Plastic Processing Flashcards
Why are plastic shaping processes so important
Variety of processes means infinite geometry possibilities
Net shape process
Processing temps are lower so less energy
Lower temps so handing product is simplified during production
Finishing is not required
Extrusion
Compression process where material is forced to flow through a die orifice to provide long continuous product whose cross section shape is determined by shape of orifice
What type of products are used for extrusion
Thermoplastics and elastomers
Process of extrusion
Pellet or powder fed into extrusion barrel where heated and melted
Fed onto rotating screw who moved material along barrel. Subsequent mixing generates additional heat to maintain melt
What are the three sections of the screw
Feed section- stock is moved from hopper port and preheated
Compression section- polymer is transformed into liquid consistency. Trapped air extracted
Metering section- melt is homogenized and sufficient pressure is developed to pump it through die opening
Slit die extrusion of sheet and film
Narrow slit as die opening. Die includes manifold that spreads polymer melt laterally before it flows through slit (die orifice).
Why do edges of film produced from slit die extrusion usually have to be trimmed
Drastic change of shape by polymer melt during flow through die and temperature and pressure variations causes problems with uniform thickness.
How is the production rate of slit die extrusion increased
Increased cooling by directing extrudate into a quenching bath of water or onto chill rolls. (Chill roll extrusion)
Blown film extrusion process
Extrusion of a tube that is drawn upwards while molten and simultaneously expanded in size by air inflated into it through the die mander. Frost line marks along upward moving bubble where solidification occurs. Air pressure must be kept constant. Air kept in tube by pinch rolls. Guide rolls and collapsing tubes used to retrain and direct it to pinch rolls. Flat Tube is then collected onto a windup reel. Air is to stretch film
Benefits of blown film extrusion
Stretch by is is isotropic which is better than when stretched in one direction
Ease in which extrusion rate and air pressure can be changed to control stock width and gage.
Produces a stronger film then slit die extrusion but thickness control and production rates are lower
Injection molding two main components
Plastic injection unit- acts like an extruder except with a no return valve at tip to prevent melt moving backwards and the screw is called a reciprocating screw
Mold clamping unit
Functions of the clamping unit
Hold two,halves of the mold in proper alignement
Keep mold closed during injection, resisting forces
Open and close mold at appropriate time
Claiming unit components
Two platens, one fixed and one moveable
Stages of injection molding
Mold is closed and clamped
Melt at dinghy temp and viscosity by heating and mechanical working of screw is injected under high pressure into mold cavity
Plastic cools and begins romsolidify when encounters cold surface of mold. Ram pressure maintained to pack melt and compensate for contraction during cooling
Screw is rotated and retracted with the non return valve open to permit fresh polymer to flow in
Mold opened and part ejected once completely solidified
Blow molding
Air pressure is used to inflate fold plastic inside a mold cavity
Blow molding steps
Fabrication of a starting tube of molten plastic called a parison by extrusion or injection molding
Inflation of tube to desired final shape
Injection vs extrusion blow molding
Injection has:
-higher production rate
-grater accuracy in final dimensions
-lower scrap rates
- less wasteful of material
However:
-larger containers can be produced by extrusion blow molding
-extrusion is more feasible for double layered bottles
Stretch blow molding (a variation of injection blow molding)
Blowing rod extends downwards into the injection molded parison in step two, stretching the sole plastic and creating a more favourable stressing. Resulting product is more rigid, higher transparency and better impact resistance
Thermoforming
Process where a flag thermoplastic sheet is heated and deformed into the desired shape. Heating usually accomplished by radiant electrical heaters located on both sides of sheet for long enough to soften sheet.
What three categorise of methods is there for the forming stage of thermoforming
Vacuum thermoforming
Pressure thermoforming
Mechanical thermoforming
Vacuum thermoforming
Negative pressure is used to draw preheated sheet into a mold cavity Plastic cools
Negative molds
Has a concave cavity
Positive mold
Has a convex shape- in thermoforming sheet is draped over and neg and pos pressure is used to force it over mold.
How to choose between a positive or negative mold for thermoforming
For a negative mold the exterior surface will have the exact surface contour of the mold cavity. Inside will be an approximation and posses finish corresponding to that of stating sheet. Visé versa for positive mold
