19 décembre 2015 ~ 0 Commentaire

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

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

Effective foam extrusion can look like art as much as science. An optimized foam process using inert gases is normally a materials and energy saver with no global-warming or ozone-depletion potential. Thermoplastic foam extrusion offers proved beneficial to extruders of rigid profiles, boards, and sheets, and cable, garden hose, and films. But six essential guidelines are generally overlooked. Such oversights can discourage converters from producing a chance of it.

At gas options are carbon dioxide and/or nitrogen, introduced via chemical foaming agents or direct gas injection, sometimes used in combination. These gases are usually preferred for moderate- and high-density foams because they’re noncorrosive, nontoxic, and non-flammable. Cost savings may result from plastic extrusion machine actually the modest density reductions that these gases promote. Included in this are significant strength reductions practical from optimizing your extrusion process for foam.

But if you expect to casually drop in magic dirt and end up with products which are lighter, faster, stronger, and all better value, you may come away disappointed and wondering what all of the talk is about. For those who have failed and attempted to foam extruded thermoplastics, here are the key principles that you might not have recognized and that many encourage you to try again. In this posting, the target is on chemical foaming of high-density sheet.

Not absolutely all CFAs meet all criteria for all polymers. The decomposition temperatures of the CFA must be close to the processing temperature of the polymer. The gas should be liberated within a particular, narrow temperature range and really should be of adequately excessive volume fairly, controllable by pressure and temperature. The gas will need to have time to disperse in the polymer completely. An extruder with 24:1 L/D is the minimum length to allow this.

The gases released during decomposition of the CFA will need to have a low fee of diffusion in the polymer, so they remain in the plastic material mass long enough for this to nice and harden. No less important is without question a carrier resin for the CFA masterbatch that is miscible in the picked resin. In general, a processing heat about 25-35° F above the start of the CFA decomposition will ensure complete activation and comprehensive irregular cell structures, without agglomerates and dispersion. Compromised cell structures can badly thermoform, leading to weak corners and limiting depth of pull.

It is commonly misunderstood that if 1% of a given CFA achieves 15% density decrease, that 2% of the same product should result in 30% reduction, but this is not usually the case. In fact, the considerably more you add, the farther you might find yourself from your own goals, because the density upward creeps. You can quickly reach a point where bubble development is uncontrollable since it exits the die, resulting in die swell and a account that fails to retain its intended condition.

Alternatively, the gas bubbles may grow excessively inside the melt, creating open cells where closed cells happen to be desired. The gas bubbles will coalesce and collapse, resulting in higher density than anticipated or desired. Ultimately, the portion won’t seem or perform since it was made to do. It’s best to determine an even of CFA which can be fed uniformly and achieve controlled performance together with your equipment.

When extruding with foaming agents underneath adequate internal barrel pressure, you shall decrease the glass-changeover temperature of the polymer. It is hard to convey this simple fact to the uninitiated, however the gasses released by a CFA become plasticizers, reducing the viscosity of the polymer. For example, a non-foamed PE sheet may be extruded at 400 F over the profile. However, in a foam procedure, that same polymer may have a profile of 340, 360, 380, 400, 380 F.

Take note of the “bell-shaped profile.

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