The injection molding process is the most commonly used manufacturing process for the fabrication of plastic parts in the world. It is so popular because of its ability to create a wide variety of items of differing sizes, shapes, and complexities. Most injection molding machines use the same process – thermoplastic conversion – to turn the plastic stock into the final molded parts, using these five basic steps:
Step 1: Melting the Thermoplastic
Melting or softening of a plastic material through shear heating and using a screw turning in a barrel, is known as plasticate. As the screw starts turning, the hopper releases the pellets from the feed section into the compression section, where the frictional heat is created. This is the section where most of the melting occurs.
As the screw turns, the molten material (known as melt) in this metering section flows ahead of the non-return valve, forming a shot, which is a pool of molten material in front of the screw used to fill the mold cavity. The melt begins to pressurize as it is unable to seep into the mold. This forces more shear and friction in the compression section of the screw, increasing the melting and mixing action of the screw. It is important to note that there is a cooling channel in the barrel (right below the hopper), that prevents premature melting which could cause a blockage of pellets (known as bridging).
Step 2: Injecting the Molten Thermoplastic
The machine is now ready to inject this shot into the mold. In injection molding, the conveying and shaping of this molten plasticate occur simultaneously. The screw starts moving forward pushing the melt into the fill gate while the check ring slides in the rear position so the molten material does not flow backward. In the fill gate, the melt rapidly flows through the nozzle, sprue blushing, runner system, and then the gate into the mold itself. Conveying of the melt continues, but the melt front is now starting to take shape of the mold cavity—step 3 has started while step 2 continues.
Step 3: Shaping the Molten Thermoplastic
A signal is sent to the controller to switch from velocity control (fill mode) to pressure control (packing mode) as the cavity is almost 95-99% full. The change of controls is called the velocity-pressure transfer point, and it happens in the blink of an eye. It is one of the most crucial parts of the molding cycle. The screw is still moving forward but at a lower rate. The plastic in contact with the mold has begun to solidify with the start of step 4.
Step 4: Cooling the Shaped Thermoplastic
Throughout the molding cycle, the coolant flows under pressure in a turbulent mode through the cooling channels of the mold. It stabilizes the temperature of the steel and quickly cools the material, by taking heat away. The gate seals with a small amount of material ahead of the screw called the cushion. Without the cushion, there would be no available pressure to force more material into the cavity. The cooling timer starts and the screw now begins recovering to prepare for the next shot. In the cavity, there is still internal pressure, but the pressure is decreasing as the part shrinks. The cooling timer ends.
Step 5: Removal of the Thermoplastic Article
The ejection cylinder is activated, the mold unclamps, and the ejector pins start pushing the part out of the mold. The part is ejected onto a conveyor, bringing it to the operator for trimming or any other function. The mold closes quickly and clamps slowly, waiting for the next shot.
Knowing the basics of the thermoplastic conversion process and how some of the machines perform will ensure better communication between the manufacturer and the client. If there is a specific conversion process you have questions about, our experienced industry professionals would be happy to address any question you may have. Contact Midstate Mold & Engineering today!