Thin section moldings, flow

TS plastic products that are injection, transfer, or compression molded combine thick and thin sections relatively easily since the hardening process is a chemical reaction (Chapter 6). Annular shapes are best made by compression to gain best dimensional control and freedom from distortion. In the compression process, the molding compound is compressed and reduced to the plastic state in the mold. During this process, portions of the material may lie in hard forms in the mold while other portions are flowing rapidly with great force. 

The process interaction in cast plastic products is mainly involved with the curing processes and with mold filling problems. Voids and porous sections are a frequent problem with castings because the mold filling is done at atmospheric pressure, or low pressure, and if the product has thin sections to fill, the flow may be a problem. 

An alloy made from hquid crystal polyester and PPS can be used in electrical and electronics connectors, thin walled parts, and multi-gated parts. The two materials complement each other, as the LCP offers good reinforcing strength and allows for good flow into thin sections without flash. The PPS reduces the alloy s cost, cuts warpage, and increases weld line strength. Also, the material can be processed in LCP molds. 

The highly crystalline nature of the polymer persists up to, ...625°P. At that temperature, the polymer undergoes a broad endotherm which most likely corresponds to the loss of order in one dimension. However, even above this transition, the flow is extremely limited and is insufficient to meike compression molding a useful tool for fabrication. Only very thin sections can be molded 1/4") and the resultant... 

Victrex 150G— Easy flow grade for injection molding of thin sections and complex parts... 

Turbulent flow of plastic from an undersized gate or thin section into a thicker mold section, as opposed to... 

Another problem in molded parts is the existence of contiguous areas of thick and thin sections in the flow direction. In some cases this problem can be controlled by placing the gate in the thicker section of the part. Where there are several thick sections multiple gates are used. There is a limitation on this approach because the weld lines produced by the joining of the several plastic flows are a weak point... 

Fig. 5.91 A reflected light micrograph (A) shows the layered structures in a molded bar aligned parallel to the flow direction (arrow). Variation in density and color reflect variation in orientation from layer to layer. Lateral, curved flow patterns are seen in a polarized light micrograph overview of a thin section (B) (color section). The flow layers are nearly normal to the flow direction (arrow) in the center of the bar (C) (color section).

Thermotropic LCP molded bars exhibit a layered structure as shown by reflected light (Fig. 5.91A) of a cut and polished bar. Thin sections of a molded bar show fine, nematic domains with superimposed flow lines (Fig. 5.91B, color section), especially near the center of the bar (Fig. 5.91C, color section). Skin-core morphologies are obvious in injected molded bars and extrudates with domains aligned in the flow direction. Complementary SEM assessment of fractured injection molded bars provides an overall view of the layered structure (Fig. 5.92A), the surface skin (Fig. 5.92B) and the internal fibrillar structures of the inner skin (Fig. 5.92C) and core (Fig. 5.92D). 

The ease of processing of LC polyester in injection molding machines is due to short cycle times, high flow in thin sections and... 

Figure 11-13. When molding an RP, thin sections sometimes lose strength, because fibers do not flow into a narrow space unless a suitable moldable material is used.

Fill rate and pressure are critical parameters for molding quality parts. Thicker sections of the part fill preferentially due to lower melt pressures required to fill. Plastic flow will accelerate in thicker sections and hesitate in a thin section. The time is a function of the material, volume of part, and mold. 

Thermotropic LCP molded bars exhibit a layered structure as shown by reflected light (Fig. 5.79A) of a cut and polished bar. Thin sections of a molded bar show fine, nematic domains with superimposed flow lines (Fig. 5.79B, color section), especially near the center of the bar... 

The different microstructure of HDPE/LDPE blends, having different compositions, are shown (Figure 9). In fact, these thin sections taken in the parallel flow direction inside the cavity of a mold and near the gate, show differences in... 

In a fiber form, LCPs are used in a similar manner as aramid fibers. In solid form, as is used in molding and extrusion, or even with thermoforming, LCPs are quite unique. In the molten phase, the molecular chains are oriented and aligned, without entanglements. This means the molten material can flow easily (like a liquid), allowing for fill of very thin sections at very low pressure. They then solidify... 

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