tools for plastic molding

The entire processing industry is dependent on mold-maker• Good tool give continuous production，excellent finish, easy ejection of molded part without distortion, close dimensions, flawless pieces, and low cost cleaning of parts after molding. A good mold will give many years of satisfactory service. The success of any molding operation may be measured by the efficiency of the mold design and the quality of the mold construction. Molding surfaces are often chrome plated for better finish and wear.

The best of molds are broken in production from time to time, and the mold-maker must be prepared to make repairs and replace broken mold sections quickly. Molds wear and frequently require dimensional correction and re-polishing and replateing after continuous service over long periods. Many times the mold-maker is required to change the design of a part and make mold changes that are complicated and involve risk.

The plastics molder of thermosets uses many tools which must be designed and built by his mold-maker. Loading fixtures are constructed to facilitate accurate and fast insertion of the right amount of material into the mold cavities. On some jobs，they are used to fix in place the metal inserts frequently incorporated in molded products. Many types of unloading fixtures are used to remove molded pieces from the molds. Unscrewing fixtures are frequently constructed to remove threaded sections from the mold. After molding, it is often desirable to place the piece on a cooling or shrink fixture in order to hold shrinkage to fixed limits. These shrink fixtures are accurately designed and built to permit contraction of the part from the mold size to the close dimension required after cooling.

Some molded parts are subjected to finishing operations after molding. These operations serve to remove “gates,” “flash,” or “fins.” The gates are the feeders which connect the molded piece to the source of material in transfer and injection molding. “Flash” is the excess material squeezed out of the mold cavity as a compression mold closes, or as the pressure is applied to a transfer or injection mold. A “fin” is formed by the material which flows into the small gap between movable parts of the mold, Fig. 1.16. This excess material must be removed from molded parts during the finishing operation. Trimming and punching dies are frequently used for this operation. These dies punch out the fins in holes and on irregular surfaces. Other special fixtures serve to perform such operations as grinding dimensions to close limits or drilling unmoldable holes. Gauges for checking dimensions, and jigs for filing and drilling, are built by the mold-maker.

Many of the molding compounds are preformed before molding. This is done in special presses with “pill” dies. These preforms or “pills” facilitate loading the molds since each unit contains a fixed amount of material. Pills are often made special for the specific material or application，therefore the mold-maker must be prepared to design and build pill  dies.

Thermal and ultrasonic bonding techniques are used for the thermoplastics and the tool-maker is often called on to provide fixtures for this and other Postmolding operations such as decorating，adding inserts, combining parts, and machining.