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