Definition: Mold (Mould) 1 A hallow form or matrix for giving a particular shape to something in a molten or plastic state. 2. That on or about which something is formed or made. Temperature 1. A measure of warmth or coldness of an object, with reference to some standard value. Thus mold temperature is that temperature of our mold. This is not the temperature a heater or chiller is set at but the actual temperature of the mold itself.
The aspect of the mold temperature has to be further defined as the steel temperature or the actual temperature of the steel in the area the plastic is molded against. Though in most cases due to surface finish it is not recommended to physically meter, touch or define this area but it is better to readjust to an area adjacent to the cavity and use that as a temperature reference point.
When molding parts one of the aspects of scientific molding is to know the steel temperature or mold temperature. Though most setup sheets will define the set points on the pump, mold heater or chiller for the real temperature what we need to monitor is the actual steel or mold temperature. In most cases this is done by a process called mapping the tool.
When we refer to mapping the tool it is that by creating a map of the mold to record actual temperature at actual locations or positions in the tool. By use of a pyrometer with a surface probe measurements of steel temperature are taken at specified locations points within the mold. Yet if we were to check all the areas specified at one time the tool temperature would change while the task was being preformed... This change is due to coolant running though the tool, air flow over the open mold, interruption in cycle. Basically the radiation, convections and conduction happening to our tool would result in a change in steel temperature which would give false results.
Thus when taking mold temperature we need to be quick, only do a few spots at a time and take our time in proceeding.
The process starts with the drawing of a map or layout of the A side and B side of the tool. (Using the plan view works well) It would than be determine how many spots we need to take, or cavities we wish to check. Once we have these determined we now take a surface probe and dependant on quickness of probe we either preheat or not so that within a normal cycle we go into the mold and touch the surface and record temperature for that area. We can only measure due to loss of heat one or two spots on a mold open cycle. The cycle is continued and the press is allowed to run for three to four cycles to stabilize and than we would interrupt the cycle and take another two points of measurement.
By going though this process what is found is how uniform is the temperature control of the steel and what variation the tool may have. If the tool temperature is non uniform than there is a possibility that the part temperature or cooling rate is non uniform leading to variations within our parts, either due to warpage, crystallinity or stress.
One way to double check our steel temperature is to check part temperature as it exits the mold. The key here is that a uniform temperature of the part coming out of the tool means that overall we shall have uniform shrinkage and stress within our part while if we do not have uniform temperature we could run into issues.
There are tools which are faster at determining what has just been explain, and they are Infra Red cameras which will measure the heat within the part and also in some cases the mold. Again we need to remember the reflection off shinny surfaces. But I have seen that the camera works fairly well given the way they are designed to operate
By mapping the mold we have now establish what the steel temperature is for a reference that can be check at any point to see that it is what it should be. This is a result of factors so for example if someone were to speed up the cycle what would be expected is that the mold would start to heat up, which now can be measured to confirm. What than could be done is to adjust the coolant temperature, to try and bring the steel temperature within the range established.
Though it is not to be discussed here is the fact that at some point the steel, plastics thickness and or the fluids heat conveyance properties a limit will be reach. What is meant is that if the steel can only convey X BTU's per second, and we can only remove Y BTU's per second a point will be reached that steel well continue to increase in temperature and the part well come out hotter and hotter no matter what we do from the processing standpoint.
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