Temperature control of plastics in injection molding usually uses rotation of screw and heating plate on material tube to heat low-temperature solid plastic particles into high-temperature liquid melt. Melt temperature will affect quality of product. If it is too high, there will be problems such as material cracking and yellowing; if it is too low, it will reduce fluidity of material and increase flow resistance. Temperature of melt cannot be directly controlled by injection molding machine, but is indirectly affected by plasticization process. There are three main conditions in plasticization process that affect melt temperature, including temperature of heating plate, screw speed and back pressure. Following will explain how plasticization process affects material temperature.

Heating plate generally has multiple different settings, which can be set according to material's sensitivity to temperature. Before plastic is melted, heating plate provides main heat. At this time, temperature of plastic will be lower than that of heating plate. As plastic melts and enters rear section of screw, shear heat gradually increases, and temperature of molten plastic may be higher than that of heating plate, as shown in Figure 1.

 

Figure 1 Relationship between heating plate and material temperature

Rotation of screw will drive plastic to flow, and shear heat generated in process will increase temperature of molten plastic. If screw speed is too fast, material temperature will deviate too much from temperature of heating plate; if speed is too slow, metering time will increase and production efficiency will be reduced. As can be seen from Figure 2, the faster screw speed, the higher temperature at the center; while the two sides are maintained at temperature of heating plate, resulting in uneven temperature of molten plastic. When increasing speed to reduce metering time, it is necessary to pay attention to impact of subsequent temperature rise on plastic.

 

Figure 2 Relationship between screw speed and material temperature
After injection molding machine is running normally, plastic particles pass through effective thread length of screw, fully absorbing heat conducted to barrel by heating coil, and change in volume of screw groove makes plastic particles compressed, friction generates heat, most of heat is shear heat and friction heat generated by rotation of screw, commonly known as screw shear heat, which generates about 60%-85% of heat.
Melting of plastic is generally due to heat generated by rotation of screw. Therefore, if screw speed is too fast, following effects will occur: thermal decomposition of plastic, shortening of glass fiber (fiber-reinforced plastic), and accelerated wear of screw or barrel.

During metering, screw will transport molten glue to the front end of material tube. Molten glue stored at the front end will generate pressure. When pressure is greater than back pressure, screw will push backward. If back pressure is too high, screw is not easy to retreat, contact time between plastic and screw becomes longer, and shear heat causes material temperature to rise; if back pressure is too low, screw retreats too fast, resulting in inaccurate metering, molten glue is not tight enough, and may contain air. Back pressure should be coordinated with screw speed to make plastic in screw for a moderate time to obtain good material temperature control.

1. Set temperature of each section of barrel, screw speed and back pressure according to recommendations of material property table, generally set to middle value of recommended range.
2. Injection molding machine starts to heat up. Most of energy required for melt to heat up comes from shear effect brought by screw rotation. A small part comes from heating ring of barrel.
3. After injection molding machine heats up, at least 10-20 molds are produced to make injection molding machine reach a stable state.
4. Injection molding machine is switched to manual mode, and a receiving tray is installed at the front end of nozzle. An insulation pad is placed at the bottom of receiving tray to catch discharged molten material.
5. Prepare insulation gloves and thermometers.
6. Manually control screw to move forward, eject molten material in barrel, and catch it with a cushion.
7. Quickly insert thermometer probe into molten material so that molten material and pin are in full contact.
8. Read the highest temperature of thermometer.
9. Record the highest temperature, which is actual temperature of molten material.