Chapter 15 Thermoforming

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Chapter 15Thermoforming

• ETPL 1100

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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

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Introduction

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

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Introduction

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

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Introduction

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

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Introduction

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

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Introduction

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

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Introduction

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

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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

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Introduction

• Competition to thermoforming
• Blow Molding
• Rotational Molding
• Injection molding

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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

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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

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Plastics and Polymers
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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

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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

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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

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Plastics and Polymers

• Important thermal properties
• Enthalpy or heat capacity
• Thermal conductivity
• Temperature dependent density

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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

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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

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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

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Plastics and Polymers

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

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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

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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

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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

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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

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Plastics and Polymers

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

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Plastics and Polymers

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

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General Forming Concepts
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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

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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

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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

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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

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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

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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

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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

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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

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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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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

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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

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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

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General Forming Concepts

• Twin sheet forming
• Competes with blow molding
• Two halves of the finished product are made then
  glued together

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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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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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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.

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Machinery for the Forming Process
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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

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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

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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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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

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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

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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

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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

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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

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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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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

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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

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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

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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

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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

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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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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

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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

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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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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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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

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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

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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

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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

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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

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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

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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

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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

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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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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

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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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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

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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

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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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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

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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

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Methods of Sheet Heating
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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.

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Methods of Sheet Heating

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

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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

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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

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Methods of Sheet Heating
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Methods of Sheet Heating
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Methods of Sheet Heating
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Methods of Sheet Heating
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Methods of Sheet Heating