The effect of fill time on the mechanical properties, surface appearance, and part dimensions of several polymers was determined. Two crystalline materials, polypropylene and nylon 6,6, and an amorphous material, acrylonitrile-butadiene-styrene (ABS), were used. In addition, the effect of the presence of glass fibers was examined using glass fiber reinforced nylon 6.6. The fill time was varied from 0.8 to 20 sec which included both the viscous flow controlled region (short fill times) where laboratory samples are ordinarily molded and the heat transfer controlled region (long fill times) where production parts are commonly molded. No large variations in tensile properties were observed for polypropylene or nylon, but a 10 percent increase in peak tensile stress and strain for ABS did indicate that molecular orientation increased with increasing fill time. However, significant differences did occur in the properties of glass reinforced nylon. Peak tensile stress increased 15 percent and flexural strength decreased 10 percent as the fill time was increased. Although no change in the flexural modulus was observed, the scatter in the modulus decreased with increasing fill time. These property variations can be attributed to differences in the glass fiber orientation of the skin and core regions of the part. The measurement of molded tensile bar dimensions indicated there was little effect of fill time on the shrinkage of the various polymers except for shrinkage in the length direction for polypropylene. The shrinkage increased from 13 to 15.4 mm/m over the fill time range, a great enough difference to affect the fit of large parts. The most dramatic change with fill time was the surface appearance of the glass reinforced nylon. The surface of samples molded at short fill times had a dark uniform color and smooth appearance while samples molded at long fill times had a lighter color and a porous surface. This surface porosity is due to crystallization prior to complete pressurization of the mold. Therefore, in addition to affecting surface appearance, other surface related properties such as aging and the ability to plate plastic parts could also be affected.

synopsisIn the design of thermal degradation processes for polymers, the energy to degrade or volatilize the materials often must be known. The heats of volatilization for six polymers were measured by differential scanning calorimetry (DSC), where the area under the degradation endotherm peak was shown to be directly proportional to the heat of volatilization. Values measured for poly(methy1 methacrylate), which yields monomer quantitatively in the temperature range investigated, agree well with theoretically predicted values. hproducibility of the method is shown by an average standard deviation of f 10% for the six polymers investigated. Caution must be used when applying data obtained by this method to thermal conditions widely differing from those employed in the DSC.

Triallyl cyanurate (TAC) and triallyl isocyanurate (TAIC) are thermosetting monomers with interesting interrelations: Calorimetry and infrared spectrophotometry were used to investigate the influence of atmosphere and initiator on the thermal events. The large extents of reaction observed during polymerization of TAC and TAIC are attributed to the formation of intramolecular rings. The higher conversions obtained with TAC are attributed to the extra length and flexibility of its allyl groups. Isomerization of poly(TAC) to poly(TAIC) is described as a three‐step process: depolymerization of poly(TAC) to form monomeric TAC, isomerization of TAC to TAIC by a Claisen rearrangement, and repolymerization of the TAIC to poly(TAIC).

The flow of polypropylene, nylon 6,6, and 33‐percent glass‐fiber‐filled nylon 6,6 into a tensile bar mold was investigated. Pressures needed to fill the cavity and runner system were measured as a function of fill time and melt temperature. The experimental results were compared to pressures predicted using the Moldflow flow‐analysis programs. Correlation between experimental and predicted pressures was good provided that accurate input data to the computer programs were used. The choice of runner diameter in the approximation of the irregular shaped runner of this tensile bar mold was found to be important, since the runner length was approximately 40 percent of the total flow length. Material properties of particular importance were thermal conductivity, viscosity, and no‐flow temperature (the temperature at which the resin will no longer flow). Viscosity/shear rate/temperature data are needed for the computer programs and two methods of obtaining the data were examined: an Instron capillary rheometer and a capillary nozzle on an injection‐molding machine. Good agreement between the two methods was found for polypropylene over a shear rate range of 100 to 10,000 s−1. Only the injection‐molding capillary nozzle could be used for the nylon‐ and glass‐filled nylon due to the thermal degradation that occurred in the Instron rheometer.

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