POLYMER PROCESSING (MOLDING)
D. JIM LIVINGSTON
Faculty of Chemistry,
St. John’s College.
MOLDING
A process in which a finely divided plastic is forced by the
application of heat and pressure to flow, fill and forms the
shape of the mold.
Compression
Extrusion
Injection
Blow
COMPRESSION MOLDING PROCESS
Polymers (fiber/resin) pre weighed to a specific size charge,
placed between movable and stationary mold.
Heat (150OC) and pressure (3000) psi is then applied so the
polymeric material becomes plastic, flows and fill the mold.
The material is compressed and compacted into the shape
inside the cavity.
The excess material flows out of the mold called as flash.
After a short time the press is opened and the molded part is
removed.
Advantages of compression molding
low mold maintenance
low capital and tooling costs
well distributed fibrous fillers
EXTRUSION MOLDING PROCESS
Extrude – expulsion or forcing something out
Polymer is poured into a hopper.
The extruder contains a motor, which turns a screw and the
polymer is propelled to the heater.
Polymer is melted into liquid and pushed through a die.
Polymer from the die is cooled rapidly
The product is cut into desired length.
Single screw extruder
Zones of extruder
Feed zone – receives the charge and transfers to next zone
Compression zone – polymer is melted and compressed.
Metering section – molten polymer is pushed to the die
Working zone – effective mixing. (optional)
INJECTION MOLDING
Polymeric material is fed into hopper at definite time intervals.
Pressure is applied through hydraulically driven piston
(reciprocating screw).
Piston pushes the molten polymer through the cylinder.
The material is then injected through a nozzle into a mold
cavity.
Mold is held under pressure till the polymer cools and hardens
Mold is opened and the article is removed.
Meanwhile the screw moves back to take the charge for the
next cycle.
Differences between injection and extrusion
Injection Extrusion
Screw moves front and back Screw moves continuously
( cyclic)
Suit to produce 3D objects Suit to produce 2D materials
The melt plastic material is Melt plastic is pressurized
injected into the mold into the die.
applications
BLOW MOLDING
also known as blow forming, is a manufacturing process by
which hollow plastic parts are formed.
Process Materials
Low Density Polyethylene (LDPE)
High Density Polyethylene (HDPE)
Polyethylene Terephtalate (PET)
Polypropylene (PP)
Polyvinyl Chloride (PVC)
Types
EXTRUSION BLOW MOLDING
INJECTION BLOW MOLDING. STRETCH BLOW MOLDING.
Processing
begins with melting down the plastic and forming it into a
parison or preform.
parison - tube-like piece of plastic with a hole in one end in
which compressed air can pass through.
pressurized gas, usually air, is used to expand the hot preform
and press it against a mold cavity
plastic is melted and extruded
into a hollow tube
parison is then captured by
closing it into a cooled metal
mold
Air is then blown into the parison
inflating it into the shape of the
hollow bottle
After the plastic has cooled
sufficiently, the mold is opened
and the part is ejected
INJECTION BLOW MOLDING
Molten polymer is injected into a heated preform mould
The preform consists of a fully formed neck with a thick tube of
polymer attached
Compressed air is blown through the core rod into the preform
material, inflating it
The blow formed polymer is allowed to cool. It is then removed
from the blow mould and stripped off the core rod.
STRETCH BLOW MOLDING
The plastic is first molded into a "preform" using the injection
molding process.
These preforms are produced with the necks of the bottles,
including threads (the "finish") on one end
These preforms are packaged, and fed later (after cooling) into a
reheat stretch blow moulding machine.
Steps involved in stretch blow molding
ADVANTAGES
Well suited to low and high production rates
Tooling is less expensive
Trim and flash reusable into finished parts
References
https://www.slideshare.net/UmerWarraich2/blow-molding
https://www.slideshare.net/habib_117/extrusion-molding
http://www.idc-
online.com/technical_references/pdfs/chemical_engineering/E
xtrusion_and_Injection_molding_of_polymers.pdf
Polymer Science – Vasant R. Gowariker
Text book of polymer science – Bill Mayer
