Due to different properties and processing techniques of various plastics, requirements for mold temperature are also different. For thermosetting plastics, it needs to be cross-linked and solidified by heating in mold, mold temperature requirements are higher; while for thermoplastics, due to difference in their properties, mold temperature requirements are different. For plastics with good fluidity, such as PE, PS, PMMA, PP, etc., mold temperature requirements are low, room temperature water is required for cooling; for plastics with poor fluidity, such as PC, polyphenylene ether, POM, etc., mold temperature requirements are relatively high, hot water, hot oil or electric heating is required to assist mold to be heated.

In order to ensure that high-quality, dimensionally stable, small-deformation plastic products are produced within an effective time, basic principles of mold temperature control should be clearly understood when designing.
1) Molds with different surface qualities and different structures require different mold temperatures, which requires pertinence in design of temperature control system.
2) Temperature of cavity is higher than temperature of core, and temperature difference is generally 20~30℃.
3) Temperature of concave mold required by fire pattern is higher than temperature of concave mold required by general smooth surface. When die needs to be hot water or hot oil, general temperature difference is about 40 ℃.
4) When actual mold temperature cannot reach required mold temperature, mold should be heated. Therefore, when designing mold, full consideration should be given to whether heat brought by molten plastic into mold can meet mold temperature requirements.
5) Mold temperature should be balanced, and there should be no local overheating or overcooling.

Structure of plastic mold cooling system depends on product shape, size, mold structure, gate location, and cavity surface temperature distribution requirements.

Common cavity cooling method is shown in Figure 1, where cooling water channel in Figure 1 a) is longer, temperature difference between outlet and inlet is large; cooling water channel in Figure 1 b) is shorter. Temperature difference between inlet and outlet is small. In order to reduce temperature difference between outlet and inlet, if necessary, multiple pairs of outlets and inlets of cooling water channel should be set on mold. Figure 1 c) is an externally connected DC circulating cooling structure, which is connected from outside with a plastic tube, which is easy to process and easy to check for blockage. When cavity depth is large and it is an integral combined structure, cooling method shown in Figure 1 d) can be used.

 

a

 

b

 

C

 

d
Figure 1 Cooling of cavity

Cooling type of core is related to structure, height, radial size and other factors of core. Structure shown in Figure 2 can be used for cooling cores with small height dimensions, structure shown in Figure 3 can be used for cooling cores with large height dimensions and radial dimensions. When radial size of core is small, structure shown in Figure 4 can be used for cooling; when radial size of core is small, structure shown in Figure 5 can also be used for cooling (heat-conducting rod-type cooling); when core diameter is very small, structure shown in Figure 6 can be used for cooling.

 

Figure 2 Cooling of cores with small height dimensions

1. Water channel 2. Seal ring 3. Core 4. Faucet

 

Figure 3 Standpipe spray cooling for cores with larger height and radial dimensions

 

Figure 4 Standpipe spray cooling for cores with smaller radial dimensions

 

Figure 5 Thermally conductive rod-type cooling of cores with smaller radial dimensions

1. Beryllium copper alloy heat conduction rod 2. Cooling water channel 3. Cooling water outlet 4. Cooling water inlet

 

Figure 6 Cooling of a core with a small diameter (cooling at the bottom of the core)
1. Beryllium copper alloy core 2. Cooling water inlet 3. Cooling water outlet

When injection molding process requires mold to work at a temperature above 80℃, a heating device must be installed in mold. There are many heating methods for molds, such as hot water, hot oil, steam, and electric heating. If medium is a variety of fluids, design method is similar to design of cooling water channels.

This method is suitable for large injection molds that need to be heated before injection molding and need to be cooled after a period of time. Heating medium can be input through pipeline on mold. Its structure and design principles are similar to design of cooling water pipelines, cooling water channel can be used to realize heating of mold. Method has a forced flow process to make temperature distribution of the entire mold uniform, which is beneficial to improve quality of plastic products, but mold temperature adjustment cycle is long, and it is not easy to realize automatic control.

Device has simple structure, convenient use, large temperature adjustment range, low heat loss, clean heating and no pollution, and is suitable for large injection molds that require high mold temperature. Disadvantage is that it is easy to produce local overheating, so you should pay attention to it when designing heating device.
Main types of electric heating devices include electric heating plate heating, electric heating jacket heating, electric heating coil heating, electric heating rod heating, etc., as shown in Figure 7. Insert (sleeve) electric heating element in appropriate part of mold, and install a thermocouple, which can be easily connected with a temperature regulator to automatically control temperature of mold, or it can be connected with a voltage regulating transformer for manual temperature adjustment.

 

Figure 7 Electric heating device

In order to make mold heated evenly and ensure that it meets molding temperature conditions, following two points must be considered comprehensively when designing mold electric heating device:
1) Take effective heat preservation measures to reduce heat conduction and radiation loss of mold. Asbestos insulation boards are usually installed between mold and fixed board of equipment and around mold, thickness of which is about 4~5mm.
2) Heating elements are distributed correctly and reasonably. For large molds, two sets of temperature control instruments can be installed to adjust temperature of center and edge respectively. Generally, power of heating element at the center is slightly lower, and power of heating element at edge is slightly larger.