermal stability of the material is poor, excessively high temperatures may cause the material to degrade and produce charred materials.

Part.2 Reasons why the extrusion temperature is higher than the melting point

In the actual production process, for example, the melting point of PP is about 165°C, but the extrusion temperature may be set to 220°C or even higher. This is to ensure that PP has good fluidity during the extrusion process. By increasing the temperature, the viscosity is reduced, making it easier to form a film. In addition to improving fluidity, it is also affected by factors such as production efficiency and film quality.

(1) Production efficiency

Higher extrusion temperatures can increase production speed because the molten resin is easier to be extruded and pass through the die, thereby improving production efficiency.

(2) Film quality

At higher temperatures, the sliding between resin molecules is easier, which helps to reduce the internal stress in the film and improve the flatness and transparency of the film.

(3) Volatile removal

If the resin contains moisture or additives, higher temperatures help remove these volatile substances and prevent bubbles or unevenness in the film.

(4) Equipment limitations

The design and operating conditions of the extruder may require operation at a temperature above the melting point to ensure the normal operation of the equipment and smooth production of the film.

In summary, although the resin begins to melt at its melting point, in order to ensure a smooth extrusion process and produce high-quality films, it usually needs to be processed at a temperature higher than the melting point.

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