When manufacturing injection-moulded parts from plastics, the mould temperature has a decisive influence on the quality of the parts and the cycle time. Optimum temperature control during the processing of thermoplastics therefore also has a direct influence on the efficiency of the production. This article explains how temperature control systems work and what benefits they provide.
The quality of injection-moulded parts essentially depends on how well the thermal conditions in the machine are controlled. These include the thermal balance of the injection mould as well as the temperature of the cylinder and screw. The temperature control of the mould fulfils two tasks:
- heating the mould up to operating temperature
- keeping the mould at operating temperature
Thermal balance of the injection mould
The injection mould is cyclically heated by the injected material. The heat is dissipated by the thermal conduction in the material and in the steel of the mould to the surface of the temperature control channel, where the heat is given off via heat transfer to the circulating heat transfer medium (water or oil). The temperature control unit dissipates the heat of the heat transfer medium and returns the cooled medium to the circulation.
Influence of the temperature control on the injection-moulded parts
Good control of the thermal processes is directly discernible from the constant high quality of the injection-moulded parts and an optimised cycle time, because the mould temperature influences the surface quality, shrinkage, warping and fluidity of the material. Here are some examples for illustration:
- Sink marks and blowholes are often typical of moulds that are too hot
- Gloss differences occur if the mould temperature is too high
- Warping can occur in the case of uneven temperature distribution in the mould or high temperature differences between material and mould
- Jetting can occur if the mould temperature is too low
The homogeneous and adapted control of the mould temperature restricts such injection errors to a minimum.
A temperature control system consists of three components: the mould, the temperature control unit and a heat transfer medium. Certain conditions have to be fulfilled so that they can work optimally with one another.
The mould itself needs an adequate temperature control channel surface area and a channel cross-section that is not too small. Cross-sections that are too small lead to a higher drop in pressure in the mould. This necessitates temperature control units with expensive pumps that can supply the necessary higher delivery pressure. There is considerable potential for savings here. It is also important for the heat transfer medium to have only small temperature differences between the inlet and outlet in the mould. Ideally, the maximum difference is 3 °C.
An optimised layout minimises the occurrence of a great many injection errors. At the same time, shorter cycle times and thus improved productivity can be achieved. For example: For each degree Celsius that the temperature increases in the mould, the cooling time is extended by about 2 percent on average.
Both effects together lower the production costs noticeably.
The temperature control unit must control the mould temperature within narrow limits. These are between 5 and 20 °C, depending on the material and component. Optimum control is necessary for this, combined with a well matched and adequate heating, cooling and pumping capacity.
Amongst other things, the heat transfer medium should have good heat transmission properties in order to be able to dissipate large quantities of heat in a short time.
Water cooling is cost-effective
To put it briefly: since water has better heat transmission properties than oil, it is preferable to use such temperature control systems. As long as no special reasons oppose it, this is possible at mould temperatures up to 90 °C. Pressurised water units can be used between 90 and 250 °C. Oil is used as the heat transfer medium at temperatures above 90 or 250 °C respectively.
When selecting the temperature control unit for injection moulding, the performance profile of the temperature control system is defined by the following criteria:
|Processed material||Mould temperature|
|Mould weight and heating-up time||Heat transfer medium|
|Quantity processed per time unit||Cooling capacity|
|Temperature drop across the mould||Delivery rate|
|Pressure and flow conditions in the mould||Delivery pressure of the pump|
What does the temperature control unit do?
The temperatures in the mould differ locally. In addition, they change periodically with the injection cycle. For the quality of the injection-moulded part it is very important that in particular the level of the periodically changing temperature remains constant over the entire mould cavity.
During production, the temperature control unit ensures that the minimum and maximum mould wall temperatures are kept constant. In addition, the temperature control system ensures that this temperature range does not drift upwards or downwards. Three control systems are available for this:
- Control of the temperature of the medium
This method is used most frequently. The control precision is sufficient in many cases.
- Control of the mould temperature
A temperature probe is installed for this. This method is necessary if the constancy of the mould temperature needs to be controlled within particularly narrow limits.
- Cascade control
This method combines the control systems described above and improves the temperature control in the mould still further.
Those who use temperature control units correctly utilise the technological advantages to the full and work economically in every phase of the production process:
- At the start of production, the mould is already at production temperature. This reduces scrap when starting up.
- Correct temperature control during production reduces scrap that would otherwise occur, for example, due to warping, blowholes, streaks or poor mould filling behaviour.
- Following breaks in production, the regular production rhythm is regained more quickly.
Overall, the processor benefits from a higher quality of the injection-moulded parts. The results are better surfaces and low-stress parts that can be produced with tight tolerances. Scrap minimised in this way lowers the costs of reworking, waste and materials.
Temperature control units operate with a closed water circulation instead of fresh water. Problems with calcification or microbial contamination are reduced to a minimum – that keeps maintenance and repair costs down. Moreover, a balanced temperature profile in the mould ensures less mould wear.
All of these factors together enable considerable cost savings in practice, while at the same time resulting in high quality components.