The term “flash” in extrusion blow molding refers to the excess plastic material that remains on the body of a new container after the container has been formed. Though flash is a side-product of the extrusion blow molding process, it can have negative effects to blow molding efficiency as well as to the plastic part or container if not properly controlled.
Extrusion blow molding is the manufacturing process whereby plastic is melted and “extruded” into a hollow form. The melted plastic, called a “parison,” is cooled in the sealed mold. Air is then blown into the parison, which presses the material into the shape of the mold. Extrusion blow molding is a significant means of manufacturing hollow parts and forms such as plastic juice and water bottles, though the full numbers of applications are extensive.
Flash is essentially the rejected waste of the process – and the problem with flash is that it can impose restrictions on the overall efficiency of the blow molding process while leading to an increased reject rate of product. The primary problem with flash is that it can attach itself to the molded part, thereby causing that part to be rejected. This poses problems associated with an increased reject rate of formed parts; leading to a slower manufacturing time with lost time and efficiency. Some of the associated problems can occur include:
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The flash pocket isn’t large enough to hold the flash material; thereby resulting in too much plastic material;
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The flash pocket doesn’t fill properly, which can happen if the pressure is inadequate. This will force plastic back to the container;
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If there are any shortcomings with the process geometrics or effectiveness, this will in turn lead to pinch-off problems.
As with any post-molding process, flash removal – whether done manually or mechanically – must be done correctly or damage can occur to the formed products. And flash can affect both the look and the usefulness of the formed part. By example, excessive flash can interfere with the seal and reliability of a cap or closure.
It might be said that the best solution(s) are achieved with use of well-designed extrusion blow molding machinery; thus minimizing or even eliminating the problems with flash before such problems can even exist. Additionally, initial solutions will likely be (1) decrease the cooling time differences for the molded part and the flash, and; (2) operate the mold at a lower temperature, which will quicken and reduce the cooling time. The challenge is that flash presents issues that are not always conveniently solved. However, some solutions can, in part, be achieved by…
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Changing parison viscosity. This will help correct the percentage of plastic material going back to the container;
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Alter the closing speed of the mold. This will help modify the amount of plastic being forced back into the container;
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Change the pressure and duration of air blowing into the mold. This too will help modify the process;
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Inspect the pinch-off areas on molds to ensure a proper gap; any overly large or small gaps will create problems;
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Experiment with different flash pocket designs to maximize process reliability and efficiency.
