Blow Molding machine & tooling
An extrusion blow-molding machine consists of an extruder that melts the plastic and forms it into a molten tube (called a parison or preform) through a conventional-type die and a split-body mold. The die closes around the parison, sealing both ends, and a blow pin is inserted into the parison to inflate it, causing it to expand and conform the shape of the mold cavity. Again, the mold is cooled and once the part has solidified, the mold opens and the part is removed. Extrusion blow-molding is a continuous process that is used to mostly to manufacture small, thin-walled parts but can produce parts as large as 44-gal drums.
An injection blow-molding machine consists of a number of stations with various devices at each station. In one such machine in the first station, the mold is closed and, with the aid of a mandrel, a hollow injection-molded preform is created. (A mandrel is a piece of steel that allows a hollow to be formed in extrusion or injection molding by filling the part of the cavity that would otherwise be filled by the melt. It is sometimes called a tongue.)
The mold then opens and the hot and soft preform is indexed to the blow station on the machine, where the final shape mold closes. Air is introduced through the mandrel to inflate the part to conform to the internal cavity of the mold. Once cooled, the mold opens, and the part is indexed to the ejection part of the machine where the finished part is removed from the mandrel.
In plunger type machines all the heat for melting the plastic is supplied by external heaters. In screw type machines the shear heating of the resin between the screw flights provides a large contribution to heating and barrel heaters are used to produce complete melting and for controlling the final temperature of the melt, as in most injection molding machines. The spreader or torpedo is used to produce uniform flow around machine component peripheries and to produce desired flow-induced molecular structure.
Blow Molding Molds
In addition to the mold cavity which determines part geometric characteristics, blow molding molds have many features that influence process operation, efficiency and effectiveness in terms of part quality. Coolant flow channels are provided to accelerate part cooling and so reduce cycle time. In blow molding the general intent is to cool the part to a suitable ejection temperature as quickly as possible. In the production of preforms in injection blow molding the coolant may be heated to a temperature lower than the melt temperature but high enough so that the preform can be directly transfered to the blowing station with no, or little, temperature conditioning.
There are raised regions on the die face to pinch off and seal the parison before blowing. Recessed regions are provided for flash to flow into, and so minimize the potential for mold separation due to flashing. Mold inserts are separate components fitted to the mold to produce specific features, e.g., a thread insert used to produce threads on the neck of a container. Vents are small channels, perhaps with a porous plug at the mold wall end, to allow air to escape from between the part wall and mold surface.
Since blow molding pressures are relatively low compared to other molding operations, mold material strength is not as important and a large proportion of molds are made from high strength aluminum alloys. However, mold wear may become a problem. Plated steel and beryllium-copper are alternative materials for molds or these more wear-resistant materials can be used for various components of aluminum molds, e.g., inserts and pinchoffs.
Product Characteristics and Process Design
In some situations product end-use and required part performance determine almost all product design requirement. This is usually the case in high-performance parts and products. In contrast, for many large lot, high production rate operations the effects of part design on process performance must be included in part design. In a very general sense, this means that part production cost is more important in some situations than in others. And, in cases in which production process costs are important part design should include explicit consideration of part design features that influence process operation and efficiency.
For example, in blow molding of large quantities of inexpensive parts process cycle time must be as short as feasible. The largest part of cycle time may be the cooling and solidification phase of the part production cycle. Minimizing cooling time implies not only effective process design and efficient process operation – e.g., material transport and cooling – but also the inclusion of cooling time considerations in part design. Such cycle time considerations lead to injection molded parts designed with thin sections and minimum wall thickness in blow molded products.
In plastic parts with mechanical performance requirements such as strength or deflection specifications, part designs are often complicated. The requirement for short cycle time leads to parts with thin sections. Mechanical behavior requirements lead to complicated shapes so that thin section parts perform adequately, e.g., ribs, webs, double curvature, etc.
Blow molded part shapes have to be simple since the the parts are hollow and pneumatic pressure is used to inflate the parison. Overall part shape and outside surface features are set by the mold and these may be complex. However, part inner surface features are limited. Available part design variables are overall shape, local part geometry, wall thickness and material properties.