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Microparts: Market growing, parts shrinkingMicroparts: Market growing, parts shrinking

October 20, 1998

10 Min Read
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Figure 1. An operating pin produced by conventional technology with a relatively large sprue component is shown above. At left, the same pin produced by the new micromolding technology is shown with the much smaller sprue.

Injection molded microparts have been going into watches, medical devices, appliances, and other applications for many years. Lately, however, demand is increasing as designers try more, smaller applications. The demand has spurred improvements in materials and machine production technology, and these improvements have, in turn, generated still more applications. The net result is a lot of growth with the promise of more to come.

Micromolding technology will be much more in evidence at K'98 than it has been at previous K fairs. Solutions are being offered by Arburg, Netstal, Krauss-Maffei, Demag, Engel, and others. Ferromatik Milacron recently announced a new system for microparts based on its all-electric machines. Battenfeld's solution is surely the most radical: The Micro-system 50 is a completely new production cell made exclusively for molding microparts (see story below).

The Austrian Micromolding Connection
In the very recent past, insufficient metering accuracy and non-homogeneity of the very small quantities of melt involved in molding microparts restricted productivity. It was nearly impossible to make parts weighing less than a gram cost-efficiently on conventional injection molding machines. The moldmaking challenges were equally intense. Parts often were approximately the size of the smallest vents. One veteran microparts moldmaker once described for IMM the sinking feeling he experienced when, during mold trials, the mold opened and absolutely nothing was inside. It went out through the vents, vaporized. It only takes a little too much heat or pressure to turn a tenth of a gram of resin into vapor.

Developing Battenfeld's Microsystem took a team effort. One member of the all-Austrian MMM (Micromolding Machinery) Project is HB-Plastic GmbH of Korneuburg, a specialist in molding small-sized, high-precision technical parts. Last year, it doubled its Austrian staff to 95 employees, and it has another 35 people at a plant in Hungary. Sales will be near DM 20 million (US$ 11 million) this year. Along with that activity, HB-Plastic is responsible for pilot applications generated by the Micromolding Project. Dieter Seidler, head of R&D, had the task of setting up an injection molding department for microparts. He spoke with IMM about producing an operating pin for a microswitch (Figure 1).

A Matter of Scale
"Metering is one of the chief problems encountered in micro-injection molding," explains Seidler, illustrating the point with an example: the smallest commercially available plasticizing units have a 14-mm screw and a metering volume of 9 cu cm. Seidler follows the general rule that controlled, reproducible melt injection is just barely feasible if a minimum of 5 percent of the metering volume is utilized. That makes the lower limit on shot volume some .5 cu cm. For polypropylene, this equals a minimum controllable shot weight of approximately 450 mg, and with LCP it's about 800 mg. Therefore, when parts weigh just a few milligrams, the sprue has to be so big that sprue-to-part ratios of 100:1 are not uncommon. Cycle time, dictated by this large sprue, is quite high relative to the part.

Conventional Technology
The micro-operating pin is made of easy-flowing liquid crystal polymer, Vectra LCP from Ticona. A four-cavity mold developed with MMM Project member Zumtobel Staff GmbH of Dornbirn yields average molded part weight differences of only .00008g on less than 0.1 mg. Until now, the pin has been molded on a BA 250/50 Battenfeld CDC machine fitted with Battenfeld's Micromelt injection unit (14-mm screw). The shot weight on this machine is 212.4 mg or a shot volume of .131 cu cm. That means 1.45 percent of the maximum metering volume of 9 cu cm is used. With four parts weighing 18.2 mg each, the sprue component is 66 percent. This is a relatively good percentage for micromolding but certainly could stand some improvement. By making the thickest point of the sprue not much greater than the part thickness, the sprue did not determine the cooling time. It was possible to optimize cycle time to 6.3 seconds from an initial 12.2 seconds. Much of the cycle is in handling and relatively slow opening and closing movements designed to prolong the service life of the slide bars.

Low Resolution Leads to Errors
The required metering accuracy is below the resolution limit of the screw stroke movement, making it impossible to inject the melt in a controlled manner. "Using a displacement-triggered switch from injection to holding pressure, attempts to correct machine control led to constant error messages and, as a result, to machine standstills," explained Seidler. It was decided to use time-dependent switchover. While this avoids the error messages, it is no longer possible to influence mold filling. The melt is injected until the mold is full, and the material freezes once the melt has come to a standstill. The holding pressure has no effect. This variation works and yields a sufficient level of reproducibility.

Figure 2. The newly developed nanomelt injection unit with a high displacement resolution can be retrofitted to standard machines.

The low shot weight could prove problematic with materials that are more sensitive than Vectra LCP. Even with short cycle times, there could be unacceptably high thermal degradation. A further problem is the specified reject rate of just a few ppm. Because the machine control system did not have sufficiently fine resolution, short shots or over-injected parts happened each time the machine attempted to correct its setting. The resulting reject rate was up to 5 percent. This stimulated development of the new screw/piston combination in the MMM Project, which can be retrofitted. The injection volume resolution of this nanomelt unit (Figure 2) is 5.5 times greater than the Micromelt unit.

The Modular Production Cell
The aim of the MMM Project from the start was an all-in-one solution, a compact, modularly-designed production cell. This is Battenfeld's new Microsystem 50, making its public debut at K'98. HB-Plastic already has considerable practical experience with this new system.

A new mold was built for this machine-and not just because of the unique connection dimensions. The melt is injected directly into the mold parting plane, making it possible to considerably reduce the sprue weight. This optimized gate design permits 60 percent material utilization instead of the former 34 percent. "I've not yet heard of any development to match this in micro-injection molding," says Seidler. There was also good news on the cycle time. Although the previous mold is the slide bar type, it had been tailored to the material and the machine features, and a cycle time of 4.7 seconds was achieved. Seidler is convinced this can be improved upon.

Zero-Defect Production
Part quality was similarly increased. Shorter flow paths make complete filling more certain. However, a guaranteed reject rate of just a few ppm can only be achieved with 100 percent inspection. Inline quality control of injection molded precision parts frequently includes weighing units, which is not possible with micro-moldings that weigh fractions of a gram. Besides the handling problems themselves, measuring accuracy of less than .1 mg cannot be achieved profitably in an industrial production environment.

Solution: quality of all parts is monitored optically via a video system. Substandard parts are eliminated at the handling stage as a quality function, and zero-error production is the standard. Because of electrostatic charging, active mold removal or part transfer is required in micromolding. In the case of these operating pins, handling is performed by suction plates. Manual intervention is not necessary, and the Microsystem supports a cleanroom module, so the parts are very clean, a benefit for medical and electronics parts.






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