Heating, Cooling & Temperature Control Systems
Precision temperature management for optimal plastic extrusion performance
Stable and precise temperature control is the prerequisite for achieving optimal extrusion molding. The organic combination of accurate temperature measuring elements and advanced control systems is an important means to reduce temperature fluctuations during the extrusion process in a plastic extruder machine. Heating and cooling enable the plastic extruder machine to ensure precise temperature control in each section.
By utilizing thermal resistors or thermocouple humidity sensors to measure temperature, and adopting digital temperature control instruments with proportional-integral-derivative (PID) regulation or programmable logic controller (PLC) temperature control modules, temperature fluctuations can be controlled within ±1℃. With more advanced fuzzy (FUZZY) temperature control methods, the temperature control accuracy can reach ±0.1℃ in a high-performance plastic extruder machine.
German manufacturer Kruss Madei has already adopted multi-point melt temperature sensors for implementing melt temperature distribution control in their plastic extruder machine systems. Currently, commonly used temperature measuring elements include copper-constantan and nickel-chromium-copper-constantan thermocouple temperature sensors. A more advanced method is the combination of deep and shallow hole temperature measurement.
Heating Systems in Plastic Extruder Machine
Heating brings the plastic extruder machine to the temperature required for normal startup and maintains the temperature needed for normal operation. There are two sources of heat in a plastic extruder machine: one is external heating; the other is the shear friction heat between plastic and the barrel, between plastic and the screw, and among plastic particles themselves.
Heat Source Distribution
The energy provided by mechanical energy accounts for 70%~80% of the total energy, while the energy provided by heaters accounts for 20%~30% in a typical plastic extruder machine. The proportion of these two parts of heat is related to the different stages of the extrusion process, the structural forms of the screw and barrel, the process conditions, and the properties of the processed materials.
In the solid conveying section, the shear friction heat is relatively small. In the homogenizing section, due to the shallow groove, the shear friction heat is large, and sometimes it is not necessary to heat but to cool to control the temperature rise in the plastic extruder machine.
1.3.6.1 Electrical Heating Methods
Electric heaters are easy to clean, convenient to maintain, low in cost, and high in efficiency, making them widely used in the plastic extruder machine industry. Electric heaters are arranged in sections along the barrel of the plastic extruder machine. Small plastic extruder machines have 3~4 sections, while large plastic extruder machines can have 5~10 sections, each controlled individually to achieve a temperature distribution suitable for material processing requirements.
Electrical heating is divided into two main types: resistance heating and induction heating, each with its own advantages and applications in the plastic extruder machine.
(1) Resistance Heating
Resistance heaters are categorized into band heaters, ceramic heaters, and cast heaters. Each type offers specific benefits for the plastic extruder machine depending on operational requirements.
Types of Resistance Heaters
- Band heaters: Compact size, easy adjustment, convenient assembly and disassembly, good toughness, and low price, but vulnerable to damage.
- Ceramic heaters: More robust than mica-insulated band heaters with a longer service life (4~5 years) and simple structure.
- Cast aluminum heaters: Small volume, easy installation and removal, and eliminates the need for mica sheets. The resistance wire is protected by a magnesium oxide powder iron tube, providing oxidation resistance, moisture resistance, shock resistance, explosion resistance, and long service life.
- Cast copper heaters: Higher heating power, long heating life, good thermal insulation performance, strong mechanical properties, corrosion resistance, and non-magnetic properties.
Fig 1-41: Resistance heater configurations
| Heater Type | Maximum Power (kW/m²) | Maximum Temperature (°C) |
|---|---|---|
| Ordinary Mica | 50 | 500 |
| New Type Mica | 165 | 500 |
| Ceramic | 160 | 750 |
| Cast Aluminum | 55 | 400 |
| Cast Copper | 80 | 400 |
(2) Induction Heating
Induction heaters, as shown in Figure 1-42, are characterized by uniform heating, small temperature gradients, long service life, and short heating times in a plastic extruder machine. For example, a 65mm diameter plastic extruder machine requires 45 minutes with resistance heating, while electric induction heating takes only 7 minutes.
Induction heaters can achieve precise temperature control with high heating efficiency and low heat loss. Although the power factor of induction heating is lower than that of resistance heating, the total power consumption is approximately 20% less than resistance heating in a plastic extruder machine.
The heating temperature is limited by the insulation performance of the induction coil. Induction heaters require a large amount of valuable silicon steel sheets and copper, resulting in higher costs and inconvenient assembly and disassembly. If one section fails, other sections must be removed for maintenance, making it inconvenient to install on the machine head. These issues have restricted the application of electric induction heating in the plastic extruder machine.
Fig 1-42: Induction heater structure
Cooling Systems in Plastic Extruder Machine
Cooling is used to reduce temperature, which represents energy loss. Therefore, cooling should be minimized during the extrusion process in a plastic extruder machine. If a plastic extruder machine requires extensive cooling during operation, it indicates that the screw structure or operating parameters are improperly set.
When the thermal energy converted from mechanical energy during extrusion is exactly equal to the thermal energy required for plastic plasticization and heat dissipation to the environment, "self-heating" extrusion can be achieved. However, heating and cooling devices are still necessary in such a plastic extruder machine.
In addition to the hopper base, which always uses forced water cooling, other parts of the barrel in a plastic extruder machine can be cooled by air, oil, or water.
(1) Air Cooling
Fig 1-43: Air cooling system and heat transfer enhancement
Air cooling is gentle, uniform, and clean, making it widely used in plastic extruder machines produced both domestically and internationally. However, the fan occupies a large space, and poor-quality fans can produce significant noise.
It is generally considered that air cooling is more suitable for small and medium-sized plastic extruder machines. To enhance heat dissipation, copper fins are connected to cast aluminum heaters to increase the heat dissipation area, although this consumes valuable copper. Currently, there are commercial plastic extruder machines with a diameter of 70mm that use copper fins to enhance the air cooling system.
(2) Water Cooling
Fig 1-44: Water cooling system and heat transfer enhancement
Water cooling offers fast cooling speed, small volume, and low cost in a plastic extruder machine, but it can cause rapid cooling, which disrupts the stable flow of plastic. If the seal is poor, leakage can occur.
Cooling systems that use water pipes wrapped around the barrel of a plastic extruder machine are prone to scale formation and pipe blockage, as well as corrosion.
The water used in water cooling systems is not tap water but chemically treated deionized water. Studies have shown that distilled water should not be used because it contains a certain amount of dissolved oxygen, which can accelerate corrosion. It is generally considered that water cooling is suitable for large plastic extruder machines.
(3) Screw Cooling
Large plastic extruder machines, as well as those seeking high plasticization quality and production efficiency, should have单独 temperature control for the screw. Screw cooling is performed in the core, as shown in Figure 1-45.
The cooling medium (water or oil) flows through a copper tube installed in the screw core to the front end of the screw in the plastic extruder machine, then flows out to the screw tail through the annular space between the copper tube and the screw core hole, and is discharged from the outlet.
The maximum cooling position with this method is at the front end of the copper tube, and the maximum cooling position can be controlled by adjusting its axial position in the screw. Another technology uses heat pipe technology, which does not require inlet and outlet pipelines but cannot achieve external temperature regulation in the plastic extruder machine.
Fig 1-45: Screw internal cooling diagram
Advanced Temperature Control in Modern Plastic Extruder Machine
The evolution of temperature control systems has significantly improved the performance and efficiency of the plastic extruder machine. By combining precise measurement devices with sophisticated control algorithms, modern plastic extruder machines can maintain temperature stability within extremely tight tolerances.
Whether through advanced PID controllers, fuzzy logic systems, or multi-zone heating and cooling configurations, the goal remains consistent: to provide the optimal thermal environment for plastic processing in the plastic extruder machine, resulting in higher quality products, reduced waste, and improved energy efficiency.