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2nd Quarter 2009 Other technologies

Whether for vacuum cleaners, hearing aids or blood filters, engineers who design the tool systems for injection moulded plastic parts face challenges familiar to everyone: 'How fast; how accurate; how long; and (like always) how much?'

In many cases, the right answer to each of these questions has been made possible by re-evaluating the role that hydraulics has traditionally played in that arena.

For generations, hydraulic cylinders were the motion control devices of choice for mouldmakers. But today, when it comes to creating the precise, rapid motions that typically happen in these moulds, electro-mechanical actuators with integrated motors (like the Exlar GSX Series) are getting more attention. According to Jeff Nichols, President of ASIC Corporation, a systems integrator headquartered in West Chester, OH, “In many respects, mouldmaking is about creating voids. It is easy to put resin where you want it – the trick is making sure nothing goes where something else needs to be later.”

Inside a GSX actuator: the actuators referenced by Jeff Nichols in this article have an internal electric motor. Because an inverted planetary roller screw drives the shaft and the internal stator rotates the planetary assembly, these actuators fit in the same space as hydraulic cylinders with the same stroke lengths

A boy-girl thing

Moulds are generally male/female assemblies where the male (or core) forms the finished part’s concave surface. It creates the all-important 'nothing'. On a laptop shell, that void houses the electronics; on a plastic plate the voids create separate places for baked beans and potato salad.

Cycle time is crucial

Volumetrics, gate design, temperature controls ... mould makers use a host of tools to fill a mould quickly. But after that is done, the mould still has to be cycled. How fast the core can be moved in and out impacts profitability.

Traditional hydraulic cylinders are fast. But ASIC reports that in some applications, an electro-mechanical actuator can enhance cycle rates by letting related operations start sooner. This is possible because this kind of actuator constantly feeds the core’s actual position and motion status back to the controller.

Longer tool life

“Every time we can replace an hydraulic cylinder with an Exlar electromechanical actuator, we can virtually guarantee a longer tool life.” Nichols continued, “That actuator lets us get the core inserted faster without just slamming the mould together violently. Less stress means longer life.”

Re-usability for savings

Especially in retrofit situations, ASIC always addresses re-use in their cost analysis. According to Nichols there are cases where an actuator specified for one mould has more power than needed because it could be re-used on another assembly that did require that additional power. “These actuators are really simple to retask. You just plug a laptop into the drive, feed it a new motion profile, and it is ready to go,” said Nichols.

Ease of retrofit

The Exlar actuators that ASIC uses are truly 'plug-and-play'. An hydraulic cylinder’s two hoses are replaced with a pair of electrical connections. The mounting flanges are all industry standard. And because the GSX has an internal motor, both have the same form factor. Thus, for comparable stroke lengths, the GSX units fit in the same space.

For more information contact David Horn, Measuring Instruments Technology, +27 (0)12 348 0569, [email protected], www.measureit.co.za

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