New technology takes micromolded parts to the milligram level

Editor's note: Micromolded parts are already very small, but the demand to make them smaller is here. In the meantime, the process needs to be more efficient. The IKV Institute for Plastics Processing in Aachen, Germany has developed technology for the molding of milligram-level parts, and a very small prototype machine to show it off.

As a reader of IMM, you have seen a growing parade of incredibly precise micromolded parts, many well under a gram, over the last few years. Everyone connected with this segment of the industry says this is just the start. In the future parts will be smaller, in the milligram range. This is already happening, but there are some stones in the path of development.

Small is Beautiful, and Relative
To produce small parts, current molding technology demands relatively large sprues to achieve the minimum shot size for the machine. That means up to 90 percent of the shot is wasted, and the large sprue increases cooling time and lengthens the cycle. If the material is costly, as it frequently is, and cannot be recycled, as it frequently cannot, the part cost can become greatly inflated.

Consider, for example, light-guiding elements for a model train made by the German company Märklin (see "PIM Raises the Bar for Model Train Collectors," November 2000 IMMC, pp. 15-16). The company makes precise HO-scale replicas of trains, and has exceptionally demanding production standards to deliver parts that are highly detailed. In this component, the weight of the sprue is more than 50 times that of the two attached parts combined. A resin pellet is larger than one of the parts. 

According to two of the scientists driving the project at the IKV, Walter Michaeli, institute director, and Alrun Spennemann, the scientist in charge, the size of the pellets is critical. In this case, the PMMA pellet requires a minimum screw diameter of 14 mm. Therefore, when the screw moves just 1 mm, about .185g of melt is injected into the mold. As of this writing, 14 mm is the smallest screw available for use in a micromolding machine.

Thinking Out of the Box
In its drive to reduce the minimum shot weight to less than .01g, and to circumvent the large-screw problem, the IKV has developed a specialized molding machine with the support of Ferromatik Milacron, hot runner supplier Otto Männer GmbH, and AGA Gas GmbH. In addition, during the several years this project has been in development, the IKV had the input of a number of cooperating molders regarding critical machine and mold factors (see Table 1). The machine the IKV has built, about the size of a shoe box, separates the plasticating and injection processes and appears to solve the large-sprue challenge.

The machine and its sequence of operation are shown in Figure 1. The small amount of material required for the shot is plasticated in an electrically heated vertical cylinder, and then moved into the horizontal injection cylinder by a plunger. A second plunger driven by an electric motor and a linear drive injects the melt into the cavity. The injection plunger's diameter is 2 mm, while the dosing plunger has a 5-mm diameter.

The nozzle and mold plates are heated to the temperature of the melt. The nozzle, an especially critical element in this process, is heated electrically. However, because the shot is so small, mechanical means are not adequate to close the nozzle after each shot. To cool and seal the nozzle, at the end of the hold period AGA Gas Toolvac technology is used to inject liquid CO₂ around the nozzle and sprue. The sprue is demolded first, followed by the part, before a new cycle starts (Figure 2).

This technology reduces the sprue to about 5 to 20 mg, and simulation testing puts the overall cycle time at approximately 10 seconds. When the prototype machine was built, each element was designed and tested separately. Testing determined that the CO₂ could reduce nozzle temperature by 40 deg C for about 7 seconds, long enough for dosing and demolding to take place. The time can be varied using more or fewer pulses of CO₂, but increasing cooling increases the reheating time and the overall cycle.

Machine tests showed the melt was homogenous, the ball-check valve in the plastication cylinder worked well, and there was sufficient contact force between the injection unit and the Toolvac system to prevent leaking. The first microstructured test parts, with a weight of 45 mg and a sprue of 5 mg, were molded successfully in polypropylene on this machine.

Work is under way to improve the efficiency of the plasticating unit, part of which involves the possible use of ultrasonics to melt the material. The IKV says this is promising because the amount of material is so small. Though more development is required, the technology appears to have promise.