Doubling throughput on existing injection molding machines typically sounds too good to be true. But that is the claim made of a double-daylight mold developed by Christoph Jaroschek of Bielefeld University for Applied Sciences, Bielefeld, Germany. Previously an application technology manager with Ferromatik Milacron, Jaroschek has dubbed the technology TandemMould. A mold was recently demonstrated on a Boy 55 A machine, but the concept is best suited on larger machines producing thickwalled parts in long cycle times.

The first commercial TandemMould went into production in early 2002 at an undisclosed German molder, and a second tool was recently delivered. Both were built by Lehmann GmbH, Bad Salzuflen, Germany, and their part applications remain proprietary.

In traditional double-daylight molds, the cavities in both daylights fill at the same time, so both the machine’s clamp and injection unit remain idle for most of the cooling time. The TandemMould enables relay-like part production from its two daylights. While the part in one daylight cools, in the other daylight, another is injected or ejected. Once the molding machine has finished plasticating, it starts a new cycle on one daylight without waiting for the part in the other daylight to finish cooling. No hot-runner or screw/barrel alteration is necessary, only control modification.

The concept is similar to the Tandem injection machine that was introduced several years ago by Husky Injection Molding Systems, Bolton, on, Canada. But with the TandemMould, the double-daylight technology is mold-driven, so it can be employed on a conventional machine. Husky’s Tandem machines, which are no longer produced, had two injection units and an additional platen with its own hot-runner system.

Two bayonet locks on rotating shafts located on either side of the mold lock and unlock the daylights. The two bayonets are linked by a belt, so that when one daylight locks, the other unlocks. Jaroschek says one daylight can be opened while the other is closed for cooling because the bayonet locking mechanism provides the necessary clamp force — this is no more than around 20% of the maximum clamp force. Control is implemented via “core pull” signals that activate a pneumatic cylinder or electric motor. Position sensoring is monitored by limit switches.

During startup, cavities in both daylights (level 1 and 2) are filled. Then, one daylight (level 1) opens, and the part is demolded while the level 2 daylight remains closed. After level 1 completes demolding, it closes and its cycle begins again — the part in level 2 has cooled. The bayonets lock level 1 and unlock level 2 for part removal.

As only one daylight opens at a time, the opening distance is the same as a standard mold, although the added height of the mold means the maximum stroke is reduced.

The TandemMould costs the equivalent of two molds and targets parts with cooling times that are longer than the total time for holding and plasticating. The higher the proportion of total cycle time that cooling accounts for, the closer production economics get to those of a stack mold.

“Although a stack mold, which generally requires larger machines and complex controls, does have a slight advantage [in] production cost and output, you have to take into account the initial high capital cost involved,” notes Jaroschek. “[It] must be based on a long-term investment to [be] viable.” Machines running stack molds need to handle higher shot volumes, as well, since they inject all levels at the same time.

The double-daylight technology is applicable to multicomponent molding and sandwich molding. Jaroschek says the next step is to develop a TandemMould for thermoplastic elastomers and silicones. These materials have longer cooling times, which present a timing challenge for the daylights.