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June 20, 2002

5 Min Read
Tandem Mold: A sum greater than the total

If a tandem bicycle delivers two riders to their destination on a single vehicle, can the same principle be applied to a tool? It can and has, as demonstrated this past April at the KMO Show in Bad Salzuflen, Germany. Prof. Christoph Jaroschek of the Bielefeld University of Applied Sciences (Germany) showcased a tandem mold design that revealed an innovative and relatively simple way to cut cycle times for certain types of molding (see Figure 1).


Figure 1: At the recent KMO Show in Germany, this tandem mold in a standard Ferromatik Milacron machine produced thick furniture handles. Its inventor says the tool also is effective when molding thin-wall parts.

Jaroschek was in charge of application engineering at Ferromatik Milacron until moving to academia in 1998. When he began to develop the tandem mold concept, his initial focus was on reducing the long cooling times required by large or thick-sectioned parts. His tests confirmed that the tandem design could increase output by up to 60 percent while reducing gross production cost by about 5 percent. As he continued working, he saw that the benefits were not limited to large and/or thick parts; the concept worked similarly well for thin-wall technical parts.

What is a Tandem Mold?
Here's how Jaroschek's design works: A three-platen tandem mold physically resembles a normal stack mold; it has a set of cavities in each of two different platens and a common center platen. However, in the tandem the two parting lines open alternately rather than simultaneously. The cooling time for one set of cavities is used to fill and pack the second set.

But rather than simply reducing cooling time, you could say Jaroschek's design makes productive use of that time. Alternately opening the two sides allows more production within a given overall cycle. In Figure 2, which shows several different mold configurations, note the length of the total cooling time of each side of the two-platen mold: It is actually longer in the tandem mold. Molded parts, therefore, can be more stable upon ejection.

The new design requires a dedicated core pull to lock and unlock the parting lines. A toothed belt (see Figure 3) moves forward on one cycle and back on the next to alternately lock and unlock the bayonet-type connections between the center and the two outside platens. A core pull is also needed on the nozzle side to handle ejection from that set of cavities.

Control software must be capable of controlling the independent opening of the sides and alternating the ejectors. Jaroschek says this is available from virtually all machine manufacturers, as it is very similar to the control software for multicomponent molding.


Figure 2: The number of parts produced per hour and the production costs show where the advantage lies with a tandem mold. The initial objective was to reduce the impact of cooling time on overall cycle, which has been accomplished. Yet, the total cooling time for each cavity set in the 4+4 tandem mold (bottom) is actually longer than with a four-cavity standard mold.

Why Go Tandem?
The tandem mold is not a panacea, but it does have potential applications in a variety of markets and offers a number of benefits, in part because the solution is mold based rather than machine based. In addition to more parts per hour and increased cooling time per part, a tandem mold does not require an injection machine made to accommodate the larger opening space required by a stack mold. Therefore, it does not require the additional capital cost for that type of machine, nor the additional floor space.

Also, since one side of the tandem opens at a time, production can be done efficiently on a smaller machine, which Jaroschek notes as another factor in lowering production cost. The injection end of the machine is standard as well, as opposed to the relatively large single-shot capacity needed by a stack mold.

The high plasticating capability needed by a traditional stack mold generally requires a larger screw/barrel combination and/or an independent plasticating screw motor working in parallel with other machine functions. Either way, it adds up to higher capital expense for the machine. The tandem mold, on the other hand, needs roughly half the plasticating and injection capacity of the stack mold to fill one cavity set—half the total number of cavities—per shot.

The tandem mold that Jaroschek demonstrated at the KMO show was molding thick furniture handles. He also sees his invention offering an alternative to molders now using family molds to harmonize production of multicomponent assemblies. Family molds are generally considered difficult to run since runners and cavities must be balanced to account for what can be significant differences in part volumes and geometries.

The tandem mold offers the option of putting smaller parts on one side and larger pieces on the other, and then controlling the shots into each set. Platens can be of different thicknesses to accommodate varying part dimensions. It may also be possible to use existing mold cavities, depending on the kind of melt distribution used.


Figure 3: The key to a tandem mold is the two end platens that open alternately. The toothed belt moves back and forth to engage, in turn, the bayonet locks on either platen.

As a rule of thumb, Jaroschek says a tandem mold's cost will be about 1.8 times that of a normal mold, but that buys twice the number of cavities. Looked at another way, you are purchasing anywhere from 30 to 60 percent more parts per hour or shift. It is necessary to look at the application to determine what the tandem will cost and then to calculate what it is really worth to your business.

A licensing policy is not yet fixed for the tandem mold concept. Jaroschek is currently making reference molds for various applications such as family molds, fast cycle applications, and technical moldings. At the same time, he is inviting moldmakers and molders to join the technology and will support anyone wanting to put it into production.

Contact information
Fachhochschule Bielefeld University of
Applied Sciences, Bielefeld, Germany
Christoph Jaroschek
+49 (521) 162 0524
[email protected]

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