Part Thicknesses

Highest thermal performance with PPS compounds requires that parts be molded under conditions leading to a high level of crystallinity. Glass-filled PPS compounds can be molded so that crystalline or amorphous parts are obtained. Mold temperature influences the crystallinity of PPS parts. Mold temperatures below approximately 93°C produce parts with low crystallinity and those above approximately 135°C produce highly crystalline parts. Mold temperatures between 93 and 135°C yield parts with an intermediate level of crystallinity. Part thickness may also influence the level of crystallinity. Thinner parts are more responsive to mold temperature. Thicker parts may have skin-core effects. When thick parts are molded in a cold mold the skin may not develop much crystallinity. The interior of the part, which remains hot for a longer period of time, may develop higher levels of crystallinity. 

Calculator An interactive process wizard that provides quick, effective material, design, processing, and cost solutions. This Engineering Calculator s capabilities include (1) Material, to select from a variety of GE Plastics materials (2) Design, which calculates minimum part thickness based upon allowable deflection (3) Processing, which calculates pressure to fill and clamp force and (4) Cost, which calculates estimated material and processing costs for the intended part. 

Mold temperature and cycle time vary with part thickness, part configuration, and hardness and the temperature is kept normally in the range of 40°C-60°C. Proper mold temperature wdl ensure... 

Another important variable to consider is the fiber orientation. This is affected by many variables such as the injection molding conditions, fiber length, resin viscosity and part thickness. The fiber orientation can be determined experimentally by optical methods [44], or it can be estimated from the modulus of the molded part as follows [45-47] ... 

High part thickness decreases oxygen diffusion in the core of the polymer and reduces degradation, as can be seen in Figure 3.42. 

K nitrate (to aid ignition) 1.25% desensitizer added methylcellulose 0.20 parts. Thickness of sheet controls ignition time]... 

Most thermosetting materials are polymerized in heated molds. Figure 9.4 shows a schematic diagram of the mold L is the part thickness, which is assumed to be much less than the other two dimensions. Therefore, the system may be modeled as a case of unidimensional heat transfer with simultaneous heat generation. 

Figure 9.4 Schematic diagram of the heated mold (Tw = wall temperature, To = initial temperature, L = part thickness). 

Differential Eqs (9.10) and (9.11), with initial and boundary conditions (9.12) and (9.13), may be numerically solved for different sets of values of the four dimensionless parameters, W3-W4 (Williams et al., 1985). To illustrate the evolution of temperature and conversion profiles during the cure, values of W2-W4 will be kept constant and Wi will be varied to simulate the influence of the part thickness. The particular case of W2 = 40, W3 = 1.5, and W4 =0.125 will be analyzed. This represents a process characterized by high values of both the activation energy and the adiabatic temperature rise. 

Decreasing the part thickness to obtain W3 =10 produces a dramatic change in the way in which the part cures, as observed in Fig. 9.8. The material located at the core cures first, generating conversion and temperature fronts advancing at a very fast rate to the wall, which acts as a heat sink (notice the short period of time in which most of the material is cured). The maximum temperature is observed at an intermediate position and is lower than m the previous case. Tq -l- A l ad Tj ax Tw + ATad. 

Crystallization and shrinkage were influenced by cooling rate (part thickness and mold temperature). While the oil-heated mold maximized crystallization, cooler (water-heatable) molds produced crystallinity levels of 25% or better (Chapter 1). Parts molded with the high mold temperature did exhibit better surface finishes. Shrinkage was relatively low for all processing conditions and design variables. 

Three general types of molds are used for CM. In the positive mold (Figure 14.3a) all the material is trapped in the mold cavity. The pressure applied compresses the material into the smallest possible volume. Any variation in the weight of the charge will result in a variation in part thickness. In multicavity molds, if one cavity has more material than the others, it will receive proportionately greater pressure. Multiple cavities, therefore, can result in density variations between parts if loading is not done with some degree of precision control.1 278 284... 

OPAC 1 TY PART THICKNESS OPAQUE [] TRANSLUCENT M TRANSPARENT [] (MILLS) EXPOSURE INDOOR HEAT STABILITY HIGH (ABOVE 500)F [] MODERATE (400-475 [] LOW (380-400) ... 

Comparison of entries 5 and 6 for neat SRT-300 in Table I highlights the dependence of anisotropy on part thickness. In both cases the volumetric CTE s are nearly identical, but the thinner (1/8 ) tensile bar has a much higher degree of anisotropy. This result is probably due to a greater core thickness in the 1/4 HDT bar, this core possibly having a 90 degree orientation relative to the flow direction. Thus, the flow direction CTE for the 1/4 HDT bar is very similar to that of the width direction. 

After filling, the charge remains in the hot mold for the cross-linking reaction to be completed and solidify. The curing time depends on several factors such as resin-initiator-inhibitor reactivity, part thickness, and mold temperature. More about curing is explained in the cure section. 

The solidification of the polymer melt in rotational molding is relatively slow, in comparison to other processes, and is estimated to be in the range of 10-30°C/min. Moreover, the melt solidification is gradual and nonuniform across the molded part thickness, leading to important variations in the morphological features, as illustrated in Fig. 9, and dictating the properties and overall performance of the final product. The effects are more dramatic for resins with slower crystallization rates, such as polypropylene, compared to that observed with polyethylene. 

Fig. 6 Seasonal cycles of SST in the Large Sea averaged over the period 1982-2000 (a) in the western part, (b) in the eastern part. Thick (thin) solid lines correspond to averaging over 1994— 2000 (1982-1993) dashed lines correspond to seasonal cycles of SST in the conventionally natural period. This figure is reproduced from [24]...

Molded samples of LCP s often form a "skin-core" structure. The phenomenon is depicted by a significant dependence of the anisotropic properties of molded parts on part thickness ( 5.). Scanning electron photomicrographs of the cross-section of a molded part reveal a highly oriented "skin" layer surrounding a less ordered inner "core." Apparently, the fraction of disordered core material diminishes as the sample thickness decreases. 

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