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PLASTIC FORMING TECHNIQUES - Coggle Diagram
PLASTIC FORMING TECHNIQUES
Calendering
calender is a machine equipped with 2,3 or more heavy internally heated/cooled rollers
Rollers
rotate in opposite directions
operate at even or uneven speeds
Advantages
producing rubber sheets of various lengths and thickness
skin coating of fabrics
shower curtains
tablecloths
laminating film
Process
A polymer mixture is heated .
Then fed between two rotating main calenders.
The pulp is pressed to a rough thickness .
Then rolled by subsequent rollers to the desired thickness.
Extrusion
Continuous process
forcing a molten materials (plastic) through a shaped die by means of pressure
e.g. melting of plastic resin + adding mixing fillers
screws are used to progress the polymer in the molten/rubbery state along the barrel of the machine
e.g. Single screw extruder , Twin screw extruder
Alternate method for making sheets/flat film
Process
Materials fed into hopper.
Then falls through a hole in the top the extruder (feed throat) which onto the screw.
The screw moves the molten plastic forward until the end of the extruder barrel to which die has been attached.
Die gives shape to molten plastics.
Cooled in water tank.
extrudate is extruded directly into the nip area between two rolls
rollers have small gap between them, and plastics is forced through this gaps by the counterrotating of the rollers
Screw Extruder
Feed zone (constant channel depth)
preheat the polymer and convey it to subsequent zones
2.Compression zone (decreasing channel depth)
expels air trapped between the original granules
heat transfer from the heated barrel walls is improved as the materials thickness decreases
density change during melting is accommodated
3.Metering zone (shallow, constant channel depth)
homogenize the melt
Compression Molding
process
Mould is held between the heated platens of the hydraulic press
A prepared quantity of molding compound is placed in the mould
The platen closes with sufficient pressure to minimize flash at mould parts line
The compounds softens and flows to shape, chemical cure then occurs as the internal mould temperature becomes high enough
Press is opened and the molding removed
advantages
low scrap arising (2-5 %)
fibrous fillers are well distributed
low residue stresses
mechanical and electrical properties are retained
low mould maintenance
capital and tooling costs are relatively low
plant and tooling are comparatively simple
Blow Molding
Manufacture of bottles and hollow-shaped parts
Advantages
cycle is very short :arrow_right: low cost
high production rates
Types
Extrusion blow molding
Injection blow molding
Stretch blow molding
process
Melting the resin- done in extruder
Form the molten resin into a cylinder or tube (parison)
Parison is placed inside a mold, and inflated so that the plastic is pushed outward against the cavity wall
The part is allowed to cool in the mold and is then ejected
The part is trimmed
Injection Molding
:
Inject molten polymer into a closed mold where it is solidifies to give the product.
2 main section
injection unit :arrow_right: plasticizing part of the process
clamping unit/press which houses the mold :arrow_right: essentially press closed by a hydraulic/mechanical systems
process
The mould is closed :arrow_right: a short of melt is ready in the injection unit
Injection occurs :arrow_right: the valve opens and the screw acting as a plunger forces the melt through the nozzle into the mold
The hold on stage when pressure is maintained during the early stages of cooling to counteract contraction
The valves closes and screw rotation starts :arrow_right: pressure develops against the closed off nozzle and the screw moves backwards to accumulate a fresh shot of melt in front
The molding in the mould has continue to cool :arrow_right: when ready the press and the mould open and the molding is removed.
The mould closes again and the cycle repeats
Transfer moulding
development of compression molding in which reservoir of the molding compound is located in the mold and upon closure is transferred via runner to the cavities.
Advantanges
give many small parts easily
reduce risk of damage/movement of thin/delicate mold parts
faster due to better heat transfer trough runners
Disadvantanges
flow usually gives unwelcome orientation on the products
increases wear and maintenance cost
tooling is rather complex :arrow_right: more expensive
runner are scrap :arrow_right: not economical
Blown Film extrusion
A continuous tube is extruded clamped between rollers at one end and blown with compressed air to form large hollow tube having a very thin wall.
The tube continuously flattened and placed on a roll for bag production or it can split opened and used for film.
no mold is required
process
The extruded materials flow through a tubular die
The melt flow around a mandrel and exits the die as a tube
A cooling ring is placed at the exit of the die to give the tube some dimensional stability
Air is introduced through the back of the die and flow upwards inside the middle of the tube of materials
The tube or bubble, continues to expand, cool and crystallize until the radial (tensile) strength of the plastics equals the pressure of the air inside
The bubble is then forced into a flat sheet by the collapsing guides, and moved into the nips rolls
Then it travels down over some rollers and it was sealed
Rotational Molding
Uses gravity inside a rotating mold to achieve a hollow form
favor more complex external geometries, larger parts, and lower production quantities
simple and inexpensive
production cycle is much longer
process
A predetermined amount of polymer powder is loaded into the cavity of a split mold.
The mold is then heated and simultaneously rotated on two perpendicular axes, so that the powder impinges on all internal surfaces of the mold, gradually forming a fused layer of uniform thickness.
While still rotating, the mold is cooled so that the plastic skin solidifies.
The mold is opened, and the part is unloaded. Rotational speeds used in the process are relatively slow. It is gravity, not centrifugal force, that causes uniform coating of the mold surfaces.
Thermoforming
Advantages
Utilize thermoplastic sheet/film
Used on large scale for the manufacture of packaging
For large parts such as swimming pool/motor vehicle parts
reshaping of plastics under the influence of heat and pressure or vacuum
process
Heat a thermoplastic sheet until it softens
Force the hot and pliable material against the contours of a mold by using either mechanical, air or vacuum pressure
Held against mold and allow to cool, and plastics retains its shape
material is not melted, lower pressures are required
Disadvantages
generates greater amount of scrap
cost of the sheet materials is raised
limited design parts
Filament Winding
products
pressure vessel
storage tanks
aerospace parts such as helicopter blades
produce a hollow (usually cylindrical) shape
The fibers may be impregnated with resin before winding (wet winding) or pre-impregnated (dry winding)
process
1.The fibers are passed through the resin bath
2.Then continuously wound onto the mandrel
3.After number of layers,curing is carried out in an oven/room temperature
Mandrel is removed
Pultrusion
process
Continuous rovings of the reinforcement are impregnated with the resins (passing through a bath of resin)
Then pulled through the heated die (compact & give shape to the composites)
Curing takes place in the heated die & oven
4.Puller used to pull the whole system with certain speed
manufacture components with continuous lengths and constants cross-sectional shape (rod, tubes, beam, etc)
Continuous process
Easily automated (production rates are high, hence it is very cost effective)
Variety of shapes are possible
