Blow molding offers many advantages, including low material waste and minimal finishing requirements
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Production Process
The blow molding process was first designed in the 1930s and was originally based on metal die casting designs. Blow molding offers many advantages to alternative manufacturing methods, including minimal losses from scrap (since scrap pieces can be melted and recycled), and minimal finishing requirements.
The process uses large blow molding machines, which advance the resins through six major processes to produce everything from computer parts to plastic Halloween spiders. Although an blow molding machine is a complex piece of equipment, it consists of two basic elements, the blow unit and the clamping unit.
The process starts with a mold, which is clamped under pressure to accommodate the injection and cooling process. Then, pelletized resins are fed into the machine, followed by the appropriate colorants. The resins then fall into an injection barrel, where they are heated to a melting point, and then injected into the mold through either a screw or ramming device.
Then comes the dwelling phase, in which the molten plastics are contained within the mold, and hydraulic or mechanical pressure is applied to make sure all of the cavities within the mold are filled. The plastics are then allowed to cool within the mold, which is then opened by separating the two halves of the mold. In the final step, the plastic part is ejected from the mold with ejecting pins.
The blow molding process requires complex calculations. Every different type of resin has a shrinkage value that must be factored in, and the mold must compensate for it. If this value is not precisely determined, the final product will be incorrectly sized or may contain flaws. Typically, this is compensated for by first filling the mold with resin, holding it under pressure, and then adding more resin to compensate for contraction.
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