Definition: Mold: form and heat exchanger for creating plastic parts. Isolation: to isolate or keep separate.
In all of the books and guides to process a set material in injection moulding the mold is always stated as steel temperature and or mold temperature. In the authors many years as a troubleshooter and many of those cases were over the phone the one question is what is the mold temperature, or in the case of extrusion the bath temperature. In many of those cases it was the set point of the heater control, not the actual temperature of the steel or bath.
Once it was explained that the actual temperature of the steel was required and that was what the process conditions needed the issue were typically solved. One really needs to understand what is happening in the mold or bath to control temperature. Again if one were to look at the mold as a heat exchanger it tends to make more sense. That is heat in the form of melted or softened plastic is injected into the mold, which contacts the mold which than transfers that heat through it to the cooling media which than try's to take it away. In some cases this is done with cool media, and others it is done with hot media, both of which help to regulate the steel temperature, along with conducting heat away from the plastic so it can cool enough to be ejected as a solid.
But in molding there is another factor, and that is the clamp or platens of the press. In placing a mold in the press and bolting it to the platen they then become one, in the sense that metal to metal contact acts as a conductor. If for example the mold is at an 85°C temperature than eventually the platen shall be at an 85°C temperature. This increase in thermal mass means that the mold temperature control or heater has to work harder to maintain temperature, not necessary a good thing. In addition it means that the platen heats up which can than lead to many issues with a press (another topic in itself).
This heat dissipation can mean longer time to achieve a proper mold temperature, and numerous other processing issues. This is where isolation of the mold comes into play. This is typically done by using thermal insulation plates made of MICA and or other thermally retarding materials. Basically what happens is that the heat tries to leave the mold and transfer to the platen it is slowed down by these thermal restrictive materials, thus the platen is not the heat sink it was, the mold does not lose its heat as quick and control of the mold is quicker and more accurate for the materials being processed. Understand that long running tools well heat up the platen as these plates only retard the transfer, and if one wishes not to heat up the platens at all than additional plate and cooling is required. (see below)
The isolation plates can also aid in the processing of hot runners in that it aids in isolation of the mold, even though the hot runner manifold should be isolated inside the mold.
In some cases it may be suggested to place the insulation between two top or bottom clamp plates so that the additional top clamp or bottom clamp plate may be cooled thus isolating the press clamp area from any heat buildup, but in most cases the insulation sheet is bolted or screwed to the top and bottom clamp plates to isolate the mold from the platen.
Areas of concern for selecting which materials should be used and or manufacture are: maximum heat, continuous heat, thermal conductivity and maximum pressure. The one usually missed point is on the thermal conductivity point in that the calculation by the product manufactures is usually done in a per inch thickness range and the sheets in plate are only 0.125" or so. Thus the thermal conductivity is higher as the sheet is not as thick. Uniformity of thickness is also a concern since this is to be flat and square. This is another reason some add an additional clamp plate so as not to damage the insulator plate.
By isolating the mold from the platen and therefore the area that one wants to control it is controlled better leading to an easier processing of materials, as the heat sink of the platens is eliminated.
This can mean a smaller or more properly sized heat control unit, or that the one that was specified now works properly in controlling the mold temperature. Since the sizing of the mold temperature control unit uses the weight of the steel as part of the calculation the elimination of the platen weight now allows for the original calculation to work.
The use can mean, quicker start-ups as the platen is not being heated, more accurate temperature control as now the mold is isolated.
A simple test to see if one has an issue is to place one's hand on the platen, (safe location) can the hand be maintained there for an indefinite time or is it too warm or even hot to maintain ones had there, if so than the platen has now become part of the mold and the temperature control has become more varied.
SL Silvey
12112012
www.silveysplasticconsulting.com
