Sliding dies for molding hollow parts



Die Sliding during Injection (DSI) was developed by The Japan Steel Works Ltd., Tokyo (JSW Plastics Machinery Inc., Santa Fe Springs, CA) to overcome problems JSW technicians believe to be inherent in other processes that are used in the production of hollow parts, such as blowmolding, lost-core molding, and welding. DSI is designed to produce high-performance hollow products with inner ribs and bosses, like high-quality/low-cost automotive parts and E/E assembly encapsulations, some of which might be impossible to make using other processes. Wall thicknesses are adjustable using DSI. Work continues to improve DSI's applicability to more general-purpose hollow parts production and multimedia molding.
Figure 1
Figure 1. How the DSI molding system works.

JSW technicians say fluctuations in wall thickness, particularly at corner areas, is a weakness in blowmolding: heterogeneous parison expansion and the inability to accurately control uniformities in specific local dimensions cause such fluctuations. As a result, blowmolded hollow products can lack strength. Regarding lost-core molding, JSW technicians say the process is unsuitable to cost-effective mass production, since preprocessing is required to mold the cores, and since postprocessing is required to remove them. What's more, additional processing is required to make lost-core molded hollow products airtight. And welding? They say welding is unsuitable for producing products with complex shapes in high volumes. Defect rates may climb.

As you can see in the accompanying drawings (Figure 1), a hydraulic unit installed on the fixed platen operates the sliding die in the DSI molding system (A). First, both halves of a product are simultaneously molded in two cavities (B), and the mold opens (C). A sliding die moves one half of the molded product to a position where the parting-line surfaces of both halves face each other (D), and the mold closes again (E). Next, at the second injection stage, what JSW technicians call a "circumference part" is overmolded to combine the two parts produced in the first injection stage (F). A single hollow product is produced. DSI cycle times can be anywhere from 1.3 to 1.5 times slower than straight injection, but costly secondary processes are eliminated and complex hollow parts with surface appearance equivalent to straight injection molded parts can be produced. Defect rates plummet. And, hollow parts can be stronger with molded-in ribs and bosses. Three part-design possibilities are illustrated in Figure 2.

DSI processing requires mold and molding machine modifications. Molds must have two cavities for molding product halves. Particular attention has to be paid to mold design, since the two molded product halves are not removed from their cavities after the initial molding sequence and are held in place. JSW has patents pending on its DSI mold design technology. Also, JSW injection molding machines equipped for DSI processing come with controllers smart enough to handle the different sets and additional sequence steps of molding conditions for molding product halves and the circumference part.

Figure 2
Figure 2. Sample product designs molded using the DSI system.

JSW offers 11 models of its E series machines, ranging from 30 to 850 metric tons, with DSI systems as standard equipment. Yet company sources say DSI can be adapted for use with other machines bearing the company marque. A wide range of commodity and engineering materials can be used, including FR and foamed grades, as can materials combinations. At IPF '96 in Japan, JSW demonstrated its latest two-shot/two-material M-DSI process on a twin-barreled 150-tonner, model J150EII-P-2M. Products with ABS as the core and a circumference seal of TPE were molded at the show. JSW also demonstrated how DSI technology can be used to produce two different types of parts designed for subsequent assembly in the same material. A 40-metric-ton vertical clamp/ vertical injection model JT40REII-20V molded two different parts in a 30 percent glass-filled LCP using two different mold faces carried on a two-station rotary table.

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