Pinch-off areas

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. 

Molds with aluminum pinch-off areas could last 1 to 2 million cycles if... 

Aluminum is used for single molds, molds for prototypes, and large numbers of identical molds, as found on wheel-type blowing equipment or equipment with multiple die arrangements. (Regarding prototyping, it is best to utilize a material that the production mold will use to duplicate heat transfer conditions.) Aluminum may tend to distort after prolonged use. Thin areas, as at pinch-off areas, can wear in aluminum. 

Barrier integrity of handle breached Too little material in handle Poor pinch-off Program more material into handle and pinch-off area... 

The cycle starts with mould preparation. The mould is cleaned and a release agent is applied. The reinforcement is then placed in the lower mould. Several layers may be applied, depending on the thickness of the mould cavity to be filled. The reinforcement is allowed to overlap the pinch-off area of the mould. 

The press is then closed and a pressure of about 5 bar is applied. As the mould halves close, the resin and reinforcement are compressed. The resin is pushed towards the perimeter of the mould, the pinch off area. Because the reinforcement overlaps the pinch-off it is compressed more than within the mould. This restricts the flow of resin but allows the passage of air. ffence as the resin flows to the pinch-off a back pressure is created which ensures that the resin flows to all parts of the cavity. 

With the wet resin technique, the closure of the die and the consequent hydraulic pressure forces the resin through the reinforcement, pushing a front of air to, and through, the pinch-off area. When SMC or DMC are used the closure of the press compresses and heats it. Its viscosity reduces rapidly and it is able to flow to the pinch-off zone of the die. 

In bottle blow molds, there will be a hardened steel insert with a land of 0.076-0.13 mm (0.003-0.005 in.), a relief angle of 20° with a total depth of 0.76 mm (0.030 in.) measured from the inside bottom of the blow mold, and then a 45° cut to the bottom of the relief section in the pinch-off area. Normally, the total of this relief section will be 90% of the parison wall thickness to be pinched (see Fig. 31). This design will also minimize residual flash. It is best to design the pinch land at 0.25-0.4 mm (0.010-0.015 in.) and have metal to remove, if the pinch is not adequate. 

FIGURE 2.21. Drop of liquid as it detaches from a capillary tube. The shape it takes on is remarkable, particularly in the pinch-off area (courtesy S. Nagel and X. D. Shi). ... 

Clamp tonnage is determined by the addition of the air blow pressure in pounds per square inch times the product s projected area in square inches, plus the pinch-off area in pounds per square inch. A safety factor of 25% is used. 

Generally, in large volume production, pinch-off inserts are made of steel (P-20 or S-7) with the other portions of the blow mold produced from a nonfer-rous metal. The pinch-off edge should not be similar to a knife edge, or it will tend to act as a cutter and will yield a V groove where the tail or pinch-off area of the parison is forced to bond. Fig. 13.20 depicts a poor weld line due to the pinch-off having a knife edge. 

There are multiple types of pinch-offs used. Single pinch-offs are used to make smaller bottles. Usually, these bottles are up to 19 liters [1]. The landing varies from 0.254 to 1.0 millimeter [1]. The land on the one half of the mold usually varies from the land on the other half of the mold for better cut off Another type of pinch off is the double compression pinch-off. This type is used for bigger bottles. The primary pinch-off at fre parting line level widths vary from 1.0 to 2.0 millimeters [1]. The secondary flash pocket, which is below the parting line, varies from. 254 to 1.0 millimeter. The width of this pinch-off area has a length that is equal or smaller. 

The second improvement would be to increase the compression pooentage of the main body of the pinch-ofF. This should allow more plastic to flow into the weld of the bottle and make it strongo-. The increased flow through the pinch-off area shonld cause a greater pressure drop in the tail end of the pinch-off. Since plastic will flow where there is the least amount of resistance, the plastic should be squeezed into the weld of the bottle. 

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