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Chapter 15 Thermoforming

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Title: Chapter 15 Thermoforming


1
Chapter 15Thermoforming
  • ETPL 1100

2
Introdution
  • Thermoforming is the process where plastic
    materials are shaped from softened sheet
  • The process involves
  • Heating the plastic sheet to a temperature range
    where it softens
  • Then stretching the softened plastic against a
    cold surface mold
  • When the sheet has cooled, it is removed from the
    mold and excess plastic is trimmed

3
Introduction
  • Thermoforming is a gemeric term for a group of
    processes
  • Vacuum forming
  • Drape forming
  • Billow forming
  • Mechanical bending
  • Match-Mold forming
  • Pressure forming

4
Introduction
  • Thermoforming is one of the oldest methods of
    forming useful articles

5
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6
Introduction
  • Markets
  • Thermoformed products are categorized as
  • Permanent( Industrial)
  • Disposable

7
Introduction
  • Typical Industrial Products
  • Equipment cabinets
  • Tote bins
  • Pallets
  • Trays
  • Head liners
  • Shelves

8
Introduction
  • Typical disposable products
  • Point of purchase containers
  • Hand and power tool cases
  • Meat and poultry trays
  • Egg cartons

9
Introduction
  • Major growth area for thermoformed products
  • Multi layer containers
  • Returnable containers
  • Modified atmospheric packages
  • Medical device equipment

10
Introduction
  • Terminology
  • Thermoforming processes are divided by thickness
    or gauge of the sheet used
  • Thin gauge for sheet less than .06in (1.5mm)
  • Heavy gauge for sheet greater than 0.12in. (3mm)
  • gray area between the thickness, depends on
    whether the material can be rolled or comes in
    sheets

11
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12
Introduction
  • Competition to thermoforming
  • Blow Molding
  • Rotational Molding
  • Injection molding

13
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14
Introduction
  • Advantages of thermoforming
  • Low temperature, low pressure required
  • Only a single surface mold is required
  • Molds are easy to fabricate and use inexpensive
    materials
  • No need for the plastic to flow
  • Can make very large surface area to thickness
    ratios

15
Introduction
  • Disadvantages to thermoforming
  • Plastic material is more expensive because the
    pellets have to be made into sheets
  • Generally more waste to reprocess
  • One sided
  • Can get a great deal of wall thickness variation

16
Plastics and Polymers
17
Plastics and Polymers
  • Thermoforming uses plastic sheet, which is
    heated, stretched, cooled and mechanically cut
  • The plastic sheet is manipulated as a rubbery
    solid or elastic liquid
  • The solid or elastic liquid properties are more
    important than the viscous properties when
    thermoforming

18
Plastics and Polymers
  • You can thermoform both amorphous and crystalline
    polymers
  • Amorphous
  • No organization, glass transition
  • PS, ABS, PVC, PMMA, PC
  • Crystalline
  • Organized region called crystals, glass
    transition and melting
  • PE, PP, Nylon, Acetal

19
Plastics and Polymers
  • Thermoforming Window
  • Temperature range over which the polymer is
    sufficiently subtle or deformable for stretching
    and shaping
  • Typically amorphous have broader window than
    crystalline
  • PS 260 to 360 F
  • PP few degrees below melting point of 330F

20
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21
Plastics and Polymers
  • Important thermal properties
  • Enthalpy or heat capacity
  • Thermal conductivity
  • Temperature dependent density

22
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23
Plastics and Polymers
  • Heat Capacity or specific heat
  • Measure of the amount of energy required to
    elevate the polymer temperature
  • One measure of energy uptake is enthalpy
  • It increase with increasing temperature
  • When a material goes through a phase change, such
    as melting, the enthalpy-temperature curve
    changes dramatically
  • When a material goes the glass transition, the
    enthalpy-temperature curve changes very little

24
Plastics and Polymers
  • Heat Capacity or specific heat
  • It takes twice as much energy to heat heat a
    crystalline polymer from room temperature to
    above its melt temperature than to heat an
    amorphous polymer the same

25
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26
Plastics and Polymers
  • Heat Capacity or specific heat
  • In general enthalpy changes can be calculated as
  • M is mass
  • Cp is heat capacity
  • T is temperature

27
Plastics and Polymers
  • Heat Capacity or specific heat
  • If going through a melting transition
  • ?H gives the energy required

28
Plastics and Polymers
  • Thermal conductivity
  • The measure of energy transmission through a
    material
  • Plastics have substantially lower rates than
    metals
  • Aluminum has 1000 times greater than PS
  • Th rate of heat transfer to the sheet is very
    important because it determines the time the
    process takes

29
Plastics and Polymers
  • Density
  • Mass per volume
  • It decreases with increasing temperature
  • Small change at glass transition, large change at
    melting temperature
  • At forming temperature amorphous has 10-15 less
    density than at room temperature
  • At forming temperature crystalline has 25 less
    density than at room temperature
  • Shrinkage must be accounted for to meet final
    specifications

30
Plastics and Polymers
  • Thermal diffusivity
  • Combination of polymer heat properties
  • Thermal conductivity/density/heat capacity
  • Fundamental polymer property in time dependent
    heat transfer to materials

31
Plastics and Polymers
  • Infrared energy absorption
  • Most commercial thermoforming heaters emit energy
    in far infrared wavelength
  • Thermoforming is concerned most with the
    wavelength range 2.5 to 15 microns
  • Very important when heating sheet

32
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33
Plastics and Polymers
  • Thermoformable polymers
  • Polystyrene is by far the most widely used
  • Other PS family
  • HIPS
  • ABS
  • SAN
  • OPS orientated PS
  • PVC
  • PMMA

34
Plastics and Polymers
  • Thermoformable polymers
  • Cellulosics
  • PC
  • PET
  • PE
  • PP
  • If a polymer can be made into a sheet it can be
    thermoformed

35
General Forming Concepts
36
General Forming Concepts
  • The simplest thermoforming process consists of
    simply heating the sheet and forcing it against a
    solid shaped mold
  • There are many variations of this simple method

37
General Forming Concepts
  • Simple heating and stretching
  • The basic thermoforming process is of
    differential stretching
  • Only the sheet that is not touching the mold
    stretches
  • As stretching continues, the sheet becomes
    thinner and thinner
  • Area last formed are the thinnest, most
    orientated and weakest
  • Final part has very non-uniform wall thickness

38
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39
General Forming Concepts
  • Drape forming
  • Earliest method
  • The sheet is heated and then manually shaped over
    the mold
  • Uses a male or positive mold
  • Yields a part that is thinner along its side
    walls, rim and corners than at the bottom
  • Used to make heavy gauge products such as signs
    and refrigerator liners

40
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41
General Forming Concepts
  • Vacuum forming
  • The sheet is heated and stretched into a female
    mold
  • Requires a vacuum system
  • The part is thinner in the bottom and corners
    than at the top
  • Used to make heavy gauge products such as signs
    and thin gauge product such as picnic plates

42
General Forming Concepts
  • Free forming
  • Billow or free bubble forming
  • Does not use a mold
  • The sheet is heated to its forming temperature,
    then air pressure is applied against the sheet,
    and the sheet expands
  • As the bubble expands it touches a shut off
    device for the air, controlling the final size of
    the bubble

43
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44
General Forming Concepts
  • Free forming
  • Because the bubble never touches a solid surface,
    it remains mar free
  • The bubble is uniform in thickness
  • Transparent polymers are most used
  • Heavy gauge, free fromed shapes are used as
    skylights and aircraft windows
  • Thin gauge, free fromed shapes are used in
    blister packaging

45
General Forming Concepts
  • Assisted forming
  • Used to improve the wall thickness uniformity for
    deep draw parts
  • Three types
  • Non uniform heating
  • Pneumatic preforming
  • Plug assist

46
General Forming Concepts
  • Non uniform heating
  • For heavy gauge froming its produces a sheet that
    is hotter in certain ares than others
  • Hotter sheet stretches more that cooler sheet
  • Regions of the sheet that would normally be over
    thinned are not heated as much as regions that
    would normally be over thick

47
General Forming Concepts
  • Pneumatic preforming
  • The heated sheet is first inflated with air
  • A mold is plunged into the inflated sheet
  • Vacuum is applied to the mold to ensure the sheet
    replicates the mold surface

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50
General Forming Concepts
  • Plug assist
  • Plugs are mechanically driven, shaped solid
    structures that are pressed into the softened
    sheet prior to forming
  • Used to locally stretch a sheet
  • Used for heavy gauge products such as tote bins
    and equipment cabinets
  • Used for thin gauge products such as drink cups

51
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52
General Forming Concepts
  • Pressure Forming
  • When the pressure difference across the sheet
    exceeds 1 atmosphere, 14.7 psi or 0.1MPa
  • Pressure forming uses air pressure up to 10
    atmospheres on the free side of the sheet and
    vacuum on the sheet surface closest to the mold
  • Pressure formed parts have surface texture and
    radii that rival injection molded parts

53
General Forming Concepts
  • Pressure Forming
  • Used in thin gauged forming to improve cooling
    cycle times by rapidly stripping the sheet from
    the plug and driving it against the cold mold
  • Heavy gauge pressure forming is used with stiffer
    filled polymers

54
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55
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56
General Forming Concepts
  • Twin sheet forming
  • Competes with blow molding
  • Two halves of the finished product are made then
    glued together

57
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60
General Forming Concepts
  • Contact forming
  • The thin sheet is brought into contact with a
    heated, non stick metal surface, then quickly
    transferred to a mold for forming
  • Its not economical to heat very thin plastics by
    non contact means

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63
General Forming Concepts
  • Diaphragm forming
  • The diaphragm is usually a high temperature
    rubber such as neoprene
  • The sheet is clamped against the diaphragm and
    heated
  • The diaphragm is for support with plastics of low
    forming strength
  • The diaphragm and hot sheet are deformed against
    the mold surface
  • Part wall thickness is very unifrom

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65
General Forming Concepts
  • Mechanical forming
  • For simple bends, strip heaters are placed
    against the sheet held in a bending structure
  • When the local area of the sheet is hot, the
    bending fixture is activated
  • The sheet is allowed to cool in its final bent
    form
  • Care must be taken to ensure that the plastic is
    sufficiently heated, otherwise some spring back
    occurs.

66
Machinery for the Forming Process
67
Machinery for the Forming Process
  • Currently the machinery market is 500 million
  • Most machines are homemade or substantially
    modified from original design
  • Low temperature, pressure process
  • Important things in commercially manufactured
    machines is to produce quality parts with high
    productivity, high reliability and low
    maintenance costs

68
Machinery for the Forming Process
  • We will look at 5 major systems
  • Thin gauge, roll fed machines
  • Heavy gauge sheet forming
  • Thin gauge form, fill and seal
  • Extrusion/forming line
  • Matched mold forming machines

69
Machinery for the Forming Process
  • Several defining characteristics shoul be
    considered when choosing a roll fed machine
  • Platen dimensions
  • Maximum depth of draw
  • Nature of the forming process
  • Type of power drive for the platen and sheet
    indexing
  • Type of heater and controls

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71
Machinery for the Forming Process
  • Sheet take off
  • The incoming sheet is delivered as a roll
  • The roll stand should be capable of handling 2000
    lbs and 6 foot diameter rolls
  • The roll stand must have passive braking
  • Rapid roll changer is an optional feature

72
Machinery for the Forming Process
  • Pin chain and pin chain rail
  • Typically the thin gauge sheet is advanced
    through the machine on pins spaced along the
    length of continuous bike-like chain
  • Self lubricating chains have advantages, but
    chains without lubricants are required for
    medical and food applications
  • Servo driven chain advancement is usually
    standard and offers smooth and constant sheet
    acceleration and deceleration rates and constant
    sheet speed during advancement

73
Machinery for the Forming Process
  • Oven
  • It is necessary to provide as uniform energy
    input to the sheet as possible
  • Many machine have modular ovens to allow more
    than one shot to reside in the oven
  • The oven can have more than one section for hard
    to heat polymers and foam
  • APET,PVC and PP have 3 oven sections
  • Low density foams have 5 sections
  • Most ovens have opening for infrared temperature
    measurement

74
Machinery for the Forming Process
  • Press
  • Once the sheet is hot, it must be quickly pressed
    against the mold surface
  • The press has many functions
  • It must close smoothly and rapidly against the
    sheet, without banging the mold
  • It must retain alignment for the platen, mold and
    any ancillary devices

75
Machinery for the Forming Process
  • Forming assist devices
  • Plug assist and pressure forming are common for
    deep draw parts such as drink cups
  • Plug assist features should include the
    capabilities for
  • Rapid replacement of individual plugs
  • Internal heating and cooling aluminum plugs
  • Relatively easy adjustment of the plug assist
    platen
  • Easy adjustment of plug travel and rate of travel

76
Machinery for the Forming Process
  • Trim means
  • With inline trimming, the sheet with the formed
    parts still attached to the web is conveyed away
    from the thermoforming machine to a separate
    inline trimming press
  • With in machine trimming, the sheet with the
    formed part is conveyed from the forming press to
    a separate in machine trimming station
  • With in place trimming, the trim die is an
    integral part of the forming die

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78
Machinery for the Forming Process
  • In machine stacking
  • Many machines include means to stack the product
  • The parts are punch separated from the web using
    either mechanical piston linkage or rack and
    pinion servo devices

79
Machinery for the Forming Process
  • Trim or web take up station
  • The excess material needs to be ground up into
    pellets, eventually
  • There are two general way of treating the web
  • One is to cut the web directly from the machine
    and feed it directly to the grinder
  • The other is to continuously wrap the web into a
    roll and send it off to be ground

80
Machinery for the Forming Process
  • Condition monitor
  • Sheet temperature should be measured using
    infrared thermocouples
  • Mold and coolant temperature should be monitored
  • Heater temperature should be closely monitored
  • Air pressure and vacuum pressure should be
    monitored
  • Photoelectric eye and alarms are recommended in
    the oven for excessive sheet sag and out of
    sheet indications

81
Machinery for the Forming Process
  • Process control
  • Most machines are equipped with PLC controllers
    and color monitors
  • Most controls are based on time, how long the
    sheet is in the oven
  • Most important are the automatic protocols for
    emergencies
  • Fire
  • Power outage
  • Safety cage breech

82
Machinery for the Forming Process
  • Heavy gauge sheet forming
  • Two general types
  • Shuttle presses
  • Rotary presses
  • Machine Criteria
  • Platen dimensions
  • Depth of draw
  • General nature of the forming process
  • Types of motive power for moving things
  • Types of heater and max energy output

83
Machinery for the Forming Process
  • Shuttle presses
  • 70 of machines
  • Very versatile and capable of forming parts of
    nearly unlimited dimensions
  • Economically inefficient

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85
Machinery for the Forming Process
  • Shuttle presses
  • Simplest thermoforming press
  • The sheet is clamped in a four sided clamp frame
  • The sheet and frame are moved into the oven
  • When the sheet reaches its forming temperature,
    the sheet and clamp are moved to the forming
    station

86
Machinery for the Forming Process
  • Shuttle presses
  • The formed part is help against the mold until it
    cools
  • The part with excess material is removed from the
    mold
  • The formed part is then trimmed in a secondary
    trimming operation

87
Machinery for the Forming Process
  • Shuttle presses
  • In this simple process, no other sheet is formed
    while a sheet is heated. Only one thing happens
    at a time
  • Two ways to overcome this inefficiency
  • Double oven press one sheet is formed while the
    other is heated
  • Use a rail system similar to that used in a
    roll-fed system

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89
Machinery for the Forming Process
  • Rotary Press
  • Very efficient
  • Requires more care in set up
  • Are limited in the size of the part that can be
    formed

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92
Machinery for the Forming Process
  • Rotary Press
  • The heart of the rotary press is the carrousel
    that carries the sheet from station to station
  • The stations are
  • A load/unload station
  • An oven station
  • A forming press station
  • A second oven station is added in a four station
    machine

93
Machinery for the Forming Process
  • Rotary Press
  • A four station machine is used when heating the
    sheet limits the process or when sequential twin
    sheet thermoforming is being done
  • The sheet is clamped on all sides at the
    load/unload station
  • It is then indexed into the oven
  • The heated sheet is then indexed into the forming
    press
  • The formed part is then rotated to the
    load/unload station, where it is removed and a
    new sheet is clamped in the frame

94
Machinery for the Forming Process
  • Rotary Press Sheet handling
  • When sheet of standard dimensions is used or when
    production runs are long, the sheet is picked and
    placed mechanically
  • Sheets are either lifted with vacuum suction cups
    or elevated with pneumatic lifting tables

95
Machinery for the Forming Process
  • Rotary Press Sheet Clamp
  • Clamp pressure should be at least 50 psi
  • The clamping area should be at least 0.5 in on .1
    in thick sheet and at least 2 in on .4 in thick
    sheet
  • The clamp frame and associated equipment should
    be able to withstand 800 F for 20 minutes
  • The clamp frame should be easily adjustable for
    varying sheet dimensions

96
Machinery for the Forming Process
  • Rotary Press Oven
  • The oven temperature should be step controlled
  • Side baffles or oven side walls should close off
    the sheet and clamp during heating to minimize
    drafts
  • Oven should be provided with ports for in oven
    infrared temperature measurement
  • Provisions should be made for emergency shut down
  • The spacing between the sheet and the top and
    bottom halves should be easily adjustable

97
Machinery for the Forming Process
  • Rotary Press Press
  • Adequate press capacity is the most critical part
    of heavy gauge machine design
  • The press should have sites for platen leveling,
    rapid platen alignment and capabilities for rapid
    mold change over

98
Machinery for the Forming Process
  • Rotary Press Pre-stretching means
  • Pre inflation or pre-stretching of the heated
    sheet is common in heavy gauge thermoforming
  • Two methods are used
  • Bubble stretching
  • Vacuum or draw box

99
Machinery for the Forming Process
  • Bubble stretching
  • The hot sheet is temporarily clamped over a lower
    mold element that acts as a blow box
  • The sheet is then inflated with extent of
    inflation controlled by an electric eye
  • Vacuum or draw box
  • The sheet is drawn down by vacuum, with the
    extent controlled by a photo electric eye

100
Machinery for the Forming Process
  • Vacuum and pressure
  • Vacuum pumps should be capable of maintaining
    28.5 in (735 mmHg or 35 torr) at their inlet
  • Surge tanks should be capable of maintaining 25
    in (635mmHg or 125 torr) at their inlet
  • The surge tank volume should be 6 to 20 times the
    combined volumes of the deepest cavity

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102
Machinery for the Forming Process
  • Vacuum and pressure
  • Pressure forming need air pressure of at least
    100 psi
  • Forming of multilayer, filled or reinforced sheet
    requires air pressure of 200 psi or more

103
Machinery for the Forming Process
  • Thin gauge form and seal
  • Many disposable, rigid, medical and food packages
    and certain point of purchase containers are
    produced with FFS equipment
  • Thin gauge sheet, typically 10 to 20 mils, is
    continuously fed from a roll to the heating
    station
  • For wide or heavy sheet, a pin and rail system is
    used

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105
Machinery for the Forming Process
  • Thin gauge form and seal
  • Non contact infrared heater are used for heavier
    sheet, while direct contact heating is used for
    thin sheet
  • The sheet is heated to temperature lower than
    traditional thermoforming temperatures
  • Mechanical pressing is used rather than vacuum or
    pressure forming
  • The formed containers, still connected to the web
    are fed beneath a filling station

106
Machinery for the Forming Process
  • Thin gauge form and seal
  • The containers are then filled with product
  • Manually in simple cases
  • Automatically in most cases
  • After filling, the containers proceed to a
    sealing station
  • May involve snapping a lid closed or heat sealing
  • The containers are then trimmed from the web
  • Then the containers are stacked and placed into
    cartons

107
Machinery for the Forming Process
  • Extrusion/Forming line
  • For dedicated thermoformed products, the
    extrusion process is often coupled with the
    thermoforming process
  • Refrigerator door liners are often produced this
    way
  • More complex process because it involves both
    extrusion and thermoforming

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109
Machinery for the Forming Process
  • Extrusion/Forming line
  • Most line are found in the thin gauge forming
    operations for the production of drink cups and
    deli containers
  • Care must be taken to match the output of the
    extruder to the through put of the thermoformer
  • The thermoforming cycle dictates

110
Machinery for the Forming Process
  • Match Mold forming machines
  • Two types
  • Low density foams because they are hard to heat
    and stretch
  • Highly filled or reinforced polymers because they
    require high clamping and forming forces

111
Methods of Sheet Heating
112
Methods of Sheet Heating
  • The first step in forming a plastic part involves
    heating the sheet too the proper forming
    temperature
  • Heat transfer and the method of heating dominate
    thermoforming technology, because its the only
    way to produce a good part and is a major
    component of the cost.

113
Methods of Sheet Heating
  • Concepts in Heat Transfer
  • There are three basic methods of heat transfer
  • Conduction
  • Convection
  • Radiation

114
Methods of Sheet Heating
  • First we must understand how much energy is
    required to heat a unit mass of sheet from room
    temperature to the forming temperature
  • Where Q is the total amount of heat needed

115
Methods of Sheet Heating
  • Q, the heat needed to raise the temperature from
    room temperature to the forming temperature is
    attained through one of the methods

116
Methods of Sheet Heating
117
Methods of Sheet Heating
118
Methods of Sheet Heating
119
Methods of Sheet Heating
120
Methods of Sheet Heating
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