Specific Process Knowledge/Back-end processing/Polymer Injection Molder/MoldTemp
Mold temperature setup
Injection moulding processes can be divided into two different types:
- Constant mould temperature processes
- Variable mould temperature (Variotherm) processes
1. Constant mould temperature processes
In this type of process the mould remains at the same temperature during the entire injection moulding cycle. The temperature should not exceed the glass transition temperature of the polymer being used. Otherwise the polymer will never get cold enough to solidify to a degree where it can be demoulded without being deformed/damaged. How close one can get to the glass transition temperature depends on several factors like e.g. polymer type, cooling time and whether the polymer has a tendency to stick to the shim (especially an issue when replicating high aspect ratio structures). With Topas 5013L-10 (which has a Tg around 135°C) one can often go up to 100°C and for samples that are easy to demold even higher (110-120°C). For polymers with very low Tg (such as e.g. flexible polyurethanes) one often has to run as cold as possible. With the mould heeting switched completely off and mould cooling water fully open, the lowest achievable mould temperature is around 15°C.
| Parameter | Benefits | Drawbacks |
|---|---|---|
| Low mould temperature | Very fast cycle times due to short cooling times | Problems with replication of small features because polymer solidifies prior to complete filling |
| High mould temperature | Reasonably short cycle times, improved replication | Problems with demoulding |
For constant mould temperature processes it is a good idea to manually close the mould cooling water valves e and g. For reproducible results it is also a good idea to make sure that you always run at the same temperature setpoint for the mould casing cooling water controllers (see Chapter 4.11 in the manual). A setpoint around 70°C for both sides works well in most cases. If the tool casing is allowed to heat up to 90°C or higher it will often result in problems because the internal parts of the mould will get stuck (especially the ejector pin system).
2. Variable mould temperature (Variotherm) processes
In this type of process, the mould changes temperature during the cycle. There are two principal ways to achieve this:
- Switch off mould heating during the moulding cycle. This case is best suited for situations where a slow and well-controlled cooling is desirable. Since cooling happens quite slowly (heat dissipates out in the tool casing) this method is usually only of practical interest when the mould temperature only has to drop slightly (maybe up to ~10-20°C drop in temperature).
- Switch off mould heating and enable mould cooling water during the moulding cycle. This is basically the same procedure as above except we also send cooling water through the mould. This allows for rapid cooling of the mold making it suitable for situation where large drops in temperature are desired (> 20°C temperature drop).