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Why switching to AC drives for plastic extrusion makes sense

Drive energy appraisal leads to 60 percent fall in energy costs and 30 percent higher output for plastic container manufacturer.

When a 23 year old blow molding machine was failing to deliver its proper output, Northern Ireland’s PrimePac faced a dilemma. The machine uses a hydraulic motor to run an extruder on a blow molder. Because the hydraulic machine had spent its life running 24 hours a day, five days a week, the hydraulic motor was operating inefficiently and suffered poor speed holding properties.

“Previously I could create a bottle in 10 seconds, delivering 50 grams of material,” explains Clifford Craig, Engineering Manager at PrimePac. “However, in recent years we had to run that bottle at 12 seconds because the extruder output wasn’t quick enough. The extruder had started to create long tails, which are basically excess plastic that needs to be trimmed and recycled.”  The excess trimmed plastic was then granulated and transferred back into a hopper where it was once again heated and re-used. However, the 18.5 kW granulator and accompanying heating process is energy intensive and the machine was now using more electricity, as Craig explains: “We have a grinder which chips the waste bottle material and conveys it back to the hopper. Gravity feeds it back into the screw. This all takes extra energy which in effect is for making the same bottle that you made five minutes earlier. You take energy to make it, energy to convey it, energy to grind it, energy to drive the screw. “I investigated refurbishing the pump and drive and other parts of the machine but was quoted £30,000 ($44,000), which would have kept it running but with the same technology and still using the same amount of energy.” However, not only was the hydraulic machine running inefficiently but the company’s electrical capacity was at its limit. And its plans to introduce an additional all-electric injection molding machine to produce a new type of medicine bottle would mean an investment of £250,000 in upgrading their 600 kVA electrical supply.

Energy appraisal reveals opportunities

It was at this stage that PrimePac’s energy and automation partner, Advantage Control Limited, and one of ABB’s Authorized Value Providers, was asked to carry out an energy appraisal of the existing machine. “We came in and logged, for one week, the energy use of the older machine against one of the existing all-electric models, both of which were making similar bottles of the same size and weight,” says David Watt, Technical Sales Adviser at Advantage Control Limited. “Our energy survey was carried out on the main incoming supply which guaranteed that we weren’t just seeing one particular motor. We looked at the entire machine and the efficiency across it. When we compared the efficiencies we could see that the old machine was consuming significantly more energy to carry out the same process.” Furthermore, the old blow molding machine’s speed holding, or speed regulation, was causing issues as the motor was running directly from the mains. This meant that there was no torque control. Speed regulation is critical to the success of the perfect plastic container. The plastic is extruded in different lengths and weights. If excess material enters the mold, this increases the weight which results in a poorly formed bottle.  The energy appraisal also revealed that the power use was erratic and consumption was high due to the hydraulic pack requiring extra torque on start up. Watt believed that ABB’s synchronous reluctance motor (SynRM) and ABB industrial drive, ACS880, package would not only reduce electricity use, improve speed holding and the quality and consistency of the finished product but would also free electrical capacity that could be used to power the new injection molding machine.

Energy saving exceeded

Advantage Control predicted between 25 to 30 percent energy savings, based on a constant torque load. However, following the installation, logged data revealed a 60 percent energy saving.  “We can see the SynRM delivering the power that is needed when it is needed. There is no drop and no overspill on power. It is consistent power use which has greatly improved the customer’s process,” says Watt.  Even with the modifications that were required to the machine, the £7,400 ($11,000) purchase of the 55 kW, IE4 SynRM package is expected to payback in under 12 months.

Speed regulation critical to success

The ABB industrial drive, ACS880, uses the fourth generation motor control platform, direct torque control (DTC), a key benefit of which is highly accurate torque control without encoders. “Encoderless feedback is very accurate and for this application that makes a big difference,” says Watt. “For instance, when a tachometer was fitted to the injection molding machine’s motor, the difference was only 0.5 rpm at 1500 rpm. “DTC is a real step forward in terms of the actual speed holding on the drive, which is much more accurate, leading to a consistent length and weight of plastic through the extruder and into the mold. This means less wastage – no more long tails - as there is no product overspill. “On this application, where you are looking for a constant speed for product consistency, this manifests itself by way of better productivity and far less wasted material.”

Improved throughput

Previously 63 percent of extrusions successfully turned into bottles. “Before, when the machine was being erratic and sporadic in its movements, we were losing a lot of plastic because the tail length of the bottle was varying so much,” explains Craig. “This resulted in the profile of the bottle changing; it could’ve been thinner at the top or thinner at the bottom depending on whether the tail went long or the tail went short. Sometimes we couldn’t even re-use the tail because it came as large lumps which couldn’t be cut up again.”  Following installation of the SynRM package, 96 percent of extrusions turn into bottles. “The missing 4 percent is purely as the machine is ramping down,” says Watt. “So it has greatly improved the productivity as over 30 percent more bottles are fit for purpose.” Because there is less waste, the 18.5kW granulator used to recycle the waste is used less. ‘’Approximately 5 percent of the energy saved comes from reduced load on the 18.5 kW granulator’’, explains Watt.  Other benefits include an improvement in the quality of the environment. “Now the operators can hear what their colleagues are saying while standing alongside the motor,” says Watt. “There are machines in the factory that are using standard AC motor technology and the SynRM package is running quieter than these comparable motors. Running cooler and quieter than the equivalent IE3 motor is an immediate demonstration of how much more efficient this motor is.”

Beating expectations

ABB’s SynRM package has not only resulted in a 60 percent energy saving on an existing plastic container blow molder, but has enabled a new all-electric injection molding machine to be installed and operated out of the energy savings. The savings avoided the need to upgrade their 600 kVA electrical supply at a cost of £250,000. Other benefits to the existing blow molder include higher product throughput, better product quality and much less waste.  “We have 600 kVA of electricity coming into the company. We couldn’t install a new machine because we didn’t have the electric power; we were already peaking,” says Craig. “But with the 60 percent energy saving we achieved on the old blow molder we have been able to install another new all-electric injection molding machine. And even that machine is still not using all the 60 percent of energy we saved. This is beyond our wildest ambitions. We have managed to massively increase our production, just by installing one SynRM package from ABB.”  

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Clifford Craig, Engineering Manager at PrimePac Ltd.

PrimePac decided to replace the old hydraulic motor used to run an extruder on a blow molder as it was not operating efficiently and speed holding was erratic. The new solution was ABB SynRM and ACS880 variable frequency drive.

The hydraulic pack consumed 68 kW, whereas after installing the SynRM package, power use is constant at 26 kW: a 60 percent energy saving. PrimePac was able to fit a new line without exceeding power restrictions in its electricity supply.

ACS880-01 drive can be orders with IP55 enclosure which enables installation close to the process.

The gained energy savings made it possible to install a new extruder without upgrading the 600 kVA electrical supply.

PrimePac Limited specializes in continuous extrusion blow molding to manufacture a range of plastic bottles, containers and jars together with closures, pumps, sprays and various accessories.

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