The entire processing industry is dependent on the plastic mold maker. Good tools 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 plastic molds are broken in production from time to time, and the plastic mold maker must be prepared to make repairs and replace broken mold sections quickly. Plastic Molds wear, as shown in Fig and frequently require dimensional correction and re-polishing and re-plating after continuous service outer long periods. Many times the plastic mold maker is required to change the design of a part and make plastic mold changes that are complicated and involve risk.
The plastics molder of thermoset uses many tools which must be designed and built by plastic 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 plastic mould. After molding finished, 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 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 un-moldable holes. Gauges for checking dimensions, and jigs for filing and drilling, are built by the mold-maker.
Heavy production schedules often justify considerable expense in devising special fixtures to expedite finishing and after-molding operations performed by the molder.
Many of the molding compound are preformed before molding. As shown in Fig. This is done in special presses with “pill” dies. These preforms or “pills” facilitate loading plastic 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 post molding operations such as decorating, adding inserts, combining parts, and machining.