Exam 2 - Thermoforming Flashcards
Thermoforming types (7)
- Straight Vacuum Molding
- Drape Forming
- Matched Mold Forming
- Plug Assist Forming
- Pressure Bubble Forming
- Other Forming Processes
- Twin Sheet Forming
Thermoforming History
Originally used horn
Tooling costs for thermoforming
inexpensive
Thermoforming (general) properties
– Thermoforming good for large parts
– Good for short runs or prototypes
– Shorter lead times than other processes
– Less internal stresses than injection molding
Thermoforming materials
Most thermoplastics can be used: Polyethylene, Polystyrene, Polypropylene, Acrylic, Polycarbonate, ABS, PET, PETG, PVC
Two types of thermoforming
– Thin gauge or roll fed (1/16” thick)
Straight Vacuum Forming Process
Vacuum in mold cavity draws in heated sheet
Most versatile and widely used TF process
Straight vacuum forming
Straight Vacuum Forming Properties
- Requires trimming and scrap reprocessing
- 2° to 7° draft angle usually needed
- Maximum draw ratio depends on material
- Edges and corners are usually thinner
Drape Forming Process
heated sheet is draped over mold and vacuum is applied through the mold
Drape Forming Properties
• 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
Matched-Mold Forming Process
similar to compression molding heated sheet is clamped between male and female dies
Matched-Mold Forming Properties
• 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
Plug-Assist Vacuum Forming Process
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
Plug-Assist Vacuum Forming Properties
• 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
Pressure-Bubble Plug-Assist Vacuum Forming Process
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
Pressure-Bubble Plug-Assist Vacuum Forming Properties
• Used for deep thermoforming
• Controlled air pressure stretches the material to
a predetermined thickness
– Thickness control is possible uniform or varied
Solid Phase Pressure Forming Process
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
Solid Phase Pressure Forming Properties
Biaxial stretching enhances
- strength
- toughness
- environmental stress-crack resistance
Vacuum Snap-Back Forming Process
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
Vacuum Snap-Back Forming Properties
Deep Draws are possible
Pressure Bubble Vacuum Snap-Back Forming Process
Similar to vacuum snap-back forming – only
pressure is used to blow a bubble prior to plug
plunge and vacuum is on plug
Trapped Air Sheet Contact-Heat Pressure Forming Process
Similar to straight vacuum forming with air pressure and vacuum used to force material into female mold
Free Forming Process
• Air pressure blows hot plastic
through silhouette of female mold
– Allows for smooth bubble
– Used for aircraft canopies
Mechanical Forming Process
• No vacuum or air pressure is
used, only the mechanical
manipulation of hot plastic is used
Twin Sheet Forming Process
two heated sheets with vacuum applied to outside and pressure applied between sheets to form a hollow part
Twin Sheet Forming Properties
• Produces hollow body, without solvent, molding additives and adhesives; no inner strains. • Materials: H.M.W. HD Polyethylene, ABS, PC/ABS, Polycarbonate
Thermoforming Equipment
• 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
PC Thermoforming Properties
– 130-180°C – Optically clear – Susceptible to solvents – Eyeglasses, Instrument cases
PP Thermoforming Properties
– 140-170°C
– Good moisture resistance
– Good solvent resistance
– Packaging
PET Thermoforming Properties
– 315-370°C
– Heat resistant
– Tough and rigid
– Food and medical trays
PMMA Thermoforming Properties
– 150-190°C
– Optically clear
– Good weather resistance
– Signs, skylights
PVC Thermoforming Properties
– 100-155°C
– Flame retardant
– Decent Optical Properties
– Blister Packs
PS Thermoforming Properties
– 130-180°C
– Good processability
– Good dimensional stability
– Toys, refrigerator lines
