Molds part thicknesses

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. 

Thus, very stiff moldings can be obtained by increasing the molded part thickness without large increases in the weight of the molded part. 

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

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. 

Design Considerations for Injection Molded Parts (Parting lines, draft angles, wall thickness, fillets and radii, bosses, ribs, opening formations, shrinkage, gating, vents, potential knit lines)... 

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... 

A number of organic pigments can cause warping of certain thick-walled, large-area, non-axially symmetrical injection-molded parts such as bottle crates, where they act as nucleating agents for partially crystalline polymers. 

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] ... 

Rigid caul plates are typically constructed of thick metal or composite materials. Thick caul plates are used on very complex part applications or cocured parts where dimensional control is critical. Many rigid caul plates result in a matched die configuration similar to compression or resin transfer molding. Parts processed in this manner are extremely challenging because resin pressure is much more dependant on tool accuracy and the difference in thermal expansion between the tool and the part. Tool accuracy is critical to ensure no pinch points are encountered that would inhibit a tool from forming to the net shape of the part. 

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). 

Fig. 13.32 Schematic comparison of the skin thickness distributions in co- and gas-assisted injection molded parts due to the negligible gas viscosity.

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... 

To aid in controlling the thickness of molded parts and/or support the pressure loads put on sections of a mold, lands in the mold are used. Examples of lands are shown in Figure 14.2. Figure 14.4 shows the land locations used in a mold that supports the split-wedge in the mold. 

When molding a product with a variable wall thickness, it is possible to vary the thickness of the preform. This is usually accomplished by baffling. Another approach that can be used is to completely block off areas where no fiber is desired. This action saves material that would otherwise be trimmed off and probably discarded. It has also proven practical to combine two or more preforms into one molded part. This technique is very useful where the thickness of the molded part prohibits the collection of the preform in one piece. 

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