Compression-molding inching

Impact Failure. Standard notched Izod impact measurements have been made over wide temperature ranges on specimens cut from compression molded %-inch thick sheets of BPA polycarbonate and two BPA carbonate-silicone block polymers (Figure 4) (see Table I for compositions and properties). In the homopolymer a ductile-brittle transition occurs at 0° to —15°C, as reported previously (4, 5). Introduction of 15 and 25% silicone lowers the transition to —45° and —110°C (block polymers A and B). As indicated in Table I, this increase in toughness at low temperature is accompanied by the reduction of modulus and yield stress. 

Injection molding requires the barrel temperature to be about 350°C with a barrel pressure in excess of 138 MPa. The mold is maintained at 110°C to ensure uniform flow and high definition, and to discourage an uneven index of refraction, birefringence. The CD is about four one-hundredths of an inch (0.5 mm) thick. For prerecorded CDs, the PC is compression-molded on a stamper imprinted with the recorder information. This takes about 4 sec. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc. Then, a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic surface and the CD is complete. This process is described later in greater detail. 

Sample Temperature Rise Owing to Irradiation. A temperature rise of 2.9°C. during irradiation was found at the center of 0.033-inch thick water-cooled polystyrene samples for a beam current of 10 fxa. This temperature was measured several times, using thermocouples compression-molded into the center of the samples. The thermocouple leads entered the sample from the direction shown in Figure 1. Several different thermocouple junction sizes were used, with the same measured results. 

Samples for irradiation crosslinking were prepared by milling in the antioxidant, followed by compression molding. Sample dimensions were 6 X 7 X 0.083 inch. Details of the milling and compression molding procedures are given in Table II. 

The additive was added gradually to the polymer fused on a two-roll mill at 170°-174°C. After addition, polymer sheets were taken off the mill and put back on the mill endwise. Several such passes were made until the sample was thoroughly mixed. The specimen was removed from the mill in thin sheets and, while hot, cut into small pieces. The polymer was compression molded at 700 p.s.i.g. and a temperature of ca. 155°C. into a 6 X 6-inch sheet of about 0.045-inch thickness. This sheet was cut into the 5 X 1/2 X 0.045-inch specimens for burning in the modified D635 test. The sample was initially evaluated with 25% additive. If the compound was effective, lower concentrations were used until the additive would not confer fire retardant activity, or until the supply of additive was exhausted. With poly (methyl methacrylate), PMMA, cast samples also were prepared. 

We have tested the following polymers polycarbonate (PC), poly-carbonate/4% polyethylene blend (PC/PE), poly (ethylene terephthal-ate) (PET), ABS, and impact modified polystyrene (HIPS). All materials except PC were compression molded into nominal Vs-inch sheets. The PC used was an Vs-inch extruded sheet heat-treated in a manner previously described (22). These PC specimens were considered to be... 

Figure 4. Temperature dependence of impact energy for Vs inch thick notched Izod specimens cut from compression molded sheets of BPA polycarbonate and two of its block polymers. A 15% silicone, B 25% silicone. Silicone DPn = 20.

Figure 7, Temperature dependence of failure stresses in Instron three-point bend tests on Vs inch notched Izod bars cut from (a) extruded polycarbonate sheet and (b) compression molded block polymer B. Crosshead rate = 0,02 inch /min. Span = 2 inches, o = net section stress = force/net cross-section at notch root, O, Craze initiation , ductile failure X, brittle failure ...

Procedure. Specimen Fabrication. The reinforcements were mixed into the polystyrene melt on a Farrell two-roll mill at 320°F. It was necessary to dry the asbestos fibers for 24 hr at 250°F prior to mixing to ensure the breakup of bundle aggregates. The milling/fluxing time was held to 8 min for all samples. The sheets obtained in milling were cut, crossplied, and compression molded in an open frame on a Wabash press. After they reached the platen temperature the material was held at 330 °F and 2000 psi for 6 min. The frame was then transferred to a cold press, and the sample was cooled under the same pressure. The test specimens were cut from ys-inch thick plates prepared in the foregoing manner. 

Stress-Strain Values Polyurethane stress-strain properties were measured with the Scott Tensile Tester operated at a jaw separation rate of 20 inches per minute. Test pieces were microdumbbells died out of 25 mil thick compression-molded sheets. The test pieces were pre-conditioned for 24 hours at 50% relative humidity and 25°C, and then tested in this environment. 

Dielectric Analysis. Samples of 0.80 0.01 mm thickness were compression molded into disks one inch in diameter. Permittivity, e and the loss factor, e", were determined at temperatures fi om -150°C to 30°C above the glass transition temperature of the samples using a TA Instruments 2970 DBA. The frequency range scanned was from 10" to 10 Hz. 

KINEL 5504 Is 65% filled with quarter-inch glass fibers, exhibits the highest mechanical properties of any KINEL (49,500 psi flexural strength and 3.25 MSI flexural m ulus at room temperature), and is generally compression molded. Current commercial end-uses include jet engine parts typical of these are the blocker doors used for retro-thrust in the Rolls-Royce RB.211 engines of the Lockheed L-1011. 

These PPS composites from fiber mats can easily be fabricated by fast compression molding similar to metal stamping operations. Three- to five-inch flows in the mold are feasible. 

These mixtures were mechanically blended for 10 seconds at room temperature and poured rapidly into a wooden mold (14" x 6" x 4") and allowed to expand freely at room temperature. After approximately 10 to 15 minutes, the resultant foam was very rigid, posessing fine cells. The residual solvent was removed from the resultant foam by placing it in a lOO C oven for 4 days. The foam had the following physical properties density, 2.82 pcf compressive strength (parallel to rise), 30.2 psi and flame test (ASTM D 1692), total length burned, 0.1 inch. 

Smooth surfaces of each polymer were also prepared (without additives) by pressing samples of the powdered polymers against a highly polished stainless steel surface in a Carver press at 16,000 p.s.i. Circular disks 1 inch in diameter and weighing several grams were formed in this way. PS and the PVeC copolymer were compressed at room temperature, PAM at 120° C.,and PMMA at 150° C. The pressure was maintained until smooth polymer surfaces were obtained. The mold and stainless steel piston were cleaned prior to use, so that contact angles could be measured on the polymer surfaces without further surface treatment. 

Equation 7.165 is comparable to Eq. 7.131 both equations are closed-form analytical solutions. However, Eq. 7.165 is more compact and easier to use. A word of caution is in order. Equation 7.165 has been experimentally verified with the screw simulator. The results of the screw simulator may not fully apply to actual melting in a single screw extruder. For instance, the screw simulator uses a molded solid polymer sample of one cubic inch. The solid bed in the extruder consists of compressed, partially sintered, polymeric particles. It is clear that an actual solid bed, as occurs in the melting zone of an extruder, may have different characteristics in terms of heat transfer properties and deformation behavior, as compared to a molded solid block of the same material. 

The early results were primitive at best. We were clamping a sample of high density polyethylene sheet about l/16th inch thick in a single cavity mold, then heating the entire assembly to just below the crystalline melt point. Finally we would remove it from the oven and blow it with compressed air. Invariably, the plastic sample would rupture. 

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