Blow molding, 170: accumulator head

This added design feature cannot solve all color change problems for large industrial blow molding accumulator heads, but tests so far have shown good improvements on time consumed. Implementation of this is simple and relatively inexpensive, and easy to operate and control. It is worthwhile for molders who change color often to look into this different approach time and material savings should result. 

Eric Dawkins, Kris Horton, Paul Engelmam, M. Monfore Method for Effective Color Change in Extrusion Blow Molding Accumulator Heads 157... 

Blow molding accumulators take one of two forms. The separate accumulator is a heated chamber which is an integral part of the blow molding machine. The capacity of such a chamber can be very large and can, if necessary, be served by several extruders. The other type of accumulator is built into the parison head (Fig. 10.36). This takes the form of an armular ram that surrounds the mandrel and torpedo. The construction of the parison head becomes quite complicated and there is the potential for melt leakage and degradation... 

The extrusion blow molding process can be continuous or intermittent, as shown in Fig. 14.16. The former, employed commonly for parts less than 1 gal, has a continuously rotating screw extruder, extmding parisons through one or more dies. The latter may use either an accumulator head with a piston-driven extrusion forming of the parison, or a reciprocating screw, such as the one used in injection molding. 

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

Figure S.7 Introduction to a continuous extruded blow molding system with its accumulator die head...

SBC can be processed on most conventional blow molding and injection blow molding equipment designed for many other types of polymers with little or no equipment modifications. This includes continuous, accumulator head, or reciprocating screw extrusion blow molding equipment. Bottles made from SBC have been used widely for applications such as honey bears. The SBC will withstand the moderate hot fill temperatures required for this application. 

Head n. (1) In any extrusion operation, the delivery end of the extruder, usually fitted with a hinged gate that may contain a breaker plate and screen pack, to which the adapter and die are attached. (2) In blow molding, the entire apparatus by which the molten plastic is shaped into a tubular parison. This may include an adapter, a parison die, and a melt accumulator. 

A growing trend in extrusion blow molding is to coextrude parisons that contain up to seven layers of different materials. However, as of this time, coextrusion is limited to machines that only use one (1) parison as on a wheel and the continuous-type machine. Since the different materials combine in the head tooling (see Fig. 35), the use of manifolds for multiple cavity is not feasible, nor are accumulator machines (see Coextrusion). 

Figure 7-53. This Navistar International truck fascia is of multiple-extrusion blow-molded parts made from Himont s HiFax polypropylene. The fascia parts, which range from 8 to 17 lbs. and come in lengths from 24 in. to 6 ft., are molded on 50-lb. accumulator head capacity machines. The ability to mold the PP in an integral color eliminates the need to paint. 

With this method, the extruder also works continuously, but the melt is stored in a tubular piston accumulator (Figure 1.139) from which the plastic is intermittently extruded to produce the tubular preform. Figure 1.140 illustrates an extrusion blow molding machine equipped with an accumulator head as it is used, for example, in the manufacture of plastic fuel tanks, barrels, liners, and other similar large structures. The removal of the finished part is done with grippers and a transfer mechanism. The accumulator heads work best with the first in, first out (FiFo) principle. This is necessary in order to accommodate for the limited thermal stability of the plastic. 

Blow molding machines with accumulator heads are used for parts with volumes from 30 1 to 10,000 1 and with shot weights from 500 g to 250 kg. Accumulator heads are also used for plastics with lower melt strength, such as polyamide, low density polyethylene (LDPE), polystyrene, and polycarbonate, as well as for a large group of engineering plastics (alloys). 

FIGURE 1.140 Schematic of an extrusion blow molding machine with accumulator head bridge construction... 

Fig. 13.7 depicts an intermittent extrusion blow molding machine with a ram (piston) accumulator remote from the die head. 

The ram pushes the accumulated material through the die head to form the parison as a shot. This type of machine is obsolete however, there are machines of this type out in the industry still in use. The extruder or plasticizer is stationary and can continuously feed heated plastic into the accumulator. The extruder may be programmed to stop when the die is filled to a predetermined volume and then start again after the parison is extruded. The extruder speed is adjusted to fill the accumulator at the proper rate so the parison is extruded almost immediately after the blow mold has opened and the product ejected. Fig. 13.8 is a photo of an original product blow molding machine of this style of equipment. 

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