INJECTION MOLDSToth Mold/Die Inc. Since 1982
Injection molded parts that have internal threads need to be unscrewed. On prototype molds and short run molds, the threads can be unscrewed manually. This option adds cost to the final part price. In high production molds, this is not a feasible option due to the final part price. Automatic unscrewing molds are used to speed the cycle time up dramatically. The cost of these injection molds are very expensive and are made for high production. Unscrewing action of these injection molds are accomplished with hydraulic or electric motors. Precision made cores have the threads ground on them. They also have bearings attached to them to keep them in position. Gears are also attached to the cores and they engage with a main drive gear which has a chain attached to it on one end and is also attached to the motor on the other end. When the mold is open, the motor starts to turn the main drive gear which turns the threaded cores. The part is held in place by the means of ribs or serrations which allows the part to automatically unscrew and eventually fall off the core. In some injection molds, a rack and pinion is used to automatically unscrew the part. Collapsible cores are also used. These cores have segments contained in them and collapse allowing the part to fall off the threads.
Toth Mold/Die Inc. has been casting internal threads for 40 years. The tolerances of most of the threads we produce are required to be within .004/in. on the pitch diameter. One of the issues of holding the tight tolerances is the part shrinking in an out- of- round condition. This condition is usually caused by the part design. The wall stock around the threads must be kept even to eliminate the out of round condition.
The material used for the cores is critical. One of the most common problems with injection molding threads with automatic unscrewing cores is the scoring between the core and the bushing. A high heat condition adds to the chance of scoring. Toth Mold/Die Inc. uses an oil impregnated grade of steel for the core and a high wear resistant metal for the bushing. Another area for concern is the alignment of the core and bearings. These components need to be bored and positioned with high precision or it will also cause scoring. It is also a good idea to make sure the injection mold making shop you choose to use to build your injection mold should have vast experience in building automatic unscrewing injection molds.