The gas remains dissolved in the melt while the melt is under pressure. As the melt is injected into the mold the pressure is reduced allowing the gas to expand the polymer. Bergen has developed a special line of CFAs for nucleation of physical blowing agent gases, such as Nitrogen, used in structural foam molding.
Co-Injection injection molding, also known as sandwich molding, allows for the injection of two dissimilar plastics. One injection unit shoots a solid plastic, the other a plastic containing a chemical foaming agent. The solid plastic injection stage begins a split second before the foamed plastic stage causing the foamed plastic to move to the inside of the solid material.
The end product displays a solid skin with a foamed core. Gas counter-pressure molding is an injection molding process where the mold is pressurized with an inert gas. This pressure pushes the injected foam melt against the mold walls forming a hard, solid skin. Screw design should build pressure across the profile of the screw, yet have relatively gentle mixing. This helps keep the gas in solution with the melt, and also prevents overworking the polymer and reducing its melt strength.
The use of screen packs is not recommended, as they cause a pressure drop and can result in premature foaming. Screen packs can also lead to foam dispersion issues as they begin to plug off. Extruder degassing or vent ports should be plugged off, as they will allow the foaming gas to escape. The real world demands exceptions to the rule for the ideal foam extruder. Excellent foams have been made on screws with barrier and mixing sections and on machines with screen packs. Foams are even made on machines with degassing zones, as in the case of foamed PVC extrusion on conical twin screws.
If screen packs are needed, coarser screens are generally better than fine screens for foam extrusion. Setting the first temperature zone after the feed throat as cool as possible reduces the chances of pre-foaming and gas escaping out the feed throat. Temperatures should peak in the zones that follow to allow for good polymer melting and complete decomposition of the selected foaming agent.
Lastly, reduced temperature at the die or nozzle of the machine allows for increased melt strength, which prevents the foam from collapsing. One of the most critical elements, yet probably one of the most commonly overlooked, is dosing.
While gravimetric feeders are preferred, higher cost makes these feeders harder for some to justify. The more common volumetric feeder can be just as precise and accurate as long as feed-rate checks are performed. It is recommended that calibration curves be generated for each volumetric feeder and for each material used on that feeder. A lot of the concepts from foam extrusion also are applied in injection molding applications.
In addition, the ideal molding press will have a shutoff nozzle to prevent drooling between shots. Gates and runners should be situated to allow for fast and uniform filling. Short flow lengths should be utilized when possible. Venting is critical to allow for foam expansion. Experience shows that vents can range from 0.
Shimming the mold is a proven method to determine vent depth and placement. Poly lactide acid or polylactide PLA is a biodegradable and biocompatible polymer produced from such renewable sources as cornstarch and sugarcane [ 1 , 2 , 3 , 4 ].
PLA foam is a competitive material among most of other thermoplastic foams due to its biocompatibility and biodegradability, PLA has been widely used in tissue engineering applications such as skin, bones, blood vessels, due to their highly porous structure as scaffolds in last [ 4 ]. The porous surface of the PLA foams enhances the biological activities of both seeded and native cells. High porosity is important for enhancing biological properties of the scaffold such as the adhesion, proliferation, and migration of the cells.
However, mechanical properties of foams decrease with the increasing of porosity. Besides, the high strength and brittle properties of PLA make it difficult to use and process it in foaming techniques. Similar to the other thermoplastics, PLA foams with uniform cell morphology are generally obtained by physical foaming agents such as carbon dioxide and nitrogen in foam injection molding and foam extrusion.
However, the poor melt strength of PLA brings difficulties in obtaining an enhanced cell morphology. Low melt strength of PLA induces cell coalescence during cell growth. Addition of chain extenders to PLA increased the rheological properties of PLA, and depending on this, cell morphology is enhanced [ 43 , 44 , 45 ]. Crystallization is an important factor in improving melt strength and foaming ability of thermoplastics. The low melt strength of PLA can be promoted by improving crystallization kinetics and the poor viscoelastic behavior of the polymer.
However, high crystallinity has negative effect on cell generation by suppressing the foam expansion. On the other hand, during foaming, cell nucleation starts around crystals [ 46 , 47 ]. Therefore, improving crystallinity can be balanced by some nucleating agents such as additives and nanofillers behave-like nucleating agents.
There are several studies on PLA nanocomposite foams that used calcite, sepiolite, and multi-walled carbon nanotube as nanofiller [ 46 , 47 , 48 , 49 ]. In these studies, nanomaterial addition was found to be nucleating agent for crystallinity and cell generation. There has been great interest to clay-reinforced PLA composite foams due to the enhanced viscoelastic behavior of clay particles in the polymer matrix which improves cell morphology [ 48 , 50 ].
As the nano clay particles increased, the cell density of the foamed samples increased. It has been reported that even a small amount addition of carbon nanotube CNT promoted the cell density due to its effect on cell nucleation [ 47 ]. This is due to the plasticization effect of the supercritical fluid phase of CO 2 [ 43 , 47 ]. Therefore, in foam extrusion and foam injection molding, the foaming agents do not only provide foaming but also disperse the particles homogenously in the matrix.
Thermoplastic foams are generally obtained by batch foaming, foam extrusion, and foam injection foaming. Batch foaming is cheaper than the others due to simple equipment, but in each method, the main aim is to promote cell morphology by providing small cell in diameter and high cell density in the polymer matrix.
The thermal properties of the polymer, its viscosity, degree of crystallinity, and melt strength are the important factors in improving the cell morphology. There are several ways to improve the cell morphology of the thermoplastics such as preparation of polymer blends, using chain extenders or using nanofillers.
Nanofiller addition is popular in last decade due to improvements in properties of the polymer foams. It is known that some nanoparticles are difficult to disperse in the polymer matrix because they tend to agglomerate seriously. However, in polymer foam processing, usage of foaming agent such as CO 2 or N 2 gases enhances the dispersion of the particles by reducing providing plasticization effect.
The homogenous distribution of the nanoparticles contributes the cell nucleation. Nanocalcite, nanomontmorillonite, nanosilicate, and carbon nanotube are the most used nanoparticles in polymer foams. Graphene-reinforced polymer foams are still under investigation. Both carbon nanotube and graphene-reinforced polymer foams have application area as thermal insulator or electrical conductive polymer foams. Nanocalcite or nanosilicate has been used for improving cell generation, increasing mechanical strength, and enhancing flame retardancy of the polymer foam.
It has been seen that small amount of nanofiller addition improved cell morphology seriously. Polypropylene and polystyrene foams are rigid foams that have wide application area in automotive and wind industries. On the other hand, polylactic acid is a promising biomaterial, and PLA foams are suitable materials for tissue engineering as scaffolds.
The high porosity of the PLA foams, as scaffolds, provides enhanced biological activities of both seeded and native cells, and they can substitute the native tissue until the native tissue heals. Licensee IntechOpen.
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Downloaded: Abstract Polymer foams have wide application area due to their light weight, resistance to impact, high thermal insulation, and damping properties. Keywords thermoplastic foams closed cell foams foam processing nanofiller cell morphology polymer foam nanocomposites. Batch foaming Batch foaming can be applied in two different methods as follows, pressure-induced method and temperature-induced method. Foam extrusion In foam extrusion, a tandem line extrusion machine is equipped with a gas supply as shown in Figure 4.
Foam injection molding Foam injection molding is similar to conventional injection molding, but an additional gas unit is integrated to the injection molding machine if physical foaming is applied Figure 5. Must be done in the previous processes such as injection molding or extrusion, etc. Composition can be changed at any time.
Nucleating agents can be introduced at anytime during processing Nucleating agents can be introduced also at anytime during processing Blowing agent supply Sample is saturated with the blowing agent until equilibrium is reached Blowing agent is metered but not more than the melt can take Blowing agent is metered but not more than the melt can absorb at a certain processing condition Tooling cost Cheaper than the others Expensive depending on the machine capacity Expensive depending on machine capacity and also mold is extra cost.
Table 1. Comparison on batch foaming, foam extrusion, and foam injection molding. Morphology of foams In batch foaming, uniform cell size and homogenous distribution of the cells can be gained.
Polypropylene-based foams Polypropylene, member of linear polyolefin group, has poor solubility of carbon dioxide and low melt strength. Polyethylene-based foams Polyethylene PE is a member of polyolefin-like polypropylene. Polystyrene-based foams Polystyrene PS is an amorphous polymer, and it has wide application area in polymer foam processing such as thermal insulation, packing material due to its low cost, ease of processing, resistance to moisture, and recyclability.
Polylactic acid—based foams Poly lactide acid or polylactide PLA is a biodegradable and biocompatible polymer produced from such renewable sources as cornstarch and sugarcane [ 1 , 2 , 3 , 4 ].
More Print chapter. How to cite and reference Link to this chapter Copy to clipboard. Cite this chapter Copy to clipboard Mihrigul Altan December 20th Available from:. Over 21, IntechOpen readers like this topic Help us write another book on this subject and reach those readers Suggest a book topic Books open for submissions. More statistics for editors and authors Login to your personal dashboard for more detailed statistics on your publications.
Access personal reporting. More About Us. Generally uniform but sometimes the cells in the core are different in size from those found at the edges.
Foaming composition is fixed from the onset. Nucleating agents can be introduced at anytime during processing. Sample is saturated with the blowing agent until equilibrium is reached. Blowing agent is metered but not more than the melt can absorb at a certain processing condition.
View all results. What makes shower and bath products foam? What is a surfactant? Different types of surfactants? Synthetic Surfactants A lot of surfactants are synthetic, however, consumers are becoming increasingly aware of the harshness of common surfactant ingredients.
Soaps Soaps are the earliest and most basic anionic surfactant. Glycerides Glyceryl esters are a group of surfactants and emollients chemically synthesized from esterification of glycerol and fatty acids mostly from vegetable oils. Lactylates Lactylates are also widely used in cosmetics or personal care products and are simply salts derivatives of fatty acids and lactic acid. Alkyl polyglucoside There is a growing focus on natural speciality surfactants like alkyl polyglucosides.
Acyl glucamides Acyl glucamides are similar to Alkyl Polyglucosides and are also derived largely from natural sources. Share this post. Previous Post. Next Post. Join the Community Want to stay up to date with all the latest news, recipes and launches straight from the Stephenson Lab?
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