Blow molding extruder, stage

Intermittent accumulator EBM machines use a normal conventional axially fixed continuously operating extruder to prepare the melt. The accumulator is a heated reservoir where the melt is temporarily stored in the intervals between parison extrusion (similar action of a two-stage injection molding machine (IMM) as reviewed in Chapter 4. Also used is a conventional reciprocating IMM that delivers melt through a die into a blow mold. 

Extrusion stretch blow molding (ESBM) is a one-stage or two-stage process using two mold/mandrel sets where one is for preblow and the other for final blow. An extruded parison is first pinched off and blown... 

FIGURE 2.21 A two-stage vented extruder with a valved bypass. (After Fisher, E. G. 1971. Blow Molding of Plastics. Iliffe, London.)... 

Injection vs. extrusion/blow-molding For clarity, processmg factors go hand-in-hand with the choice of a clarifier. Relatively thick-walled, injection-molded cPP homopolymer or random copolymer parts have hmits on the clarity they can achieve, perhaps down to 7%-10% haze. Only thin extruded or injection-stretch blow-molded (ISBM) cPP may achieve lower haze levels (l%-2%) that allow it to compete with PET. For one-stage ISBM of random copolymer PP, for example, newer clarifiers may work better than sorbitol-based agents... 

Extrusion blow molding is the process employed in the production of large containers such as drums, as well as complex automotive parts. This process is widely used to produce containers of various sizes and shapes. It is also adapted to make irregular, complex, hollow parts, such as those supplied to the automobiles, office automation equipment, and pharmaceutical sectors. In this process, the extrudates swell [5], as well as sag or draw down during parison extrusion stage. 

Extrusion stretch blow molding is a two-stage process using two mold/mandrel sets — one for pre-blow and the other for final blow. An extruded parison is first pinched off and blown conventionally in a relatively small pre-blow mold to produce a closed-end preform. The preform is then transferred to the final blow mold where an extending stretch rod within the blowing mandrel bears on the closed preform end to stretch it axially. The stretched preform is then blown to impart circumferential stretch. Standard blow molding machines can be converted for extrasion stretch blow molding. 

The dip BM process bears some resemblance to IBM in that it is a single-stage process performed with a preform on a core/blow pin.The difference is in the way the preform is made. The process uses an accumulator cylinder that is fed by an extruder. The cylinder has an injection ram at one end while the other is a free fit over the blow pin. The blow pin is dipped into the melt so that a neck mold on the pin seals the end of the accumulator cylinder. The injection ram is advanced to fill the neck mold then the blow pin is withdrawn at a controlled rate so that it is coated with a melt layer extruded through the annular gap between the pin and the accumulator cylinder. The thickness of the coating can be varied or profiled to an extent by varying the speed of the blow pin and the pressure on the injection ram. After trimming, the preform is BM in the same manner used for IBM. 

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