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Composites m sc new class

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  • 1. COMPOSITES: A WONDER MATERIAL FOR THE FUTURE Sreekumar P.A
  • 2.  
  • 3. COMPOSITE: ?
    • Example of perfect partnership
    • Consists of fibre and matrix with distinct interface.
    • Each partner share the load according to one’s mite.
    • In case of exigencies, the stronger partner share the load
    • and prevent the weaker partner from failure.
    • The partners share a perfect bond.
    • But each partner retain ones physical and chemical identity
    • throughout the life time of the composite.
    • Combination produces properties that cannot be achieved
    • with either of the constituents alone.
    • Here the fibre is load carrying matrix keeps them in the desired position.
  • 4. Composite materials Fibre reinforced composites Particle reinforced composites Single layer Multi layered Composites (Angle ply) Laminate Hybrid Continuous fibre Discontinuous fibre Unidirectional Bi-directional Random orientation Preferred Orientation Random Preferred
  • 5. Fibre Natural Man Made Regenerated Synthetic Mineral Plant Animal Leaf Bast Fruit Wool Mohair Silk
  • 6. Natural fiber classification
  • 7. REASONS FOR THE USE OF NATURAL FIBERS
    • Annual growing raw material up to two crops/a
    • Low costs 0.5 to 0.9 €/kg compared to 2 €/kg for glass fibers
    • Low density 1500 kg/m 3 , glass 2500 kg/m 3
    • Fibers act non-abrasive
    • Low energy consumption one-fifth of fiber glass production
    • Good specific mechanical properties
    • Physiological harmlessness no skin irritation
    • CO 2 -neutrality
    • Residual free thermal utilization
    • Safer crash behavior (high stability and absence of splintering)
  • 8. LIMITS OF NATURAL FIBERS Natural fibers vs glass, carbon, etc .
    • High moisture adsorption
    • Poor microbial resistance
    • Low thermal resistance
    • Local and seasonal variations in quality
    • Demand and supply cycles
    • Fast absorption/desorption of water
    • Good thermal conductivity Biodegradability
  • 9. Factors influencing the mechanical properties of the composite Strength, modulus and chemical stability of the fibre and the resin matrix
    • Choice of the material depend on final requirement of the product.
    • The function of the resin matrix in a fibrous composites will vary,
    • depending upon how the composite is stressed.
    • For compressive loading the matrix prevents form buckling, and
    • therefore a very critical part of the composite without it the reinforcement
    • could not carry any load.
    • A bundle of fiber can sustain high tensile strength without matrix.
    • Resin prevent the fibre from corrosion as well from the abrasion.
    • Resin also provides stores transfer medium so that when individual
    • fibre breaks it does not loses it load carrying capacity.
  • 10.
    • The physical properties of the resin influences the
    • behavior of the composites includes the following
    • Shrinkage during cure.
    • Modulus of elasticity
    • Ultimate elongation
    • Tensile or flexural strength
    • Compression strength
    • Ineterlaminar shear strength
    • Fracture toughness
    Factors influencing the mechanical properties of the composite
  • 11. Factors influencing the mechanical properties of the composite Critical Fiber Length The minimum length per given fiber diameter essential for high tensile fracture stress.When the length of the fibre is below critical fibre length, the maximum fibre stress may never reach the ultimate fiber strength      f l           l< l c l = l c l > l c
  • 12. How to calculate critical fibre length? Consider an infinitesimal length distance dx at a distance x form one of the fibre ends the force equilibrium for this length is (  /4.d 2 f ) (  f + d  f ) – (  /4. d 2 f  f ) -  d f dx.  = 0 (1) Which on simplification gives d  f /dx = 4  / d f (2) Where  f = longitudinal stress in the fibre at a distance x from one of its ends.  = Shear stress at the fibre/matrix interface. d f = Fibre diameter. Factors influencing the mechanical properties of the composite
  • 13. Assuming no stress transfer at the fibre ends, i.e  f =0 at x=0, and integrating equation (2) the normal stress distribution in the fibre ends as  f For simple analysis, it is assumed that interfacial strength is constant. Then equation becomes  f = (4  I / d f ) x Where = interfacial shear stress. The maximum fibre stress that can be achieved at a given load is (  f ) max = 2  I (l t / d f ) Where x= l t /2 = load transfer length at each fibre end . Factors influencing the mechanical properties of the composite 3 4 5
  • 14. l c = (  f / 2  I )d f
    • For l f <l c , the maximum fibre stress may never reach the ultimate
    • fibre length. In this case either the fibre/matrix interfacial bond
    • or the matrix may fail before fibre achieve their potential strength.
    • For l f >l c , the maximum fibre stress may reach the ultimate fibre
    • strength over much of its length. Over a distance of l c /2 the fibre
    • remains ineffective
    Factors influencing the mechanical properties of the composite Where l c = minimum fibre length required for the fibre stress to be equal to the fibre ultimate strength  f = ultimate fibre strength
  • 15. Factors influencing the mechanical properties of the composite Fibre content
    • As fibre content increases mechanical properties such as tensile and
    • flexural strength,Young’s an flexural modulus for the composite
    • also increases up to a optimum fibre load beyond that limit decreases.
    • At lower fibre loading dispersion of fibre is very poor so that stress
    • transfer will not occur properly. At higher fibre loading there is a
    • chance for fibre-fibre interaction and poor wetting of fibres and thereby
    • reducing the effective aspect ratio.
    • Crack initiation and its propagation will be easier at higher loadings .
  • 16. How to calculate fibre content and composite density? Factors influencing the mechanical properties of the composite Where R is the resin content in composite, r is the sisal fibre vol%, D is the density of the resin d is the density of sisal fibre. T d =100/(R/D + r/d) V f = W f /  f W f /  f + (1-W f ) /  m Where W f is the fibre weight fraction ( 1-W f ) is the matrix weight fraction  m is the resin density  f is the fibre density
  • 17. Interfacial Adhesion How to improve interfacial adhesion? By Chemical Methods By Physical methods By using Coupling agents Factors influencing the mechanical properties of the composite fibre matrix interface
  • 18. Treatment Methods
    • NaOH treatment
    • Sisal fibres were fully immersed in 5, 10 and 15 % of NaOH solution for
    • 30 minutes. After that fibre is taken out, washed several times with
    • distilled water. Finally it is washed with water containing little acid and dried
    • Heat treatment
    • Sisal fibres were heated at 150 0 C in an air-circulating oven for
    • 4hrs continuously. The fibre was then cooled to room temperature
    • Permanganate treatment
    • Sisal fibres were soaked in KMnO 4 solution in acetone at a concentration
    • of 0.02% for 3 minutes. After that fibre is taken out, washed many times
    • with distilled water and dried in an air oven
  • 19. Treatment Methods
    • Benzoylation
    • Sisal fibres were soaked in 5% of NaOH solution for half an hour
    • and agitated well with 50ml of benzoylchloride. The mixture was
    • kept for 15mts, filtered, washed thoroughly with water. The fibre
    • is then soaked in ethanol for 1hr to remove unreacted
    • benzoylchloride and finally washed with water and dried.
    • Silane treatment
    • Sisal fibres were dipped in alcohol water mixture (60:40)
    • containing Vinyl tris (2 ethoxymethoxy) silane as coupling agent.
    • The fibres were washed in distilled water and dried.
    • Gamma Irradiation
    • Sisal fibres were exposed to gamma radiation from 60Co
    • at a dose rate of 1 Mrad for 4 hours.
  • 20. Proposed Reaction Mechanism During treatment
    • NaOH Treatment
    • Silane Treatment
  • 21. Proposed Reaction Mechanism During treatment
    • Silane Treatment
  • 22. Proposed Reaction Mechanism During treatment
    • Permanganate Treatment
    • Benzoylation Treatment
  • 23.   Influence Of Fibre Orientation Longitudinal Transverse
    • Longitudinally aligned fibrous composites are anisotropic
    • in that maximum strength is achieved in the direction
    • fibre alignment.
    • Transverse direction fracture usually occurs
    • at low tensile stress.
    Factors influencing the mechanical properties of the composite
  • 24. Continuous and aligned fibre composites Longitudinal loading There are composites in which the fibers are aligned in the direction of applied stress.Assume that all the filaments are perfectly bonded to the matrix. Where  c = composite strain  f = fibre strain  m = Matrix strain Factors influencing the mechanical properties of the composite  f =  m =  c The total tensile force applied on the composite lamina is hared by the fiber and matrix P =P f +P m Since load = stress x area: then
  • 25. Rule of mixtures Factors influencing the mechanical properties of the composite  c . A c =  f . A f +  m . A m  c . =  f (A f /A c ) +  m (A m /A c ) Where  c . = Average tensile strength A f = Area of the fibre A m = Area of the matrix A c = A f + A m Since V f = A f /A c and V m = Am/A c  c . =  f V f +  m V m This equation is known as rule of mixtures
  • 26. For transverse loading In this type the load is applied at 90 0 angle. Under this situation Stress to both phases are exposed in the same time E c = E m .E f /V m .E f + V f . E m Factors influencing the mechanical properties of the composite E c = elastic moduli of the composite E f = elastic moduli of the fibre E m = elastic moduli of the matrix V f = elastic moduli of the fibre V M = elastic moduli of the matrix
  • 27. For randomly oriented fibre composites are composed short and discontinuous fibre. Under these circumstance the expression for the elastic modulus K= Fibre efficiency parameter which value is lees than unity Modified rule of mixture Factors influencing the mechanical properties of the composite E c = KE f V f +E m V m
  • 28.   Voids
    • During the incorporation of fibers into matrix or during the
    • manufacturing, of laminates, air or other volatiles may be
    • trapped in the material.
    • Voids destroy the integrity of the composite and as they grow
    • and interact with each other, initiate cracks and promote
    • specimen failure
    How to calculate void content? Factors influencing the mechanical properties of the composite where T d is the theoretical composite density, M d is the measured composite density . V= 100(T d -M d )/T d
  • 29. Processing techniques
    • Thermoset Composites
    • Hand Lay Up, Spray Lay up
    • Vacuum Bag Moulding
    • Compression Moulding
    • Filament Winding
    • Pultrusion
    • Resin Transfer Molding
    • Structural Reaction Injection
    • Moulding
    • Thermoplastic Composites
    • Calendaring
    • Sheet Moulding
    • Film Casting
    • Injection moulding
    • Extrusion
    • Blow Moulding
    • Rotational Moulding
    • Thermoforming
  • 30. HAND LAY UP Processing stages
    • The mould is cleaned and a mould releasing agent is applied.
    • A gel of UPE resin containing pigment and curing additives
    • is brushed over the mould surface.
    • After the gel coat becomes stiffened layers of glass reinforcement
    • and resin are applied.
    • The glass layers are fully wetted and impregnated with resin by rollers.
    • When it is cured it is tripped from the mould an trimmed to size,
    • usually with power saw
  • 31. FEATURES OF HAND LAY UP
    • Extremely large parts can be made in a single moulding
    • The moulds are not pressurized and extremely large parts
    • can be made from single moulding.
    • 3. Moulds can be made from cheap materials
    • 4. Thin areas in the moulding and sharp corners often become resin rich.
    • 5. Only one surface is moulded, the other is being rough.
    • 6. The process is very operator-dependent and a consistent resin-glass
    • ratio is difficult to achieve.
    • It may require post curing to develop optimum strength.
    • Void content is high.
    • Fibre damage can occur during processing.
    • Open Moulded method
  • 32. SPRAY LAY UP
    • In this feeding a stream of chopped fibres into a spray of liquid
    • in a mould cavity
    • A specialized spray gun is used to apply the chopped fibre and resin
    • to the tool.
    • The direction of fiber is random.
    • Uniformity for the surface occurs.
    • Void content is lees when compared to handlay up.
  • 33. COMPRESSION MOULDING Moulding through the force of compression is another very common industrial process. The materials used are melamine, phenol and urea formaldehyde, Polyesters etc. Process Description The mould is held between the heated platens. A 'slug' or piece of the plastic is placed into the mould . The hydraulic press closes with sufficient pressure. The Compound softens and flows to shape. If necessary cooling is done. The press is opened and the moulding removed
  • 34. Classification of Moulds Positive Mould Semi-positive mould Flash Moul d
  • 35.
    • Filament Winding is the process of winding resin-impregnated fiber
    • or tape on a mandrel surface in a precise geometric pattern.
    • This is accomplished by rotating the mandrel while a delivery head
    • precisely positions fibers on the mandrel surface.
    • By winding continuous strands of carbon fiber, fiberglass or other
    • material in very precise patterns, structures can be built with properties
    • stronger than steel at much lighter weights.
    • Filament winding machines operate on the principles of controlling
    • machine motion through various axes of motion .
    FILAMENT WINDING
  • 36. The filament winding process was originally invented to produce missile casings, nose cones and fuselage structures, but with the passage of time industries other than defense and aerospace have discovered the strength and versatility of filament winding. Picture
  • 37. Advantages
    • The highly repetitive nature of fibre placement.
    • The capacity to use continuous fibre over the whole component
    • area and to orient fibres easily in the load direction.
    • Ability to fabricate structures that are larger than most autoclaves.
    • Obtainablity of high fibre volume fraction.
    • Lower cost for large quantity of components.
    • Relatively low material cost.
    Disadvantages
    • Difficulty in winding reverse curvature
    • Inability to change fibre path easily
    • Need for mandrel which can be complex or expensive
    • Poor external surface finish
  • 38. Characteristics of resin
    • Low temperature for curing
    • Viscosity should be lower
    • Pot life should be as long as possible
    • Toxicity should be low
    Resin Provides
    • Retaining the filament in proper position
    • Transferring the load form filament to filament
    • Protecting the filaments from abrasion
    • Controlling the electrical an thermal properties
    • Providing the interlaminar shear strength
  • 39. Impregnation method Prepreg: Wet Rerolled
    • A controlled volume of resin is impregnated on the controlled
    • length of fibre reinforcement and then respooled.
    • Preservative and solvents are not required
    Wet winding
    • This is accomplished by either pulling the reinforcement
    • through a resin bah or directly over a roller that contains
    • a metered volume of resin controlled by a blade.
    • Widely used in the case of epoxy and polyester resin
  • 40. Winding Patterns Helical
    • The mandrel rotates more or less continuously while the fibre feed
    • carriage traverses back and forth at a speed regulated to generate
    • the desire helical angle.
    • After the first circuit is applied fibre are not adjacent, additional
    • circuits must be traversed before the patterns..
    • The mandrel revolution s per
    • circuit vary with winding angle
    • band width and overall length
    • of the vessel.
    • Any combination of diameter and
    • length may wound by trading
    • off winding angle
    Picture
  • 41. Polar The fibre passes tangentially in the polar opening at one end of the chamber.Reverses direction, and passes tangentially to the opposite side of the polar.It is simple and winding speed can be maintained Hoop patterns High angle helical winding that approaches an angle of 90 o. They are generally combined with longitudinal windings to produce a balance structure
  • 42. Surface Considerations
    • Use thinner tows on the last few outer hoop over wraps
    • Do not squeegee the last hoop layers
    • Over wrap with shrink tape or teflon-coated cloth and remove after
    • curing
    • Confine the last few layers to hoops only.
    • Use high wind angles as opposed to low wind angles
    How to avoid slipping?
    • Pins to control the movement of the fibre
    • Powders or tackfying agent on wet filament wound parts
    • A tacky prepeg or wet rerolled tow to control movement
  • 43.
    • Pultrusion is a manufacturing process for producing continuous lengths
    • of FRP structural shapes.
    • Raw materials include a liquid resin mixture (containing resin, fillers and
    • specialized additives) and reinforcing fibers.
    • The process involves pulling these raw materials through a heated steel
    • forming die using a continuous pulling device.
    • The reinforcement materials are in continuous forms such as rolls of
    • fiberglass mat or doffs of fiberglass roving.
    • As the reinforcements are saturated with the resin mixture (&quot;wet-out&quot;) in
    • the resin impregnator and pulled through the die, the gelation.
    • (or hardening) of the resin is initiated by the heat from the die and a rigid,
    • cured profile is formed that corresponds to the shape of the die.
    PULTRUSION
  • 44.
    • The reinforcement must be located properly within the
    • composite and controlled.
    • The resin impregnator saturates (wets out) the reinforcement
    • with a solution containing the resin, fillers, pigment, and
    • catalyst plus any other additives required.
    • The interior of the resin impregnator is carefully designed to
    • optimize the &quot;wet-out&quot; (complete saturation) of the
    • reinforcements.
    • On exiting the resin impregnator, the reinforcements are
    • organized and positioned for the eventual placement within
    • the cross section form by the preformer
    • The preformer is an array of tooling which squeezes away
    • excess resin as the product is moving forward and gently
    • shapes the materials prior to entering the die
    Precautions to be taken
  • 45.
    • In the die the thermosetting reaction is heat activated
    • (energy is primarily supplied electrically) and the composite
    • is cured (hardened).
    • On exiting the die, the cured profile is pulled to the saw for
    • cutting to length. It is necessary to cool the hot part before
    • it is gripped by the pull block (made of durable urethane foam)
    • to prevent cracking and/or deformation by the pull blocks.
    Advantages
    • High strength to weight ratio
    • Dimensional Stability is high
    • Wire, Wood can be encapsulated on a continuous basis
    • Wide variety of reinforcement can be used.
    • Pultured shape can be made as large as required
    • Cost of die is less
  • 46. Desired resin characteristics and Matrix used
    • Low Viscosity less than 200cps
      • must remain liquid as it is held in the reservoir prior to injection
      • must impregnate fiber preform quickly and uniformly without voids
      • must gel as quickly as possible once
      • impregnation occurs must possess sufficient hardness
      • to be demoulded without distortion
    • Matrix
      • Vinyl ester
      • Polyester
      • Epoxy resin
      • Phenol formaldehyde resin
    RESIN TRANSFER MOULDING
  • 47. Preform Tool Injection Cure Demould SCHEMATIC REPRESENTATION
  • 48.
    • Mold filling process
    • Proper mold designing
    • Resin characteristic
    • Reinforcement characteristic
    • Mold temperature
    • Vaccum state of system
    • Resin flow rate
    Factors affecting the RTM Process
  • 49. Different Aspects Of Mold Filling Process?
    • Fibre washing
    • The unexpected movement or displacement of reinforcement in the closed mold. It leads to the failure of RTM due to fibre displacement and interrupt the uniformity of predetermined reinforcement distribution
    • Edge flow
    • In RTM due to small clearance there exist a path for resin flow during mold filling.This edge flow can create dry spots or spillage of resin.
    • Mould filling
  • 50. Is fibre washing is related to injection pressure ? How? Fibre washing increases when pressure is increased
  • 51. Is fibre washing is related to fibre content? How?
        • Fibre washing distances reduces with more number layers of fibre and
        • virtually reduces to zero due to
        • -Higher clamping forces occurs. It can be increased by prelaying of narrow nonwoven strips along the edge to intimate contact with the mold
  • 52. Schematic diagram of edge flow
  • 53. Factors governing the edge flow? Injection pressure Only a marginal increase for mould filling at the edge with increasing edge pressure
  • 54. Preform permeability
      • More layers of fibre layer results in high flow resistance
      • and slow flow and slow impregnation
  • 55. Proper mold designing what it means?
    • Shape of the mold
    • Proper positioning of gate and vent
    • Gate and vent should be opposite in direction
    • Number of gate and vent
    • Pressure at the vent must be lesser than that from the gate position
  • 56. What is “Dry Spot”? How it forms ?
    • Region of composite part which is devoid of resin.
    • Due to the presence of inserts , ribs, cores, edge flow etc
    • the resin flow may branch and merge around the inserts or
    • low permeability areas .
    • Flow front merges in the absence of a vent air get entrapped
    Voids can form in RTM process
    • During processing
    • In the rein before processing
    • During injection
    • During curing
  • 57. Importance of Dry spot….
    • Reduces
        • Tensile strength
        • Compression strength
        • Flexural strength
    • Impair surface quality
    • Reduces the water resistance
    Is there any way to prevent dry spot?
    • By vacuum assistance.
    • To use gas which is easily dissolved into the resin.
    • To keep the resin flowing through a completely filled mould.
    • High pressure in curing stage of processing.
    • Use of more homogeneous reinforcement
    • Better wetting between the resin and fibre must be done
    • To continue filling after reinforcement has been completely wetted by resin
  • 58. Advantages of RTM
    • Low capital investment
    • Good surface quality
    • no air entrapment if properly designed (tooling, preform, and resin)
    • low tooling cost
    • Large, complex shapes
    • Ribs, cores and inserts
    • Range of available resin systems
    • Range of reinforcements
    • Controllable fiber volume fraction
  • 59. CALENDERING It is employed to produce continuous film and sheets .
    • It consists of set of highly polished metal rollers rotating in opposite direction.
    • There is provision of precise adjustment of the gap between them.
    • The gap determines the thickness of the sheet.
    • The sheet are maintained at an elevated temperature.
    • Emerging sheet is cooled by passing through cold rollers.
    • Finally it is wound up.
    • Eg: PVC, ABS, rubbers.
  • 60. SHEET MOULDING COMPOUNDS The material is composed of a filled , thermoset resin and a chopped or continuous strand of glass fibre. Advantages
    • High volume production.
    • Weight reduction.
    • Excellent design flexibility.
    • Minimum material Scrap.
    • Low labor requirement .
  • 61. FILM CASTING Used to produce polymeric films
    • In this polymer in a suitable solvent is allows to fall at a
    • precalculated rate on an endless metallic belt of high finish
    • moving at a constant speed.
    • Continuous sheet of polymer solution is formed.
    • The solvent is evaporated under controlled condition.
    • The film is removed by stripping.
    • Eg: Cellophane sheet, photographic films.
  • 62. INJECTION MOULDING It involves forcing or injecting a fluid plastic material into a closed mould where it solidifies to give the product Two basic categories: Thermoplastic; Thermosetting In former material is melted and force through an orifice or gate into a cool mould . In later a reacting material is injected into a warm mould in which the material further polymerizes into a solid part
  • 63. Schematic Diagram
  • 64. Process
    • Feeding of the compounded plastic as granules through
    • a hopper at definite interval of time.
    • It is softened and pressure is applied.
    • Through a hydraulically driven piston to push the molten
    • material through a cylinder into a mould fitted at the end
    • of the cylinder.
    • While moving the ‘torpedo’ helps to spread the plastic
    • material uniformly around the inside wall and ensures
    • the uniform heat distribution.
  • 65. Screw move back and check valve opens Stage 4 Stage 1 Stage 3 Stage 2 Mould open Mould clamped cavities filling with melt. Mould clamped cavities full, melt freezing. Screw almost stationary Screw move forward and check valve closed Reservoir full Frozen moulding in clamped mould. MOULDING STAGES
  • 66. EXTRUSION MOULDING
  • 67. Blow Moulding
    • The plastic is fed in granular form into a 'hopper' that stores it.
    • A large thread is turned by a motor which feeds the granules
    • through a heated section.
    • In this heated section the granules melt and become a liquid
    • and the liquid is fed into a mould.
    • Air is forced into the mould which forces the plastic to the sides,
    • giving the shape of the bottle.
    • The mould is then cooled and is removed.
    • The process is similar to injection moulding and extrusion.
    • The process is also known as injection or extrusion blow moulding.
  • 68. Rotational Moulding It is used t o produce small to large hollow items with very uniform wall thickness Heating while rotating Cooling while rotating Part removal
  • 69. Processing stages
    • A hollow thin wall mould with good heat transfer characteristics
    • is first charged with an amount of plastic that is equal to desired
    • part weight.
    • Mould is then attached to a mechanism that generally rotating it
    • simultaneously about two axes that are at 90 o angles to each other.
    • During rotation the material inside the the mould tumbles to the
    • bottom creating a continues path that covers the mould surfaces equally.
    • The material is normally heated by rotating the mould in an oven.
    • After the proper time and temperature, the mould is removed and are
    • cooled to room temperature.
    • The mould is then opened.
  • 70. Rotational Moulding Three arm indexing machine
  • 71. VACUUM FORMING
    • Vacuum forming is a technique that is used to shape a variety of
    • plastics. In school it is used to form/shape thin plastic, usually plastics
    • such as; polythene and perspex. Vacuum forming is used when an
    • unusual shape like a ‘dish’or a box-like shape is needed.
  • 72. THE STAGES INVOLVED IN VACUUM FORMING
    • First, a former is made from a material such
    • as a soft wood. The edges or sides are shaped
    • at an angle so that when the plastic is formed
    • over it, the former can be removed easily.
    The former is placed in a vacuum former
  • 73. A sheet of plastic (for example, compressed polystyrene) is clamped in position above the mould. The heater is then turned on and the plastic slowly becomes soft and pliable as it heats up. The plastic can be seen to 'warp' and 'distort' as the surface expands . After a few minutes the plastic is ready for ‘ forming’ as it becomes very flexible.
  • 74. The heater is turned off and the mould is moved upwards by lifting the lever until it locks in position. The 'vacuum' is turned on and this pumps out all the air beneath the plastic sheet. Atmospheric pressure above the plastic sheet pushes it down on the mould. At this stage the shape of the mould can be clearly seen through the plastic sheet. When the plastic has cooled sufficiently the vacuum pump is switched off.