SPE Thin Gauge Guide 4

This guide is provided to assist in determining the cause of the most common
Thermoforming Troubleshooting Guide problems during the thermoforming process. It does not take into account
problems that may occur as a result of defective or hard to form materials and it
For Thin Gauge Thermoplastic (Roll-Fed) assumes that the equipment being used is up to modern standards. It is intended
for this guide to be used by thermoforming professionals to assist in the training
of operators, technicians and tooling personnel. There is no guarantee that the
www.thermoformingdivision.com solutions in this guide will solve all problems encountered in the process.
Problem Probable Cause Course of Action Problem Probable Cause Course of Action Problem Probable Cause Course of Action
Poor Detail Webbing, bridging, Plug Marks
· Material too cold · Increase heating time · Too much vacuum and/or · Plug holes and redrill with smaller bit · Plug coming loose · Check and tighten plug
· Increase oven temperature compressed air · Check vacuum system
wrinkling · Plug depth too deep · Reduce plug depth
· Check seal for air leak · Uneven cooling · Add water channels to mold
· Check for uniform heat · Mold and plugs not aligned · Realign
· Check for plugged water flow
· Preheat sheet
· Improper plug assist temperature · Reduce plug or sheet temperature
· Material too cold · Increase heating time
· Check for drafts
· Use Syntactic foam plug assist
· Increase oven temperature
· Air Pressure too low; air leak in · Increase psi · Teflon-coat plug
· Check for uniform heat
the seal, tool or sheet clamp · Check air timing · Use mold release
· Preheat sheet
· Air eject holes clogged · Sand and polish plug surfaces
· Check for drafts
· Check effect air pressure
Stress/Shrink marks, · Material too cold · Increase heating time
· Thin spot in sheet · Check with sheet supplier
· Timing on eject air
· Increase oven temperature (number of heaters)
sheet whitening
· Add air holes, increase injection pressure · Incorrect platen speed · Check and adjust speed
(PET/RPET)
· Check O ring imprint · Mold open before sheet advances · Delay sheet advance until sheet clears
· Incorrect timing of form air · Check and adjust timing
· Clamping frame too cold · Preheat clamp frame · Poor mold design · Increase number of vacuum holes
· Incorrect rail setting · Check and adjust as needed · Check for plugged vacuum holes
· Insufficient vacuum and/or · Check vacuum holes for clogging
· Mold radii should be at least the thickness
· Bad sheet clamp spring · Check and replace as needed
compressed air · Increase number of vacuum holes
of material
· Check vacuum system for minimum 25 inches Warpage, distortion, · Bad thermocouple, relay or · Check and replace as needed · Reduce depth of draw
of Hg pressure blown fuse · Increase draft (taper) of mold
excessive shrinking
· Remove any 90-degree angles in vacuum
· Uneven part cooling · Add water channels to mold
system · Part left on mold too long · Remove part from mold as soon as it
· Check for plugged water flow
· Increase size of vacuum holes stabilizes
· Check for plugged vacuum holes
· Check for vacuum or air leak
· Mold surface too smooth · Grit blast mold surface with #30 grit
· Enlarge vacuum line and valves · Poor part design · Break up large flat surfaces with ribs where
· Part shrinking away · If mold allows, apply 20-30 psi air pressure
practical or make concave/convex
· Material too hot · Reduce heating cycle
Vacuum holes and on part opposite mold surface
· Reduce heater temperature
insert lines are · Mold too cold · Increase mold temperature · Add moat to mold just outside trim line
· Increase heater distance · Preheat mold
showing Splits and tears
· Check for uniform heat · Closing speed between mold · Reduce rate of closure
· Provide uniform heating of mold
· Preheat sheet and sheet
· Check temperature control system
· Screen center of sheet, allowing edges to
· Bad sheet (not complete, · Check with sheet supplier
· Mold too hot · Reduce mold temperature
heat first
homogenous)
· Increase cooling cycle
· Too much vacuum and/or · Plug holes and redrill with smaller bit · Lower surface temperature of sheet · Incorrect timing of sheet advance · Adjust timing of sheet advance
compressed air · Use slot vacuum
· Poor mold design · Increase number of vacuum holes · Poor detail on the trim locators · Repair or redesign as needed
· Check vacuum system for too much Hg
· Check for plugged vacuum holes
pressure · Plug assist temperature too low · Increase plug assist temperature
Chill marks, mark-off
· Mold radii should be at least the thickness
· Reduce vacuum surge and/or pump capacity · Use mold release
lines, flow lines
of material
· Teflon-coat plug
· Air pressure too high · Reduce air pressure (use 20-50 psi) · Increase draft (taper) of mold
· Use Syntactic foam plug assist
· Check timing
· Poor wall distribution · Improve pre-stretching or plugging · Reduce plug height
· Air eject holes clogged
· Use plug assist
· Check eject air pressure · Mold too cold · Increase mold temperature
· Check temperature uniformity
· Timing on eject air · Preheat mold (200-260 degrees)
· Check sheet gauge
· Provide uniform heating of mold
· Material too hot · Reduce heating cycle
Webbing, bridging,
· Too much/too little sheet · Ask sheet supplier to reduce/increase · Check temperature control system
· Reduce heater temperature
wrinkling orientation · Polish plug surface
· Check for proper ventilation
· Check for uniform heat · Clamp frame too cold · Preheat clamping frame · Inadequate mold temperature · Increase number of water cooling tubes or
· Preheat sheet control channels
· Part left on mold too long · Remove part from mold sooner
· Check for plugged water flow
· Melt strength of resin too low · Use lower melt index resin
· Air Pressure too low · Increase psi (use 20-50 psi)
(sheet sag too low) · Use minimum sheet temperature possible · Material too hot · Reduce heating cycle
· Air eject holes clogged
· Profile temperature of sheet · Reduce heater temperature
· Check eject air pressure
· Check for proper ventilation
· Too much/too little sheet orientation · Reduce/increase orientation · Timing on eject air
· Increase heater distance
· Insufficient vacuum and/or · Check vacuum holes for clogging Poor material · Improper forming temperature · Check and adjust forming temperature · Check for uniform heat
compressed air · Increase number of vacuum holes · Preheat sheet
distribution · Hot/cold spots in sheet · Reduce/increase heating cycle
· Increase size of vacuum holes
· Reduce/increase heater temperature · Water leak · Locate and correct water leak
· Check for vacuum or air leak
· Check for proper ventilation
· Check vacuum system for minimum 25 inches · Insufficient vacuum and/or · Check vacuum holes for clogging
· Increase/reduce heater distance
of Hg pressure compressed air · Increase number of vacuum holes
· Check for uniform heat
· Remove any 90-degree angles in vacuum · Increase size of vacuum holes
· Preheat sheet
system · Check for vacuum system for minimum 25
· Variations in sheet thickness · Consult supplier inches of Hg pressure
· Sheet clamps inadequate · Add sheet clamps if none present
· Remove any 90-degree angles in vacuum system
· Stray drafts and air currents · Enclose or otherwise shield or screen heating
· Blank too small for mold · Leave minimum (2 inches) of material · Enlarge vacuum line and valves
around machine and forming areas
around mold · Check clamping frame air cylinders for leaks · Sheet touching mold on the index · Check clearances
· Too much sag · Use screening or other temperature control · Air leak blowing on sheet · Locate and correct air leak
of center areas of heater banks
· Use lower melt index resin
· Use more orientation in sheet
· Mold too hot · Reduce mold temperature
· Increase cooling cycle
· Lower surface temperature of sheet
WHAT TYPE OF PLASTIC IS IT?
A Quick and Easy Guide to Identifying Thermoforming Sheet
1. Determine Specific Gravity by weighing a sheet 12” x 12”.
Weight (lbs) divided by gauge (in thousandths of an inch) x 5.28 = Specific Gravity
Specific Gravity of 16 common materials (will vary depending on source)

Polypropylene 0.9 Polycarbonate 1.2
HDPE 0.96 PETG 1.27
HMWPE 0.96 PET 1.33
HIPS 1.04 PVC 1.34
ABS 1.05 PVC/Acrylic 1.35
ABS/PVC 1.07 RPET 1.33
Cast Acrylic 1.18
Extruded Acrylic 1.19
2. Apply a flame to the corner of the sheet for a few seconds, then remove.
Material Keeps Burning Smell Flame Color Smoke Drips Other Tips
Polypropylene (b) yes, slowly like blue trace of yes Shows a transparent hot area
asphalt white smoke when burning
HDPE (a) yes, slowly paraffin blue, yellow tip trace of yes Scratches easier than HMWPE.
white smoke Floats in water. See below
HMWPE (a) yes, slowly paraffin blue, yellow tip trace of yes Feels harder than HDPE.
white smoke Floats in water. See below
HIPS (b) yes, rapidly floral yellow dense+soot yes Illuminating gas when burned

ABS (b) yes acrid yellow, blue edges black+soot yes Bubbles when burning
rubbery
THERMOFORMING A probable cause and
ABS/PVC (b) no acrid yellow, blue edges black+soot no Burn rate depends on amount of
PVC content. Heavier than ABS
TROUBLESHOOTING
solution guide for
Cast Acrylic yes, slowly fruity blue at source grey no Flame may spurt if rubber
predominantly yellow modified
GUIDE
Extruded Acrylic yes, slowly fruity blue at source grey yes Flame may spurt if rubber
Thin Gauge/Roll Fed
predominantly yellow modified
Polycarbonate no, chars sweet orange dense yes Metal-like ring when struck with
faint smell black, soot hard object
thermoplastic
PETG yes, rapidly yellow, spurting black no Will crack and break under
no soot stress
PET yes, rapidly yellow, spurting black no Will crack and break under
no soot stress
RPET yes, rapidly yellow, spurting black no Imperfections in the clarity and
no soot transparency will be noticeable
PVC no acrid yellow, green spurts chars+melts no
smell
PVC/Acrylic no fruity blue, yellow tip no

 

3. Some other clues
(a) HDPE vs HMWPE: Cut a sliver 6” long from edge of sheet. Try to stretch it. The HDPE will break. The HMWPE will stretch before breaking.
(b) Generally not available in transparent.
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