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October 20, 1998

10 Min Read
Microparts:  Market growing, parts shrinking

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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.

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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.



Micropart molding cell

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The Microsystem 50 is a compact production cell that incorporates all the modules from injection, via inspection, through to packaging.

In 1985, Battenfeld introduced an injection unit for parts weighing between 0.5 and 4g: the Micromelt. Working with five development partners in the MMM Project, Battenfeld developed a production cell for parts less than 0.1g: the Microsystem 50. At K'98 in Düsseldorf, it will mold a medical application (micro-housing, .022g in PC), a watchmaking part (micro-cogwheel, .0008g in POM), and an automotive part (micro-operating pin weighing .018g in LCP). IMM had a look at the new system.

There are seven modules to the production cell: clamping, injection, rotary mold table, removal/handling, quality assurance, cleanroom, and packaging. The injection module includes the obliquely aligned rigid screw that plasticizes the material and places it in front of the metering piston. The metering piston, which comes down from above, precisely measures the melt prior to each shot and positions it in front of the injection piston. Contrary to conventional injection molding, it is the piston that performs the injection. The horizontal injection piston has a 5-mm diameter. To compare, Battenfeld's Micromelt unit injects the melt via a 14-mm screw. Parting line injection and hot runners mean not only savings on what can be expensive materials but considerably faster cycle times thanks to the absence of a sprue that can be 90 percent of the shot.

ArticleImage4570.jpgIn the injection process, the extruder dispenses resin into a shooting pot at an angle and a horizontal plunger does the injection at high speed.

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After injection, when the parts are ready to be ejected, the mold plate rotates.

The Microsystem 50 is completely servoelectric-driven. The advantages include extremely high precision, a basic pre-disposition for cleanroom applications, and the cycle time reductions offered by electric drives performing machine movements in parallel rather than sequentially. The machine reduces cycle time for microparts by about 50 percent, according to Battenfeld, and is designed for sub-five-second process times. Cleanroom capability can be up to class 100 and easily class 1000. The enclosed nature of the cell was not only for cleanroom support, however. It also protects parts from static electricity or from a stray breeze that can take away an hour's worth of production in an instant.

With the rotary table, injection at the first station can run parallel with ejection at the second station. The handling module is designed for the application using suction, air pressure, electrostatics, and more. Each part is optically monitored by a video-imaging camera with a monitor mounted outside the cell. The quality monitoring software is Windows-based. Because microparts are virtually always intended for later assembly, the packaging module is designed so the parts are assembly-correct, spread out, correctly spaced, and oriented the right way for automated assembly. The packaging can be a blister pack, a belt, or a roll of film to protect the parts in transport.

ArticleImage6570.jpgAfter the mold plate has turned, a robot can empty half the cavities while the other half are filled.

ArticleImage7570.jpgThe quality monitor shows a visual comparison of the part to pre-set standards, gives tolerance deviations, and records rejects with supporting data.

All this is computer controlled, with interfaces for quality and productivity monitoring. The Microsystem controller is also new, one of the latest B series of Battenfeld's Unilog controllers. Based on a 32-bit Risc processor, these controllers are language-independent. The interface is completely graphical. There is no keyboard. Functions are accessed through simple symbols. Machine settings are displayed and supported by a process analysis display with colors used to reflect machine functions.




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Contact information
Machinery:
Battenfeld Kunststoffmaschinen
Harald Bleier
A-2542 Kottingbrunn, Austria
Phone: +43 (2252) 404-431;
Fax: +43 (2252) 404-402
Pilot applications:
HB-Plastic GmbH
Dieter Seidler
A-2100 Korneuburg, Austria
Phone: +43 (2262) 74339-55;
Fax: +43 (2262) 74744
E-mail: [email protected]
Tooling:
Zumtobel Staff GmbH
Mr. Gerd Hollenstein
A-6850 Dornbirn, Austria
Phone: +43 (5572) 390-668;
Fax: +43 (5572) 390-185
E-mail: [email protected]
Handling:
Inocon Technologie GmbH
Mr. Gerhard Schwankhart
A-4800 Attnang-Puchheim, Austria
Phone: +43 (7674) 62526-0
Fax: +43 (7674) 62526-27
E-mail: [email protected]
Laboratory (testing):
Fachhochschule Wiener Neustadt
Dr. Christian Kukla
Johannes-Gutenberg-Strasse 3
A-2700 Wiener Neustadt, Austria
Phone: +43 (2622) 22859-31
Fax: +43 (2622) 22859-17
Coordination:
IFWT-Institut für Feinwerktechnik, TU Wien
Mr. Helmut Loibl
Prof. Helmut Detter
A-1040 Vienna, Austria
Phone: +43 (1) 504 1431-24:
Fax: +43 (1) 504 1431-17
E-mail: [email protected]

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