News in LSR Molding at K 2016
Two-color molding with one injection barrel and pumping system.
One of the more unusual liquid silicone molding demonstrations at K 2016 was molding a two-color LSR-on-LSR egg cup using an Arburg machine with just a single barrel and a single LSR pump. The technique was demonstrated by LSR materials supplier Momentive Performance Materials Inc., Waterford, N.Y. The pumping system from Elmet of Austria (Lansing, Mich.) supplied the barrel with A and B components of Momentive’s Silopren LSR 2670, plus a white colorant. A T-coupling diverted a side stream of the two components plus a yellow colorant to a second static mixer and a side-mounted pneumatic piston pump. The latter delivered the smaller yellow shot. Then the rotating core on the 4 + 4 cavity tool (also from Elmet) indexed the part to the second cavity where the larger white shot was added. Using one pump and injection barrel, instead of two of each, reduced cost and footprint.
The mold and process were designed with the aid of Sigmasoft Virtual Molding simulation from Sigma Engineering GmbH (Schaumburg, Ill.). Simulation helped determine the filling time, injection pressure needed, and the optimal material grade. Sigma’s highly developed LSR simulation can show such details as air temperature at the vents, formation of flow lines during fill, and percent cure to indicate when parts can be ejected.
Meanwhile, Elmet introduced its TOP 5000 P pumping system, which makes loading drums faster and easier through access from the front, both sides, and even at 45° angles. This is said to be valuable for cleanroom applications, where space is limited. Open-access design also makes the system easier to keep clean.
Also new is a step-by-step instruction guide on the touchscreen, which minimizes the amount of operator training necessary. Other new features include automatic air venting from drums, and piston pumps that provide pulsation-free metering. A new static-mixer unit uses clamps instead of threads, so there is no risk of damaged threads. There’s no more need for a large wrench—a simple wrist action with a special tool is sufficient.
The reason you dry certain plastics is to get the moisture out. But why does the moisture have to be taken out before processing?
One of the important process parameters to establish and record for any injection molded part is its injection or fill time. Research research reveals the limitations of popularly taught methods of establishing this critical parameter.
It may seem like a dull topic, but it will overcome the emotional experience that follows when you put a new mold into a machine and you find out there is not enough barrel capacity to make a full shot.