SPE Heavy 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 Heavy Gauge Thermoplastic (Sheet-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
Sheet wrinkling · Sheet too hot · Cooler heater temperature Poor definition, · Sheet too cold · Increase heating cycle Excessive wall · Improper sheet sag · Mount mold on top platen
· Shorten heating cycle · Increase heater temperature · Use snap back method
webbing bridging detail of forming thinning
between tools
· Poor vacuum · Add vacuum holes · Poor vacuum · Check for clogged vacuum holes · Hot cold spots in sheet · Check heat system for consistency
· Check vacuum system for 25 inches of Hg · Add more vacuum holes, increase size of · Use screens/heater zones for consistent
current vacuum holes heat temperature
· Poor tool design · Redesign tool · Check for vacuum leaks
· Stray drafts on heated sheets · Eliminate drafts in forming area
· Improper drawer ratio for mold/ · Add assist block in wrinkled area · Cold damp frame · Preheat clamp frame
improper mold layout · Use recessed pocket in waste area near web · Cool tool · Increase tool temperature
· Increase draft and/or radii if possible · Slow vacuum, sheet not drawn · Check system for 25 inches of Hg
· Use mechanical assist fast enough · Check for vacuum leaks · Too much sheet sag · Use screens/zone heaters to control center
· If more than one tool, increase spacing · Check for plugged vacuum hose. Increase size sheet temperature
between tools of vacuum hose. Increase size of vacuum holes
Thin corners in
or add more holes · Sheet too thin · Use heavier gauge material
Blisters, voids, pits, · Heating too rapidly (over-heating) · Cooler heater temperature · Add vacuum pump capacity deep draw
· Slower heating cycle · Sheet temperature variation · Check heating system for consistency
bubbles
· Additional pressure · Use plug or frame assist · Use heating zones or screens for sheet
· Moisture in sheet · Pre-dry sheet · Use 20-50 psi air pressure on opposite side of consistency
· Heat sheet from both sides tool surface
· Cooler heater temperature · Tool temperature · Heat tool temperature just below forming
Mark off lines,
· Use material immediately after pre-dry · Tool temperature too low · Increase tool temperature, do not exceed window
forming window of material
chill marks
· Uneven heating · Use screening method to even · Relieve mold corners · Forming technique · Use plug assist to pre-stretch material
heat distribution · Check temperature control of tool, if using · Increase corner radii
· Check for defective heaters heated tool
Shrink marks in
· Adjust heater zones · Poor vacuum · Check for plugged vacuum holes
· Cool plug assist temperature · Increase plug assist temperature · Check for vacuum leaks
corners (inside radii
Excessive · Sheet too hot · Cooler heater temperature · Add vacuum holes
of tool)
· Shorter heating cycle · Sheet too hot · Reduce heater setting · Add vacuum pump capacity
sheet sag
· Lower surface temperature of sheet with
· Sheet too large · Use screens or zone heaters to forced air before tool contact · Tool surface too smooth · Sand smooth tool surface
cool center of sheet · Heat sheet slower
Part sticking to tool · Wooden tool · Lower tool surface temperature
Shiny streak, spots · Overheated sheet in area · Use screens to cool shiny area of sheet · Poor vacuum · Add vacuum holes · Use tool release agent
Surface marking on
· Cooler heater temperature · Check for vacuum leaks
on formed parts formed part
· Shorter heating cycle · Check for vacuum pressure · Tool too hot · Lower surface temperature

Part Warpage · Uneven cooling · Cool entire part consistently · Air entrapment over smooth · Lightly sand surface of tool to create scratch · Sheet too hot · Lower sheet temperature before contact
· If water cooled, add more channels tool surface lines for trapped air to escape of tool
or tubing/Check for plugged water flow
· Tool too hot · Lower tool temperature · Tool undercut · Increase air eject pressure
· Poor tool design · Add vacuum holes · Remove tool ASAP and use cooling
· Check for plugged vacuum holes · Tool too cool · Increase tool temperature fixture on formed part
· Create moat below trim line of tool · Redesign tool to breakaway
· Dirt on tool · Clean tool surface
· Wall thinning · Use plug assist pre-stretch material · Poor draft angle · Use female tool
before tool contact · Dust in atmosphere · Use ionized air gun · Increase draft angle
· Increase sheet gauge · Isolate forming area · Remove tool ASAP and use cooling fixture on
· Check for uniform heating of sheet formed part
· Dirt on sheet · Use ionized forced air, clean sheet
Sheet tears · Tool too cold · Raise tool temperature just below · Improper plug assist temperature · Cool plug temperature
sheet forming window · Improper tool composite · Use aluminum when possible · Use tool release compound
when forming
· Avoid heat sink tool materials
Sheet sticking to
· Part removed from tool too fast · Increase cooling time · Heating of sheet uneven · Use screens for uniform sheet heat
plug assist
· Tool surface too rough · Sand tool surface with fine grit sandpaper · Check for malfunctioning heaters
· Large flat surface · Use ribs, or make part concave, convex · Change tool material compound · Check for air leaks in clamp frame

Pre-stretch bubble
Sheet whitening
· Sheet too cold · Increase heating cycle or heater temperature · Sheet stretching beyond yield · Increase sheet temperature, vacuum, · Air draft · Enclose area around forming machine
· Check for uniform heating of sheet point because of under-heating speed of drape from draft
non-uniform
· Use baffle in pre-stretch box
· Sheet too hot · Decrease heating cycle or heater temperature Nipples on tool side · Sheet too hot · Reduce heater temperature
· Check for uniform heating of sheet · Reduce heat cycle time · Inconsistent air blow · Check air blow cycle
of formed part
· Tool design · Increase radius of corner(s) · Vacuum holes too large · Reduce sheet temperature before tool contact · Insufficient sheet temperature · Increase sheet temperature
· Plug holes and re-drill
· Feed rate of tool · Slow down rate of speed when tool Part cracks or · Stress concentration · Reduce stress by increasing forming time
Sag variation
is pushed into sheet · Sheet variation temperature · Check for air drafts during heat cycle and slower plug speed
breaks in use
between blanks
· Poor material selection · Change materials
Excessive shrinkage · Removed part too soon · Increase cooling cycle
from tool · Use cooling fixture
or part distortion
after tool removal
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
Heavy Gauge/Sheet 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.

 
www.thermoformingdivision.com

Leave a Reply