How to stop granulators chewing through energy use

Energy-efficiency expert Dr Robin Kent of Tangram Technology says there is an unseen energy drain in injection moulding – granulators. He discusses solutions with Mark Holmes

Effective and efficient handling of reground material is particularly important in any injection moulding facility. Used inefficiently, granulators can be a financial drain on an operation. However, by taking a few key steps, this can be greatly minimised, according to Dr Robin Kent, Managing Director of UK-based Tangram Technology, consulting engi- neers for energy management in plastics processing. “Granulators for injection moulding do not use

large amounts of energy but the good thing is that much of [the energy they do use] is discretionary and can be saved by simple actions,” says Kent. “However efficient the site, there will be a need for some type of regranulation to allow the re-use of waste pro- duced by the process, even if it is only the start-up scrap – either for re-use in the process or for size reduction before disposal. Most sites have no control over the use of granulators and this costs significant amounts of money – granulators running in the background are common at plastics processing sites and, like compressed air leaks, are taken to be part of the environment. The energy cost of even a small granulator can be around £10,000 per year (US$14,200/€11,600). Failure to control granulators will use large amounts of energy and controlling granulators can be a rewarding exercise.” Kent’s first recommendation is to stop granulators

idling. The power trace of a 30 kW central granulator is shown in Figure 1 (on next page) and for most of

the 20 minutes of monitoring the granulator was idling and doing no effective work. The full load power was around 23 kW – it was not really working hard, but this was only for about 25% of the time. The idling load was around 17 kW, and this was drawn even when the machine was idling and doing no productive work. At an energy cost of £0.1/kWh, this is £11,000 (US$15,600/€12,800) of wasted energy. It is possible to fit most types of granulators with controls to turn them off automatically when they are not needed. This can be by simple current detection and turning off the granulator when the current indicates that it is idling. Kent offers some further tips when operating

granulators. These include: n Do not turn the granulator off immediately but pause to allow the throat of the granulator to clear. If this is not done, then it is possible that it will stall when restarted.

n Consider a control which uses vibration detec- tion as a signal to turn off the granulator. Start-up is controlled by a photocell which detects feeding of the granulator. This is simple, effective and available as a retrofit kit (120V) for around £800 (US$1,140/€930).

n Train staff to recognise that granulator use is not ‘free’ and provide them with the techniques and tools to minimise energy use. Another suggestion is to make sure that you have

June 2021 | INJECTION WORLD 25

Main image: Production scrap needs to be granulated efficiently


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