the right sized granulator. Kent says that the granulator shown in the example was 30 kW but the actual regranulation process required a maximum of 6 kW and the rest of the energy was wasted. He adds that similar waste is seen in machine-side granulators which are poorly sized for the application. At one site, two moulding machines were producing exactly the same products, but were fitted with different granulators: n Machine 1: 5.5 kW granulator with 0.55 kW blower to the blender. Running cost was around £6,235 per year (US$8,850/€7,230).

n Machine 2: 7.5 kW granulator with 2.5 kW blower to the blender. Running cost was around £10,306 per year (US$14,600/€12,000), which is £4,071 per year (US$5,780/€4,720) higher. “There was no difference in the requirements, but the site had the 7.5kW granulator with the 2.5kW blower available and simply used it without thinking about the energy cost of the larger motors,” says Kent. “The savings from using the correct size granulator and blower were more than enough to pay for new, and correctly sized, equip- ment, with a payback of less than six months. This site had 25 granulators so the total savings were over £100,000 per year (US$142,000/€116,000). Always make sure that the granulator is the smallest size possible for the job. Never use the largest granulator, just because it is there.” Another energy-saving technique he recom- mends is to make the granulator responsive to moulding activity. Granulators should be linked to the injection moulding machine motions so that when the machine stops operating, the granulator runs on for a small time to clear the throat and then switches off. In addition, the granulator can be set to only start when the machine motions start – it will be up to full speed before the first material enters the throat and the risk of jamming due to high start-up torque is low. Kent says that granulators are designed to be

sturdy workhorses but he nonetheless makes a number of usage and maintenance suggestions: n Use soft-start and other motor controllers to prevent high starting currents.

n Sharp blades are needed for efficient operation and good regranulation. Sharpen them regularly to reduce energy, noise and fines.

n Always check and maintain the gap between the rotating and fixed knives – it should be around 0.2-0.3 mm.

n Do not overfeed granulators. Overfeeding will increase current spikes and can causing jamming.

n Check the drive mechanism. If it uses V-belts, then consider replacing these with cogged belts.

Figure 1: Granulator power trace Source: Tangram Technology

n Make sure the motor is a high-efficiency motor and possibly replace it if it is not.

n Granulation is a dusty process and bearings are subject to dirt and dust collection. Clean and lubricate bearings regularly.

n If the granulator is not automatically emptied, then always make sure that the collection bin is regularly emptied. Full collection bins will back up and the granulator will eventually fail.

n Granulation of purgings has always been a problem. However, there are solutions such as the Purging Recovery System from Maguire, for example, which shaves purgings down to a size where they can be regranulated. A sound management approach is called for.

“The cost of closed loop regranulation for injection moulding should be critically examined,” Kent says. “The cost of running a granulator is largely fixed and reducing the amount of sprues and runners can make closed loop recycling expensive. In one case, the cost of the closed loop recycling was £0.80/kg (US$1.14/€0.93) and this excluded the investment and operational cost of the sprue picker/robot. The continuous operation of ma- chine-side granulators for small numbers of sprues and runners, often as low as 1 kg/hour, can make it more economic to turn the granulator off and collect the sprues and runners until an economic amount is collected. The granulator can then be run for a short time to clear the backlog.” He concludes: “A final tip is to calculate the cost

of closed loop recycling, especially when the sprues and runners are small and the granulator is large and compare this to the cost of the material being recycled. It may well be more economic in energy and cost terms to centrally granulate at night when energy costs are lower.” �

June 2021 | INJECTION WORLD 27

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