Unscrewing Molds

Parts made from these types of tools are easily recognizable by the serrated edges of the round part. The main idea behind understanding how an unscrewing mold works is study the actual movements involved in the process of removing a closure from a bottle. When studying the motions used you will be surprised to learn that while you are turning the cap your hand will automatically rise up to the pitch of the threads. A cam action on the stripper bar must do the same thing in order for the part to be removed from the core. Unscrewing molds consist of a rack, motor or hydraulic cylinder and a gear rack. The stripper plate will need an outside mechanism to cam the stripper plate in at the same rate as the thread pitch. This keeps the pressure on the underside of the part so the teeth on engaged to keep the cap from rotating with the unscrewing core. Another feature the cam mechanism has that pushes the part of the serrations is a bump on the end of the cam bar that accelerates the stripper plate creating inertia that pushes the closure off the serrations. Unscrewing molds are routinely considered as a viable method of closure (Le. screw-on bottle caps, etc.) manufacturing. If you are looking at a closure and have ever wondered what kind of mold was used to manufacture it the serrations on the bottom of closure will reveal if a part has been unscrewed or stripped. Unscrewing molds typically will have a longer cycle time than a stripped closure mold or collapsible core mold, which may cause you to ask why any are made this way. There are Two Basic Reasons: Unscrewing molds are usually necessary when the thread profile and the material chosen cannot flex due to the lack of elasticity in the material. The closure is tightened to provide a seal that would be leak proof. The strip thread mold design is not robust enough to provide a leak proof result. There are Primarily Two Types of Unscrewing Molds: The first type is known as a reciprocating core. This type does not require the stripper action to take part in unscrewing. This type of unscrewing mold requires racks, gears and hydraulic cylinders. It also must have serrations along the parting line of the open end of the closure deep enough to keep it from turning while the core is unscrewing and retracting, yet shallow enough to allow easy ejection. The second type is known as unscrewing stripper mold. In this type of design the core does not reciprocate but simply turns. The outside cam lifter bar of the mold drives the stripper up at the same rate of the thread pitch. This design requires that hydraulic cylinder and the thread cores move simultaneously. If the thread pitch and the cam bar angels are not matched it will cause the threads to strip. Both types of molds require a hydraulic cylinder that has alignment coupler as well as gears, and gear racks. The hydraulic cylinder must be long enough to turn the core enough revolutions to complete the unscrewing action and should be in line with the pull direction for best operating characteristics. An alignment coupler must be used to prevent the force of cylinder from cocking the rack and binding the mold. The flow rate of the oil, along with the design of the cylinder is most important to determine the best operation. A cylinder that is too large will have slower operating speeds. A cylinder that is too small may not have enough force to operate the mold action. Remember also that the maximum speeds and forces will likely be different from one direction to the other. Conventional rotary core molds are known to have some significant disadvantages. In particular, these molds require rotary seals in order to provide cooling to the cores. These seals are prone to wear and leakage and require significant maintenance time. Further, rotary seals limit the size of cooling lines and result in slower cycle times. The tapers between a stripper plate and the core also tend to get worn quickly due to the core rotation. In response to molders cycle time and maintenance concerns, certain suppliers to the injection molding industry have developed patented technologies to improve unscrewing technology. Most recently, the patented “Rotating Ratchet Ring” system by Husky Injection Molding Systems was introduced using “dog teeth” imbedded into the part to rotate the part off of the core of the mold. The “ratchet ring or stripper ring” is rotated using a hydraulic cylinder driving pinion gears and in turn driving the ratchet ring. This system allows for stationary cores, improved cooling and virtually eliminates taper wear.