MANUFACTURING WITH PLASTICS Machinery's Handbook, 31st Edition
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Manufacture of Plastics Products The main processes for manufacturing large quantities of plastics products are ex- trusion, injection molding, blow molding, and sheet thermoforming for thermoplastic materials; compression molding, transfer molding, prepreg molding, and pultrusion for thermosetting materials.
Free Expansion
Assembly with Metal
L L = Length of Beam
Change in Any Linear Dimension L = L T
Stress in Plastics T = ( m – p ) E P T
Relative Motion
d
d
Shoulder Screw
Plastics
Washer
d = Diamter of Hole
d = d T
Metal
Restricted Expansion
Relative Motion Between Parts at Shoulder Screw L rel = ( p – m ) L T
L
Stress
Stress T = T E P
T
T =
=
T
L
Fig. 12. Thermal Expansion Equations for Various Combinations of Materials and Situations
Fig. 13. Placing Most of the Material Far from the Centroid Achieves Far Higher Bending Stiffness per Unit Weight of Plastics
More tonnage of thermoplastics pass through extrusion machines than any type of plastics forming machines. The dominant type of extruder is the single-screw extruder. A feed hopper supplies small plastic granules to a thick-barreled, heated cylinder, inside which turns a sturdy, precisely contoured screw. A powerful variable-speed motor drive turns the screw at speeds typically between 10 and 1000 rpm. At the delivery end is attached a die, a block of metal with a specially shaped orifice. The screw has two main jobs: 1) to melt by friction and thoroughly mix the feed, transforming it from cold, solid particles into a homogeneous hot, viscous melt; 2) to develop the
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