Parison extrusion

Single-stage EBM produces a blown product in a single integrated process cycle. BM immediately follows parison extrusion and relies on the melt condition of the parison for the deformation and flow necessary to take up the shape of the mold. There is no reheating of the parison before molding. The principal variants are the continuous extrusion and the intermittent extrusion processes. Extrusion in this distinguishing sense applies to extrusion of the parison rather than the operation of the extruder. 

Two or more molds can be used in a shuttle arrangement, so that one or one set is open for parison extrusion while the others are performing the blowing and cooling cycle. There is also a method of mold rotary movement that results in very high production rates. In such machines,... 

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. 

The low melt index grades of PPE blends with PS or HIPS Noryl , Luranyl ) are most suitable for blow molding. The optimal processing conditions depend on the article type and machine and thus should be experimentally optimized. The typical temperature range for parison extrusion is 250-260°C whereas it is 230-255°C for the adapter and accumulator. HOPE should be used for purging. 

The process of parison extrusion can be continuous or intermittent. For intermittent extrusion, the melt from the continuously rotating extruder may be fed into an accumulator, from which it is periodically ejected, or a reciprocating extruder like those used for injection molding may be used. Continuous extrusion is preferred for most packaging applications. It provides higher productivity and reduces thermal degradation, since the melt is not held up inside the hot extruder. Intermittent... 

Either reciprocating screw or continuous parison extrusion may be used. These resins have less diameter die swell than HDPE and have a higher melt temperature range (195-215 C). 

The intermittent extrusion blow molding process is generally used in the fabrication of large volume articles (e.g., > 20 liter bottles). An accumulator is employed to increase the parison extrusion speed (see diagram below). After the mold closes there is no additional mold movement. Refer to Injection Stretch Blow Molding. 

Both processes involve the inflation of a parison (extrusion blow molding) or a preform (injection blow molding) into the final part. 

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. 

The intermittent parison extrusion blow molding machine using a reciprocating screw is often referred to as shot extrusion. The screw rotates and retracts, plasticizing or melting toe resin as it moves back, then charging toe shot in front of toe screw. The screw then rams forward via hydraulic or electric means. 

At this point, the blow molds close. Parison extrusion is rapid, compared to a continuous extrusion. For a typical blow molding machine producing gallon size HDPE milk containers the parison extrusion time is under 1.5 s. Uniloy blow molding machines produced by Milacron can have from four up to eight... 

Figure 13.5 Intermittent parison extrusion blow molding machine using a reciprocating screw.

Resin temperature affects viscosity and part quality by causing variations in the parison extrusion time, length, thickness, and cooling time. This temperature is a critical parameter that must be monitored carefully and the source of any variations must be accurately identified. Extruder factors that can cause resin temperature variations include ... 

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. 

Extrusion is one of the most used processing methods. It is well-suited to the production of tubes, pipes, sheets, and filaments. The polyma- melt is extruded with extremely high pressure through the nozzle of a die whose cross-sectional shape determines those of the resulting parison. Extrusion applies preferentially to thermoplastic materials, but it can also be adapted to the processing of thermosetting resins. 

Figure 12 Principal steps of extmsion blow molding, (a) Parison extrusion (b) encapsulation of parison (c) insertion of blowing needle (d) inflation and eooling (e) ejection.

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