Parison head

Standard Extrusion Blow-molding 2-Parison Head 4-Fold Stretch Blow Molding PVC (2) Single-parison Heads 4-Fold Stretch Blow Molding PET... 

The parison head, sometimes called the die head or simply the die, is a specialized form of tube extrusion die (Chapter 17). Its function is to deliver a straight parison in the correct diameter, length, wall thickness, and at the correct temperature for BM. Prior to being clamped in the mold, the parison is suspended unsupported in free air. To avoid undue deformation, it is necessary to extrude the parison vertically downwards. 

The BM extruder is almost always arranged in a horizontal attitude, so the first task for the parison head is to turn the melt flow stream through a right angle. This is basically difficult and undesirable but necessary to achieve in a way that meets the essential requirement for a constant flow rate at every point in the annular die gap. A second and related requirement is that the parison should carry as little evidence as... 

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. 

The many variations of the blow molding process are discussed later in this chapter, with an emphasis on those most important for processing fluoropolymers. Two features—the extruder and parison head—are common to many blow molding processes and will be discussed separately. 

The continuous extrusion blow molding process is the most widely used for working with fluoropolymers. In this process, the parison is extruded continuously from the parison head, between the open mold halves (Fig. 10.35). When the required length of parison has been produced, the mold is closed, trapping the parison which is severed by a hot knife. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic air-tight object. Compressed air is introduced through the blow pin into the interior of the sealed parison which expands to take up the shape of the mold cavities. The cooled mold chills the blown object which can then be ejected when the mold opens. 

Relative movement between the parison head and mold is necessary so that parison extrusion can proceed continuously while the mold is closed. This is achieved in many different ways. The mold may be lowered, moved aside laterally, or swung aside on an accurate path. Alternatively, the extruder may be moved while the mold remains stationary. Often, two molds are used in a shuttle arrangement, so that one is open for parison extrusion while the other is performing the blowing cycle. One method of mold movement results in very high production rates. In such machines, generally known as wheel machines, a number of molds are mounted on a rotary table. Movement of the table carries the closed mold away and presents a new open mold to the die head, allowing extrusion to continue. 

In the parison extrusion phase, the extruder die block valve is opened and the screw performs the action of a ram by moving forward in the axial direction without rotating. This forces the accumulated melt at the forward end of the screw through the parison head, where it is extruded at a relatively high flow rate. In practice, the flow rate is limited by the onset of shear phenomena such as sharkskin and melt fracture. 

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... 

Figure 10.36 Example of accumulator parison head by Bekum.P i...

The extrusion-blowing of hollow bodies also requires pressurized air, but this process is discontinuous. A tube (or parison) is extruded into an open mold. After closing the mold, a blow mandrel (not shown in Figure 13.14) is inserted into the parison head through which pressurized air is blown this causes the parison to adopt the shape of the mold the blow mandrel is then lifted and a cutting blade separates the hollow body from its top, ejecting the molded body upon opening... 

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