| June 13, 2023
Plastic injection molding was the height of technology when it was first introduced to the market decades ago. This manufacturing technique is simple just melt the plastic and squirt it into the mold, correct? Not so fast, managing polymer movement and phase change is more complicated than thought. I’ll outline an approach to optimize the 3 critical portions of the injection molding process (Fill – Pack -Recover)
First, what is two-stage injection and why is it needed?
If only one pressure (stage) is used it will need to be relatively high because the polymer is being pushed through a runner system and needs to fill the entire mold. We use 2 stages so we can protect the mold by lowering the pressure to a packing level after the mold is full. Typically, the lower 2nd stage pressure is used to continue adding volume to compensate for the expected volume shrinkage occurring as the polymer cools.
Let's start this discussion by examining the screw Recovery phase, where the material is loaded for the next cycle before injection.
Material dosage is accomplished by the rotation of the screw, which conveys the pellets auger-style from the feed throat, through the heating chamber (barrel) towards the nozzle at the front of the barrel. When the screw turns, the non-return valve attached to the front tip of the screw is forced open by the volume augured forward, allowing the material, which is melting through a combination of conductive heat and friction from the screw, to flow through it into the nozzle area creating volume to be injected the next cycle.
As the volume in the nozzle area increases a pressure (back pressure) is formed which causes the screw to be pushed back until it reaches a sufficient volume to fill the mold during the next injection. Back pressure regulates the density of the material being prepared for injection; without it, air bubbles or other inconsistencies would be in the melt.
There should always be a little extra material in the dosage so that during the inject, excess material is available to either be packed into the mold or transfer the pressure into the mold. This is called a cushion, without it the screw bottoms out losing its ability to pack parts consistently.
When injection begins, the screw comes forward and the pressure of the plastic pushes the non-return valve closed preventing any back flowing of material into the barrel and forcing it all towards the mold.
During the fill (1st stage) our goal is consistently getting from point A (Injection start position) to point B, the transfer position between the first stage and second stage set typically where the mold is about 95% full. The transfer position between the 2 stages should always be set just before the mold is completely full. We do this so the actual fill pressure is a measurement of the resistance to push the polymer at that given viscosity but doesn’t include the spike that would be seen if the polymer deadheads against a full mold (transferring too late). We control the velocity by ensuring the fill pressure setting is high enough to carry the velocity setpoint and produce repeatable fill times. If set correctly, the fill time will be consistent every cycle, with changes in material viscosity instead seen in the monitored actual fill pressure measuring the resistance going from point A to point B.
Setting the proper transfer position is critical in this procedure, this can be accomplished by removing any post-transfer pack or hold pressure and adjusting the transfer position until a visual of a 95% full cavity is observed, at which time the pack or hold pressure can be re-applied. Plastic has a higher volume when it is in a melt state so as soon as the mold is full, and the melt comes in contact with the mold walls, it will begin to shrink.
During the pack or hold (2nd) stage, post transfer our objective is to compensate for the shrinkage that will take place. By maintaining a pack pressure after the mold is filled, we force more plastic through the gate and still molten center of the part to force the collapsing walls outward against the mold with more volume. This is highly effective in reducing sink but can also introduce complex below-the-surface molded-in stresses that sometimes create warp.
It is best to keep pack pressure applied until the gates have frozen. A simple test for this is to keep adding pack time until a leveling out of part weight is detected. Gate freeze was accomplished at the leveling point because no more volume was able to be forced in the mold, so any additional pack or hold time beyond that point is without merit. This also ensures your part weight and volume will be consistent, a large factor in dimensional control.
When the machine is running and stable, we focus on the monitor screen feedback for consistent cycle data for fill time, recovery time, fill pressure, and cushion. By taking a snapshot of these variables when a job is running well, important clues as to the root cause of a problem can be detected during future troubleshooting efforts when that data changes.
From commodity to engineered thermoplastic resins, we can help you in your specialized and high-volume end market applications, such as mobility, E&E, healthcare, packaging, and more.
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Charles Kritz | Senior Technical Service Representative
Charles has over 40 years in the plastics industry, and over 16 with Nexeo Plastics. Charles has a degree in polymer technologies and was a former instructor of plastics technology at Erie Community College in Buffalo, NY. His strengths include scientific injection molding, process validation, process troubleshooting, and quality systems management.
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