2. • A polymer is any substance made up of many
of repeating units, building blocks, called mers.
• When in form ready for further working, they
are called resins.
• Polymers are seldom used in their neat form,
most often compounded with various
additives. The resulting material is usually
referred to as a plastic.
• Frequently, polymers, resins, plastics are used
interchangeably.
4. Molecular weight (MW)
Degree of polymerization (DP)
Chain structures
Crystalline and amorphous polymers
Glass transition temperature (Tg) and melting
temperature (Tm)
Mechanical properties
5. Thermoplastic: the resin is heated to make a viscous
liquid and then processed into a usable object without
much additional chemistry. Example: polyethylene,
polystyrene.
Thermoset: upon heating, further reaction occurs to
make molecules “set up” into a useful product.
Chemistry occurs, so these are sometimes called
“reactive polymers”. The resin may be provided as
either small molecules or “prepregs”—partially
polymerized stuff. Example: polyurethanes, phenol-
formaldehyde, melamine-formaldehyde, epoxy glue.
8. In this process, a polymer is melted and the melt is pushed ahead by a
rotating screw continuously along the extruder barrel and across a metal
die that shapes the melt into the desired form.
9. Feed zone – picks up the polymer or the compounded granules from the hopper
and push them forward into the plasticating zone, where the granules melt under the
influence of shearing action and heat.
The molten mass then passes through a compression zone to the metering zone
The metering zone helps in maintaining a uniform flow rate at the die point & build
up enough pressure to force the molten mass out of the die
10. The lab-scale twin
screw extruder
The polymer
extrudate, obtained
from the die
The polymer in
pellet form
11. Chapter 15 -11
Processing Plastics – Extrusion
Adapted from Fig. 15.25,
Callister 7e. (Fig. 15.25 is from
Encyclopædia Britannica, 1997.)
12.
13. The extrusion machine forms the basis of
nearly all other polymer processes.
Basically involves melting polymer pellets and
extruding them out through a two
dimensional die.
Produces long, thin products
Coating for electrical wire
Fishing Line
Tubes, etc.
14. ME 355 W. Li
Single screw and Twin Screw extruder
Twin screw extruders involve much higher
shear energy, and care is needed when
processing thermally-sensitive polymeric
materials such as thermoplastic polyurethane
and PVC, as aggressive melt processing
inevitably leads to a loss in molecular weight
and reduction in properties and performance.
15. The technique of injection moulding of polymers was developed to
achieve high speed moulding of thermoplastics.
In this process, a molten thermoplastic is injected or shot into a steel
mould cavity under high pressure with the help of plunger system.
The injection units consists of a barrel of appropriate length to allow
melting of the polymer/plastic compound as it runs down its length
through successive zones of higher temperature under the pressure
applied through a ram, plunger or a screw.
As the screw rotates, it pumps the molten polymer forward, the screw
itself moving backwards allowing accumulation of enough material to fill
the mould. The screw stops rotating as the mould is closed and then as it
moves forward axially, it plays the role of an injection plunger. The two
kinds of motion of the screw are controlled by a hydraulic system.
The filling of the mould is accomplished by the flow of the polymer melt
under high injection pressure along the nozzle, runners and gates into the
cavities.
18. Injection moulding is used for the
formation of intricate plastic
parts with excellent
dimensional accuracy. A large
number of items associated with
our daily life are produced by
way of injection moulding. Typical
product categories include
housewares, toys, automotive
parts, furniture, rigid packaging
items, appliances and medical
disposable syringes.
19. Very similar to die
casting
Must control heat
transfer and fluid flow
- by controlling
temperature and
pressure
ME 355 W. Li
20. Gas Assist Injection Molding is a low-pressure process utilizing nitrogen
gas to apply uniform pressure throughout the molded plastic part.
It is a method of making use of the gas pressure to make the problems of
common molding, such as sin mark, warping to the lowest level.
21. ME 355 W. Li
Gas assist injection technology is applied to a variety of plastics, such as
plastic chair, plastic hanger, computer, TV, car shell, bathroom furniture,
household electrical appliances and daily supplies, various types of plastic
boxes and so on.
Advantages:
Elimination of sink marks
Reduced in mold pressure by up to 60%
Reduced clamp tonnage, so can use smaller injection molding
machine, less power consumption
Reducing stress and warpage
Cycle time reduction
Permitting different wall thickness
22. ME 355
The rapidity of moulding & other advantages of injection moulding process
prompted efforts toward developing similar techniques for the processing
of thermosetting compounds. Thermosets undergo a chemical crosslinking
that produces the solid structure. The process must be so designed &
controlled as to prevent premature hardening of the thermosetting mass
in the injection unit or the plasticating chamber as it is preheated there, but
at the same time to heat it sufficiently to enable it flow rapidly and fill the
mould under positive pressure.
23. 2 types of methods
a) Jet Moulding
b) Reaction Injection Moulding
A modification of injection molding designed for moulding thermosets.
The moulding powder is heated in the plasticating cylinder just enough to
permit only flow under high pressure but allowing no measurable chemical
reactions to take place in the plasticating cylinder. The plasticated mass at a
relatively low temperature is then forced or injected at a high speed through the
jet or nozzle into the mould which is kept at a much higher temperature, leading
to fast curing inside the mould.
JET MOULDING
24. ME 355 W. Li
This technique is of a more recent development. It involves injection of
low viscosity monomers or low polymers into the mould.
Two highly reactive liquid monomers are carefully metered, brought
together in a mixhead, and immediately injected into a heated mold under
low pressure.
The process is suitable for the production of polyurethane moulding by
rapid injection of metered liquid streams of a diol and a diisocyanate into
an impingement chamber, where the 2 reactants get mixed intimately.
Fast polymerization occurs inside the mould cavity under the influence of
catalyst, heat and pressure employed.
(See Video)
25. Blow Molding (see video)
Polymer process that creates hollow plastic parts
An important polymer processing technique for manufacturing hollow
articles such as bottles and the like, and the technique is borrowed from the
glass industry.
This process begins with melting down the plastic that is to be blow molded
in to a parison
Parison (preform) - is a tube-like piece of plastic with a hole in one end
through which compressed air can pass.
The parison is then clamped into a mold and air is pumped into it. The air
pressure then pushes the plastic out to match the mold. Once the plastic
has cooled and hardened the mold opens up and the part is ejected.
Mainly limited to processing of thermoplastics
Most plastic bottles are processed this way
There are three main types of blow molding: 1) Extrusion blow moulding
2) Injection blow moulding
3) Stretch blow moulding
29. Thermoforming is a manufacturing process where a
plastic sheet is heated to a pliable forming
temperature, formed to a specific shape in a mold, and
trimmed to create a usable product. The sheet, or "film"
when referring to thinner gauges and certain material
types, is heated in an oven to a high-enough
temperature that it can be stretched into or onto a mold
and cooled to a finished shape.
30.
31. Process used to shape thermoplastic sheet into
discrete parts
Basic principles
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
34. Thermoforming process is differed from other
plastic processing because the material is not
melted, lower pressures are required to
thermoform (the mold materials is less sturdy
materials compared to other processings)
Disadvantage; generates greater amount of
scrap, cost of the sheet materials is raised
(because of separate sheet-forming step),
limited design parts (parts with sharp bends
and corners are difficult to be produced),
process results in internal stresses
37. 1998 – Approximately 20% of plastic waste is
recycled (optimistic estimate)
1998 – Polymers account for approximately
18% by volume of material to landfills
38. Conservation of non-renewable fossil fuels -
Plastic production uses 8% of the world's oil
production.
Reduced consumption of energy.
Reduced amounts of solid waste going to
landfill.
Reduced emissions of carbon-dioxide (CO2),
nitrogen-oxide (NO) and sulphur-dioxide
(SO2).
39. Mechanical recycling of plastics refers to
processes which involve the melting, shredding
or granulation of waste plastics.
Plastics must be sorted prior to mechanical
recycling.
At the moment in the UK most sorting for
mechanical recycling is done by trained staff
who manually sort the plastics into polymer
type and/or colour.
40. Following sorting, the plastic is either melted
down directly and moulded into a new shape,
or melted down after being shredded into
flakes and than processed into granules called
regranulate.
41. • PET (polyethylene
terphthalate) beverage
containers, boil-in food
pouches, processed meat
packages
• HDPE (high density
polyethylene) milk bottles,
detergent bottles, oil bottles,
toys, plastic bags
• PVC (polyvinyl chloride)
food wrap, vegetable oil
bottles, blister packaging
• LDPE (low density
polyethylene) shrink-wrap,
plastic bags, garment bags
• PP (polypropylene)
margarine and yogurt
containers, caps for
containers, wrapping to
replace cellophane
• PS (polystyrene) egg
cartons, fast food trays,
disposable plastic silverware
• Other multi-resin
containers
42.
43. COMPOSITES
The word composite means a macroscopic combination of two
or more distinct materials having an identifiable interface
between them, but more pointedly, the definition is confined
to only such materials as would contain a fibrous or
particulate reinforcement supported by a binder or matrix
material.
Polymers are particularly attractive as matrix materials
because of their relatively easy processibility, low density
and good mechanical and dielectric properties.
POLYMER MATRIX COMPOSITE (PMC)
44. The resin systems used to manufacture advanced composites
are of two basic types: thermoplastic and thermosetting.
Thermosetting resins predominate today, while
thermoplastics have only a minor role in advanced
composites manufacture. Plastics are used so frequently in
composites today and the development of plastics technology
had to precede the development of suitable composites.
However, they are also other polymer types include
elastomers (or rubbers) and thermoplastic elastomers (TPEs).
45. Usually, fiber is used to reinforce a thermoset. The fiber
increases the tensile strength of the composite, while
the thermoset gives it compressional strength and
toughness.
The fiber is most often glass, but sometimes Kevlar, carbon
fiber, or polyethylene. The matrix is usually a thermoset
like an epoxy resin, polydicyclopentadiene, or a polyimide.
46. ME 355 W. Li
Fibers dominate composite properties
48. Fabrication of polymer-matrix composites
Open-mold processes
Pultrusion
Closed-mold processes (Matched-die molding)
ME 355 W. Li
49. Open Molding (Contact Molding) is the simplest method of
fabrication of Polymer Matrix Composites.
Open Molding is usually used for manufacturing large individual
parts (swimming pools, boat bodies).
Open Molding method is mostly used for fabrication Glass fiber
reinforcing Polymers (Fiberglasses) with polyester
(sometimes epoxy or vinylester) matrix.
The methods of open mold fabrication:
I. Hand Lay-up
II. Spray-up method
III. Tape Lay-up
IV. Filament Winding
V. Autoclave Curing
OPEN MOLD PROCESSES
50. Hand Lay-up
The most popular type of Open Molding is Hand Lay-up process. The
Hand Lay-up is a manual, slow, labor consuming method, which
involves the following operations:
The mold is coated by a release anti-adhesive agent, preventing
sticking the molded part to the mold surface.
The prime surface layer of the part is formed by applying gel coating.
A layer of fine fiber reinforcing tissue is applied.
Layers of the liquid matrix resin and reinforcing fibers in form of woven
fabric, rovings or chopped strands are applied. The resin mixture may
be applied by either brush or roll.
The part is cured (usually at room temperature).
The part is removed from the mold surface.
The disadvantages of the Hand Lay-up method are: low concentration
of reinforcing phase (up to 30%) and low densification of the
composites (entrapped air bubbles). Also a slow process and the
resulting composite depends on the skill of the labor.
51. ME 355 W. Li
Spray-up
In Spray-up process liquid resin matrix and chopped reinforcing
fibers are sprayed by two separate sprays onto the mold surface.
The fibers are chopped into fibers of 1-2” (25-50 mm) length and
then sprayed by an air jet simultaneously with a resin spray at a
predetermined ratio between the reinforcing and matrix phase.
The Spray-up method permits rapid formation of uniform
composite coating, however the mechanical properties of the
material are moderate since the method is unable to
use continuous reinforcing fibers.
52. ME 355 W. Li
Filament Winding
Filament Winding method involves a continuous filament of
reinforcing material wound onto a rotating mandrel in layers at
different layers. If a liquid thermosetting resin is applied on the
filament prior to winding the, process is called Wet Filament Winding.
If the resin is sprayed onto the mandrel with wound filament, the
process is called Dry Filament Winding.
Besides conventional curing of molded parts at room temperature,
Autoclave Curing may be used.
53. Control the orientation
of the fiber using a
relative motion of
rotation and linear
motion
Example Products:
Pressure Vessels
Fishing Poles
Light Poles
54. Pultrusion is an automated, highly productive process of fabrication
of Polymer Matrix Composites in form of continuous long products of
constant cross-section.
Pultrusion is used for fabrication of Fiberglasses, Carbon Fiber Reinforced
Polymer Composites and Kevlar (aramid) fiber reinforced polymers.
55. 1. Reinforcing fibers are pulled from the creels. Fiber (roving) creels may be
followed by rolled mat or fabric creels. Pulling action is controlled by the
pulling system.
2. Guide plates collect the fibers into a bundle and direct it to the resin bath.
3. Fibers enter the resin bath where they are wetted and impregnated with
liquid resin. Liquid resin contains thermosetting polymer, pigment, fillers,
catalyst and other additives.
4. The wet fibers exit the bath and enter preformer where the excessive resin is
squeezed out from fibers and the material is shaped.
5. The preformed fibers pass through the heated die where the final cross-
section dimensions are determined and the resin curing occurs.
6. The cured product is cut on the desired length by the cut-off saw.
A scheme of the process is presented on the diagram:
56. ME 355 W. Li
Closed Mold methods are used when mass production of identical parts with
both smooth surfaces is required.
Closed Molding methods are used for fabrication of Fiberglasses, Carbon
Fiber Reinforced Polymer Composites and Kevlar (aramid) fiber reinforced
polymers.
Example:
Compression Molding
Injection Molding
Transfer Molding
CLOSED MOLD PROCESSES
57. Compression Molding
Compression Molding is a Closed Mold process in which a
molding charge is squeezed into a preheated mold taking a shape
of the mold cavity and performing curing (cross-linking) due to heat
and pressure applied to the material.
The method uses a split mold mounted in a hydraulic press
58. Injection Molding
Injection Molding is a Closed Mold process in which molten polymer
(commonly thermoplastic) mixed with very short reinforcing fibers (10-
40%) is forced under high pressure into a mold cavity through an opening
(sprue).
Polymer-fiber mixture in form of pellets is fed into an Injection Molding
machine through a hopper. The material is then conveyed forward by a
feeding screw and forced into a split mold, filling its cavity through a
feeding system with sprue gate and runners.
Screw of injection molding machine is called reciprocating screw since it
not only rotates but also moves forward and backward according to the
steps of the molding cycle.
It acts as a ram in the filling step when the molten polymer-fibers mixture
is injected into the mold and then it retracts backward in the molding step.
Heating elements, placed over the barrel, soften and melt the polymer.
The mold is equipped with a cooling system providing controlled cooling
and solidification of the material.
The polymer is held in the mold until solidification and then the mold
opens and the part is removed from the mold by ejector pins.
59. Resin Transfer Molding is a Closed Mold process in which a
pre-weighed amount of a polymer is preheated in a separate
chamber (transfer pot) and then forced into a preheated mold
filled with a reinforcing fibers, taking a shape of the mold cavity,
impregnating the fibers and performing curing due to heat and
pressure applied to the material.
The method is used primarily for molding Polymer Composites
with thermosetting resin matrices, but some thermoplastic
parts may also be produced by Transfer Molding.
61. The Resin Transfer
Molding Process:
• Fiber reinforcement is placed in
the mold
• The mold is closed and clamped
• Resin is injected into the mold
cavity under pressure
• The resin cures
• The part is removed from the
mold
Pressurized
Resin
Air Air
The RTM Process
Steps of RTM Process (see video)
62. Open Molding
Only one sided mold
Limited temperature
control
Lower capital cost for
molds
Closed molding – or
compression molding
Two sided mold – part is
sandwiched between two
mold halves
Pressure applied
Improved material
properties (fewer voids)
Higher Capital cost
