Thin-walled molding

RM is a simple, basic, four-step process that uses a thin-walled mold with good heat-transfer characteristics. Its closed mold requires an entrance for insertion of plastic and, most important, the capability to be opened so that solidified products can be removed. These requirements are no problem. Liquid or dry-powder plastic equal to the weight of the final product is put into the mold cavity(s), which rotates simultaneously about two axes located perpendicular to each other (Fig. 8-68). These two rotation speeds can be varied to permit more... 

Rotomolding. Nylon-6, nylon-11, and nylon-12 can be used in rotomolding and are generally supplied for these applications as a powder or with a small pellet size. The process involves tumbling the resin in a heated mold to form large, thin-walled moldings. Nylon-11 and nylon-12 use mold temperatures of 230—280°C and nylon-6 is processed at over 300°C. An inert gas atmosphere is preferred to avoid oxidation. 

Thermoplastics have non-Newtonian melt flow characteristics it means that their viscosity will change dependent on their velocity or the amount of shear that occurs in the melt (Chapter 1). This non-Newtonian characteristic is a key in thin wall molding. As in any molding setup one cannot just simply ram the melt into the cavity. It s flow characteristic, gate size as well as position, and venting have to be balanced in order to obtain the desired structural part and meet tight tolerance requirements. 

Thin wall molding and injection compression molding are processes utilized to obtain very thin molded... 

Fassett, J. Thin wall molding how its processing considerations differ from standard injection molding. Plast. Eng. 1995, 51, 35-37. 

The properties of nylons are considerably affected by the amount of crystallization and by the size of morphological structures, such as spherulites, which in turn, are generally influenced by the processing conditions. Thus, a molding of nylon-6, slowly cooled and subsequently annealed, may be 50-60% crystalline, whereas a rapidly cooled thin-walled molding may be only 10% crystalline. 

With thin-walled molding and short cycle times, orientations could be fixed by crosslinking. This possibility is not, however, treated in the published literature, according to which orientations, even in thin-walled structural components, relax during processing without anisotropy. 

The production of thin-walled moldings is nothing new. Cups made of PS or PP with wall thicknesses of 0.5-0.6 mm were being injection-molded over... 

KINEL 5514 is 50% filled with eighth-inch glass fibers and is specially formulated for thin wall moldings. Typical end-uses Include high-temperature electrical connectors and high precision moldings. 

Usually 2 tsi. (31MNm-2)) is sufficient but if thin walled moldings are being produced then this may have to be doubled (e.g. 4 tsi or 62MNm-2). 

Passett, J., Thin Wall Molding, GE Plastics, Pittsfield, Mass., April 21, 1998. 

Vents in the mold s cavities and runners permit air in the cavities to leave the mold ahead of the melt front. The absence, poor location, or clogging of vents can produce short shots (incomplete filling of the cavity) accompanied by char marks from burning of the trapped air (commonly referred to as dieseling). Because thin wall molds are filled very rapidly, vacuum is applied to the vents to improve removal of air in the mold. 

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