Blowing pin

Processing temperatures should not exceed 180°C, and the duration of time that the material is in the melt state should be kept to a minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates. Similarly, injection moulds are recommended to be held at 60 5 C. 

Hoechst has developed a method moulding PP foam automobile air ducts that optimise air flow and also provide good sound and heat insulation. Machines are adapted to suck the parison onto the mould walls rather than to blow mould it. The process also uses a blowing pin operating in reverse mode to create a partial vacuum inside the moulding in order to roughen its inner surface. HOECHST AG... 

Fig. 14.15 Schematic representation of the blow molding process, (a) The extruder head with the blowing pin and open mold (b) the extrusion of the parison (c) the mold closed with the parison pinched in the bottom and sealed at the top (d) the inflated parison forming a bottle.

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. 

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 The injection ram is advanced to fill the neck mold then the blow pin is withdrawn at a controlled rate so that it is coated with a melt layer extruded through the annular gap between the pin and the accumulator cylinder.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. 

The body mold closes on the parison that is blown normally by a neck calibrating blow pin. Immediately, with the mold still closed, the liquid contents are injected through the pin. The pin is then withdrawn and the neck is formed and sealed under vacuum by the neck-forming members. Both mold parts then open to eject a filled and sealed container. Small containers may be formed entirely by vacuum rather than blowing. 

The third mode is the shuttle method, where usually two or more sets of molds are used. Each set of molds can have at least two or more molds. Their blowing stations are around the periphery of the extruder die head and parisons. One set of molds in the open position is located under the die. With the proper length of the parisons (a parison for each mold), the open molds underneath close. After the molds are closed, parisons are cut, usually with an electrically charged hot wire, and quickly shuttled to a blow station, where blow pins are inserted into the parison openings. Blow-molding parts solidify and are released from the molds when they open. In the meantime, the parisons continue to be extruded as another set of open molds is positioned around these parisons. Thus, the molds alternately shuttle producing molded parts. 

In general, the plateout material from a fluorescent color concentrate is composed of lower molecular weight fluorescent pigment carrier resin fractions, color concentrate carrier resin, and other additives and materials that are present during the molding process. This mixture of plateout normally appears as colored deposits on the mold face, or condenses on the blow pin assembly of blow molding equipment. 

For the second test, a quantitative measurement is made of the amount of plateout material that builds-up on the blow-pin of a extrusion blow molding machine. The testing procedure consists of the following ... 

The following color concentrate samples were prepared and evaluated for both blow-pin and injection mold plateout. All of the samples were produced under the same processing conditions, i.e., Killion 1 1/4", single screw extruder 375°F melt temperature, and 120 RPM. 

Table 1 contains the different color concentrate samples and their compositions. Table 2 displays the results of the Blow pin plateout and Injection mold plateout testing procedures. 

Sample Blow pin plateout test, mg Injection mold plateout test... 

Fig. 4-1. Basic extrusion blow molding process. A = Parison cutter, B = Parison, C = blow mold cavity, D = blow pin.

When the parison exits the die and reaches a preset length, a split cavity mold closes around it and pinches one end of it. Usually a blow pin is located opposite the pinched end of the tube. Compressed air inflates the parison against the female cavity of the mold surfaces. Upon contact with the relatively cool mold surface, the blown parison cools and solidifies to the part shape. Next the mold opens, ejects the part, and then repeats the cycle by again closing around the parison, shaping it, and so on. 

Various techniques are used to introduce air. It can enter through the extrusion die mandrel (as with most pipe lines. Chapter 3), through a blow pin over which the end of a parison has dropped (Fig. 4-1), or through blowing needles that pierce the parison. The wall distribution and thickness of the blown part are usually controlled by parison programming, the blow ratio, and part configuration. 

There appears to be an unending series of new developments in BM to improve productivity and reduce costs. For example, with cooling there are reliable CO2 systems, air chillers that reduce the temperature of the blown air to around — 95°F ( —70°C), and blow pins that permit heated air in the blown part to exit, so that a continuous flow of fresh/cool air enters the part. With such systems, the output can be increased 10 to 30 percent. 

Fig. 4-7. Mold movement can locate the blow pin in any required horizontal position.

Moils not separating from neck finish Cutting ring is dull Poor contact between cutter ring and striker plate Sharpen or replace cutting sleeve Increase overstroke and downward pressure of blow pin... 

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