Rapid mold heating and cooling

At higher mold surface temperatures, the surface quality of the part improves, but the time required to cool the mold down will increase, resulting in an increased cycle time. Rapid mold heating and cooling is a technique that is used to improve the surface quality of a part, while keeping the cycle time as short as possible. Since the mold temperature is constantly changing, the simulation of this analysis is achieved using the transient cool solver.

The mold surface is heated, ideally to a temperature above the glass-transition temperature (Tg) of the material, prior to injection, then injection starts. Once the mold is filled, the mold is cooled, rapidly, to solidify the part before ejection. The mold is then heated up to temperature again, before the next injection occurs. By heating the mold up above the transition temperature of the polymer, the molding pressure can be minimized, reducing chain scission due to shearing and improving impact strength and heat resistance. The high temperature also prevents formation of the skin-like layer that creates residual stresses and impedes polymer flow, resulting in better surface quality. Rapid mold heating is usually restricted to one side of the mold, giving the part a smooth shiny surface on one side. For transparent parts, however, both sides of the mold will be heated.

There are two main categories of heating system used in the rapid heating of molds:
Surface heating
This category includes those techniques in which an insulation layer is coated onto the mold base, and then a heating layer is applied to the insulation layer as the cavity surface. The heating layer can be heated using a variety of different devices, including electrodes, infrared, electromagnetic induction, etc. In this approach, following ejection of the part, the mold is heated to temperature before closing. The advantage of this approach is that the mold surface can be heated very fast, thereby reducing the cycle time. The disadvantage is the complexity of the equipment.
Volume heating
This category includes those techniques in which one or both sides of the mold is heated. Channels are drilled into the mold, which can be used for both heating and cooling, or dedicated to one or the other. For example, rapid heating could be achieved using electric heater cartridges located beneath the mold surface on the core side of the mold, as illustrated in the figure below.



Simplified illustration of the rapid mold heating/cooling process, using heater cartridges

where the left-hand plate is the fixed plate, the center plate is the heating plate, the right-hand plate is the moving plate, the red circles are the heater cartridges and the blue circles are the cooling channels.

In the first image, the mold is open showing a small insulation gap between the heating plate and the moving plate. The heater cartridges are on and heating the mold. In the second image, the mold is closed, the heater cartridges are off, and the cavity is filled with hot polymer. In the third image, the cooling channels are filled with cold water and the part is being cooled. Finally, in the last image, there is no heating or cooling occuring, the mold is opened and the part is ejected.

The same effect can be achieved using the cooling channels to heat the mold, instead of heater cartridges. The cooling channels may carry hot water, or high temperature steam as heating rods for the filling phase, and carry low temperature coolant as cooling channels for the packing and cooling phases. In this case, to improve the heating efficiency after the cooling step is finished, air is forced into the channel system to remove all traces of water and ensure good contact of steam on the walls of the channel.



Schematic showing the simulated mold temperature changing during the rapid heating/cooling process

where (a) is cycle 1; (b) is cycle 2; (c) is cycle 3; A is the ejection temperature; B is the polymer glass transition temperature; 4->1 is the injection/packing stage; 1->2 is the cooling stage; 2->3 is the mold open/ part ejection stage; and 3->4 is the heating stage.

Note: The simulated injection molding cycle starts with the injection step. Clearly, this means that for the mold rapid heat/cool simulation, the first cycle does not reflect mold heating accurately.