Thermoforming Process

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Thermoforming Process
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Thermoforming

• Process used to shape thermoplastic sheet into
  discrete parts
• Basic principles
• Heat a thermoplastic sheet until it softens
• Force the hot and pliable material against the
  contours of a mold by using either mechanical,
  air or vacuum pressure
• Held against mold and allow to cool, and plastics
  retains its shape

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• Thermoforming process is differed from other
  plastic processing because the material is not
  melted, lower pressures are required to
  thermoform (the mold materials is less sturdy
  materials compared to other processings)
• Disadvantage generates greater amount of scrap,
  cost of the sheet materials is raised (because of
  separate sheet-forming step), limited design
  parts (parts with sharp bends and corners are
  difficult to be produced), process results in
  internal stresses

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Summary of advantages disadvantages of
thermoforming process
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Forming Process

• The techniques of forming process can be grouped
  into several types
• Fundamental Vacuum forming
• Pressure forming
• Plug-assist forming
• Reverse draw forming
• Free forming
• Matched-die forming, etc

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Forming Process Fundamental Vacuum forming

• The term Fundamental is used to emphasize that
  this is the simplest method of vacuum forming
• Glossary Sag point- the condition (temperature,
  time, thickness, etc) at which a sheet begins to
  sag inside the thermoforming oven

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Forming Process Fundamental Vacuum forming

• The material is clamped into a frame
• Material is heated, begin to sag, the center of
  materials moves downward and seal against the
  mold
• Vacuum is applied to the back of the mold
  (outside pressure of the air pushes the plastic
  against the mold)

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• Uneven wall thicknesses are observed in this
  technique
• As the sheet touch the mold, the thickness at the
  location become fixed, further stretching occur
  in the area that have not yet touched the mold,
  typically at he corner.
• Therefore the thinnest area occur at the corner,
  near the clamp

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Forming Process Pressure forming

• Positive air pressure from the top of the plastic
  used to force the materials against the mold

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Pressure forming

• Methods
• The materials is clamped and heated
• Softening sheet is transferred to the mold area
  and the seal is made
• Air pressure is introduced and force the plastic
  against the mold
• The pressure is applied quickly to prevent the
  sheet from cooling

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Pressure forming

• Advantages
• Mold cycle can be faster than fundamental vacuum
  forming
• Sheet can be formed at lower temperature, because
  the forcing pressure is high
• Greater dimensional control

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Plug-Assist Forming

• In this system, a plug is used to force the
  material into the mold
• Generally the plug will not pushed the material
  completely into the mold, then a vacuum is
  applied to draw the materials against the cavity
  walls
• Instead a vacuum, air pressure could be used to
  force the parts against the walls

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Plug-Assist Forming
new thermoplastic composite material that can be
used to make thermoforming plug assists, it
displays tremendous dimensional stability and
low thermal conductivity. Designed to replace
Syntactic foam, machined aluminum and wood for
plug assists.
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Plug-Assist Forming

• Better wall thickness uniformity than obtained
  from vacuum and pressure forming
• Plugs can be made of metal, wood or thermoset
  materials
• The plug should be 10 to 20 smaller in length
  and width than the female cavity

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Reverse draw forming

• Is used when very deep draws are needed
• In this technique, the material is heated to the
  sag point, and then blown away from the mold
• The purpose of blowing is to thin the material in
  the center of the sheet
• The size of the bubble is controlled by an
  electric eye. When the buble reachest the correct
  size, the pressure is topped and the plug press
  the material to the mold
• The vacuum is activated through the mold to draw
  the plastic against the mold wall

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(b) plug-assist and vacuum Forming step
(a) Reverse draw step to Pre-stretch the material
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Reverse draw forming Plug assist air slip forming
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Free forming

• The forming is done without a mold, especially
  those where very high optical quality is required
• Touching the mold can result in undesirable
  changes in the surface quality of the part
• Complexity of shapes of parts is limited
• Often used for canopies

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Free forming

• The part is expanded with air pressure
• The size of the bubble is monitored by an
  electrical eye
• When the bubble reach the desired shape, air
  pressure is reduced to a level that maintains the
  size while the part cools

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Matched die forming

• Some parts with very complicated shapes, can best
  be formed if both plug and cavity mold are used
  together
• These mold must be matched so that it can be fit
  together

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Matched die forming

• Material is heated to the sag point, and then
  transferred to the molding station, where the
  plug and cavity molds are brought together to
  squeeze the material
• No vacuum or air pressure is applied during the
  forming process
• Material is mechanically pressed into the shape
  defined by matched molds, and allow to cool while
  the mold continue to press against it

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Thermoforming Machine

• Not as complicated or costly as other plastic
  processing machine that melt the plastic
  materials
• Element of the machine heating chamber or oven,
  method for removing the material from the oven,
  and locating it so that the sheet and the mold
  can be brought into airtight contact
• Machine can be single-mold batch system or
  continuous, highly automated machine

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Single-station thermoforming machine

• Heating the sheet
• Clamping the sheet
• Moving the sheet and mold into proper
  relationship for forming, a vacuum or pressure
  system

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Critical operation parameters

• The most important sheet property is thickness
  (variation in thickness over the sheet should be
  kept under 5)
• Uniformity from sheet to sheet is desired, so
  that the processing parameter can be kept
  constant, i.e. melt index, if one sheet has a
  lower melt index than another, the amount of heat
  to achieve the same formation will be higher than
  in the sheet with lower melt index
• Other variables that might change from sheet to
  sheet are density, molecular orientation, etc

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Critical operation parameters

• Key parameters during the forming operations
• Speed of vacuum application- vacuum should be
  applied as quickly as possible
• Temperature of the mold- the mold is normally at
  room temperature, or temperature below
  solidification point of the plastic
• Size of the bubble
• Plug size

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Male and Female mold

• Female mold- a mold in which the part is pressed
  into a cavity

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Draw ratio

• Is used to estimate the amount of wall thickness
  variation that might occur
• High draw ratio result in excessive thinning and
  wall nonuniformities
• Draw ratio depth of part / width of part

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Area ratio

• Give some approximation of the amount of thinning
  that will be experienced by the plastic sheet
  when it is transformed
• Area ratio area of the sheet before forming
  area of the part after forming
• If the sheet is 200 cm2, and will be thermoformed
  into a part that has total area of 400cm2, the
  area ratio is 12
• Area ratio x desired thickness of finished part
  minimum original thickness (thickness of the
  blank)
• The overall average thickness of the part will
  therefore be one-half of original thickness
• The area ratio is often used to calculate the
  size of the unformed sheet that must be used to
  make a particular part

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• Determine the thickness of the blank for
  fundamental vacuum thermoforming, assuming that
  the thickness of the finished part is uniform.
  The finished dimensions of the part to be made is
  60 x 50 x 5 cm prepared by 2mm thick blank.
  Consider the allowance for clamping and mold
  clearance 2 cm on each side

• Dimension of blank (with allowance) 64 x 54 cm
• Calculate surface are of the part
• 50 x 60 x 1 side 3000 cm2
• 50 x 5 x 2 sides 500 cm2
• 60 x 5 x 21 sides 600 cm2
• Total area of finished part 4100 cm2
• 3. Thermoforming area of the blank 60 x 50
• 3000 cm2
• 4. Area ratio 3000/4100
• 5. Thickness of the blank
• 2 mm/0.7317 2.73 mm

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