Thermoplastic foam extrusion has proved beneficial to processors. But six essential rules are commonly overlooked.
Powerful foam extrusion can seem like art as much as science. An optimized foam method using inert gases is a energy and materials saver with no global-warming or ozone-depletion potential. Thermoplastic foam extrusion provides proved advantageous to extruders of rigid profiles, boards, and sheets, together with cable, hose, and films. But six essential guidelines are generally overlooked. Such oversights can discourage converters from producing a go of it.
Among the many 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 medium- and high-density foams because they're noncorrosive, nontoxic, and nonflammable. Cost savings may foam extrusion manufacturer result from the modest density reductions these gases promote even. Among them are significant strength reductions conceivable from optimizing your extrusion method for foam.
But if you be prepared to casually drop in magic dirt and end up with products that are lighter, quicker, stronger, and all at a lower cost, you may come away disappointed and wondering what all the talk is about. If you have attempted and failed to foam extruded thermoplastics, here are the key principles that you might not have been aware of and that lots of encourage you to try again. On this page, the emphasis is on chemical foaming of high-density sheet.
Not all CFAs meet almost all criteria for most polymers. The decomposition heat of the CFA must be close to the processing temperature of the polymer. The gas should be liberated within a specific, narrow temperature range and really should be of adequately great volume fairly, controllable by pressure and temperature. The gas will need to have time to disperse in the polymer extensively. 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 amount of diffusion in the polymer, so they remain in the plastic material mass long enough 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 determined resin. Generally, a processing temperatures about 25-35 F above the start of the CFA decomposition will ensure complete activation and detailed without agglomerates, irregular cell structures and dispersion. Compromised cell structures can badly thermoform, leading to weak corners and limiting depth of attract.
It is commonly misunderstood that if 1% of confirmed CFA achieves 15% density reduction, that 2% of the same product should result in 30% reduction, but this is not the case usually. In fact, the even more you add, the farther you will probably find yourself from your own goals, because the density upward creeps. You can quickly reach a point where bubble growth is uncontrollable since it exits the die, resulting in die swell and a profile that does not retain its intended condition.
Alternatively, the gas bubbles may grow inside the melt excessively, creating open cells where closed cells are desired. The gas bubbles will coalesce and collapse, resulting in larger density than expected or desired. Ultimately, the portion won't seem or perform since it was designed to do. It's best to determine a level of CFA which can be fed uniformly and accomplish controlled performance with your equipment.
When extruding with foaming agents beneath adequate internal barrel pressure, you will certainly reduce the glass-transition temperature of the polymer. It is hard to convey this simple fact to the uninitiated typically, but the gasses released by way of a CFA become plasticizers, decreasing the viscosity of the polymer. For instance, a non-foamed PE sheet might be extruded at 400 F across the profile. However, in a foam method, that same polymer might have a profile of 340, 360, 380, 400, 380 F.
Observe the "bell-shaped profile. ?By feeding into a cool zone, you prevent premature foaming that would result in gas escaping away the feed throat. At area 4, you have totally activated the CFA; and by the die exit, you will be forming your sheet while currently starting the cooling method. The melt temperatures at the exit ought to be optimized to permit the expansion without distorting the shape.
The screw is the primary method of pumping and transforming the CFA and resin right into a homogeneous melt. Conventional screws are divided into three distinct doing work segments: Feed section, transition section, and metering section.
Barrier screws are occasionally employed to prevent materials from flowing back to the feed section, however they shall operate against your foam course of action. The barrier shall act as a decompression zone causing gas damage or early cell generation, resulting in poor density reduction or ugly blowouts on the sheet surface area. Close the extruder vents for the same explanation.
Or at least use a coarse screen. Display packs are usually useful to build pressure and/or prevent agglomerates but are not usually wanted in a foam method. A screen pack that is too excellent can rupture cells and disrupt bubble composition. The display could be removed entirely, but a mesh of 20/40/20 is acceptable if a screen can be used usually.
As resin is conveyed along the screw, the die style becomes the crucial aspect in generating high-quality foam. For sheet, a slit die can be used, with a "coat-hanger commonly?manifold. In a typical, non-foaming extrusion process, the die's primary function is to shape the extrudate after plastication. However in a foam procedure, the die's position shifts to preventing expansion of the foam before it exits, just guiding a general condition to be defined through a calibrator.
Foam die angles and terrain lengths are optimized to minimize pressure drops that could cause early foaming. A foam die exit is smaller compared to the finished component dimension typically, allowing it to grow upon exiting the die outward. If the goal is to achieve a 20% density reduction, the die should 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 stage. Adequate cooling after expansion shall minimize gas diffusion and solidify the sheet in its intended dimensions.