Polymers and composites

Question 1

Draw and label a diagram of extrusion moulding

Answer 1

Question 2

In terms of Polymer processing, explain what occurs in the metering, compression and Feed zone of an extrusion moulder and how the pressure changes.

Answer 2

1. Feed zone =
   - Feeds solids forward
   - Packs material
   - Melt film begins to form (gel point)
2. Compression zone =
   - Flight depth decreases
   - Pressure Increases
   - Compaction of polymer
   - Entrapped air squeezed out via a hopper
   - High pressure to control flow
3. Metering zone =
   - Melt homogenisation
   - Uniform flow at constant temperature and pressure

Question 3

What two ratios affect a polymer extrusions screw characteristics and give example values

Answer 3

Screw characterisation =

L/D ratio ie length to diameter ratio

Compression ratio =

The depth of flight at feed zone [H]

The depth of flight at metering zone [h]

Typical values =

• L/D = 15 to 35
• Compression ratio 2:1 to 4:1

Question 4

Describe the process of polymers melting within the screw and draw a diagram related to this process.

Answer 4

Polymer Processing - Extrusion

1. Melting begins at the end of the feed zone.
2. A thin film of molten polymer forms at the barrel wall (heat source).
3. Advancing screw flight scrapes molten film off the wall.
4. Melt pool accumulates in front of the advancing flight (high-pressure side).
5. Solid bed accumulates behind advancing flight (low-pressure side).
6. As the material is transported downstream melt pool increases in size at the expense of the solid bed.
7. Melting should be completed before entering the metering zone.

Question 5

Explain the functions of the Breaker plate and screen pack system

Answer 5

Breaker plate and screen pack system:

Breaker plate double function of:

1. breaking up the rotational flow of the melt, converting it into translational flow into the adapter zone.
2. supporting the filter pack, and preventing the fine mesh wires breaking under the pressure developed in the melt by the screw.

Screen pack acts as a filter for coarse particles and contaminants but more importantly creates a back-pressure without which control of the flow of the melt would not be possible. This flow control is essential for uniform production.

Question 6

What are vital factors that determine the effectiveness of extrusion moulding

Answer 6

In theory each polymer under different processing conditions requires a different screw. This is not feasible on a cost basis.

No processor would have more than two screws for any one extruder, so a compromise must be reached. Some factors to be considered would be:

1. the form in which the polymer is fed to the extruder

i.e. granules, powders, continuous ribbon, regran etc.

2. the melt viscosity

i.e. the polymer type, temperature, hydrostatic pressure etc.

3. thermal degradation

i.e. processing temperature and dwell time

Question 7

Identify the important features in an extrusion die design. Include a diagram outlining the parts of the die

Answer 7

The important features in die design are :

1. The adapter and die system must give a smooth flow of melt with no dead spots.

2. The approach channel to the final parallel should taper gradually to maintain compression and assist flow.

3. The die parallel must be long enough to exert back pressure to control uniform flow.

4. The die faces must be aligned precisely.

Question 8

Draw and label an injection moulder used for polymer processing

Answer 8

Question 9

(a) Describe the polymer manufacturing process of extrusion. (b) Identify the typical properties which can be determined using thermo-analytical techniques and why these are important?

Answer 9

(a). The extruder is essentially a screw conveyor: Carries cold plastic granules (or powder) forward Compacts them under pressure at high temperature Feeds the material forward into the form shaping die as a uniform and homogeneous melt (b) Thermo-Analysis (studying polymer properties as they change with temperature)  Differential scanning calorimetry (DSC) Chemical changes Melt  Thermogravimetric analysis (TGA) Physical changes: Weight loss and gain Moisture storage Degradation  Thermomechanical analysis(TMA) Expansion/contraction in the mould Mould design etc By carrying out these types of tests, you are attempting to simulate manufacturing processes. Determine times polymer can be in the barrel or mould before changes take place. Differences in off-line / industrial techniques. Helps reduce development time for new materials/products

Question 10

What factors of the in-coming raw polymeric material need to be assessed for quality control purposes?

Answer 10

To gain the most comprehensive understanding of the raw material, the following should be assessed:  Flow characterisation under simulated processing conditions  Thermal response  Granule size variation  Residue content, e.g. By ashing or TGA Note that the cost implications of this can be quite significant.

Question 11

How does melting take place in an extrusion process?

Answer 11

1. Melting begins at the end of the feed zone. 2. Thin film of molten polymer forms at the barrel wall (heat source). 3. Advancing screw flight scrapes molten film off wall. 4. Melt pool accumulates in front of the advancing flight (high pressure side). 5. Solid bed accumulates behind advancing flight (low pressure side). 6. As material is transported downstream melt pool increases in size at the expense of the solid bed. 7. Melting should be completed before entering the metering zone. Melting takes place due to: 1. Transfer of heat from the barrel walls. 2. Dissipation of mechanical energy into heat through the deformation of the solid plastic.

Question 12

(a) Identify the major components of an injection-moulding machine. (b) Discuss the effect and type of defects in injected moulded parts.

Answer 12

(a) . Material is fed into a heated barrel, mixed, and forced into a mould cavity where it cools and hardens to the configuration of the cavity. Injection moulding machine consists of two major components: The Injection unit and the mould assembly and clamping unit
(b) Shrinkage and mould defects

 Short shots – polymer solidification prior to the mould being filled.

 Flashing – excess polymer squeezed out at the parting line in the tool.

 Sink marks and voids – for thick mould sections because of the specific volume change during cooling if the solidified skin is thin then the internal stresses cause the surface to deflect causing a depression on the mould surface to accommodate the volume change. On the other hand if the skin is stiff and resists deflection then the volume change due to cooling must be accommodated internally and hence voids are formed.

 Weld lines – when polymer is injected into a mould at more than one point a number of flow fronts move throughout the cavity until ultimately these meet up. The flow fronts on confrontation move transversely (parallel) to each other with no mass flow across the boundary forming a weakness between the two fronts. Under load this can be readily fractured. Careful component and tool design is required to ensure that this is not situated in a critical part of the component.

Question 13

Discuss the moulding assembly and clamping arrangement for an injection moulder.

Answer 13

Mould assembly is made up of at least two platens, which support the precision- engineered tool (mould). One half of this is movable so that the mould can be opened and closed to eject the solidified mouldings (components). The clamping unit can be toggle, hydraulic or hydromechanical mechanisms providing high pressures to resist the high injection pressures associated with this process, keeping the mould closed so that flash free components can be produced which require minimum trimming after ejection.

Question 14

What are the three types of composites? – Give examples of each.

Answer 14

1. Particulate: Concrete, cemented carbide (tungsten carbide particles in cobalt)
2. Fibrous: Wood, bone, glass fibre reinforced polymer, carbon fibre reinforced polymer
3. Laminate: Plywood

Question 15

What is the role of the textile in a composite material?

Answer 15

 Textile reinforcements using engineering fibres for composites have good tensile strength and are lightweight but have poor performance in terms of compression or stiffness.  This necessitates the use of a matrix to encapsulate the fibres, thus:  Protecting them from damage (mechanical and/or environmental)  Enhancing the performance of the composite, in particular overcoming some of the weaknesses of textiles.

Question 16

What is the definition of a structural composite?

Answer 16

Structural composites can be defined as products which:

 Use fibre reinforcements; e.g. carbon, aramid or glass, 50-70% by weight  Very high strength and stiffness

 Made with polymeric metal and other matrices The matrix binds the reinforcing fibres together, forming a cohesive structure. Applied stresses transferred from one filament through the matrix to the adjacent filament. Polymeric matrices give low densities with very high specific properties i.e. high strength/weight and high stiffness/weight ratios.

Question 17

Describe the steps involved in the manufacture of high structural composites by use of autoclave and prepreg technology.

Answer 17

 The plies are hand laid into the thoroughly degreased and clean moulding tool in the correct sequence and orientation. Constant inspection and signing-off of the lay-up at each stage is necessary to ensure performance and quality.  Where the component is comprised of a large number of plies frequent de-baulking is required i.e. the lay-up is compressed under vacuum, after which a further series of plies are laid-in.  A balanced lay-up, i.e. symmetry of lay-up about the neutral axis, minimises the extent of spring-back. Once the lay-up is completed, a layer of release film is placed on top of the plies, breather cloth placed on top of the release film and the whole assembly is bagged and sealed as shown.  Vacuum is then applied to the complete assembly.  After confirming the integrity of the seal, the bagged assembly while still under vacuum, is placed in a computer controlled autoclave which is programmed to follow a particular processing cycle of both temperature and pressure.

Question 18

What are the advantages and disadvantages of the autoclave prepreg manufacturing process?

Answer 18

Advantages: • High fibre volume fraction, (60%) - quality composite • Low void content (< 1%) - quality composite • Uniform fibre and resin distribution (controlled) • High performance Disadvantages: • Low temperature storage required for prepreg. • Wastage of high value materials during ply cut-out (gerber cutter). • Labour intensive and regular inspection required during ply lay-up. • Health and safety precautions due to handling of prepreg • Bottleneck in composite production

Question 19

What are the advantages and disadvantages of composite manufacture by liquid moulding?

Answer 19

Advantages: • Low capital investment although may be complex (expensive) • Dry preforms used – no cold storage required • Shelf-life constraints are not relevant • Near net-shaped preforms eliminate wasteful cutting out • Preforms laid into tool – no handling of plies

Disadvantages: • Lower pressures • Poorer consolidation

Question 20

Identify the two basic types of polymer and give examples of them

Answer 20

There are two basic types of polymeric materials, (excluding rubbers). These are:

• Thermoplastic polymers are solid at room temperature but become a viscous liquid at elevated temperature. This is a reversible process through cooling and heating without a significant change to the material.

Examples: polyethylene (PE)

polypropylene (PP)

polyamide or nylon (PA)

poly (vinyl chloride) (PVC)

• Thermosetting polymers when initially heated soften and flow for moulding but also undergo a chemical reaction (cross-linking) which hardens the material into a solid. Reheating of the material causes degradation and char. This process is irreversible

Examples: epoxy

polyester (thermoset)

phenolic

Question 21

The inherent viscosity of polymeric material is a function of:

Answer 21

• molecular architecture
• molecular chain length
• molecular weight distribution
• additive system employed

Question 22

State the 5 properties reinforced fibres should have

Answer 22

Reinforcing fibres should have the following properties:

1. High modulus of elasticity in the direction of the fibre.
2. High ultimate strength in the direction of the fibre.
3. Low variation in mechanical properties between fibres.
4. Stability and retention of mechanical properties during handling and fabrication.
5. Uniform fibre cross-section.

All fibres used in polymer engineering can be divided into two categories, namely synthetic and natural fibres, carbon, glass, and Aramid fibres represent the most important. As far as their natural counterparts are concerned, plant fibres are the most commonly used.

Question 23

Probably the biggest single factor is making a decision on which processing technique to use is the working temperature requirement of the composite component. Systems for producing composite structures fall loosely into 3 temperature areas. Identify those three areas:

Answer 23

• Low temperature range: up to 60^oC. For general use, including the majority of components for marine applications. Sheet Moulding Compounds (SMC), wet lay-up, and RTM are the conventional manufacturing techniques employed. Composite tooling systems include polyester based resins and epoxies.
• Middle temperature range: 60^oC – 125^oC. Composite component manufacture for special uses. Some in marine components, where high quality polyester based resins and epoxies give greater resistance to water as well as temperature. Techniques for component manufacture include wet lay-up and RTM. Composite tooling systems include wet lay-up laminating techniques and low temperature prepregs.
• High temperature range: 125^oC – 180^oC. Apart from high temperature working properties, this range of systems also give maximum chemical resistance and mechanical properties. Systems for this temperature range are large epoxy of phenolic base, and at the top end of the range are used for military aircraft components. Civil aircraft components generally fall into the category of systems at the lower end of the range, e.g. 150^oC. Wet-lay-up techniques are possible but prepreg manufacture is more usual at this temperature range. RTM novel fibre technology is also an increasingly important option. Composite based tooling on epoxy resins, both wet lay-up and prepreg, is produced from systems in this range.

Question 24

Discuss the use of autoclaves used in the production fo composites

Answer 24

Autoclave manufacture has been the chosen method of manufacture for high performance composite within the aerospace industry since the introduction of composites for structural applications. Manufacture by this method generally involves the building up of a thickness of pre-impregnated (prepreg) carbon fibre in a laminate configuration on a very rigid mould tool. This part of the manufacturing process can be completed by hand or automatically employing specially designed tape laying or ply laying robots. The lay-up of the initial structure is a long process that may also involve a number of debulking routines. Debulking is the application of pressure on the laminate to consolidate it before more plies are added. Consideration in the lay-up process must also be given to such things as ply drop-off, cores and inserts. After the lay-up is completed the laminate is “bagged” and placed in an autoclave. The autoclave applies pressure to the bagged laminate this may be in the range of 5-14 Bar. A heating routine is also completed to ensure resin flows from layer to layer in the laminate and subsequent laminate curing.

The autoclave process, when controlled, produces a

High fibre volume fraction,

Low void content composite.

However autoclave manufacture has a:

• High-cost factor brought about by the labour intensive preparation, autoclave pressurizing and heat cycles.
• In addition to this the initial set-up costs for manufacture by this route are very high.

Question 25

Discuss the use of liquid moulding in the production of composites

Answer 25

As far back of the 1960’s hand lay composites were being challenged by “closed mould” liquid moulding, of composite materials.

Liquid moulding is the introduction of resin to a closed or semi-closed mould tool by vacuum or positive pressure. In all cases of liquid moulding, the resin, after entering the mould tool must flow through the reinforcement and finally exit the tool at an appropriate point. Once the resin has permeated through the reinforcement the inlet to the mould tool and the outlet to the pressure pot are sealed and the composite cured.

The curing of the composite material can take place in an oven or in some cases can be at room temperature. When curing has been completed the component is removed from the mould tool. Whereas autoclave moulding is generally used for high performance composites, liquid moulding is used across the range of composite components from low performance car panelling to structural automotive components. The advantages, over traditional autoclave production methods, created by the use of liquid moulding for composites components are; reduced cycle times, lower initial set-up costs, increased production rates, more complex parts geometries are possible and the possibility of using advanced textile preforms.

Question 26

Discuss the use of RTM in producing composites

Answer 26

RTM is prevalent in electronics, corrosion resistant, recreational vehicle and consumer markets because it

produces two finished sides and class A finishes – the highest quality surface technically available.

A relatively low pressure (vacuum to 100 psi)) process,

RTM moulds near net shapes in 30 to 60 minutes. The production capabilities of RTM have long been known.

Advancement of the technique for the production of structural components with high fibre volume fractions has increased with the use of RTM in the production of aerospace components including landing gear on the NATO NH90 helicopter, surely a good demonstration that RTM is suited to advanced composites as well as low cost mass produced products.

Further advantages of RTM include:

Lower cost tooling – low injection and compaction pressures result in smaller forces acting on the tool and therefore lighter, cheaper tooling can be used. This tooling does not have to be in line with the high strength steel tools used for autoclave processing.

Shorter Cycle times – rapid injection and cure of the resin coupled with thermal analysis and simulation of cure cycles has resulted in reduction of processing times.

Reduced Labour – closed mould technology has an inherently lower amount of preparatory work than the lay-up vacuum bagging technique.

Reduced worker exposure – utilisation of dry reinforcement reduces the exposure of the operative to dangerous resin/prepreg systems

Increased productivity – faster moulding cycles and reduced turn around time for tool usage lead to increased productivity

RTM Processing Parameters

The RTM process can be described as having 5 stages.

Design of Preform

Preform manufacture/preparation

Tool Loading - preform consolidation

Injection of resin - mould filling/resin flow

Resin cure

Question 27

State some common uses of polymers

Answer 27

• Household furnishings

Carpets, curtains and wallpaper

• Electrical Fittings

Wire Insulation, casings for electrical goods, printed circuit boards

• Household Fittings

Drain pipes, kitchen fittings, window frames, mastics

• Surgical Prostheses

Implants, tooth fillings, contact lenses

• Transport

Bicycles, cars, trains, planes, space-craft

• Sports Materials

Clothing, shoes, athletic tracks

• Domestic

Utensils, containers, non-stick coatings

Question 28

What properties would make up an ideal Engineering polymer

Answer 28

• Low cost and low density-polymers are bought by weight and sold by volume and if the material has a high density then more of it will be required to make a moulded component;
• Easy processing, low mould shrinkage and dimensional stability in use – this is so that components may be made quickly and accurately, and so that components will retain their dimensions in service;
• Transparency – if a material is transparent then the colour range is only limited by the thermal stability of the colourant system;
• High strength, stiffness and impact strength – these are obviously useful in metal replacement applications.
• Fatigue and creep resistance – a major limitation of many polymers is that they will creep, or deform, under continuous loads (which need not be very large);
• Resistance to changes in temperature – what limits the use of many polymers is that components distort, or change shape, on heating. An EP should have good resistance to thermal degradation and to deformation; it should also not become brittle when the temperature is lowered;
• Flame resistance – a major problem with many polymers is their ease of burning and the degradation products produced on burning (e.g. smoke, burning drops etc.);
• Wear resistance – this is important because of the application areas of many EP components e.g. in bearing applications.

Question 29

State the advantages and disadvantages of thermosetting and thermoplastic polymers

Answer 29

Thermosets

FOR

• Cold cure resins simplify processing
• Low-pressure moulding means cheaper tooling
• Contact moulding suitable for large mouldings and low-volume production
• Good temperature and fire resistance

AGAINST

• User must control chemical reactions and cure process
• Liquid resins have limited shelf-life
• Health hazards from resin handling
• Recycling not easy
• Resins can be brittle, giving composites low toughness

Thermoplastics

FOR

• Can be processed quickly by hot pressing or injection moulding
• Minimal knowledge of resin chemistry needed
• Available as solid pellets which are safe to handle with a long shelf-life
• Ductility gives tougher composite materials
• Waste can be recycled
• Good environmental resistance

AGAINST

• High-temperature, high pressure moulding requires expensive tooling and sensitive controls
• Expensive tooling is only cost effective for high-volume production
• Resins soften and may burn at high temperature
• Temperature and chemical resistance varies widely

Question 30

State the 6 major steps of the injection moulding process, access the use of injection moulding and identify the considerations that have to be made when designing new moulds.

Answer 30

In summary:

There are six major steps in the injection moulding process:

1. Clamping

An injection moulding machine consists of three basic parts; the mould plus the clamping and injection units. The clamping unit is what holds the mould under pressure during the injection and cooling. Basically, it holds the two halves of the injection mould together.

2. Injection

During the injection phase, plastic material, usually in the form of pellets, are loaded in to a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorised screw that mixes the molten pellets and forces them to the end of the cylinder. Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mould through a sprue, while the pressure and speed are controlled by the screw. Note some injection moulding machines use a ram instead of a screw.

3. Dwelling

The dwelling phase consists of a pause in the injection process. The molten plastic has been injected into the mould and the pressure is applied to make sure all of the mould cavities are filled.

4. Cooling

The plastic is allowed to cool to its solid form within the mould.

5. Mould Opening

The clamping unit is opened, which separates the two halves of the mould.

6. Ejection

An injecting rod and plate eject are finished piece from the mould. The un-used sprues and runners can be recycled for use again in future moulds.

Advantages and Disadvantages of Injection Moulding

Advantages of Injection Moulding

• High Production Rates
• High Tolerances are repeatable
• Wide range of materials can be used
• Low labour costs
• Minimal scrap losses
• Little need to finish parts after moulding

Disadvantages of Injection Molding

• Expensive equipment investment
• Running costs may be high
• Parts must be designed with moulding consideration

Examples of injection molded parts in the automotive industry include sail panels, radiator end caps, door panels, lamp housings, and fuel rails

It is use for the high-volume production of small articles, often with an intricate shape, such as valves, pumps and fans.

Considerations for New Injection Moulding Designs

There are many important points to consider when designing a new injection moulded product. Before starting, consider the following requirements and parameters:

• Strength (weight/strength ratio)
• Price (weight/price ratio)
• Colour, shape, and sales appeal
• Processability
• Temperature properties
• Electrical and heat insulation properties
• Pollution and energy demands
• Life expectancy
• Safety

Question 31

Discuss the situations when traditional machining processes are not appropriate (not economic, feasible or even possible).

Answer 31

Traditional methods for the removal of material by chip formation, abrasion, or microchipping.

There are situations when these traditional processes are not satisfactory, economical, or even possible in the following situations:

• The hardness and strength of the material is very high (typically above 400 HB) or the material is too brittle.
• The workpiece is too flexible, slender, or delicate to withstand the cutting or grinding forces, or the parts are difficult to fixture – that is, t clamp in workholding devices
• The shape of the part is complex, including such features as internal and external profiles or small-diameter holes in fuel-injection nozzles.
• Surface finish and dimensional tolerance requirements are more rigorous than those obtained by other processes.
• Temperature rise and residual stresses in the workpiece are not desirable or acceptable.

Question 32

Discuss the following advanced machining processes:

1. Abrasive jet machining
2. Electrochemical grinding

Answer 32

In abrasive-jet machining (AJM), a high velocity jet of dry air, nitrogen, or carbon monoxide, containing abrasive particles, is aimed at the workpiece surface under controlled conditions. The impact of the particles develops a sufficiently concentrated force to perform operations such as:

• Cutting small holes, slots, or intricate patterns in very hard or brittle metallic and non-metallic materials
• Deburring or removing small flash from parts
• Trimming and bevelling
• Removing oxides and other surface films
• General cleaning of components with irregular surfaces

•The gas supply pressure is on the order of 850kPa and
the abrasive-jet velocity can be as high as 300m/s. and is
controlled by a valve. The hand-held nozzles are usually made of tungsten carbide or sapphire. The abrasive size is in the range of from 10 to 50 μm. Because the flow of the free abrasives tends to round off corners, designs for abrasive-jet machining should avoid sharp corners; also, holes made in metal parts tend to be tapered.

• There is some hazard involved in using this process because of airbourne particulates. This problem can be avoided by using the abrasive water-jet machining process
• Electrochemical Grinding (ECG) combines electrochemical machining with conventional grinding. The equipment used is similar to a conventional grinder, except that the wheel is a rotating cathode embedded with abrasive particles. The wheel is metal-bonded with diamond or aluminium-oxide abrasives, and rotates at a surface speed of from 1200 m/min to 2000 m/min.

The abrasives have two functions:

1. to serve as insulators between the wheel and the workpiece,
2. to mechanically remove electrolytic products from the working area.

• A flow of electrolyte solution (usually sodium nitrate) is provided for the electrochemical machining phase of the operation. Current densities range from 1 A/mm2 to 3 A/mm2.
• The majority of metal removal in ECG is by electrolytic action, and typically less than 5% of metal is removed by the abrasive action of the wheel; therefore, wheel wear is very low. Finishing cuts are usually made by the grinding action but only to produce a surface with good finish and dimensional accuracy.
• The ECG process is suitable for applications similar to those for milling, grinding, and sawing. It is not adaptable to cavity-sinking operations. This process has been successfully applied to carbides and high-strength alloys. It offers a distinct advantage over traditional diamond-wheel grinding when processing very hard materials, where wheel wear can be high. ECG machines are now available with numerical controls, improving dimensional accuracy, repeatability, and increased productivity.
• Electrochemical honing combines the fine abrasive action of honing with electrochemical action. Although the equipment is costly, the process is as much as five times faster than conventional honing, and the tool lasts as much as ten times longer. It is used primarily for finishing internal cylindrical surfaces.

Question 33

Discuss the capabilities and applications for photochemical blanking

Answer 33

• Photochemical blanking, also called photoetching, is a modification of chemical milling. Material is removed, usually from flat thin sheet, by photographic techniques.
• The process is also used for etching
• Typical applications for photochemical blanking are fine screens, printed-circuit cards, electric-motor laminations, flat springs, and masks for colour television. Although skilled labour is required, tooling costs are low; the process can be automated; and it is economical for medium- to high-production volume.

•

• Photochemical blanking is capable of making very small parts where traditional blanking dies are difficult to produce. The process is also effective for blanking fragile workpieces and materials.
• The handling of chemical reagents requires precautions and special considerations to protect the workers against exposure to both liquid chemicals and volatile chemicals. Furthermore, the disposal of chemical by-products from this process is a major drawback, although some by-products can be recycled.

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Question 34

Discuss the continuing development of processing structural, advanced composites providing comparisons where appropriate (ie. how to increase mechanical performance, materials, tooling development).

Answer 34

• Autoclave manufacture has been the chosen method of manufacture for high-performance composite within the aerospace industry since the introduction of composites for structural applications. Manufacture by this method generally involves the building up of a thickness of pre-impregnated (prepreg) carbon fibre in a laminate configuration on a very rigid mould tool. This part of the manufacturing process can be completed by hand or automatically employing specially designed tape laying or ply laying robots. The lay-up of the initial structure is a long process that may also involve a number of debulking routines. Debulking is the application of pressure on the laminate to consolidate it before more plies are added. Consideration in the lay-up process must also be given to such things as ply drop-off, cores and inserts. After the lay-up is completed the laminate is “bagged” and placed in an autoclave. The autoclave applies pressure to the bagged laminate this may be in the range of 5-14 Bar.
• The autoclave process, when controlled, produces a
• High fibre volume fraction,
• Low void content composite.
• However autoclave manufacture has a
• High-cost factor brought about by the labour intensive preparation, autoclave pressurizing and heat cycles.
• In addition to this the initial set-up costs for manufacture by this route are very high.
• Liquid moulding is the introduction of resin to a closed or semi-closed mould tool by vacuum or positive pressure. In all cases of liquid moulding, the resin, after entering the mould tool must flow through the reinforcement and finally exit the tool at an appropriate point. Once the resin has permeated through the reinforcement the inlet to the mould tool and the outlet to the pressure pot are sealed and the composite cured.
• The curing of the composite material can take place in an oven or in some cases can be at room temperature. When curing has been completed the component is removed from the mould tool. Whereas autoclave moulding is generally used for high performance composites, liquid moulding is used across the range of composite components from low performance car panelling to structural automotive components. The advantages, over traditional autoclave production methods, created by the use of liquid moulding for composites components are; reduced cycle times, lower initial set-up costs, increased production rates, more complex parts geometries are possible and the possibility of using advanced textile preforms.

​

• RTM is prevalent in electronics, corrosion resistant, recreational vehicle and consumer markets because of it
• Produces two finished sides and class A finishes – the highest quality surface technically available.
• A relatively low pressure (vacuum to 100 psi)) process,
• RTM moulds near net shapes in 30 to 60 minutes. The production capabilities of RTM have long been known.
• Advancement of the technique for the production of structural components with high fibre volume fractions has increased with the use of RTM in the production of aerospace components including landing gear on the NATO NH90 helicopter, surely a good demonstration that RTM is suited to advanced composites as well as low-cost mass-produced products.

Question 35

What are the main drivers/benefit of using composites in the Boeing 787 (dreamliner)

Answer 35

• Benefits the of the 787 (aka. “Dreamliner”)
• I. Lightweightt
• a. Fuel efficient
• b. Longer range than comparable aircraft
• II. Reduced maintenance costs
• a. $30-40 million in savings
• i. High reduction in fatigue
• ii. Highly corrosion resistant

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• III. Increased passenger comfort
• a. Increase in cabin pressure
• b. Increased humidity = Result of high corrosion resistance
• c. Bigger windows due to increased strength
• d. Less noise
• e. Front engine cowl intake is made of a single piece of composite, reducing drag
• IV. Decreased assembly time

a. parts arrive from the suppliers as net-shape
b. Components are pre-installed in parts a supplier factory

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Cost- Benefit Analysis of the Boeing 787

• I. Boeing estimates that 787 will consume $5 million less in fuel on a comparable route than 767
• a. Savings = Price of plane
• II. Potentially longer life
• a. Not proven yet, but likely due to the high reduction in corrosion and fatigue

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Question 36

Define a composite material

Answer 36

‘ A material system comprised of two or more physically distinct

phases whose combination produces aggregate properties which

are different and indeed superior to its constituents’

Question 37

Discuss the use of composites in comparison to metals

Answer 37

• Properties are not uniform in all directions
• Strength and stiffness can be tailored to meet loads
• Possess a greater variety of mechanical properties
• Poor through the thickness (i.e. short transverse) strength
• Composites are usually laid up in essentially two-dimensional form, while metal may be used in bars, forgings, castings, etc
• Greater sensitivity to environmental heat and moisture
• Greater resistance to fatigue damage
• Propagation of damage through delamination rather than through-thickness cracks

Advantages of composites over metals:

• Light weight
• Resistance to corrosion
• High resistance to fatigue damage
• Reduced machining
• Tapered sections and compound contours easily accomplished
• Can orientate fibres in direction of strength/stiffness needed
• Reduced number of assemblies and reduced fastener count when co-cure or co-consolidation is used
• Absorb radar microwaves (stealth capability)
• Thermal expansion close to zero reduces thermal problems in outer space applications

Disadvantages of composites over metals:

• Material is expensive
• Lack of established design allowables
• Corrosion problems can result from improper coupling with metals, especially when carbon or graphite is used (sealing is essential)
• Degradation of structural properties under temperature extremes and wet conditions
• Poor energy absorption an impact damage
• May require lightening strike protection
• Expensive and complicated inspection methods
• Defects can be known to exist but precise location cannot be determined.

Question 38

Advantages and disadvantages of using autoclaves for composite processing

Answer 38

Composites Processing – autoclave

Advantages:

• high fibre volume fraction, (60%) - quality composite
• low void content (< 1%) - quality composite
• uniform fibre and resin distribution (controlled)
• high performance

Disadvantages:

• low temperature storage required for prepreg.
• wastage of high value materials during ply cut-out (Gerber cutter).
• labour intensive and regular inspection required during ply lay-up.
• health and safety precautions due to handling of prepreg
• bottleneck in composite production

Question 39

Provide summaries of the following composite processes:

Open moulding

Vacuum bag moulding

Pressure bag moulding

Autoclave Molding

Answer 39

Open moulding:

žTwo variations; hand layup & Spray-up

žHand Layup-used for the production of parts of any dimensions such as technical parts with a surface area of a few square feet.

žSpray Lay-up: is used to make parts with large dimensions and complex shapes.

žApplied with a pressure roller brush, sprayer device, or manually

žDone at room temperature and atmospheric pressure.

Vacuum bag moulding:

žTwo-sided mold set.

žShapes both surfaces of the panel.

žLower side is a rigid mold

žUpper side is a flexible membrane or vacuum bag

žBag made of silicone material or an extruded polymer film.

• Performed at either ambient or elevated temperature
• ambient atmospheric pressure acts upon the vacuum bag
• Most economical way uses venturi vacuum and air compressor or a vacuum pump.

Pressure bag moulding:

žA solid female mold is used along with a flexible male mold

žReinforcement is placed inside the female mould along with resin.

žResin is brushed into the mold and the mold is clamped to a machine with male mold.

žFlexible male membrane is then inflated with heated compressed air.

žExcess resin is forced out along with trapped air.

žExtensively used in the production of composite helmets.

žLower cost of unskilled labor

žCycle times is 20 to 45 minutes

žFinished heated shells require no further curing.

Autoclave Molding:

žTwo-sided mold set

žLower Side rigid mold

žUpper Side flexible membrane made from silicone or an extruded polymer film

žReinforcement materials can be placed manually or robotically

žInclude continuous fiber forms fashioned into textile constructions

žUse of autoclave pressure vessel

žprocess generally performed at both elevated pressure and elevated temperature

želevated pressure facilitates a high fiber volume fraction

žElevated pressure yields low void content for maximum structural efficiency