Comparing Water-Cooled and Air-Cooled Extruder Temperature Control

Comparing Water-Cooled and Air-Cooled Extruder Temperature Control

Temperature control is one of the necessary conditions for the extrusion process. The heating and cooling systems of extruders are designed to ensure this condition.

There are two sources of heat in the extrusion process. One is the heat supplied by the external heaters of the barrel, and the other is the frictional and shear heat generated by the relative motion between the plastic and the barrel inner wall, the plastic and the screw, and the plastic itself. The former is converted from electrical energy by the heaters, while the latter is converted from mechanical energy transmitted to the screw by the motor. The proportion of these two sources of heat depends on the structure of the screw and barrel, process conditions, material properties, and the stage of the extrusion process (such as the startup stage, stable operation stage).

Temperature control in extruders: what are the differences between water-cooled and air-cooled systems?

Temperature control is crucial for the extrusion process, and the heating and cooling systems of extruders are designed to ensure this control.

There are two sources of heat in extrusion: heat supplied by external heaters of the barrel and heat generated by frictional and shear forces between the plastic and the barrel inner wall, the plastic and the screw, and within the plastic itself. The proportion of these heat sources varies in different stages of extrusion. In the feeding stage, where the material is not yet compacted, frictional heat is minimal, and heat mostly comes from the heaters. In the homogenization stage, where the material is melted and at a higher temperature, and the screw grooves are shallower, more heat is generated by friction and shear, sometimes requiring cooling rather than heating. In the compression stage, the heat transfer is a transition between the previous two stages, with more heat from friction than in the feeding stage but less than in the homogenization stage. Therefore, extruders often have segmented heating and cooling systems.

From an energy perspective, extrusion presents a heat balance issue. Part of the heat from the heaters and friction is used to change the plastic's state, while the rest is lost. Despite many factors affecting this balance, under stable extrusion conditions, this balance should be maintained.

1. Heating Methods:

- Liquid Heating: Heating a liquid (water, oil, biphenyl, etc.) and then using it to heat the barrel. Electric heating is the most common, while steam heating is rarely used except in some rubber extruders.

- Electric Heating: Divided into resistance heating and induction heating, electric heating is the most commonly used method.

2. Cooling:

- Extruders are equipped with cooling systems to maintain the plastic at the required temperature during extrusion. Frictional heat generated by screw rotation can exceed the heat needed by the material, leading to excessive temperatures in the barrel. Without proper heat dissipation, this can cause material decomposition, especially in heat-sensitive plastics, and hinder the molding process. Cooling is therefore necessary for the barrel and the screw.

- Barrel Cooling:

- Air Cooling: Gentle, uniform, and clean, air cooling is widely used in extruders worldwide. However, the large space occupied by the fan and potential noise issues are considerations, making it more suitable for medium and small extruders.

- Water Cooling: Faster, smaller, and cheaper than air cooling, water cooling can cause rapid cooling and disrupt stable plastic flow if not carefully managed. Sealing issues can lead to leaks. Water-cooled systems require chemically treated water to prevent scaling and corrosion, typically using softened and deoxygenated water. Water cooling is considered suitable for large extruders.

2.1 Cooling of the Screw

Purpose of Cooling the Screw:

2.1.1 Achieving Maximum Solid Conveying Rate:

According to solid conveying theory, the solid conveying rate is related to the difference between the friction coefficients of the material against the screw and the barrel. A higher friction coefficient between the barrel and the material and a lower friction coefficient between the material and the screw favor the conveying of solid material. Apart from increasing the smoothness of the screw surface by opening longitudinal grooves on the inner wall of the feeding section of the barrel, controlling the temperature of the barrel and the screw can also achieve this goal.

This is because the frictional properties of solid plastics are greatly affected by temperature. In some cases, the friction coefficient of a plastic increases with temperature, while in other cases, it decreases. Therefore, by controlling the temperature of the barrel and the screw in the solid conveying zone for a specific plastic (even for the same type of plastic with different brands), we can maximize the difference between the friction coefficient of the barrel and the material and the friction coefficient of the screw and the material to achieve the maximum solid conveying rate.

2.1.2 Cooling the screw to control product quality:

It has been verified that cooling the screw in the homogenization section can improve the plasticization of the material and enhance the quality of the products. However, this can reduce the extrusion rate, and the lower the outlet temperature of the cooling water, the lower the extrusion rate. This is because cooling the homogenization section of the screw makes the material near the screw surface less viscous and less flowable. It is equivalent to reducing the depth of the grooves in the homogenization section. For materials with high viscosity, special attention should be paid to ensuring that the outlet temperature of the cooling water is not too low, as this can cause the screw to break. From the perspective of energy utilization, cooling the screw results in some heat loss.

Cooling Medium:

Oil and air can also be used as cooling media. The advantages of oil and air are that they do not corrode, the temperature control is relatively precise, and they are less likely to clog pipes. However, water cooling is more effective for large extruders.

• Cooling of the Hopper Seat:

The temperature of the plastic in the feeding section should not be too high, as it can form a "bridge" at the feeding port, making it difficult to add material. Therefore, it is necessary to cool the hopper seat. Additionally, cooling the hopper seat can prevent the transfer of heat from the extrusion section to the thrust bearing and gearbox, ensuring their normal operating conditions. Water is commonly used as the cooling medium for the hopper seat.