1. Chapter 15 Thermoforming
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

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

12. Introduction Competition to thermoforming Blow Molding
Rotational Molding
Injection molding

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 it’s 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