SPECIAL NOTE: Cooling lines in the mold (and heat exchanger, for that matter) MUST be kept clean and open to allow the temperature control system to operate efficiently. Tests show that a common 1/4" waterline with a scale buildup of only 0.015" (1/64") will result in a drop of 60% cooling efficiency. To clean, recondition, or maintain your waterlines you must flush them periodically with an acid solution. Even if you use chemical additives in a closed-loop water program, you must periodically flush the mold's waterlines with the acid solution.
The temperature that you set for the mold will depend on manufacturer's specifications and the properties that you want for the final product.
The material manufacturer or supplier is responsible for providing the recommended temperature of the mold into which we are injecting the molten plastic material. The theory followed is that we are heating a plastic up to a specific melting temperature, and then injecting it into a mold in which we wish the plastic to cool down to a solid again. However, every plastic has a specific rate at which it gives up heat. If we try to take heat away too quickly, it may not attain the maximum degree of physical, chemical, thermal, and electrical properties that are available. The manufacturer runs tests on each material developed to determine the best mold temperature needed to produce the highest quality parts.
When to vary Mold Temperatures
While the material supplier may recommend a specific mold temperature for a specific material, it is possible to run the mold at a higher or lower temperature as long as it is understood that property values will suffer more as you stray from the recommended temperature. For instance, if you are using a polypropylene material for molding disposable flower pots you may be able to run the mold at 60 to 80 degrees (F) which is 40 to 60 degrees lower than the recommended 120 degrees (F) because you do not need the high level of property values that are available. But, if you are molding electrical components from the same material you should not vary far from the recommended 120 degrees (F) because you need to attain the maximum electrical properties possible or you risk product failure in the field.
It is important to understand the final use of the product being molded to determine the range of mold temperature you can use. In any case, there should be no more than a 10-degree (F) difference between any two points of the steel area of the mold that actually forms the parts. If there is more than that difference, there will be a thermal shock condition that will induce a large amount of stress into the molded part. This can be found as warp, bowing, brittleness, and other similar defect conditions.
The Table (below shows the recommended mold temperatures (as determined by the suppliers) for some common materials. The mold temperature should be checked by using a surface probe on a fast-acting pyrometer and checking the area of the mold where the molten plastic will be formed. The other areas do not count. Please remember that water control units cannot effectively maintain mold temperatures above 190 degrees (F). You will need the assistance of cartridge heaters in the mold or the use of an oil control unit instead of the water control unit for those situations.
Recommended mold temperatures (F)