Parison Flashing

In addition to the accumulator head, a number of other machine advances have taken place in the past 20 years. These are in-mold prefinish, automated flash trimming, leak detection, and conveying to filling station. Also, dual accumulators allow up to four parisons to be extruded simultaneously and molded on the clamping unit. 

The blow molding process consists of a sequence of steps leading to the production of a hollow tube or parison from a molten thermoplastic resin. This is then entrapped between the two halves of a mold of the desired shape. Air, usually at about 100 psi, is blown into the soft parison, expanding it against the contours of the cold mold cavity. The part is cooled and removed from the mold, and where necessary the excess plastic material or flash accompanying the molded part is trimmed and reclaimed for reuse. 

Coextrusion blow molding is often used to produce multilayer plastic containers. A multilayer parison is produced, and subsequent operations are identical to ordinary blow molding. Reuse of flash is more difficult since it will be multilayer. In many cases, the flash is used in an inner layer in the container, either alone or combined with other recycled plastic. High-density polyethylene (HDPE) laundry product bottles, for example, routinely are made with a three-layer structure in which the inner layer contains flash blended with postconsumer recycled HDPE. 

Scrap skrap [often attributive] [ME, fr. ON skrap scraps akin to ON skrapa to scrape] (14c) n. All products of a processing operation that are not present in the primary finished articles. This includes flash, runners, sprues, excess parison, and rejected articles. Scrap from thermosetting molding is generally not reusable. That from most thermoplastic operations can usually be reclaimed for reuse in the molder s own plant or can be sold to a commercial reclaimer. 

In bottle blow molds, there will be a hardened steel insert with a land of 0.076-0.13 mm (0.003-0.005 in.), a relief angle of 20° with a total depth of 0.76 mm (0.030 in.) measured from the inside bottom of the blow mold, and then a 45° cut to the bottom of the relief section in the pinch-off area. Normally, the total of this relief section will be 90% of the parison wall thickness to be pinched (see Fig. 31). This design will also minimize residual flash. It is best to design the pinch land at 0.25-0.4 mm (0.010-0.015 in.) and have metal to remove, if the pinch is not adequate. 

Extrusion Blow Molding. This is simple and economical. It can produce a variety of shapes, with fairly good control over wall thickness. The process is multistep. In the first step an extrusion of a tube of partially molten polymer is extruded down into a mold area. Then, the two halves of the mold close in on the tube of polymer. This cuts it from the extruder and captures the bubble of polymer inside. A blow pin (a hollow tube) is inserted and air is blown into the mold. The air forces the parison to fill the cavity (2). The flash or excess is then cut from the container ground and recycled when possible. 

Parison size is very important—too small a parison can result in a thick pinch-off, which can cause rocker bottoms and warpage, and too high blow ratios to make the product without excessive thinning or blowouts. Too large a parison may result in a thicker wall than wanted at the ends of the pinch-off, excessive flashing, trimming problems and excessive cooling requirements. The correct parison lies somewhere between these two scenarios. 

Fig. 27.7 depicts an extrusion blow molding process, in which a cylinder of semimolten plastic, called a parison, is extruded downward between two open mold halves. Once a parison of sufficient length is extruded, the mold is closed and a blow pin pressurizes the inside of the parison. The air pressure forces the parison to inflate until it contacts the entire surface of the mold cavity. The heat from the formed plastic is then transferred through the mold to the cooling lines. Once the plastic is sufficiently rigid, the mold is opened, the product is removed, and any flashing is trimmed. 

Recent developments in parison handling equipment and in blow mold design now make it possible to manipulate a relatively small parison into the complex mold cavity. The result is a blow molding largely free of flash and scrap and offering considerable process savings. There are many such techniques, some of them proprietary property, and they are collectively known as 3D blow molding.( ig. 10.42). 

In order to keep the inserts a constant size, the overall dimensions will not be changed. The Laser Mike and pillow mold will be used to set the parison thickness at 0.3048mm. The thickness of the compression region of the flash pocket will be modified four times, keeping the length of the pocket constant. The optimum thickness is determined and three more inserts will be machined with the optimum depth as a constant dimension and vaiying the length of the first pocket. The HDPE parts will then... 

One section of the pinch-off r on is the flash pockrt. Varying the depth of this area ean affect the strength of the weld line. This r on of the pineh-off has the greatest contribution to the sfrength of the weld line [4]. The purpose of the flash pockrt is to squeeze the parison as the mold closes which causes the plastie to flow upwards to the weld line and out of the mold. It also transfers heat from the material to the cooling chaimels. 

One cmrent rule is the depth of the pocket is determined by multiple factors. These factors are plastic density, weight of part and parison, parison diameter, parison orientation, and pre-blow pressme. When the flash pocket is shallow, it causes extreme pressme at the parting line and machine pressure section and it causes difficulties in trimming off the flash. When the flash pocket is deep, it causes poor cooling. The poor cooling can cause a hot flash which softens and weakens the weld line [1]. 

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