Accumulator head

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. 

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. 

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. 

In an accumulator head, there are many different shapes, sizes and configurations of flow-chaimel sections coimected to one another, either in series or in parallel. At the same time, melt is being split, shaped, turned, directed, bent, distributed, re-knitted, and finally, is collected inside a large diameter. Pressure is the only prime mover that forces the stream of melt through all these passages. 

Dual 25 accumulator heads (Rear head shut-off)... 

Add insulation to cover top, sides and ends of manifold also the infeed section of the accumulator heads. 1) through 3) Same as above, (improve color change)... 

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... 

Fig. 4-3. Schematic of blow molder using an accumulator head.

Fig. 4-4. Schematic of an accumulator head with programmable process controller controls melt characteristics (interrelates with extruder performance), rate of melt flow to form parison, and profiling thickness of parison as it extrudes from die.

Processing capability developments in the past involved commodity resins, as they were predominantly used. Now there is more focus on engineering resins. Overlapping the melt streams in accumulator heads to obtain uniform wall thickness distribution is now a more fundamental requirement. The accumulator heads that are available differ in their feed channel designs, and they are frequently protected by patents so one must be careful when purchasing them. In a parison head, the melt is divided into separate streams by the mandrel or spiders. Weld lines form where the flow fronts reunite. As the parison is deformed differentially in BM, these weld lines are potential weak sections in areas of extreme deformation. 

In a ram- or plunger-type machine, the extruder feeds the material to an accumulator/head device. The plunger pushes the material rapidly through the head-die assembly once the desired volume has accumulated. The transfer of material does not need to occur via a blowing station in the mould clamp mechanism. The part is blown, cooled and removed from the mould, and the next parison is only extruded after the part has been removed. 

The accumulator head (see Fig. 20) has replaced the ram accumulator in its application for heavy parts. The tubular reservoir is a part of the extrusion head itself Plastic melt that enters the head first is first to leave. A tubular plunger quickly extrudes the melt from the head annulus with a low, uniform pressure, which helps reduce the stresses found in other systems. 

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. 

The constant pressure of the continuous extrusion enables the manufacture of materials with different viscosities and compressibilities into an optimal plastic tube laminate. In the accumulator head system (intermittent parison generation), pressure differences of materials with different viscosities and compressibilities would lead to layer thickness problems. 

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... 

Clamp in open position (2) hydraulic power pack (3) extruder motor (4) extruder (5) accumulator head (6) supporting frame (7) supporting columns... 

The orifice angle of the cone can signiflcantly influence the flow anomalies that are frequently found with high molecular polyethylene (PE). In such cases, changing the melt temperature has little effect. Computer programs are available to calculate the distribution of shear rate in the extrusion flow channel. It is important to ensure that the critical shear rate occurs near the die exit resp. is confined to as narrow a region as possible. With the accumulator head, the effects of flow anomahes can be influenced by the extrusion velocity. To achieve the shortest cycle times with continuous extrusion, the plastic usually exits from the orifice at close to critical shear rate. 

The ram extrusion process has been replaced by accumulator head machines as built by Kautex, Har-tig, Graham, Milacron, Bekum, Automa, Battenfeld, and others. Fig. 13.9 is a photo of a Hartig with a 50-lb accumulator head used to produce automotive fuel tanks. 

Figure 13.9 Hartig machine with 50-lb accumulator head used to produce automotive fuel tanks.

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