11Thermoforming Process(Vacuum Forming Process)2ManufactureUnit ManufacturingProcessesUnit ManufacturingProcessesAssembly and JoiningAssembly and JoiningMarketResearchFactory, Systems & Enterprise•Welding•Bolting•Bonding•Soldering•Machining•Injection molding•Rapid prototyping•Stamping•Chemical vapor depositionConceptualDesignDesign forManufacture3Outline Overview Process Steps Process Equipment Design for Manufacturing4Overview Polymersthermoplastic Applicationspackaging, container, housing, etc. MaterialsABS (~15%), PMMA (~15%), PS (~20%)25Typical Production Flow:In-line Arrangement1. roll stock 2. nip rollers 3. electric eye 4. forming station 5. trimming station 6. scrap wind 7. part stacking6Three-station Thermoforming Machine1. loading and unloading station2. heating station3. forming stationMOLD3)OVEN2)CLAMP FRAME1)LOAD / UNLOAD STATION7Processing Steps Sheet Heat Form Cool Trim8Plastic Sheet Making:Extrusion1. feed hopper 2. extruder screw 3. extruder barrel 4. die5. roller stack39Heating Methods: Radiant Heating1. thermoplastic sheet 2. reflector3. tubular heater element 4. direct heat 5. reflected heat6. actual heat distribution on the plastic sheet10Other Heating Methods Convection Heating Contact Heating11PolymersAmorphous vs Semicrystalline12Forming: Vacuum or Pressure+ + + + + + + + Positive air pressure (14.5 to 300 psi) Faster mold cycle Lower temperatures with higher forming pressure413Forming: Match Mold+ + + + + + + + +14Plug-assist Vacuum FormingA: preheated sheet prior to formingB: sheet stretched with moving plug C: sheet vacuum formed into female cavitya - plug b - preheated, clamped, sheet c - female mold with vacuum holes d - moving plug e - vacuumABCabcde15Plug-assist Pressure FormingA: preheated sheet prior to forming B: sheet stretched with mechanical plug advance C: sheet air-pressure formed into female mold a - pressure box b - plug c - preheated, clamped sheet d - female mode with vent holes e - moving plug f - applied air pressure g - venting airABCabcegdf16Free BlowingA: preheated sheet prior to formingB: free-blown sheet: bubble height determined by photocell monitor.a - preheated clamped sheet b - pressure box c - proportional photocell monitor d - signal to air pressuree - hold-down ring f - air pressureABabcdef517Billow Vacuum FormingA: preheated sheet prior to formingB: sheet prestretched with air pressure C: sheet vacuum formed into female cavitya - hold down ring b - preheated clamped sheet c - female mold with pressure/vacuum holes, d - applied pressure e - vacuumABabdcCeBetter thickness uniformityDeep drawLonger cycle time18Vacuum Reverse Draw with Plug-assistA: preheated sheet prior to formingB: formation of bubbleC: plug moves into billow, air pressure continuesD: vacuum applied pulling sheet into female mold a) plug, b) hold-down ring, c) preheated, clamped sheet, d) female mold, e) plug motion activated when bubble touches it, f) applied air pressure, g) continuing air pressure as plug advances, h) vacuumACgBfabcdeDh19Pressure Reverse Draw with Plug-assistA: preheated sheet prior to forming B: sheet prestretched into bubble with air pressure C: plug moves into sheet while air pressure still on D: sheet vacuum formed into female mold a) pressure box, b) plug, c) preheated, clamped, sheet, d) female mold with air pressure/vacuum holes, e) plug begins to move when billow touches it, f) applied air pressure, g) air pressure, h) plug moving into billow, i) continuing air pressure, j) vacuumACiBfabcdeghDj20Forming Mechanism621Thickness Uniformity22Forming Considerations: Part Thickness Draw ratio- depth of part / width of part Draw ratio should be less than - 2:1 for female molds- 7:1 for male mold Area ration for blank sheet size estimation Draft angle : .5 to 5 degree23Forming Considerations: Progressive Draw-DownRR/2R/4R/824Forming Considerations: Detail Lossfemale formmale form725Forming Considerations: UndercutMOLDSHEETUNDERCUTBASE26Forming Considerations: Vacuum holesSELF SEAL SELF SEALVACUUMSOURCEATMOSPHERIC PRESSURE27CoolingAmorphous vs SemicrystallineTgTg+60°CGlassy solidRubberyMeltVˆTgTmBrittleTough and flexibleMeltVˆ(a)(b)28TrimPunch and die trim tools with different types of punches1. broad punch 2. shearing punch3. double-angle shearing punch4. hollow-ground punch1234DIEPUNCH829Design for Thermoforming Uniform thickness (~10%) Simpler shapes (avoid under cuts, etc.) Rounded corners (1t min, 4t ideal) Draft angle for removal (.5 – 5 degree) Depth of draw ratio (< 1:1) Stretch ratio (< 2:1) Shrinkage Design for holes and trim lines30Cost - Thermoforming Initial Cost • Equipment cost is low to moderate, but can be high if automated• Tooling cost is low to moderate depending on the complexity Variable Cost• Labor cost is low to moderate• Moderate to low material utilization : unformed part of the sheet are lost31Rate - Thermoforming Development time • Die design time : a few days to weeks Cycle time• Shorter than melting process : 10 to 60 seconds Production rate• Usually very fast : but vary with batch size32Quality - Thermoforming Dimensional • Affected by viscoelastic spring back : rate of change affects spring back• Shrinkage• Surface finish is good and related to the condition of mold surface Mechanical Property• Good toughness : orientation related Defects• Corners tend to become excessively thinner : pre-stretch in opposite direction and apply pressure933Flexibility - ThermoformingModerate : Die needs to be