New Technique Molds Multiple Cavities--One Part at a Time.

A new twist on sequential valve gating is said to offer numerous technical and economic benefits in multi-cavity injection molding of large parts. It opens and closes hot-runner valve gates to fill first one part and then another, controlling the sequence by cavity pressure. The technique reportedly lowers clamp-tonnage requirements, potentially allowing parts to be molded on a smaller press. It also provides more uniform part quality from all cavities and can trim cycle times, says the developer, MHI Injection Molding Machinery Inc. in Addison, Ill. MHI is the U.S. division of Mitsubishi Heavy Industries in Japan.

MHI's new Sequential Cavity Separation (SCS) process also permits molding parts of different dimensions on the same tool or filling just selected cavities in a multi-cavity mold, says Dave Voisard, MHI special projects manager and co-developer of the process along with Doug Smith, also a special projects manager. SCS runs on Mitsubishi MSJ or MMJ series presses with modifications to the machine controls. Standard injection rates, pressures, and temperatures are used in the process, for which a patent is pending.

Control by fill pressure

Unlike conventional sequential valve-gate molding, SCS doesn't rely on either screw position or time to trigger valve-gate actions. That eliminates variability caused by changes in material viscosity or density, Voisard points out. "The biggest problem in plastic molding is the plastic," he says, citing material variations as one reason why cavities in a geometrically balanced mold can fill differently--even to the extent of one flashing while the other yields a short shot.

SCS works by combining sequential valve gating and cavity-pressure control in a novel way. Sequential valve gating, in which selected gates are opened in sequence, is generally used in large-part molding, where all of the valves fill a single cavity. With the SCS process, each valve gate serves a separate cavity.

Each mold cavity is fitted with two pressure transducers, one placed just behind the gate to sense the start of filling in that cavity, and one near the final point of filling of that cavity. The gate pressure sensor controls the process, whereas the end-of-fill sensor is used solely for QC monitoring.

In the case of a two-cavity mold, the process starts with one valve gate open and the other closed. The first cavity is filled, and when the gate sensor detects a preset pressure, the sensor signals to the machine's controller to close the first valve and open the second. Valve changeover time can be measured in milliseconds, so there is no pressure spike while injection continues through the transition, say MHI sources.

One immediate benefit of this approach is that the clamp tonnage required to fill one cavity at a time is much less than when multiple cavities are filled simultaneously. The tonnage requirement is the same as for a single-cavity mold. Thus, SOS can permit use of a smaller machine, which implies considerable cost savings. In the case of a two-cavity mold for a 15.5-in.-diam. polycarbonate auto wheel cover, the 187 sq in. of projected area would normally require a 1450-ton press. But MHI has molded the parts successfully with SCS on a 720-ton machine. In the case of a family mold for dissimilar parts--an application to which SCS lends itself--the machine tonnage is reduced to that required for the largest part being molded. "How small a machine you can use is limited mostly by platen-size requirements," says Richard Stratman, regional sales manager at Mill's Wixom, Mich., technical center.

Although it may appear that sequential cavity filling would take longer than conventional concurrent filling, Voisard says that is not the case. In fact, the two-cavity wheel-cover mold fills 1 sec faster, he reports. The reason is that SCS eliminates pack and hold time. Conventional molding fills the mold only 98% during and then finishes the job with packing and holding pressure from the screw, Voisard explains. "In SCS, the cavity is filled 100% and then the valve gate is dosed. This way, the cavity is fully packed, so there is no need to use the screw for hold time." Voisard notes that MHI has used SCS only with two cavities. He cautions that sequential filling of more than two cavities could add a few seconds to overall cycle time.

Voisard adds there is also no issue of platen deflection caused by the cavity-to-cavity molding process, which does impose off-center filling pressures. Deflection is avoided through the use of SCS only on MHI presses with four locking tiebars.

Control requirements

MHI says SCS will only be run on Mitsubishi machines with its newer controls such as the MAC-VII system that is standard on its MSJ and MMJ series. SCS is also offered on the new all-electric ME series, which has brand-new MAC-VIII controls. Both types of controls give the molder more flexibility to program multiple injection profiles for SCS. Upgraded software provides 16 injection profile steps instead of the usual six. This allows operators to program the injection profile for each cavity in a four-cavity mold with up to four steps in each profile. Users can enter screw position, speed, and available pressure for each of the 16 profile steps. The control screen also features a new set-up window where a user can sequence the opening and closing of each valve. Green and red LEDs have been added to the control panel to show the status (open or closed) of each valve gate during the cycle.

First commercial use

Although MHI is initially targeting automotive applications like wheel covers for SCS, the first commercial use is an all-polypropylene notebook binder that is designed to replace conventional binders of vinyl-covered cardboard and metal rings. The single-material Uni-Keep binder from Altabind, part of Uni-venture, Inc. in Columbus, Ohio, is designed for greater recyclability. The binder has two living hinges for the book flaps and three molded-in rings that snap together. The binder is molded using SCS by Kitty Hawk Molding Inc., Carlisle, Ohio, with a two-cavity tool built by Eagle Mold Inc. in Carlisle. Two binder sizes with a weight difference of 10-15% are molded in the same tool, an unbalanced application to which SCS is well suited. The mold runs on a 610-ton Mitsubishi MMJ press in a cycle time of about 30 sec.