Click Image to Enlarge
Controlling valve gating with cavity-pressure monitoring to cure flow imbalances in multi-cavity tools was one of the processing highlights at the Molding 2007 International Conference and Exhibition, sponsored recently in Austin, Texas, by Executive Conference Management. Machinery improvements for liquid silicone rubber (LSR) injection molding and lower-cost access to the benefits of six-sigma quality were also addressed at the meeting.
Injection molders commonly use valve gates in multi-cavity molds, family molds, and large cavities with multiple drops. Valve gates help control gate vestiges and reduce cycle times by closing off the cavity at the end of packing to allow screw recovery to begin. Molders can realize additional benefits if they sequence gate opening and closing and control that sequence with cavity-pressure monitoring, says Art Schubert, chief engineer at RJG Inc. Some benefits include improved balance in hot-runner molds, more consistent part dimensions, reduction of knit lines, and lower clamp-tonnage requirements.
RJG discussed three approaches that use melt volume and cavity pressure to determine when to operate the valve gates. Each cavity has its own valve gate(s) and a cavity-pressure sensor located close to a sensitive feature on the part. The controller opens the gate at a screw position calculated in terms of melt volume delivered, and closes the gate at a cavity-pressure setpoint. RJG conducted tests on a three-cavity family tool whose center cavity is nearly three times larger than the other two, while the remaining cavities are close but not identical in size.
The first approach, Independent Cavity Sequencing, opens and closes valves at different times in order to cause all cavities to finish filling at the same time. To accomplish this, the valve gate feeding the largest cavity is opened first. When it is partially filled, the gates serving the two smaller cavities open, while filling continues in the large cavity. Although the cavities may be filling at the same time and rate, they will not necessarily pack to the same pressure—without help. Thus, cavity pressure is used to determine when to close the valve gate in each cavity.
Because of the compressibility of the melt in the runner system, RJG recommends slowing the injection ram just before the cavities are completely filled. “This lets the melt decompress as packing begins, which in turn allows for much faster filling. Faster filling reduces sensitivity to speed changes, such as when the inject stage moves from a fast filling to a slow packing speed,” says Schubert. He explains that if the rate of the injection remained constant, as each valve gate closed off a cavity, the rate of filling and packing pressure in the remaining open cavities could increase rapidly. “Future controls may provide feedback to the press to adjust speed as cavities are closed, but most presses do not currently support this feature,” says Schubert. He also notes that cavity pressure at the gate may rise due to the forward movement of the pin, which pushes more material into the mold. This can produce localized stress in the part.
This process uses the volumetric screw position instead of linear stroke so that the tool can be transferred to presses with different barrel sizes and still run with the same settings. The technique also can be used with multiple identical cavities. Here, all of the valve gates would be set to open simultaneously.
The second approach, Alternate Cavity Sequencing, is suited to filling dissimilar cavities or problematic family tools in which some parts are many times thicker than others or have thin and thick sections at different distances down the flow path. The Independent Cavity Sequencing approach would not work here because the parts do not have similar flow restriction and wall thicknesses, hence the material flow may “stall” if several or all of the valve gates are open for simultaneous filling.
The second method fills and packs one cavity at a time. In some cases there is a slight overlap of filling and packing from one cavity to the next. Explains Schubert, “Because the material in the first cavity is shrinking after the gates close, the force needed to keep the clamp closed may be reduced. It is best to fill the cavities in a manner that balances the opening forces on the mold and does not cause the mold to rock open as pressure builds on one side of the mold that is filling while pressure is dropping on the other side of the mold that has filled.”
RJG tested a three-cavity mold in which the two outer cavities were of similar size and the center cavity was three times the volume of the other two. “The velocity settings for a fast-fill/slow-pack sequence are again applied, and speed changes are made by the machine at specific screw positions set on the machine controller,” says Schubert. “However, it is obvious that synchronizing the screw speed and position could be challenging, with the screw slowing to pack one cavity and speeding up again to fill another cavity.” Cavity-pressure sensing makes the sequencing easier.
One challenge of this approach is that when the first cavity or cavities close after packing there may still be a lot of shot volume in front of the screw. The compressibility of that large melt volume can reduce the ability to control packing in the first-to-fill cavities. Users of this method should also be aware that the first-to-fill cavities will have more time to cool than the last cavity, which may have an impact on part quality.
RJG’s third approach, Sequential Gate Control, is used with long parts whose aspect ratio is so high that the part cannot be filled with a single drop. The most commonly practiced sequence produces a “cascade” of melt with a single flow front that eliminates weld lines. Filling typically begins at the central gate, and as the flow front passes a pressure or temperature sensor at each gate, it detects the flow front and opens the gate as the melt passes by. As the cavity is packed, the valve gate closes when its sensor reaches a set pressure. A fast fill and slow pack sequence is also used.
What’s new in LSR
The growing popularity of LSR injection molding was reflected in several machinery developments presented by Mark Hammond, technology manager at Engel Canada. One of the most significant was integrating the two-part LSR pump-system control with the press control. This is a new twist, says Hammond, which provides a single point of control and allows all pump settings to be stored and viewed together with the mold settings. It also eliminates a separate controller cost for the pumping system.
At the feed throat, Engel recommends a new combination material shutoff, filter, and static-mixer assembly that delivers more exacting amounts of A and B material. Engel recommends adding a pressure transducer at the feed throat to verify pumping consistency and filter pack efficiency, as well as to verify the check-disc assembly performance.
Engel has also integrated the filter and water-flow meter for cold-runner systems. “Water-flow monitoring prevents the cure-up of cold-runner blocks,” says Hammond. “Mold heats cannot be turned on unless there is a minimum set water flow.”
Engel also replaced an analog vacuum gauge with a digital gauge mounted on the moving platen. It monitors the vacuum draw on each cycle to ensure a stable vacuum is created. It can be used to observe trends such as vacuum-seal wear, says Hammond.
Also new is a hydraulic stroke-limiter for Engel’s hydraulic presses, which automatically locks the screw position after the refill cycle. This keeps the screw from moving if excessive pump pressure occurs—something that should be avoided through proper pump setup, Hammond adds.
Six-sigma training online
Six-sigma quality training on the internet is newly available from one of the original architects of the concept. Called MindPro, it was developed as a more affordable training and testing medium by Dr. Mikel Harry and is being offered through the Six Sigma Management Institute. A single user license costs under $3000. An unlimited license for training many employees can run as low as $75 per employee.
Six Sigma formerly required up to 160 hr of intensive classroom training away from the job while the employer incurred a large training expense and loss of that worker’s productivity. Training could be inconsistent, as it depended on the trainer, says Hermann Plank, CEO of the Institute.
The new on-line method utilizes 5-min video clips and an Over-The-Expert’s-Shoulder method featuring narration by Dr. Harry. MindPro is offered in an open-enrollment virtual classroom that combines online training with consulting support.