Polyacetal injection molding

There are a variety of different process plant designs used by polyacetal producers [1, 6]. Polyacetal resins are typically compound-extruded with property enhancers (additives) added during extrusion. The extrudate is water-quenched and pelletized. A state-of-the-art process can in-line extrude and injection-mold, using robotics and automation. Otherwise, the pelletized polyacetal is stored. 

Bowman [192] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. Barrel temperature effects were studied as they are known to influence both microstructure and mechanical properties [193]. Increased barrel temperature was shown to reduce the outer skin layer while increasing the extent of the equiaxed, unoriented core, resulting in a decreased tensile yield strength parallel to the injection direction. 

Bowman [157] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. 

Polyacetals have a low ceiling temperature, and are readily depolymerized by unzipping at low temperature (0.4-0.8% min at 222°C for POM). Owing to this low thermal stability, polyacetals can be used only if end-capped with stable groups (acetate or ether). This inherent thermal instability is exploited in an industrial method known as metal injection molding , which allows fine metal powder mixed with a polymer binder to be processed by injection molding, in much the same way as thermoplastic materials [31]. In a procedure based on POM, the binder is removed by thermal devolatilization according to Eq. (62)... 

One advantage over conventional systems is the minimized control complexity for multi-cavity applications. Only one control point per manifold is needed. These systems are suitable for processing polyolefin, styrene, and acetal. Applications for other plastics may also be possible after consultation with the manufacturer. Basically, shot weights are possible from 0.05 to 2 g per nozzle tip. The product made from polyacetal, which is shown in Figure 2.61,hasaproductweightofonly0.1 gand is produced in a four-cavity mold on a mini-injection molding machine babyplast . 

Tartaric Acid 5 23 365 9 —0.2 99 Delrin DuPont Specimen 127X12.7X3.2 mm (5X0.125X0.5 in) unstressed injection molded bars 10 23 0-360 8 Resistant-slight changes Ultraform BASF in mass or dimensions no irreversible change 7 Acceptable minor effect Polyacetal BITS... 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.35). They depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the rate of cooling of the melt. The skin surface in the microtomed section (top in Fig. 5.35A) is birefringent, non-spherulitic and highly oriented. The molecular chains are oriented parallel to the injection direction. The central portion of the bar consists of a core (bottom Fig. 5.35A) with randomly oriented spherulites (Fig. 5.35C). It has no preferred molecular or lamellae orientation. There are usually one or more layers between the skin and core that are transitional shear zones with intermediate... 

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