plastic mold

Why and where to use plastic injection molds

 

A plastic injection mold is any tool made in whole or in part of a plastics material (excluding industrial thermosetting laminates), and in which the plastic contributes substantially to the utility of the tool. Such tools range all the way from those made entirely of plastics to metal tools to which a plastics working surface has been applied.

There is only one proper approach to evaluating the suitability of plastics for a tool. That is to consider plastics as an independent family of engineering materials with unique combinations of engineering properties, which indicate how the materials will behave under specific service conditions. Under some conditions plastics provide outstanding benefits in terms of lower cost, more rapid plastic injection mold making, etc.; under other conditions, they should not be used at all.

The success of a particular plastics mold is solely dependent on the degree to which the plastic mold manufacturers understand the plastic injection mould design requirements and the capabilities of plastics materials. The second area is the one causing most of the problems in injection plastic molding today.

Picture if you will, a plastic mold maker specifying a material for a particular plastic mold making detail. He knows exactly where he stands with iron and steel on making plastic molds. He has ready material on hand that gives him weights, characteristics, and costs. He also has his experience which tells him what to expect from a particular metal. On the other hand, he knows very little about plastics as a die material. Naturally, he will specify that with which he is most familiar.

Becoming “familiar” with plastics is relatively straightforward; understanding them to the degree that their full capabilities can be realized is more difficult. Each member of the growing plastics family offers a relatively broad range of properties; but within each group, specific types offer more specific combinations of properties. In using metals, a standard alloy can be specified which is supplied within guaranteed compositional limits and often with guaranteed minimum strengths.

In specifying and using plastics, no such standard grades exist. Also, for the most part, tooling plastics are “made” in situ; that is, the resin is catalyzed and cured in the tooling shop in the final shape of the tool. Since the way in which the material is formulated, catalyzed, fabricated and cured determines to a large degree end properties of the material, the plastic mold manufacturing engineer must have a greater understanding of the technology of plastics than he does of metals. The plastic mold making engineer unfamiliar with plastics should work closely with the materials supplier at the start. Reliable, reproducible physical properties can be obtained in plastics, but only by processing the materials properly.

Epoxies are the most widely used tooling plastics. They provide an excellent balance of physical and mechanical properties, low shrinkage, and compatibility with glass fiber reinforcements. Other plastics should not be overlooked. Many of them can provide distinct advantages in tooling for many plastic injection molding companies if properly used. Phenolics provide high heat resistance; polyesters provide low materials cost and excellent processability; room-temperature vulcanizing silicone rubbers (not actually plastic ) provide heat-resistant, flexible mold materials; foams of phenolic, epoxy and urethane provide high bulk with low weight; and so on. Such specialty applications as the use of flexible polyvinyl chloride for molds for concrete, nylon mandrels for the machine- and hand-bending of aluminum tubing, and thermoformed thermoplastic sheet for molds for forming urethane foams indicate that other plastics also can offer benefits.

 

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Post time: Aug-10-2015
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