When using injection molding for mass production, manufacturing engineers clearly have a vested interest in maximizing the number of parts per hour in order to reduce costs and get products to market as quickly as possible.
Contrast this role with the priorities of the process control technician in the factory and you’ll notice they differ greatly. Process control technicians are responsible for carefully balancing out the various forces within the injection molding machine to control quality and ensure consistency. These forces primarily consist of injection, packing and back pressure. Modulating these is essential to getting good results but their variability tends to work at odds with a theoretically optimal cycle time. It might help part designers, therefore, to understand more about how this process works and why faster is not always better.
For those not familiar with the process, it might be tempting to crank up the injection pressure to full volume to fill the mold cavity as quickly as possible. Faster filling time means more parts per hour, right? Well, not quite, though at the beginning of the cycle it might seem like this is the case.
There is little resistance to flow when the mold cavity is empty, so it will fill without much difficulty at the beginning. But, when the cavity starts to become 90 to 95% full, there will be a sudden spike in pressure that can cause flashing or even damage the tool. To prevent this from happening, injection pressure must be reduced during the changeover point to packing or holding pressure.
Holding pressure is decreased from the initial injection pressure when the last 5 to 10% of the tool cavity must be filled.
This pressure cycle is held for a longer time for several reasons. First, trapped gases in the mold must have time to escape through the vents. Second, holding pressure helps the resin to achieve the highest density possible. Third, deep sections or thin-walled areas of the part need additional time to fill completely. And four, holding pressure helps to stabilize internal stresses in the molded part.
Of course, this pressure must be balanced against another limitation. The process engineer must calibrate forces carefully within a short period of time so that the cavity is filled before the gate freezes off and no more packing is possible.
Back pressure is a deliberate resistance that is created when the operator controls a valve on the injection molding machine to restrict the rate at which hydraulic fluid can be released. This, in turn, resists the reciprocating screw in the barrel from being pushed back to its starting position.
This deliberate delay is needed for the flutes on the screw to mix the next shot of resin. As with other limitations, back pressure should be controlled by the process engineer so that the resin isn’t over mixed, which can degrade it due to excessive sheering.
Taken together, balancing out these pressures requires a careful orchestration of forces within the machine. That’s why a mold tool cavity cannot—and should not—be filled as quickly as possible. Rather it should be filled as quickly as practicable to achieve consistent, quality results—a very different thing.
Gordon Styles is CEO and President of Star Rapid, a provider of rapid prototyping, rapid tooling and low-volume production services. Utilizing his background in engineering, Styles founded Star Rapid in 2005 and under his leadership the company has expanded to 250 employees. With an international team of engineers and technicians, Star Rapid combines advanced technologies—such as 3D printing and multi-axis CNC machining—with traditional manufacturing techniques and high-quality standards. Prior to Star Rapid, Styles owned and managed the United Kingdom’s largest rapid prototyping and rapid tooling company, STYLES RPD, which was sold to ARRK Europe in 2000.
Image courtesy Itsanan/Adobe Stock.