Why do properties of some polymers depend on the way in which the specimens are prepared. For ex, the extruded Polyvinyl Acetate sheets are much stronger (at least an order of magnitude) than the solution cast films.
Based on experience with the casting and extrusion of ceramics, I would say that this is largely due to the concentration of the polymer molecules and the flow regime of the extrusion process. Extensional flow in the extrusion process tends to orientate the polymer molecules more effectively than in casting, whilst an extruded product will tend to have far fewer inclusions and defects than a cast product as a result of the high concentration of the solid particles / polymer molecules.
Based on experience with the casting and extrusion of ceramics, I would say that this is largely due to the concentration of the polymer molecules and the flow regime of the extrusion process. Extensional flow in the extrusion process tends to orientate the polymer molecules more effectively than in casting, whilst an extruded product will tend to have far fewer inclusions and defects than a cast product as a result of the high concentration of the solid particles / polymer molecules.
Agreed with Jon, in polymers this is mainly due to molecular orientation. The melt blown films are much stronger than extruded films/sheets, and the extrusion process produces stronger and tougher films than those made by the casting technique.
It is generally agreed that mechanical working processes produces more compact structure than cast one. It is no matter what the material is. All the mechanical working processes give rise to a compact structure and hence lead to superior properties than cast product.
From my knowledge experience the casting and extrusion of ceramics, it is due to the concentration of the polymer molecules and the flow regime of the extrusion process. Extensional flow in the extrusion process tends to orientate the polymer molecules more effectively than in casting, but extruded product will tend to have far fewer inclusions and defects than a cast product as a result of the high concentration of the solid particles.
I agreed with the above statements that polymer molecules are aligned due to extensional flow during extrusion process. Its generate a compact structure and leads strong structure in comparison to cast structure.
I agree that extrusion produce stronger structure, and by the way it is also the most common way that induistries use for production of plastic. Casting may be used for research but considering that when shifting to extrusion you can get very different results. Casting in industry is used when absolutely necessary, extrusion is chepaer and easier to control.
Generally speaking, the answers above are true because the question intuitively leads them. But if the question was put in another way – Does extrusion always give greater strength of the material compared to the casting technology – the answer may be in favour of the second technique. Such a result is possible if the transverse direction strength of the extruded material is compared with the isotropic properties of the same material reached by casting.
I have a suggestion for a so far unacknowledged point.
If you perform solution casting your solvent has to be removed from the final film. In this process the polymer molecules are first highly mobile and can interact in the solvent but with decrease of the solvent concentration some molecules are forced in a solid condition but then they are not able to attain a favourable entanglement as their mobility is lower.
It is difficult to generalize. For example,when cast Perspex acrylic and extruded Perspex acrylic sheets are compared, the extruded acrylic sheet has reduced mechanical properties. The thermoforming range for cast sheets is wider. They can be reworked hot, which is not always possible with extruded sheets. The surface finish, flatness and optical properties of cast acrylic are all superior to those of extruded acrylic sheets. Of course, the aforementioned has exceptions. When we worked with some acrylic amide polymers, they hardly formed films upon casting & the films crumbled very easily.
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