Hot-molded foams

The molded foam process can be classified into two types—the hot-molded foam process and the cold-molded foam process. The two processes are classified by the molding temperature and oven temperature. 

Hot-Molded Foam and Cold-Molded Foam. Hot-molded foams are jH oduced by using either conventional polyeflier polyols (3,000 MW,... 

In comparison with cold-molded foams, hot-molded foams have disadvantages of higher mold temperatures and longer mold-retention times, but have the following advantages higher ratio of load-bear-ing/density, lower cost, and easier molding of complicated products. An example of a formulation of hot-molded foams is shown in Table 19. 

Table 19 Formulation for Hot Molded Foam (4 inch thick)...

Uses Wide processing-latitude surfactant for use in flame-retarded foam systems such as PU slabstock and hot molded foam Properties Gardner 2-4 cl. to si. hazy liq. sol. in polyol, fluorocarbon, water-amine streams sp.gr. 1.03 vise. 1000 cs flash pt. (CC) 60.6 C. 

FOAMED CROSSLINKED LDPE/PP BLENDS MADE BY HOT MOLD INJECTION MOLDING... 

The mechanical characteristics of the epoxy and polyester cold-cured syntactic molded foams do not differ much from the hot-cured materials (Table 12)1),... 

In recent years, HR foams have been prepared by using almost the same molding conditions as cold-molded foams, i.e., low mold temperature using the MDI-TDI blend formulation. However, in the development stage of HR foam, only TDI-based formulations were used, which required hot-mold conditions. For these reasons HR foams are not exactly equal to cold-cure foams. However, in recent years, HR foams have been produced by blends of MDI and TDI. The resulting foam systems can be molded at the same molding conditions as those for cold-molded foam systems. 

The blowing agent for microcellular elastomers is water. The amount of water should be accurate, and its accuracy can be obtained by a water-containing solution, such as liquid sodium sulfonate of vegetable oils containing a small amount of water. The catalysts to be used are those used in urethane foams, e.g., tertiary amines, and tin catalysts. The above ingredients are mixed and poured into a hot mold and cured in a defined period of time. After demolding, a post cure is applied to complete the polymer-formation reactions. 

It was necessary to prepare IPN foams using a hot mold method. One component contained Nlax 31-28, DC-193 and three different surfactants (L-540, L-5303 and L-5614) from Union Carbide, T-12, 2,4,6-trls(dlmethylamlnomethyl) phenol (DMP-30, Rohm Haas), BCla-amlne complex (XU-213, ciba-Gelgy), Freon llA, and water. The other Included Isonate-143L, DER-330, and Freon llA. The two components were mixed and poured Into a hot mold and cured at 9O C for 2-3 hours. The foams were post-cured at 90°C for 16 hours, and conditioned at 25 C and 50X relative humidity for three days. 

For instance, Soykeabkaew et al. (2004) prepared cellulose liber-reinforced starch-based composite foams (SCFs) by baking process. SCFs were prepared successfully by baking starch-based batters incorporating either jute or flax fibers inside a hot mold. Starch is an alternative material for making foams. Batters of starch and water can readily be baked in a closed, heated mold where the starch granules gelatinize and the evaporation of water causes the starch to foam out and take up the shape of the mold. Foams made from pure starch have major drawbacks on their brittleness and sensitivity to moisture and water. Since both the fibers and the starch matrix were naturally polar and hydrophilic, strong interaction between them was expected. 

According to data supplied by plastic manufacturers, the mold temperature for low-density PUR foams (MDI basis) should be between 40 and 50 °C 1.5 °C. The so-called hot cure foams (TDI basis) require considerable addition of heat depending on the PUR system used, temperatures from 140 to 180 °C are needed. Two methods are being used successfully ... 

As a direct consequence of equipment requirements such as venting or automatic opening and closing of the mold, or formulation aspects such as faster reacting times, molded flexible PU foam production is quite more complex than that of slabstock foams and considerably more expensive. Depending on the type of curing, two major mold-foaming processes are industrially available the hot-cure and the cold-cure. 

Kraft fiber or kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high-quality paper where strength, whiteness, and resistance to yellowing are important. The kraft process is an industrial method amply used to extract cellulosic pulp from wood. Kaisangsri et al. (2012) mixed kraft fiber with cassava starch solution, later chitosan solution was added, and a hot mold baking was used to form the cassava starch-based foam by using an oven machine with controlled temperature. 

In addition, the breaking bubble from free flow surface will be easily ironed or repaired by hot mold surface. On the other hand, hot mold may not work for the smooth surface of crystalline material foam. This is because more crystallization on the hot mold surface will create high nonuniformity of cell structure, and then, rough surface, which was also verified in the study by Saeed, etc., [5]. A balance between percentage of crystallinity and small cell structure with warm mold may result in better surface quality of crystal material microcellular foam. 

This closeness of 0 to zero explains the existence of a gas-oversaturated solution area in the polymer melt, when P < Pg, but the entire volume of gas remains in the solution. The degree of oversaturation, particularly upon free foaming (not in flow) can be 2- to 3-fold. In real polymer compositions, there are always solid admixtures, which have poor wetting areas. This reduces the degree of oversaturation at the interface melt-molding tool. Moreover, bubble nuclei can result from fragmentation of gas bubbles in the polymer [16]. Another factor that promotes the formation of bubble nuclei is the presence of localized hot points in the polymer melt they act as nuc-leation centres. Hot points appear either after a chemical reaction in the melt polymer [17], or in overheated areas on the surface of metal equipment [18]. Density of nucleation can be improved via introduction of various agents that reduce tension of the polymer [19]. 

Process particulate pickup may significantly increase the risk of boiler foaming. This kind of problem happens in the crossover of hot to cold water during manufacturing processes, as in rubber tire factories, food can cookers and retorts, plastic injection molding, and the like). 

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