Cold forming of plastics part I. Draw forming of thermoplastic sheets

Abstract

An experimental procedure is outlined to examine the potential of thermoplastic sheets in draw forming. Experiments carried out on a variety of materials indicate that the following requirements must be fulfilled for a thermoplastic sheet to be cold formable: (1) The glass transition of polymer should be above ambient temperature and above the temperature of forming, (2) tensile elongation at break should equal or exceed 30%, (3) ratio of tensile to compressive yield stress should equal or exceed 1.6 and (4) sheet must not yield locally (neck) when strained in tension. An experimental method has been developed to determine the compressive, friction and bending forces which oppose the drawing force exerted by the punch. It is shown that the compressive force is, in most cases, largest. A stress analysis is carried out leading to an expression correlating the maximum depth of draw as a function of basic properties of sheets such as tensile strength (St*) and compressive yield stress (Sc). The effect of rolling on drawability is examined and interpreted in terms of the ratio St*/Sc. The cold formed items have a lower heat distortion temperature than their thermoformed counterparts.

Deformation Phenomena of Thermoplastic Materials in the Solid and Semifluid States

“…Once a polymer has been cold formed, Johnson has shown that the deformation of unmelted particles can lead to the development of nonuniform microstructures, which may result in parts with high residual stress, reduced mechanical properties and fatigue . In fact, Li et al have described that cold formed polymers have a lower heat distortion temperature than their thermoformed counterparts, where cold formed components readily deform below their T g due to the high amount of residual stresses induced by the deformation process .…”

“…With respect to crystallinity, Martins et al observed that high crystallinity is counterproductive due to the resulting high stiffness and limited ductility, while low and medium crystallinities offer low spring back and high ductility, respectively . In terms of processability of the polymers before and after deformation phenomena, Li et al argue that the processability of brittle polymers considered not formable can be improved by performing a preorientation by means of cold rolling, which improves their formability . Once a polymer has been cold formed, Johnson has shown that the deformation of unmelted particles can lead to the development of nonuniform microstructures, which may result in parts with high residual stress, reduced mechanical properties and fatigue .…”

“…h) and TPE, as already mentioned, displays an unfavorable tendency to creep. Li et al have analyzed the response of different polymeric sheets in draw forming, finding that materials such as polystyrene, rigid PVC and PMMA, having a processing behavior similar to what we observe for PK, are not formable . Li et al also stated that polyolefins like polyethylene, chlorinated polyethylene, and polypropylene are formable, albeit with poor shape retention, also similar to what is observed in this study for TPE .…”

“…Li et al have analyzed the response of different polymeric sheets in draw forming, finding that materials such as polystyrene, rigid PVC and PMMA, having a processing behavior similar to what we observe for PK, are not formable . Li et al also stated that polyolefins like polyethylene, chlorinated polyethylene, and polypropylene are formable, albeit with poor shape retention, also similar to what is observed in this study for TPE . Finally, Li et al indicate that dry nylon 6 and dry nylon 66 are formable with a good shape retention, similar to what we observe for PA12 .…”

“…Some of the techniques used in cold forming include bending, rolling, cold drawing, die drawing, and deep drawing . Li et al have performed an extensive study on the cold forming of many thermoplastics, finding that optimal cold forming requires a polymer having a glass transition temperature ( T g ) higher than the processing temperature, where the material should have a high tensile elongation at break with a ratio of tensile to compressive yield stress larger than 1, along with a high resistance to necking . With respect to crystallinity, Martins et al observed that high crystallinity is counterproductive due to the resulting high stiffness and limited ductility, while low and medium crystallinities offer low spring back and high ductility, respectively .…”