Chapter 19: Forming and Shaping
Plastics and Composite Materials
Faculty of Engineering
Plastics ~ polymers
Plastics are engineered materials
Made from natural or synthetic resins and compounds
Low density, low tooling costs, good corrosion resistance, low cost
Can be molded, extruded, cast, or used for coatings
Plastics are very versatile materials and are used more than steel,
aluminum, and copper combined in the United States
Used as food and beverage containers, packaging, signs, housewares,
foams, paints, toys, etc
An important group: reinforced plastics (composites)
Molecular Structure of Polymers
Double or triple covalent
bonds may also be present
Smallest repeating unit
Basic structure of some polymer molecules:
(a) ethylene molecule; (b) polyethylene, a
linear chain of many ethylene molecules;
(c) molecular structure of various polymers.
These molecules are examples of the basic
building blocks for plastics
Some Plastics Terminology
Thermoplastic (TP) – Polymers that can be shaped when heated
and regain original hardness & strength upon cooling
Have a linear or branched structure (weak secondary bonds)
Process is reversible
Acrylics, cellulosics, nylons, polyethylenes, polyvinyl chloride
Thermoset (TS) – Polymers that become permanently set when
Have a cross-linked structure (strong secondary bonds)
Process is irreversible
Epoxy, polyester, urethane, phenolics, silicones
Elastomer (Rubber) – Elastic; low elastic modulus
Tires, footwear, gaskets, flooring, weatherstripping, hoses
Improve strength, stiffness, and toughness
Reduce shrinkage and weight
Common fillers: wood flour, silica flour, clay, powdered mica
Added in small amounts to reduce viscosity
• Stabilizers and antioxidants
Retard the effects of heat, light, and oxidation
• Colorant (organic dyes or inorganic pigments)
• Flame retardants
Facilitates part removal
Forming and Shaping Processes for Plastics, Elastomers,
and Composite Materials
Raw materials in the form “if thermoplastic pallets” granules, or powder,
placed into a hopper and fed into extruder barrel.
The barrel is equipped with a screw that blends the pallets and conveys them
down the barrel
Heaters around the extruder’s barrels heats the pellets and liquefies them
Screw has 3-sections
Melt or transition section
Extrusion Die Geometries
Common extrusion die geometries: (a) coat-hanger die for extruding sheet;
(b) round die for producing rods; and (c) and (d) nonuniform recovery of the
part after it exits the die.
Extrusion of Tubes
Extrusion of tubes. (a) Extrusion using a spider die and
pressurized air. (b) Coextrusion for producing a bottle.
Production of Plastic Film and Bags
(a) Schematic illustration of the production of thin film and plastic bags from tube
– first produced by an extruder and then blown by air. (b) A blown-film operation.
This process is well developed, producing inexpensive and very large quantities
of plastic film and shopping bags
Similar to extrusion barrel is
Pellets or granules fed into
Melt is forced into a split-die
Molten plastic pushed into
Pressure ranges from 70 Mpa
– 200 Mpa
Typical products : Cups, containers,
housings, tool handles, knobs,
electrical and communication
components, toys etc.
Products Made by Injection Molding
Typical products made by injection molding, including
examples of insert molding.
Mold Features for Injection Molding
Illustration of mold features for injection molding. (a) Two-plate
mold with important features identified. (b) Four parts showing
details and the volume of material involved.
Types of Molds used in Injection Molding
Injection molds have several components such as
runners, cores, cavities, cooling channels, inserts,
knock out pins and ejectors
Three basic types of molds
Cold runner two plate mold
Cold runner three plate mold
Hot runner mold
High production rates
Good dimensional control
Cycle time range 5 to 60 sec’s
Mold materials- tool steels, beryllium - Cu, Al
Mold life- 2 million cycles (steel molds)
10000 cycles ( Al molds)
Horizontal or vertical machines
Clamping – hydraulic or electric
Mixture of resin with 2 or more reactive fluids
forced into the mold cavity at high speed .
Applications : Bumpers, tenders, thermal insulation,
refrigerators and freezers, water skis, stiffness
A 2.2-MN (250-ton) injection molding machine. The tonnage is the force applied to
keep the dies closed during the injection of molten plastic into the mold cavities and
hold it there until the parts are cool and stiff enough to be removed from the die.
Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.
Injection Molding: Sources of Defects
Weld lines (similar to cold shut in metal casting)
Unfilled die cavity if have premature solidification due to
Form flash if dies do not mate properly
Sink marks form at thick sections due to uneven cooling
causing local shrinkage
Avoid defects by:
Simulate processes using computer software
Reaction-Injection Molding Process
In reaction-injection molding (RIM), a mixture of two or more reactive fluids is forced
under high pressure into the mold cavity. Chemical reactions take place rapidly in the
mold and the polymer solidifies, producing a thermoset part. Major applications are
automotive bumpers and fenders, thermal insulation for refrigerators and freezers, and
stiffeners for structural components. Initial injection pressures typically are much lower
than traditional injection molding.
Modified extrusion and Injection
A tube extruded then clamped to mold
with cavity larger than tube diameter.
Finally blown outward to fill the
Other Blow Molding processes
Injection Blow molding
Multi layer Blow molding
Schematic illustrations of (a) the
extrusion blow-molding process for
making plastic beverage bottles; (b) the
injection blow-molding process; and (c)
a three-station injection molding
machine for making plastic bottles.
Rotational Molding Process
Rotational molding is used for large plastic parts.
The thin-walled metal mold is a split female mode
made of two pieces and is designed to be rotated
about two perpendicular axes.
A premeasured quantity of finely ground plastic
material is placed inside a warm mold. The mold is
then heated, usually in a large oven, while it is
rotated about the two axes.
The action tumbles the powder against the mold
where heating fuses the power without melting it.
In some cases, a cross linking agent is added to
the powder, and cross linking occurs after the part
is formed in the mold by continued heating.
Typical parts are tanks, trash cans, boat hulls, buckets, housings, toys, carrying
cases, and footballs. Various metallic or plastic inserts may also be molded into the
Plastisols are used in slush molding
Plastic materials are forced against the inside walls of
the heated mold by tumbling action.
The part is cooled while it is still rotating and then
removed by opening the mold
In thermoforming, a plastic sheet is heated in an oven to the sag point but not to
the melting point. The sheet is then removed from the oven and placed over a
mold and through the application of a vacuum is pulled against the mold.
Typical parts are advertising signs, refrigerator liners, packaging, appliance
housings, and panels for shower stalls. The parts cannot have openings or
holes or the vacuum cannot be maintained.
Various thermoforming processes for a thermoplastic sheet. These processes commonly are
used in making advertising signs, cookie and candy trays, panels for shower stall, and
Pre-shaped charge ,pre-measured volume of powder and viscous
mixture of liquid resin and filler material is placed directly into a
heated mold cavity.
Compression mold results in a flash formation which is an excess
Typical parts made are dishes, handles, container caps fittings,
electrical and electronic components and housings
Materials used in compression molding are thermosetting plastics &
Curing times range from 0.5 to 5 mins
3- types of compression molds are
Types of compression molding – a
process similar to forging: (a)
positive, (b) semipositive, and (c)
flash, which is later trimmed off. (d)
Die design for making a
compression-molded part with
Transfer molding is an improvement if compression
Uncured thermosetting material placed in a heated transfer
pot or chamber, which is injected into heated closed molds
Ram plunger or rotating screw feeder forces material into
mold cavity through narrow channels
This flow generates heat and resin is molten as it enters the
Typical parts : Electrical & electronic components, rubber and
Sequence of operations in transfer molding for thermosetting plastics. This process
is suitable particularly for intricate parts with varying wall thickness.
Processes for Plastics and Electrical
Conventional casting of thermo plastics :
Mixture of monomer, catalyst and various
additives are heated and poured into the
The desired part is formed after
polymerization takes place.
Centrifugal casting :
Centrifugal force used to stack the material onto the mold
Reinforced plastics with short fibers are used
Processes for Plastics and Electrical
Potting & Encapsulation
Casting the plastic around an
electrical component to embed it
in the plastic is potting
Used to coat transformers,
Plastic can serve as a nonconductor
In both the plastic is Dielectric
Polystyrene beads are placed in a mold and heated
Beads will expand up to 50 times their original size
Changing the bead size will determine the density of
the finished foam part
Used to make styrofoam cups, insulating blocks and
Processes such as rolling ,deep drawing extrusion closed
die forging ,coining and rubber forming can be used for
thermoplastics at room temperatures
Typical materials used : Poly propylene, poly carbonate,
Abs, and rigid PVC
Sufficiently ductile material at room temperature
Non recoverable material deformation
Sheets produced by
process also is used
in the production of
and rubber products.
In calendering, sheets of plastic are laminated together by rolling through
heated roller. Basically, warm or molten plastic (usually from an extruder)
is fed through a series of heated rolls as in this figure. The gaps between
the rolls determine the final sheet size. Each additional roll would reduce
the sheet thickness further. Then, once the laminated sheet is the correct
thickness, the sheet is then stripped off.
Processing Polymer-Matrix Composites
PMCs have high
strength/stiffness to weight
ratio and excellent creep
They consist of the polymer
and reinforcing fibers,
bonded together in various
Reinforced-plastic components for a
Honda motorcycle. The parts shown are
front and rear forks, a rear swing arm, a
wheel, and brake disks.
Polymer-Matrix Composites: Fiber Impregnation
Fibers can consist of fiberglass, graphite, boron, ceramic
Prepregs are made by dipping continuous fibers in resin.
(a) Manufacturing process for polymer-matrix composite tape. (b) Boron-epoxy prepreg
tape. These tapes are then used in making reinforced plastic parts and components with
high strength-to-weight ratios, particularly important for aircraft and aerospace applications
and sports equipment.
Polymer-Matrix Composites: Fiber Impregnation
Sheet-molding compounds are made by dropping randomly
oriented pieces of fiber on a layer of resin paste, under
which there is a thin sheet of polymer (carrier film).
Schematic illustration of the
manufacturing process for
plastic sheets. The sheet
still is viscous at this stage
and later can be shped into
Polymer-Matrix Composites: Filament Winding
Fibers are dipped in resin bath
They are then wrapped around an object by means
of rotating mandrel
Used to strengthen pressure vessels.
Polymer-Matrix Composites: Pultrusion
Fibers are run continuously through a resin bath before
being pulled through a set of dies.
Used to make golf clubs, ski poles, ladders.
(a) Schematic illustration of the pultrusion process. (b) Examples of parts made by pultrusion.
The major components of fiberglass ladders (used especially by electricians) are made by this
process. Unlike aluminum ladders, they are available in different colors but are heavier because of
the presence of glass fibers. Source: Courtesy of Strongwell Corporation.
Processing Metal-Matrix and Ceramic-Matrix
Liquid Phase Processing
Liquid metal-matrix and solid reinforcement are either cast or
Metal-matrix is usually aluminum or titanium
Solid reinforcement is usually graphite, aluminum oxide or
Solid Phase Processing
Powder-metallurgy techniques are used
Example: tungsten-carbide reinforced tools.
Design Considerations: Plastics and Composites
Compared to metals, plastics have lower stiffness and
Dimensional tolerances, except with injection
molding, are higher than with metals
For casting, ensuring proper flow into mold cavities
Variations in section thicknesses or abrupt changes in
geometry should be avoided.
Design Modifications to Minimize Distortion in Plastic Parts
Examples of design modifications to eliminate or minimize distortion in plastic parts: (a)
suggested design changes to minimize distortion; (b) stiffening the bottoms of thin plastic
containers by doming – a technique similar to the process used to shape the bottoms of
aluminum beverage cans; and (c) design change in a rib to minimize pull-in (sink mark)
caused by shrinkage during the cooling of thick sections in molded parts.