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processing feature | Top tips for twin-screws

Figure 6: A variable-depth melt probe can provide extremely valu- able data for process

optimisation 0 6 Minimizing die-lip build-up

almost always a gradient, generally with higher melt

temperatures some distance in be negligible, or it can be 40o

from the pipe wall. This gradient may C or even

higher. With any extrusion system making a finished product, a temperature gradient can cause

big problems, mainly from varying viscosities entering the die at different points across the flow bore. The simplest tool available for measuring the melt

temperature gradient is a variable-depth melt probe (Figure 6). These devices generally have a bad reputa- tion in the industry because by their nature they have a short lifespan – polymer can creep up inside the moveable centre probe within a few days and seizes it in one position. Even with this limitation though, it’s an extremely useful tool. If the probe only works even one day you can run a series of experiments and acquire data that’s very valuable and that can’t be obtained any other way. One immediate way that gradient data can be used is

to find out what temperature to set the melt pipe zone zones. For instance, the melt pipe zones may have been set at 180o

flow is at 225o

C to reduce the temperature C may be higher than

C but you find that the centre core of the melt C, which is a significant gradient. You’ll

generally attain far more uniform die flow by raising the pipe set-points to 220-225o gradient. Even though 225o

Figure 7: Die-lip build-up can be reduced in a number of different ways

desired, the variable probe shows that most of the flow is at that temperature anyway regardless of the pipe wall set-point. This is just another demonstration of the fact that colder melt pipes can’t be used to cool the melt, because of laminar flow and the fact that most polymers are outstanding insulators.

Die-lip build-up is a very common issue, with some materials being worse than others. With a strand die (Figure 7), such build-ups may only be a minor annoy- ance, but in film and sheet extrusion it is a major problem. Here are five ways to minimize build-up: l Keep the die exit and outer face ground smooth. If the die has baked-on layers of old polymers and pigments, this will encourage new material to adhere. Remove the die from the machine, and take it to a machine shop that has a surface grinder. Have all the outer surfaces near the material exit ground. This will remove all the old baked-on material deposits, as well as clean up nicks and gouges in the steel. After grinding it should look like a new steel surface. l Coat outer lip surfaces with mould release. Mould release is designed to prevent plastic materials from adhering to steel. It does a good job, but doesn’t last long on the surface. Operators have to get in the habit of reapplying it often. l Install a vacuum box to suction away vapours. Some materials cause a lot of smoke and solvent vapours to be released at the die exit. If these vapours aren’t taken away rapidly, they will leave a sticky residue on the die face, which is the beginning phase of die-lip build-up. The way to take smoke and vapours away quickly is to position a hood or vacuum ductwork over the die. The suction duct should be positioned as close as possible to the die exit, and should be designed with enough airflow to cause vapours to be vacuumed away quickly. l Reduce die/melt temperature. With most materials, high melt temperatures make a build-up problem worse, and cooler temperatures make it better. A cooler melt is generally less sticky and volatile, which causes less build-up. l Try various additives. Materials suppliers which sell additives sometimes have a special package to aid in preventing die lip build-up. Stearates have been known to be especially effective.

0 7

Change to a high pressure pump for more efficient barrel cooling

In the study of fluid dynamics, it’s well known that turbulent flow in a pipe will induce far greater heat transfer from the pipe wall, compared to laminar flow[3] (Figure 8). Laminar flow is present in the case of low fluid velocities, caused by low pressure delivery. In a laminar flow situation, a stable boundary layer devel- ops, which acts just like an insulator between the main fluid flow and the pipe wall. “Layers” of fluid slide over adjacent layers, without crossing over each other. What


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