Thermoplastic foam extrusion has proved beneficial to processors. But six essential rules are commonly overlooked.
Effective foam extrusion can look like art just as much as science. An optimized foam method using inert gases is a energy and materials saver with no global-warming or ozone-depletion probable. Thermoplastic foam extrusion offers proved beneficial to extruders of rigid profiles, boards, and sheets, together with cable, hose, and movies. But six essential rules are commonly 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 generally preferred for medium- and high-density foams because they are noncorrosive, nontoxic, and nonflammable. Cost savings may result from possibly the modest density reductions that these gases promote. Among them are significant plastic sheet manufacturer strength reductions conceivable from optimizing your extrusion procedure for foam.
But if you be prepared to casually drop in magic dust particles and end up with products which are lighter, more quickly, stronger, and all at a lower cost, you may come aside disappointed and wondering what all of the talk is about. Assuming you have failed and tried to foam extruded thermoplastics, here are the key principles that you might not have recognized and that lots of encourage you to make an effort again. In this posting, the focus is on chemical foaming of high-density sheet.
Not absolutely all CFAs meet almost all criteria for most polymers. The decomposition temp of the CFA must be close to the processing temperature of the polymer. The gas should be liberated within a specific, fairly narrow temperature assortment and should end up being of adequately substantial volume, controllable by pressure and temperature. The gas must have time and energy to disperse in the polymer thoroughly. An extruder with 24:1 L/D is the minimum length to permit this.
The gases released during decomposition of the CFA will need to have a low charge of diffusion in the polymer, so they remain in the plastic material mass long plenty of for this to amazing and harden. Believe it or not important is normally a carrier resin for the CFA masterbatch that's miscible in the selected resin. Generally, a processing heat range about 25-35 F above the start of the CFA decomposition will ensure complete activation and complete without agglomerates, dispersion and irregular cell structures. Compromised cell structures can poorly thermoform, leading to weak corners and limiting depth of attract.
It really is commonly misunderstood that if 1% of confirmed CFA achieves 15% density lowering, that 2% of the same product should result in 30% reduction, but this is not usually the case. In fact, the more you add, the farther you might find yourself from your goals, because the density upward creeps. You can quickly reach a spot where bubble growth is uncontrollable as it exits the die, leading to die swell and a profile that does not retain its intended form.
Alternatively, the gas bubbles may grow excessively within the melt, creating open cells where closed cells are desired. The gas bubbles will then coalesce and collapse, resulting in larger density than expected or desired. Ultimately, the component won't appear or perform as it was designed to do. It's best to determine a level of CFA which can be fed uniformly and gain controlled performance with your equipment.
When extruding with foaming agents within adequate internal barrel pressure, you shall reduce 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 act as plasticizers, decreasing the viscosity of the polymer. For example, a non-foamed PE sheet could be extruded at 400 F across the profile. Nevertheless, in a foam method, that same polymer may have a profile of 340, 360, 380, 400, 380 F.
Observe the "bell-shaped profile. ?By feeding into a cool area, you prevent premature foaming that would result in gas escaping out the feed throat. At area 4, you have activated the CFA completely; and by the die exit, you will be forming your sheet while already starting the cooling process. The melt temperatures at the exit ought to be optimized to allow the growth without distorting the shape.
The screw may be the primary method of pumping and transforming the resin and CFA into a homogeneous melt. Conventional screws are divided into three distinct working segments: Feed section, transition section, and metering section.
Barrier screws are occasionally employed to prevent material from flowing back again to the feed section, however they will continue to work against your foam method. The barrier will act as a decompression area causing gas damage or early cell technology, resulting in poor density reduction or ugly blowouts on the sheet area. Close the extruder vents for the same rationale.
Or at least use a coarse screen. Display screen packs are usually useful to build pressure and/or stop agglomerates but aren't usually wanted in a foam method. A screen pack that is too good can rupture cells and disrupt bubble composition. The display screen can totally be removed, but a mesh of 20/40/20 is normally acceptable if a display must be used.
As resin is conveyed along the screw, the die design becomes the crucial aspect in generating high-top quality foam. For sheet, a slit die can be used, with a "coat-hanger commonly?manifold. In a typical, non-foaming extrusion method, the die's primary function is to condition the extrudate after plastication. However in a foam procedure, the die's position shifts to preventing expansion of the foam before it exits, only guiding a general form to be defined through a calibrator.
Foam die angles and land lengths are optimized to reduce pressure drops that could cause early foaming. A foam die exit is normally smaller compared to the finished part dimension, allowing it to grow upon exiting the die outward. If the target is to achieve a 20% density reduction, the die ought to be 20% smaller compared to the intended final shape. A brief die land will allow an individual rapid expansion as the sheet enters the cooling period. Adequate cooling after expansion shall minimize gas diffusion and solidify the sheet in its intended dimensions.