Injection Mould Cooling
Injection Mould Cooling serves to dissipate the heat of the molding quickly and uniformly, fast cooling is necessary to obtain economical production and uniform cooling is required for product quality. Adequate mold temperature control is essential for consistent molding. The layout of the cooling circuit warrants close attention especially if you consider cooling typically accounts for two third if an injection molding cycle time.
Optimal properties if engineering plastics can be achieved only when the right mold temperature is set and maintained during injection molding processing. The mold temperature has a substantial effect on:
- mechanical properties
- shrinkage behavior
- surface quality
- cycle time
- flow length in thin walled part
In particular semicrystalline thermoplastics need to cool down at optimal crystallization rate. Parts with widely to deform because of local differences in the degree of crystallization, additionally the required mold cooling time increases rapidly with wall thickness, this calculation is shown in cooling system equations.
In general, injection mold cooling system will be roughly drilled or milled. Rough inner surface enhance turbulent flow of coolant, thus providing better heat exchange, turbulent flow achieves 3 to 5 times as much heat transfer as does non turbulent flow, cooling channels should be placed close to the mold cavity surface with equal center distance in between. The mechanical strength of the mold steel should be considered when designing the injection mould cooling system.
Some thermoplastic may require mold temperatures of 100 degree or higher for optimal processing and properties, effective mold insulation is advised to minimize heat loss between the injection mold and machine mounting platens. Insulation boards with low thermal conductivity and relatively high compressive strength are commercially available.
Guidelines for mold temperature control include:
- independent symmetrical cooling circuits around the mold cavities
- cores need effective cooling
- short cooling channels to ensure temperature differences between in and outlet do not exceed 5 degree
- parallel circuits are preferred over serial cooling
- avoid dead spots and/or air bubbles in cooling circuits
- head exchange between injection mould and machine should be minimized
- differences in flow resistance of cooling channels, caused by diameter changes, should be avoided.
Mold parts that are excessively heated, like spure bushings and areas near the gates, must be cooled intensively. Rapid and even cooling is enhanced by the use of highly conductive metals, such as berylliumcopper. These metals are used to full advantage in places where it is impossible to place sufficient cooling channels. Copper transfer twice as much heat as carbon steel and four times as much heat as stainless steel. This does not mean copper mold will run 4 times faster than a stainless steel mold but they will run some thin walled parts significantly faster.