Six Rules to Keep You Out of Trouble In Foam Sheet Extrusion

Thermoplastic foam extrusion has proved advantageous to processors. But six essential guidelines are overlooked commonly.

Effective foam extrusion can seem like art around science. An optimized foam procedure using inert gases is a energy and supplies saver without global-warming or ozone-depletion potential. Thermoplastic foam extrusion offers proved advantageous to extruders of rigid profiles, boards, and sheets, in addition to cable, garden hose, and films. But six essential rules are generally overlooked. Such oversights can discourage converters from producing a chance of it.

Among the many gas options are skin tightening and and/or nitrogen, introduced via chemical foaming agents or direct gas injection, used in combination sometimes. These gases are generally preferred for moderate- and high-density foams because they're noncorrosive, nontoxic, and nonflammable. Cost savings may derive from also the modest density reductions that these gases promote. Included in this are significant plastic compounding machines strength reductions possible from optimizing your extrusion process for foam.

But if you expect to casually drop in magic dirt and end up with products that are lighter, more rapidly, stronger, and all better value, you might come away disappointed and wondering what all of the talk is about. In case you have failed and tried to foam extruded thermoplastics, here are the main element principles that you may not have been aware of and that many encourage you to make an effort again. On this page, the concentration is on chemical foaming of high-density sheet.

Not all CFAs meet all criteria for all polymers. The decomposition heat range of the CFA must be close to the processing temperature of the polymer. The gas must be liberated within a specific, fairly narrow temperature collection and should end up being of adequately big volume, controllable by heat and pressure. The gas must have time to disperse in the polymer carefully. An extruder with 24:1 L/D may be the minimum length to allow this.

The gases released during decomposition of the CFA must have a low level of diffusion in the polymer, so that they remain in the plastic mass long enough for it to great and harden. No less important is certainly a carrier resin for the CFA masterbatch that is miscible in the chosen resin. Generally, a processing heat about 25-35° F above the beginning of the CFA decomposition will be sure complete activation and detailed dispersion, irregular cell structures and without agglomerates. Compromised cell structures can thermoform poorly, leading to weak corners and limiting depth of draw.

It is commonly misunderstood that if 1% of confirmed CFA achieves 15% density reduction, that 2% of the same product should bring about 30% reduction, but this is not usually the case. In fact, the more you add, the farther you will probably find yourself from your own goals, because the density creeps upward. It is possible to quickly reach a point where bubble progress is uncontrollable as it exits the die, resulting in die swell and a profile that fails to retain its intended form.

Alternatively, the gas bubbles may grow within the melt excessively, creating open cells where closed cells are desired. The gas bubbles will coalesce and collapse, resulting in larger density than desired or expected. Ultimately, the portion won’t look or perform since it was made to do. It’s better to determine a level of CFA which can be fed uniformly and achieve controlled performance together with your equipment.

When extruding with foaming agents below adequate internal barrel pressure, you will reduce the glass-changeover temperature of the polymer. It is often hard to mention this simple fact to the uninitiated, but the gasses released by way of a CFA become plasticizers, reducing the viscosity of the polymer. For instance, a non-foamed PE sheet might be extruded at 400 F over the profile. Even so, in a foam procedure, that same polymer may have a profile of 340, 360, 380, 400, 380 F.

Observe the “bell-shaped account.â€?By feeding into a cool area, you prevent premature foaming that could result in gas escaping out the feed throat. At zone 4, you have totally activated the CFA; and by the die exit, you are forming your sheet while beginning the cooling process currently. The melt heat range at the exit should be optimized to permit the growth without distorting the form.

The screw may be the primary method of pumping and transforming the CFA and resin into a homogeneous melt. Conventional screws are split into three distinct functioning segments: Feed section, transition section, and metering section.

Barrier screws are sometimes employed to prevent materials from flowing back again to the feed section, however they shall function against your foam course. The barrier shall become a decompression zone causing gas reduction or early cell technology, resulting in poor density decrease or ugly blowouts on the sheet surface. Close the extruder vents for the same cause.

Or at least use a coarse screen. Display screen packs are usually utilized to build pressure and/or prevent agglomerates but are not usually needed in a foam method. A screen pack that's too great can rupture cells and disrupt bubble framework. The display could be removed entirely, but a mesh of 20/40/20 is normally acceptable if a display screen must be used.

As resin is conveyed across the screw, the die style becomes the crucial aspect in generating high-top quality foam. For sheet, a slit die is used, with a “coat-hanger†commonly?manifold. In a typical, non-foaming extrusion procedure, the die’s primary function is to form the extrudate after plastication. However in a foam process, the die’s function shifts to preventing growth of the foam before it exits, only guiding a general condition to be defined through a calibrator.

Foam die angles and territory lengths are optimized to reduce pressure drops that could cause early foaming. A foam die exit is without question smaller than the finished portion dimension typically, and can grow after exiting the die outward. If the target is to accomplish a 20% density reduction, the die should be 20% smaller than the intended final form. A brief die land shall allow a single rapid expansion because the sheet enters the cooling phase. Adequate cooling after growth will lessen gas diffusion and solidify the sheet in its intended dimensions.

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