The Expert's Corner

Problem:

A manufacturing plant makes plastic pipe and pipe fittings from epoxy resin reinforced with glass fibers. In preparing the plastic for molding, they had been using a conventional paddle blade mixer which combined the materials well but did not completely eliminate voids in the finished product.

The thoroughness of the mixing operation that blends the fibers with the resin determines how strong the fittings will be. Voids weaken the finished plastic fittings and can lead to leaking or weeping, both serious problems to users of plastic pipe.

Solution:

A 40-gal double planetary mixer with a vacuum hood was selected by the manufacturer because, after the fibers have been combined with the resin, a vacuum will draw bubbles of air out of the material. Trapped air bubbles in the resin mixture and fibers are the source of voids in molded plastic products. By evacuating the mixing chamber, the mixer allows the resin to wet out the fibers completely. Void-free parts are produced.

Bundles of the short glass fibers are poured into the mixing chamber through two charging ports on the vacuum hood. The tiny fibers will tend to stick together resisting thorough distribution throughout the resin. Since the stirrers on the double planetary mixer move at low speed, the fibers are not damaged or broken when the bundles are loosened. The previous mixer needed to operate at high speeds in order to wet the fibers. The double planetary mixer's low speed, high torque mixing action turns the fibers loose without damage.

The vacuum hood on the double planetary mixer can operate under very high vacuum. The manufacturer draws a vacuum to 29.5" Hg using a standard vacuum pump that was already on hand at their plant.

Multiple mixing vessels are available to make the best use of the double planetary mixer's time. While one vessel is in process, additional cans can be unloaded, cleaned or prepared for the next batch. The only interruption to mixing is the time it takes to remove one vessel and put a new one in place.

Results:

Since its installation, the Ross double planetary mixer with vacuum hood has increased processing efficiency and improved product quality. Before, it had taken 15 hours to prepare enough plastic molding material for one day of work. Now, with the double planetary vacuum mixer, the manufacturer has cut the preparation time to just 4 hours.

The double planetary mixer's 40-gal vessel allows bigger batches to be processed. To provide one day's worth of material with the paddle blade mixer, the manufacturer had to run 10 batches. With the double planetary mixer, only 2 batches are needed.

With the previous mixer, the plastic fittings could not be made consistently with totally solid walls, but the thoroughness of the double planetary mixer operation assures complete combination of the fibers in the resin. The low-speed, high-torque mixing action breaks the fibers out of the bundles without damage, and the vacuum feature eliminates voids in cast parts.