neck insert making
The mold body is usually the portion that carries some means of maintaining alignment of the mold halves, A minimum of two sets of hardened dowel pins and bushings can be used for this purpose. For mold materials such as zinc and aluminum, a shoulder type pin and bushing is desirable to prevent them from pulling out of the mold. Aluminum and zinc have so little elasticity that a press fit cannot be relied upon to hold pins and bushings in place. The size of the pin and bushing should be determined by the operation of the blow molding machine. Some machines operate with platen bushings having large clearances and the mold pins and bushings are expected to align the platens at the time the molds close. Obviously, this puts considerable strain on the mold alignment, and the pins and bushings should be large. For machines having well supported platens, pins measuring 3/s to V2 in. in diameter can be used; the larger pin being used for larger molds. The neck and base inserts are attached to the mold body with suitable fastenings and means of alignment registration. Dowel pins can be used for parts alignment, and are most suited for inserts which will not be interchanged regularly. For ease of interchanging inserts a register diameter is recommended.The same principles of cooling channel size and location mentioned for inserts apply for the mold body.
Many different designs have been used in the pinch-relief sections. Design A is probably the one most widely used forpolyethylene. In some instances, however, where the mold must pinch on a relatively thin portion of the parison and next to this pinching edge the parison must expand a large amount, the plastic will thin down and may even leave a hole on the parting line. This defect is sometimes seen near the finger hole on containers having handles. The shallower angle of 20° has a tendency to force plastic to the inside of the blown part and increase the wall thickness at the parting line rather than pushing the excess material back into the pinch relief. Another method used for increasing the wall thickness at the parting line employs a restriction or dam in the pinch relief similar to that.
In attempting to minimize the residual flash left after trimming, the pinch land is sometimes made sharper than .015/.010 in. Figure 11.44 illustrates a typical design. It should be noted, however, that pinch lands of .003 in. width should only be used on hard, tough mold materials such as beryllium copper.
For certain cavity shapes additional venting may be necessary in addition to that noted previously. In these cases vent holes and/or slots are added.
Sharp concave depressions or grooves in a cavity will trap air at their deepest point. In these areas, holes ranging in size from .005 to .009 in. in diameter are drilled, permitting the air to escape to the outside of the mold. If air entrapments occur at the parting line, then simple slots approximately V2 in, wide and .002/.0015 in. deep are milled across the entire face of one mold half at frequent intervals.
Needless to say, the cost of drilling holes .005 to .009 in. in diameter can be very great and they tend to become clogged during molding so their use is usually kept to a minimum. If sharp, square-faced, letters are required on a finished blow part, the engraving in the cavity must be vented. With some letters of the alphabet requiring as many as six vent holes to provide proper reproduction, the cost of drilled vent holes could become prohibitive. As an alternative, the mold can be inserted with a porous metal piece in the area to be engraved. Trapped air can then vent through the engraving by way of the porous material and some suitable connecting passage to the outside of the mold. Not only can this porous metal method of venting be less costly than drilled holes, but with proper selection of particle size used in the porous metal, the vent openings will not show on the finished plastic article. This is not necessarily true of drilled holes. With a practical minimum of approximately .005 in. diameter, drilled holes could show a projection on the finished part depending on temperature and type of plastic being blown. The disadvantage of a porous metal insert would be visual; a line would always show at the matching surfaces of the mold and insert.
The mold design discussion thus far has been concerned with the pinch-tube method of blow molding. Most of the design considerations can, however, be applied to all methods of blow molding. For the neck ring process, a wear-resisting and self-registering pocket must be provided in the mold to accept accurately the neck ring on each cycle.
With the continuous tube process, air must be introduced through the side of the parison rather than through its center. This is necessitated by the fact that the parison is pinched shut at both the top and bottom of the mold. This is done to allow separation of one article from another at the completion of molding. Air is usually injected by means of a hollow needle which pierces the parison. Thus the neck insert design must provide for this needle together with some means of actuating it, such as an air cylinder. The neck insert would also carry a pinching edge to close off the upper end of the parison. With the horizontal continuous tube process, ejector pins are usu ally required to free the finished part from the lower mold half.
In addition to providing pincher slides and their means of actuation in y e trapped air process, much consideration must be given the actual opposing pinching faces on the slides. A seal, trapping the air within, can be effected either by the meeting of two pinching surfaces or by crowding the parison into a cavity which is small enough to collapse the parison and cause it to seal itself. Pinching edges must be designed to provide sufficient wall thicknesses at the parting line. Otherwise there is danger of the trapped air bursting the blown article at a thin spot as it expands due to the heat of the plastics material before the article can be cooled.
