Mold Design and Construction
An important part of any plastics molding job is the mold itself. It is the heart of the operation, for it defines the size and shape of the molded article. If the product is not right, the first place to look for the cause of the trouble is the mold. Molds are usually made of steel to withstand the pressures used in the molding operations, but other metals and even plaster, rubber, or wooden molds can be used for some molding jobs, especially for special types of plastics materials or short runs. Thermosetting plastics molds have been used in place of metal molds for molding some of thethermoplastic compounds.
In the design of molds for plastics, molding techniques, mechanics, tool design, heat control and economics enter into the problem. For example, if the method is restricted to compression molding, there are eight important considerations to be given to the mold design, namely:
- type of plastics material in order to determine the bulk factor and mold shrinkage;
- size of press to be used, especially the size of the platens and the maximum opening between them;
- molding pressure to be used with the plastics material;
- parting line on the part to determine the dividing point between the two halves of the mold;
- size and shape of the part, for this will determine the number of cavities per mold;
- inclusion of any metal inserts requiring provisions for pulling side core pins and unscrewing threaded pins;
- heat requirements for proper spacing of hot oil or steam coils, or electrical resistance wires for fast and uniform heating of the mold; and
- ejector assembly for removing the molded part easily from the mold.
Economy in mold making (especially for crate mould making)may become extravagance in the production of molded parts. A well designed, durable mold is essential for any successful molding operation. The length of the run, the rate of production, and the accuracy of the detail and dimensions of the part, are some of the major factors to be considered in the selection of a suitable mold. Because of the specialized nature of mold making, a large number of mold and die makers have been established throughout the industry for producing the molds and dies for the plastics molders. However, in the larger plastics companies, one of the more important departments is a well equipped and manned mold and die shop. This requires specialized machines for the hobbing，milling and routing of the massive blocks of steel into the shape of the molds, furnaces for the hardening of the molds, and electroplating baths for the chrome-plating of the molds. All of these machining operations utilize well trained and experienced tool and die makers. Special steels are needed for the molds, and steel companies produce a variety of steels for this purpose. For many of the molds, a general purpose hot-rolled machine steel can be used. It is easy to machine and can be readily surface hardened by carburizing. Softer steels are available for hobbing, which is a method of making mold cavities by pressing a very hard steel core of proper shape into the softer steel. This is a convenient way of quickly duplicating molds in order to speed up production. Chromium plating of molds provides not only smoother surfaces, but also corrosion resistance and thus prevents deterioration of the mold on long runs. Hard chromium plating is made electrolytically by deposition directly onto the hardened steel surfaces of the mold. In general practice, a deposit of 0.3 to 0.5 mil thickness is suflB-cient. Where excessively corrosive molding compounds are to be used, a stainless steel mold is recommended. It will stand up well for runs of a year or two, whereas the usual mold will have to be replaced every six months or so, due to pitting.
In a large plastics crate molding plant, duplicating machines are standard equipment in the mold making department. Using these, patterns or models are clamped to one side of the machine and a tracer is moved over the surface of the pattern. This movement acuates a cutter spindle which cuts a duplicate shape in a block of steel on the other side of the machine. Some handfinishing is usually required, but for routine work this operation saves a great deal of the toolmakers’ time.
There are three types of molds used for compression molding: the positive, the semipositive, and the flash mold. the correct amount of charge of molding powder must be carefully measured out so that when melted and compressed, it just fills the cavity. The plunger or male part of the mold fits closely in the cavity part of the mold and the full pressure is exerted on the piece. There is no escape for any excess material. In the case of the semipositive mold, the plunger is seated with a shoulder landing which prevents full pressures being exerted on the material. A clearance of a mil or two is provided as an escape opening when the plunger is fully seated. In the flash mold, the excess material is pushed out of the mold cavity when the male plunger is inserted. This results in a fin of plastics material at the parting line between the two parts of the mold, which must be trimmed off the molded part. It is wasted material in the case of thermosetting plastics and cannot be used again, as is true of the thermoplastics. The flush type mold is frequently used to avoid excessive wear on a close fitting plunger.
In the selection of the proper molding equipment, there are many factors to be considered in addition to the type of machine to be employed. The larger plastics molding companies will have many presses adaptable for all kinds of molding operations, some of which are rather specific, while others may accommodate several types of operations as well as plastics materials. Listed below are some of the requirements for the major types of molding equipment, to be considered in making a selection. In addition, the names of some of the well known manufacturers are included, who can assist in the setting up of the exact specifications for the correct machine to be used for a specified molding operation.