Little shot

Late last year, US-based Medical Murray introduced a injection moulding machine that could mould twenty parts from a single pellet.

Manufacturing via injection moulding occupies a large segment of plastics production today. As long as the hoppers of raw material are kept filled, the machines can, almost unceasingly, crank out components for toys, cases for laptop computers and cell phones, interior panels for automobiles, lawn furniture, picnic forks, airline food trays, and on and on. For the medical marketplace, injection moulding helps keep costs down for production of single-use items.

Most of the raw stock that feeds into these machines is in pellet form, each pellet volumetrically about 1/3 the size of a mechanical pencil’s eraser. Late last year, a company introduced a machine that could mould 20 parts from a single pellet.

Enter sesame

Medical Murray is a modest-sized firm in Buffalo Grove, IL, specialising in medical device product design and development. Phil Leopold, president and founder, bears experience in development and manufacture of equipment and automation systems for a variety of electromechanical applications in medical and dental industries, while son and vice president Andy carries expertise in areas of conceptual development, prototyping, clinical testing, packaging, and marketing of disposable and reusable medical devices. Phil has also co-ordinated design, installation, and startup of numerous highly automated plastic moulding manufacturing systems, and has worked with fibre composites, thermoset and thermoplastic resins, metals and ceramics.

Andy suggested that there was a need for intermoulding the joints in catheter assemblies, using injection moulding. In some medical applications, these assemblies can become complex – Andy describes one as going ‘from a three-lumen extrusion to a two-lumen extrusion with a piece of stainless steel tubing inside of the lumens.’ The joints may be transitioning between a soft material and a hard material. To create the bond between these segments, he continues, is ‘very labour intensive process.’ After being turned away by several plastics moulding vendors, he and his father decided to develop their own machine.

Introduced at the 1998 Medical Device & Manufacturing show, the appropriately named Sesame moulding machine produces parts from 0.05 to 20 mm3. To introduce the machine, they created 20 objects the size of sesame seeds from a single pellet – each seed measuring 2 mm3 and weighing only 0.0022g. The parts carried the company logo in 0.3-mm-high moulded-in letters, legible only under a microscope.

Meeting specs

The Sesame’s design needed to fulfill two key requirements. Minute amounts of plastic must be precisely delivered at a rate fast enough to fill the mould before the plastic starts to cool, within a narrow band of time to minimise air bubbles, overheating of the plastic, and many other potential problems. Moreover, the material requires a short residence time in its melted-delivery cylinder to prevent degradation.

Because of the small scale of the Sesame machine, the injection cylinder is driven by a brushless linear servomotor. The motor selected, a 410-4 from Trilogy Systems in Webster TX, is capable of handling continuous forces of 100 lb and peak forces of 450 lb for periods under one second. Installed in Sesame, it provides the machine with a 0-60 mm injection stroke at 1250 mm/sec and total injection times as short as 0.02 second, with two profile speeds and two holding pressures. This allows for filling of extremely thin wall sections approaching 0.025 mm thick, at injection pressures up to 50,000 psi. The Trilogy motor incorporates a magnetic field position sensor within itself, called a Linear Encoder Module, which Andy says had superior accuracy than LVDTs used on the first prototype, ‘making life a lot easier for us.’

The built-in LEM did not have the alignment and packaging issues of conventional linear encoders, and optical encoders would have experienced greater failure risk from the heat and grit of an injection moulding environment. Overall, the Trilogy package combined a motor, encoder and position switches, plate, rails and bearing blocks, even the cable track and mounting hardware, into a single bolt-in assembly.

Material degradation problems can occur even when screws are 12 mm in diameter. That is because, with micromoulding shot sizes, some 1000 or so shots can be held in the barrel for several hours. To solve this problem, the Sesame uses a pneumatically driven vertical plunger for plasticising, coupled with the linear servomotor-driven injection needle. A high-pressure valve is positioned between them. Material loading is automated on the Sesame with a special stepper motor driven pellet feeder. The system is designed to feed as little as one pellet per reload cycle, if necessary. When combined with the reloading controls for the plasticising plunger, residence time can be precisely matched to the part and the material being molded.

The servo control and sequencer overseeing all of these systems are products from Delta Tau, Chatsworth, CA, with a control panel from Exor, Cincinnati, OH, providing a familiar PC-based GUI. The clamping unit is driven by dual self-aligning Festo pneumatic cylinders with force and speed control, applying a standard clamp force of 600 lb with an option to clamp as high as 4000-lb. Festo also helped by designing the manifold block that is controlled by the system computer.

‘We were looking for reliability, as well as a metric component,’ says Andy. ‘Festo met our needs, and gives great service.’ He adds that the Delta Tau computer was selected for its ability to handle the speeds involved during injection, and its ability to handle the sequencing tasks. Clamp motion and injection unit alignment are aided with the use of linear bearings from THK, Schaumburg, IL. Parts ejection is also handled by pneumatics, and vertical clamping is an option.

The Sesame was constructed with an eye towards its being used in cleanrooms – the structure is made of stainless steel and anodised aluminium, the systems use no hydraulics or lubricants, and components like the Trilogy motor are cleanroom-rated. The entire unit occupies a space of 24- x 54-in. with a height of 64 in., and its operation requirements of 15A 115 VAC and 90 psi of shop air make Sesame a benchtop plastics shop.

Other parts applications

According to Andy, the speed and efficiency of the Sesame machine can make it useful in applications other than catheter parts. There are new bioabsorbable materials on the market that could work with the machine to create implantable carriers for medicines – after injection, the carrier breaks down and is passed out of the body. Outside of medical products, Sesame can produce parts with holes down to 1.5 thousandths of an inch for low-pressure orifices, and components such as miniature guides and slides for disk drives and switches.

Medical Murray has already passed along the manufacturing and sales of the Sesame machine to Hull Corporation, Warminster, PA. But that’s not the end of Medical Murray’s involvement -not only are they handling refinements and upgrades to the machine, but they use also use Sesame as a tool for their primary business of medical device development.

This article was reproduced by kind permission of Designfax. Copyright © 2001 by Nelson Publishing, Inc.