Richard Brown, chair of Judges Design Innovation in Plastics Awards, addresses the overlooked importance of cooling systems and mould tool design in injection moulding.
Key Highlights:
- In many mould shops reduction in the effectiveness of cooling over time is overlooked all too often, and only comes to realisation when components get returned from the customer for distortion or poor fit and function issues.
- The cooling cycle generally is 60 to 80% of the process and coolant flow is critical to remove heat from the mould tool, so it is very important that coolant temperature should be controlled and monitored.
- Testing the coolant quality at predefined intervals is part of preventive maintenance as is testing coolant flow rates over time to determine if any reduction of flow is occurring.
There are three key areas where temperature control is crucial to the injection moulding process, these being the three ‘M’s’ of Material, Machine and Mould. In many mould shops reduction in the effectiveness of cooling over time is overlooked all too often, and only comes to realisation when components get returned from the customer for distortion or poor fit and function issues.
The process of injection moulding relies on the activity of heat transfer. The raw material (usually dried by heat) is fed into the injection moulding screw which mixes and heats the material to soften it prior to feeding it into the mould tool.
Mould tools are designed with internal cooling channels through which are fed a flow of circulating coolant at a predefined temperature. The coolant is pumped into the tool via a temperature control unit (TCU). The coolant that leaves the tool is key to managing the heat transfer from the mould tool and is key to achieving a steady temperature on the cavity surfaces of the mould tool.
When the material is injected into the mould tool, the initial purpose of the cooling system is to maintain an even temperature on the mould tool surface to ensure the tool does not overheat as the injected material takes shape in the tool cavities. The excess heat is removed to cool the mould tool which enables the injected material to harden and form the finished component.
The coolant exiting the mould tool is directed to a chilling system to remove the heat from it, the coolant is then recirculated back to the TCU and through the mould tool again.
The cooling principle explained in this manner seems fairly straightforward forward but temperature control in practice is far more complicated, and there are many cooling-related challenges that are faced daily within the industry.
The cooling cycle generally is 60 to 80% of the process and coolant flow is critical to remove heat from the mould tool, so it is very important that coolant temperature should be controlled and monitored.
Temperature impacts every step of the injection moulding process and temperature control is fundamental to ensure that the moulding process is effectively maintained at the required temperature.
As soon as the material is fed into the machine's barrel it is heated and mixed, so it is essential that the temperature profiles of the barrel zones are set correctly to ensure it is melted and mixed without causing material degradation. The temperature zones of the barrel increase along the barrel with the hottest zone being towards the check valve prior to material being injected into the mould tool.
The temperature that the heating bands maintain around the barrel is set to depends on the material being processed and can be found on the material supplier's specification sheet and used as a guide. If excessive or inadequate temperatures are used in the barrel it can result in quality issues in the finished component. If the barrel temperature is too high it can cause burning and degradation of the material properties, if too low it can impact the finished component.
As the material enters the mould tool the temperature of the tool is controlled by the coolant which is either water or oil. Water cooling is used to lower the temperature of the tool whereas oil (sometimes backed up with cartridge heaters) is when the mould tool must be held at an optimum temperature.
The importance of mould tool temperature and cooling profile
Processing temperatures and cooling rates vary for different polymer materials, so it is important that process technicians/engineers understand the requirements. It is critical that the coolant gets to the surface of the tool cavities and achieves the correct temperature to ensure the resin flows into the tool correctly and cools at the required rate to solidify with the expected quality.
However, if you are working with a semi-crystalline material such as PEEK you cannot use water as a cooling medium as it would reduce the tool temperature too quickly. Oil is usually used to allow the material to cool at a more gradual rate to allow the consistent material crystallisation of the part.
Coolant flow rate
As stated, maintaining the correct mould tool temperature during processing is achieved with the coolant set at the correct temperature, but the flow rate of the coolant is also a critical factor.
The coolant should flow at the rate required and with sufficient pressure to ensure turbulence in the coolant flow. Baffles can be incorporated into coolant channels to offset laminar flow to create turbulence. Turbulent flow forces the coolant to the walls of the coolant channels whereas laminar flow only enables a portion of the water to be in contact with the walls of the coolant channels. Therefore, laminar flow insulates the centre of the coolant flow preventing it from making full contact with the channel walls and thereby wasting its heat transfer potential. The rate of coolant flow is determined by how much heat energy needs to be removed from the tool based on the heat transfer of water and the size of the coolant channels in the mould tool. The turbulence of coolant flow can be calculated using a Reynolds (Re) number calculation. The aim is to achieve a coolant flow between 4,000 and 8,000. The key to generating good flow and turbulence is central to the pump size in your TCU.
Mould tool design
If you are struggling to achieve the required coolant flow it might be that the tool design may not be adequate with respect to the cooling channels. There are a number of areas that can cause problems:
- Cooling channel diameter is too small
- Cooling channels are too distant from the tool cavity surface.
- Coolant channels have become reduced due to scale build-up or corrosion.
- Coolant does not flow adequately due to the coolant channel layout
If conventional cooling channels are unable to reach some areas of the mould tool surface it is possible to use bubblers or baffles to divert the coolant at a 90° angle from the main coolant channel to the area of the tool needing better temperature removal.
The aim is to circulate coolant through the mould tool to deliver it evenly. This can be achieved by using a manifold on the incoming flow side of the tool to an output manifold. Ideally coolant flow should be designed to make a single pass through the mould tool utilising a balanced manifold to achieve similar flow rates to remove comparable amount of heat.
It is though not always possible to achieve an ideal coolant flow leading to temperature, pressure and heat transfer imbalances. Some of the factors that can cause this are:
Cooling channel lengths vary causing heat and pressure imbalance.
Coolant that's taken from a cooling channel output and redirected back into another cooling channel for a second pass through the mould tool may causing heat imbalance.
Another cause of not being able to hold a consistent mould tool temperature, despite in the past achieving it, could be due to adding extra cooling demand to your system maybe with the addition of new equipment. This may mean that additional cooling capacity will be required, an area that is often overlooked.
However, a further cause for not being able to maintain a constant temperature may lie in the mould tool, being caused by a coolant leak within the mould tool. This can be caused by fractures in tool steel or failure of a seal in the coolant channels. A solution often used, instead of addressing the root cause, is to run the cooling on negative pressure where a TCU equipped with a negative pressure option pulls rather than pushes water through the mould tool channels.
Coolant quality
The lowest cost solution to use for heat transfer is water, this though has its problems as ‘clean’ water is loaded with minerals which vary dependent on which part of the world you are in. These minerals are attracted to the channel walls or the internal pipes of a TCU causing scale deposits to form. These scale deposits have two effects, they can reduce flow and heat transfer over time.
Testing the coolant quality at predefined intervals is part of preventive maintenance as is testing coolant flow rates over time to determine if any reduction of flow is occurring. Also flushing tools with chemical descaling agents will help reduce scale build up.
Conclusion
Process monitoring and stable process control are essential prerequisites to achieve quality components. Therefore, you can conclude from this that temperature control in the process is a critical component, and should not be underestimated as to the impact poor temperature control can have during the manufacture of components.
It is therefore crucial to identify and control the temperatures at every step through the injection moulding process. There are a number of companies that offer solutions to this, Engel with their digital product iQflow, Burger and Brown Smartflow solutions, and RJG Inc Edart and CoPilot process control systems to name a few.