Cooling systems in injection molding
Efficiently brought to the right temperature
Cooling systems play a key role in plastics injection molding – and there are naturally many different options available here for optimizing the process as a whole. The goal is to only cool for as short a time as is necessary so as to keep the production cycles as short as possible too. At the same time, the highest possible part quality is to be guaranteed on a permanent basis.
Correct temperature decisive for a perfect part
Plastics react highly sensitively to temperature during processing. The temperature has a major influence on the flow behavior of the melt and also affects the solidification behavior of the material – and hence its optical and mechanical properties. Heating and cooling significantly affect shrinkage and warpage and hence the dimensional stability of plastic parts.
The fundamental rule of thumb is that the cooling time increases by the square of the thickness of the molded part. Particularly with regard to thick parts, it cannot be dismissed that the question of efficiency is always closely linked to the factor of time. In the case of complex parts, the precise points at which cooling takes place are important, since heat builds up in certain areas. Uniform cooling is indispensable for avoiding warpage and ensuring dimensional accuracy. Two different cooling concepts are applied in injection molding processes as a function of the individual requirements.
1. Mold-based temperature control concepts
Mold-based temperature control concepts frequently take the form of straight cooling channels that are drilled into the mold as deep as necessary. Changes in direction are achieved through a bore at an angular offset, often at 90 degrees to intersect another bore. Plugs ensure a controlled flow of the cooling fluid.
If the right-angled cooling channels match the shape of the part, this is a highly efficient method, since the cooling channels are close to the part and can eliminate the heat in a targeted manner. Shrinkage and warpage are kept to a minimum. A high part quality can be achieved.
Another option for mold-based cooling is a conformal cooling. Other methods of production need to be chosen, e.g. selective laser sintering, or SLS for short. This is applied in cases where complex internal structures are involved and where dimensional accuracy is important. A 3D structure is created in the metal – this is not, however, practical for the entire mold but only for inserts with complex heating/cooling channels. One variant is vacuum brazing: this involves cooling channels being milled into plates or cones, creating a layered shell structure with cooling channels in the surface, such as for the inside of a cup. These layers are then joined by brazing to achieve a leak-proof component.
To optimize the process, it is best to use materials that are good conductors of heat. Copper or its alloys are frequently used here. A copper pin that is connected to a cooling channel at one end accelerates the cooling process. Cooling with refrigerating agents or liquid carbon dioxide instead of water is also highly efficient. This is a highly specialized and technically complex solution, however, since refrigerating agents are harmful to the environment and to health.
2. Process-based temperature control concepts
The mold-based temperature control concepts described so far involve continuous cooling as standard. Process-based temperature-control concepts extend the range, with a distinction generally being made between three variants.
- Quasi-continuous cooling: If a mold is not to be too cold during the molding process – this is especially the case with small structures – cooling is stopped during injection. The temperature of the mold then initially rises. As soon as the mold has been completely filled, the cooling is activated again and the heat can be eliminated.
- A combination of two cooling systems with different temperature levels in a single mold: The hot phase is switched on for the injection phase, making it easier to mold the material. Alternating the switch, the cooling phase is activated.
- Dynamic or variothermal temperature control: An additional heater is used here. The mold is opened and the structures inside it are actively heated locally at the decisive areas, either through infrared, laser or induction heating. The mold is then closed again and the plastic injected. The heat is subsequently eliminated again via the “normal” temperature control system.
A wide range of options thus exist, with a high potential for shortening cycle times and securing part quality. When deciding on the correct cooling method, simulation software is a decisive aid, since this allows numerous variants to be run through at an early stage of development – long before the design phase has been completed. This then permits a well-founded judgement without costly and time-consuming iteration loops using real prototypes.