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Managing Director at Sumitomo (SHI) Demag UK
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All-electric or hydraulic? In injection moulding, the question may not be a new one, but it is one which is becoming increasingly relevant to greater numbers of processors. Why this interest? Since the beginning of the 1990s, when the first all-electric machines were snapped up by early adopters in European markets, margins have shrunk for many. Meanwhile, the cost of energy has escalated across the board. And at the same time, companies are looking to improve repeat accuracy in their manufacturing.
Over recent years, sources including Euromap, the European association of plastics and rubber machinery manufacturers, have estimated that while some 80% of injection moulding equipment sales in Japan were all-electric, and around 50% of US sales (possibly more), the European figure was closer to 20%.
Of course, it could be argued that end-use applications and market conditions are different in Europe. But might it be that many moulding businesses are only now starting to take a closer look at the relative efficiency and profitability of different systems?
In today’s climate, would it make sense for more end users to focus on where their North American and Asian counterparts are benefitting from a shift to all-electric? I think it would.
It is true that the complexity of injection moulding equipment markets is much greater than it was a few years ago, with larger numbers of hybrid systems, more efficient hydraulic machines and specific options and devices for energy saving. It is also true that, for a serious processor willing to put in the research, there is more detailed and accurate data available from manufacturers and organisations than ever before.
Whatever the conclusions of individual processors, the all-electric story remains a compelling one. Direct drive machines have frequently been shown to offer major improvements in efficiency, giving processors many advantages over conventional hydraulic machines. These advantages include a reduction of up to 75% in energy usage during operation and improved repeatability and fast cycle times.
Electrical drives consume less power for the main movements of plasticising (which typically accounts for around a half of energy consumption), injecting and opening the mould. Because less heat is generated, cooling requirements are reduced, too.
There is much talk about the efficiency of electrical direct drives. But what does this mean in practice? By avoiding the need for gearboxes, they also avoid the friction and loss of energy that these entail. And importantly, kinetic energy can be recovered through the use of multiple frequency drives installed on a common power bus. This means that the braking energy of one drive can be passed on to another via the inverter, rather than being lost. So, for example, while the clamping unit is braking, the energy generated can be used for the energy-intensive step of plasticisation.
Hand-in-hand with electrical drives comes digital control. Delivered via encoders, this fine control ensures that the movements of the machine are more precise. Both machine precision and maintenance requirements are helped by the fact that all-electric systems have fewer moving parts. Additionally, there are fewer variables in the process, with no stretching of hoses, sticking of valves and no hydraulic fluid to heat up or compress.
Once set up correctly, the injection moulding machine runs without the need for any adjustments. So, for instance, the screw position during filling and hold is digitally controlled, allowing just the right amount of material through, optimising resin usage and preventing unnecessary stress to the mould.
Accurate dosing of the mould is doubly important in the production of high-precision plastics, and the non-return valve has often constituted a weak point in the process. Traditionally, systems only closed passively through the pressure applied during the injection phase. In fact, the most advanced all-electric systems, including our own, have addressed this issue through an active closing mechanism on the non-return valve.
This means that, once dosing is complete, the valve can be closed, positively and securely, during the injection and hold stages. In this way, fluctuations in the melt viscosity do not need to have a knock-on effect on closing behaviour, melt cushion, shot weight or – ultimately – the quality of the final part.
Certain sectors have recognised the specific benefits of an all-electric approach to moulding. For precision industries such as medical appliances, tight tolerance requirements are key, and a combination of electric drives and digital control can turn injection moulding into a predictable and precise operation. Weight variations in critical parts can be as low as 0.0001g, which means less waste and happier customers.
There are other advantages for hygiene-sensitive food and medical production environments. Unlike traditional hydraulic machines, all-electric systems have no consumables, such as oil, and no filters requiring regular replacement. There is no risk of fluids leaking, nor of resulting contamination. Consequently, ‘housekeeping’ requirements and downtime on electric equipment can be as little as half those demanded by hydraulic machines. This type of consideration also means it is far easier for producers of all-electric systems such as ourselves to design equipment suitable for clean room operations.
It would be wrong to portray all-electric injection moulding as a universal panacea. Naturally enough, there are types of production which are better-suited to hydraulic and hybrid systems, notably those higher tonnages typically demanded by the automotive and other heavier engineering sectors.
But although the capital cost of an electric machine may be higher than the hydraulic equivalent, increasing numbers of precision-moulding customers are realising that payback times are not what they used to be. Based on energy consumption alone, savings can easily justify the higher initial investment over a 10-year period, or even less.