Molded polystyrene

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Foamed plastics (qv) were developed in Europe and the United States in the mid-to-late 1930s. In the mid-1940s, extmded foamed polystyrene (XEPS) was produced commercially, foUowed by polyurethanes and expanded (molded) polystyrene (EPS) which were manufactured from beads (1,2). In response to the requirement for more fire-resistant ceUular plastics, polyisocyanurate foams and modified urethanes containing additives were developed in the late 1960s urea—formaldehyde, phenoHc, and other foams were also used in Europe at this time. 

The most important use of polystyrene is in packaging. Molded polystyrene is used in items such as automobile interior parts, furniture, and home appliances. Packaging uses plus specialized food uses such as containers for carryout food are growth areas. Expanded polystyrene foams, which are produced by polymerizing styrene with a volatile solvent such as pentane, have low densities. They are used extensively in insulation and flotation (lifejackets). 

The inner part of typical fuze consisted of two sections, as shown in Fig E7 a) The upper section, called the switch block, was molded polystyrene which has been machined to take various plunger contacts, the trembler switches, and in some cases long delay igniter bridge... 

G. D. Gilmore and R. S. Spencer, Role of Pressure, Temperature and Time in the Injection Molding Process, Mod. Plast., 27, 143 (April 1950) also, R. S. Spencer and G. D. Gilmore, Residual Strains in Injection Molded Polystyrene, Mod. Plast., 28, 97 (December 1950). 

Figure 10.2 Deformation under various loads for 48kg/m3 (lb/ft3) molded polystyrene foams [31]. A, 224 kPa B, 193kPa C, 103kPa D, 38kPa. To convert kPa to psi, multiply by 0.145. Reproduced from W.B. Brown, PlastProg., I960, 1959, 149...

Table 10.4 Minimum load to cause long-term creep in molded polystyrene foam"...

We conclude from findings on moisture absorption and freeze-thaw resistance of various insulations, and the effect of moisture on thermal performance, that in protected-membrane roofing applications the order of resisting moisture pickup is extruded polystyrene > polyurethane > molded polystyrene [57]. Water absorption values for insulation in use for 5 years were 0.2vol% for extruded polystyrene, 5vol% for polyurethane without skins, and 8-30 vol%... 

Fig. 9 Residual stress distributions in injection molded polystyrene bars in the as-molded state and after 14 weeks of ultraviolet (UV) exposure in the laboratory. The bars were approximately 3 mm thick and the distributions are shown as a function of depth from the (exposed) surface. (See Ref. )...

Figure 18.3. Orientation function vs. volume loading of talc in compression molded polystyrene. [Data from Kim K J, White J L. J. of Non-Newtonian Fluid Mechanics, 66, Nos.2/3, 1996, 257-70.]...

Injection mold machines are rated by their ability to mold polystyrene in a single shot [3], Mold pressures can range from 55 to 275 MPa and cycles as low as 15 s. It is possible to injection mold a thermoset resin, although it is very difficult. Compression molding or transfer molding is used in these cases ... 

Table II. Effect of Polystyrene Block Size on Mechanical Properties of Compression Molded Polystyrene-Polydimethylsiloxane Block Copolymers Containing 30% Polystyrene...

Fig. 13 Representative TSC curve of a lOOpm-thick compression molded polystyrene film. Heating rate was 200 K/h...

Acids are shipped in containers containing six 5-pint bottles surrounded by molded polystyrene. This container can be sawed into six pieces to make six quite suitable "Dewar flasks". Acetone is not used as the liquid, because acetone dissolves polystyrene. Isopropanol and dry ice work quite well in these "Dewars". 

Injection-molding machines are rated by their capacity to mold polystyrene in a single shot. Thus a 2-oz machine can melt and push 2 oz of general-purpose polystyrene into a mold in one shot. This capacity is determined by a number of factors such as plunger diameter, plunger travel, and heating capacity. 

Crystal polystyrene is a misnomer because the polymer is an amorphous material. The term "crystal" refers to the high optical clarity of the molded polystyrene homopolymer. It is one of the top five commodity polymer resins in the world. Because most chemical modification methods are expensive, they are impractical for low scale production. For low volume production, polymer blending is the most effective way to modify polystyrene. 

Expandable polystyrene is the usual name for the beads used to make molded polystyrene foam parts. Parts made with these beads are usually made in a two-step process. The first step consists of pre-expansion of the beads by heat. After this step, the beads are stored for 6-12 h to allow them to reach equilibrium. The beads are then conveyed to the mold, where they expand to the final dimensions. Steam is the preferred mode of heating, either by introducing live steam through perforations in the mold or by the means of steam probes, which are withdrawn as the beads are expanding. Expandable polystyrene foam can be distinguished from expanded polystyrene foam by the round bead outlines on the surface of the part. The expanded foam has a smooth surface. 

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