Chapter 19: Forming and Shaping

Plastics and Composite Materials

Faculty of Engineering

Mechanical Dept.

Introduction

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

• Hydrocarbons

CnH2n+2

Covalent bonding

Double or triple covalent

bonds may also be present

• Monomer

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

heated

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

Polymer Additives

• Fillers

Improve strength, stiffness, and toughness

Reduce shrinkage and weight

Common fillers: wood flour, silica flour, clay, powdered mica

• Plasticizers

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

• Lubricants

Reduce friction

Improve moldability

Facilitates part removal

Forming and Shaping Processes for Plastics, Elastomers,

and Composite Materials

Extruder Schematic

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

Feed section

Melt or transition section

Pumping 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

(b)

(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

Injection Molding

Similar to extrusion barrel is heated

Pellets or granules fed into heated cylinder

Melt is forced into a split-die chamber

Molten plastic pushed into mold cavity

Pressure ranges from 70 Mpa – 200 Mpa

Typical products : Cups, containers, housings, tool handles, knobs, electrical and communication components, toys etc.

Injection Molding Sequence

Products Made by Injection Molding

(b) (a)

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.

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

Types of Molds used in Injection Molding

Process capabilities

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)

Machines :

Horizontal or vertical machines

Clamping – hydraulic or electric

Injection-Molding Machine

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

Injection-Molding Machine

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

narrow runners

Form flash if dies do not mate properly

Sink marks form at thick sections due to uneven cooling

causing local shrinkage

Avoid defects by:

Temperature control

Proper pressures

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.

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.

Modified extrusion and Injection Molding process.

A tube extruded then clamped to mold with cavity larger than tube diameter.

Finally blown outward to fill the cavity

Pressure 350Kpa-700Kpa

Other Blow Molding processes

Injection Blow molding

Multi layer Blow molding

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

parts.

Slush-molding

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

Thermoforming Process

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

packaging.

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.

Compression molding

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 material.

Typical parts made are dishes, handles, container caps fittings, electrical and electronic components and housings

Materials used in compression molding are thermosetting plastics & elastomers

Curing times range from 0.5 to 5 mins

3- types of compression molds are

Flash type

Positive type

Semi-positive

Compression Molding

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

external undercuts.

Transfer molding is an improvement if compression molding

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 mold

Typical parts : Electrical & electronic components, rubber and silicone parts

Transfer Molding

Transfer Molding

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

Assemblies

Conventional casting of thermo plastics :

Mixture of monomer, catalyst and various additives are heated and poured into the mould

The desired part is formed after polymerization takes place.

Casting

Centrifugal casting :

Centrifugal force used to stack the material onto the mold

Reinforced plastics with short fibers are used

Processes for Plastics and Electrical

Assemblies

Casting the plastic around an electrical component to embed it in the plastic is potting

Used to coat transformers,

transistors, etc.

Plastic can serve as a non-

conductor

In both the plastic is Dielectric

Potting & Encapsulation

Foam Molding

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

packaging materials

Foam Molding

Cold forming

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

Considerations :

Sufficiently ductile material at room temperature

Non recoverable material deformation

Schematic illustration

of calendering.

Sheets produced by

this process

subsequently are

used in

thermoforming. The

process also is used

in the production of

various elastomer

and rubber products.

Calendaring

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

resistance.

They consist of the polymer

and reinforcing fibers,

bonded together in various

ways.

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 and kevlar.

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

producing fiber-reinforced

plastic sheets. The sheet

still is viscous at this stage

and later can be shped into

various products.

Polymer-Matrix Composites: Examples

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

Composites

Liquid metal-matrix and solid reinforcement are either cast or

pressure-infiltration cast

Metal-matrix is usually aluminum or titanium

Solid reinforcement is usually graphite, aluminum oxide or

silicon carbide.

Liquid Phase Processing

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

strength.

Dimensional tolerances, except with injection

molding, are higher than with metals

For casting, ensuring proper flow into mold cavities

is important

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.