Exam 2 - Thermoforming

Question 1

Thermoforming types (7)

Answer 1

• Straight Vacuum Molding
• Drape Forming
• Matched Mold Forming
• Plug Assist Forming
• Pressure Bubble Forming
• Other Forming Processes
• Twin Sheet Forming

Question 2

Thermoforming History

Answer 2

Originally used horn

Question 3

Tooling costs for thermoforming

Answer 3

inexpensive

Question 4

Thermoforming (general) properties

Answer 4

– Thermoforming good for large parts
– Good for short runs or prototypes
– Shorter lead times than other processes
– Less internal stresses than injection molding

Question 5

Thermoforming materials

Answer 5

Most thermoplastics can be used: Polyethylene, Polystyrene, Polypropylene, Acrylic, Polycarbonate, ABS, PET, PETG, PVC

Question 6

Two types of thermoforming

Answer 6

– Thin gauge or roll fed (1/16” thick)

Question 7

Straight Vacuum Forming Process

Answer 7

Vacuum in mold cavity draws in heated sheet

Question 8

Most versatile and widely used TF process

Answer 8

Straight vacuum forming

Question 9

Straight Vacuum Forming Properties

Answer 9

• Requires trimming and scrap reprocessing
• 2° to 7° draft angle usually needed
• Maximum draw ratio depends on material
• Edges and corners are usually thinner

Question 10

Drape Forming Process

Answer 10

heated sheet is draped over mold and vacuum is applied through the mold

Question 11

Drape Forming Properties

Answer 11

• Can be used with both male and female dies
– Female dies preferred for multi-cavity allows for tighter spacing
• Can produce draw ratios of up to 4:1

Question 12

Matched-Mold Forming Process

Answer 12

similar to compression molding heated sheet is clamped between male and female dies

Question 13

Matched-Mold Forming Properties

Answer 13

• Water-cooled molds can 
achieve very good detail and 
dimensional accuracy 
• Multiple male cavity molds 
must be placed at distance 
equal to or greater than their 
height to prevent webbing

Question 14

Plug-Assist Vacuum Forming Process

Answer 14

same as straight vacuum forming except a plug is used to push the sheet into the die to a required depth before vacuum is applied

Question 15

Plug-Assist Vacuum Forming Properties

Answer 15

• Used to help prevent corner of periphery from thinning
• Allows for deeper drawing
• Permits shorter cooling times
• Allows for better control of wall thickness
• Requires close temperature 
control 
• Can also be used with plug 
pressure–bubble plug–assist 
pressure forming

Question 16

Pressure-Bubble Plug-Assist Vacuum Forming Process

Answer 16

same as plug-assist except the mold releases air to raise the heated sheet into a bubble before the plug begins to push it back down into the mold and the pressure is reversed into a vacuum

Question 17

Pressure-Bubble Plug-Assist Vacuum Forming Properties

Answer 17

• Used for deep thermoforming
• Controlled air pressure stretches the material to
a predetermined thickness
– Thickness control is possible uniform or varied

Question 18

Solid Phase Pressure Forming Process

Answer 18

Similar to plug assist molding except starting material is a blank that is pressed into a sheet before a plug and applied pressure forces sheet into the mold

Question 19

Solid Phase Pressure Forming Properties

Answer 19

Biaxial stretching enhances

• strength
• toughness
• environmental stress-crack resistance

Question 20

Vacuum Snap-Back Forming Process

Answer 20

a heated sheet is drawn into a vacuum box, a plug lowers over the sagging sheet, then vacuum applied though plug and vacuum is released in vacuum box. the sheet forms around the plug

Question 21

Vacuum Snap-Back Forming Properties

Answer 21

Deep Draws are possible

Question 22

Pressure Bubble Vacuum Snap-Back Forming Process

Answer 22

Similar to vacuum snap-back forming – only
pressure is used to blow a bubble prior to plug
plunge and vacuum is on plug

Question 23

Trapped Air Sheet Contact-Heat Pressure Forming Process

Answer 23

Similar to straight vacuum forming with air pressure and vacuum used to force material into female mold

Question 24

Free Forming Process

Answer 24

• Air pressure blows hot plastic
through silhouette of female mold
– Allows for smooth bubble
– Used for aircraft canopies

Question 25

Mechanical Forming Process

Answer 25

• No vacuum or air pressure is
used, only the mechanical
manipulation of hot plastic is used

Question 26

Twin Sheet Forming Process

Answer 26

two heated sheets with vacuum applied to outside and pressure applied between sheets to form a hollow part

Question 27

Twin Sheet Forming Properties

Answer 27

• Produces hollow body, without solvent, 
molding additives and adhesives; no 
inner strains.
• Materials: H.M.W. HD Polyethylene, 
ABS, PC/ABS, Polycarbonate

Question 28

Thermoforming Equipment

Answer 28

• Main machine can range from single station to continuous feed
– In all instances some type of heating and forming stations 
included
– Can include filling and sealing stations in some cases
• Dies
– Die material dependent on 
production volume and 
lifetime
– Wood - Hundreds
– Fiberglass/Renshape - Thousands
– Metal - Millions
• Secondary Operations
– trimming

Question 29

PC Thermoforming Properties

Answer 29

– 130-180°C
– Optically clear
– Susceptible to solvents
– Eyeglasses, Instrument 
cases

Question 30

PP Thermoforming Properties

Answer 30

– 140-170°C
– Good moisture resistance
– Good solvent resistance
– Packaging

Question 31

PET Thermoforming Properties

Answer 31

– 315-370°C
– Heat resistant
– Tough and rigid
– Food and medical trays

Question 32

PMMA Thermoforming Properties

Answer 32

– 150-190°C
– Optically clear
– Good weather resistance
– Signs, skylights

Question 33

PVC Thermoforming Properties

Answer 33

– 100-155°C
– Flame retardant
– Decent Optical Properties
– Blister Packs

Question 34

PS Thermoforming Properties

Answer 34

– 130-180°C
– Good processability
– Good dimensional stability
– Toys, refrigerator lines