What You Put in Your Mold Matters

Moldmakers have incorporated technology into today’s injection molds that take performance to a whole new level. Examples include servo drives for high-cavitation unscrewing molds in cleanroom applications and flexible mold technologies for quick changeover.

Servo motors. The servo motor approach takes advantage of the superior speed, precision, cleanliness and energy efficiency that is available. For unscrewing molds in cleanroom environments, the cleanliness aspect represents a dramatic improvement over conventional unscrewing technology, which most commonly uses a large hydraulic cylinder to drive a rack and pinion arrangement. For example, Canadian mold manufacturer StackTeck used servo motors for a recent medical cleanroom application that included multiple molds with 16 cavities. 

The drive system uses a simple yet robust arrangement of pulleys, pinions and a drive belt. This view in Figure 1 shows the reverse side of a core plate, which is fully encapsulated by a mating mold frame plate. Four drive pinions are shown at the center of the assembly, which deliver rotational torque to 16 stripper rings on the opposite side of the core plate. This arrangement offers significantly reduced friction in the system and improved energy efficiency compared with the conventional approach.

The precision of a servo drive arrangement provides more consistent part ejection, resulting in a tight grouping of parts falling free from the mold, shortening the machine clamp opening stroke which reduces cycle time. The rotary speed of the servo is 200 percent faster than a conventional hydraulic cylinder, and so the time required for core rotation is reduced by two thirds. For a typical closure unscrewing application, the overall cycle time improvement expected is typically between 0.5 and 1 second.

The drive arrangement with belt and pinions was chosen for its simplicity, expected reliability and scalability for bigger, higher cavitation molds. The belt-driven approach also eliminates the issues of gear backlash that are associated with rack and pinion systems that need to reverse directions on every cycle. It is possible that this design can be used for applications that require rotating cores or stripper rings, and it is suitable for a range of different part geometries. 

A simple approach was taken for the servo controls. They are housed in a stand-alone cabinet on wheels with a five-button pendant (Figure 2) and a single, standard robotic interface to the injection molding machine. The initial setup of servo speed ramps is set at the factory during mold qualification in the shop’s test room. To date, the company has sourced all servo-related components and drive hardware from a leading global servo motor supplier.

Quick product change. StackTeck has offered flexible mold technologies for quick changeover for many years. Its quick product change (QPC) configuration enables the molder to keep the mold frame and hot runner in the machine while the QPC modules can be exchanged without necessary alignment work or service connections. For a two-level stack mold (Figure 3) in a 600-ton machine, the QPC changeover time is 30 minutes or less, which represents a 90-percent reduction as compared with changing complete molds.  

Early QPC designs incorporated a hot runner in the mold frame with no flexibility to change mold pitch or cavitation. StackTeck recently developed the hot runner in a QPC module format as well. This offers complete flexibility to change the number of cavities, pitch or both (see Figure 4). For optimal changeover times, the hot runner needs to be pre-heated offline, prior to installation in the molding machine.  

For some applications, individual QPC hot runner modules are used in family stack molds and two-material injection molding machines. Their use enables molding of dissimilar parts, using different resins and completely independent process control in each face of the stack mold. This technology is particularly beneficial for molding matched pairs of parts to be assembled or packed together. This approach also enables the high-volume economies of scale created by stack mold systems for some low-volume products—for example, when two low-volume products are produced from a single face of a stack mold.

Mold designs for flexible output are another way to build injection molding systems. For example, StackTeck’s Cascade tooling approach enables a single-face mold to be converted into a stack mold with two, three or four levels (see Figure 5). This yields dramatic output increases for a single molding system and sometimes offers the potential to quadruple system output in the same machine. 

Mold designs for flexible output are another way to build an injection molding system.

The upcharge for the initial single-face mold is approximately 5 percent of the mold’s value. There are also related costs for specifying a molding machine to handle the higher throughputs, unless there is flexibility to switch machines.

These mold technology developments demonstrate the value of continually thinking of ways to better serve customers. Each has helped improve injection molding system productivity and increase manufacturing flexibility for mold changeover times and mold conversions, thereby enhancing system output.