Plastics Technology

JUL 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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In the last few months, I have seen a number of problems related to barrel capacity. I touched on this topic in a column that appeared in the August 2011 issue, but it is time to review the subject and hone in on how much barrel capacity you should utilize. In that 2011 column, I stated that your process should take somewhere between 25% and 75% of the barrel or shot capacity. Today, the maximum I recommend is 65%. This may seem a bit odd. Consider a barrel with 230 mm (10 in.) of stroke; why not use it all? What is wrong with using 80% or more (~180 mm or 8 in.)? If you do, you are inviting Murphy to the party, and he will bring along his friends. The reason has to do with how the screw works and the type of resin you are processing—amorphous or semi-crystalline. First, let's review the basic components and their function for a 20:1 L/D general- purpose (GP) screw. Typically screws have three sections (see Fig. 1): 1. Feed section, which comprises normally 50% of the flight length (10 flights). The purpose of this section is to feed the material to the transition zone, not melt it. It starts the melting process by com- pressing the granules together, which pushes the air that enters the deep flights back out the feedthroat. Also, it transfers some energy (heat) to the granules, prepping them for the transition zone. Two points to remember here: First, the feed section does not melt the plastic granules; second, the air that enters with the granules needs to be vented out the feedthroat. The feedthroat must be warm enough to allow the air and volatiles coming off the resin to reach the atmosphere. If it's too cold you will condense some of these volatiles inside the barrel. But if you have this section of the barrel too hot, you can develop a melt film between the flights and the barrel wall, which traps the air/volatiles and prevents them from venting, which causes splay or bubbles in your parts. Are You Using More Than 65% Of Your Barrel Capacity? So run the feed throat warm, not cold. You can also ask your most grizzled operator—I call him Old Crusty Senior—how many times he has seen the hopper blown to the ceiling to get an idea of the amount of pressure that can build up. Do not stick your face in that hopper without eye protection. 2. Transition section (also known as the compression section), which normally accounts for 25% of the flight length (five flights). The purpose of this section is to uniformly melt the plastic granules. Your shot has to be uniformly melted in these five flights. In this section, the root diameter expands, tapering to the root diameter of the metering zone. Thus, the flight depth decreases, compressing the plastic up against the barrel wall under significant pressure, which, in combi- nation with the friction of the pellets on the barrel wall, mechani- cally drives enough energy into the plastic granules to melt them. Generally 70% to 90% of the energy necessary to melt the plastic comes from this compressing, friction, and screw-rotation combi- nation. The flights of the screw shave the melt film on the barrel wall and push it forward to the final section of the screw. 3. Metering section, which generally makes up 25% of the flight length (five flights). The purpose of the metering section is to pump plastic forward through the check valve, developing the pressure to counteract the backpressure to push the screw backwards to develop the shot size. The metering section is not designed to melt plastic granules. It acts as pump to develop shot size. If you are, you are inviting Murphy—as in trouble—to the party. Get more insights on Injection Molding from our expert authors: short.ptonline.com/moldingKH Learn more at PTonline.com KNOW HOW INJECTION MOLDING FIG 1 Basic Components of a Screw Diameter Conventional screws have three sections: Feed, transition, and metering. Feed (50%) Transition (25%) Metering (25%) Spline Flight Land Flight Depth Flight Length Pitch 20 JULY 2017 Plastics Technology PTonline.com K now How INJECTION MOLDING By John Bozzelli

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