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Download Thermoforming : improving process performance by Stanley R. Rosen PDF

By Stanley R. Rosen

This e-book describes the roll-fed thermoforming procedure, plastic fabrics, layout of thermoformed items, thermoforming machines, trim presses, mould layout, parts of a whole mildew procedure, mould format and computer base layout, mould rate estimating, knife-like trim dies, thermoforming offline punch and die trimming

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Thermoforming : improving process performance

This publication describes the roll-fed thermoforming procedure, plastic fabrics, layout of thermoformed items, thermoforming machines, trim presses, mould layout, parts of a whole mildew approach, mildew format and computing device base layout, mildew fee estimating, knife-like trim dies, thermoforming offline punch and die trimming

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For polypropylene, on the other hand, extrusion generally leads to a reduction in molecular weight (rise in MFR) without formation of branches, and this causes reduced melt strength when high levels of regrind are used. When the molten sheet exits the die and contacts the polishing roll stack, takeoff speed is adjusted to the desired final sheet thickness. Increasing the takeoff speed of the rolls causes the sheet to become thinner, and also increases tension on the molten sheet. This tension causes the molecules to orient in the machine direction.

Raising melt temperature or going to a higher meltflow-rate material can reduce the melt viscosity of the polymer. When the extruded sheet is reheated in the oven before being formed, its temperature must be raised to the point where it is soft enough for forming to occur. For amorphous materials, this requires a temperature above the glass-transition temperature, while for crystalline polymers, the sheet temperature must often equal or exceed the melting point of the polymer. For amorphous materials, the softening process occurs gradually as the glass-transition temperature is exceeded.

Ordinary silica glass is an example of an inorganic amorphous polymer. When a glassy material (amorphous) is heated, it gradually begins to soften as molecules gain more and more mobility. However, unlike a crystalline material, there is not an abrupt change from a rigid solid to a fluid melt. If a glass rod or tube is held over a flame, it eventually becomes soft enough to bend or blow into a bottle, but unless it is overheated, it will not appear to melt. The temperature where large-scale motion of molecules becomes possible, and significant softening of the material occurs, is known as the glass-transition temperature.

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