Technical Support & Solutions

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Thermoforming Troubleshooting Guide

Sheet wrinkling webbing bridging between tools

Possible Causes and what to do:

The Sheet is too hot
Cooler heater temperature.
Shorten the heating cycle.

The vacuum is poor
Add vacuum holes.

Check the vacuum for leaks in the hose and fittings.

Poorly designed tools
Redesign the tool.

There is an improper drawer ratio for mold or improper mold layout
Add a take-out block in wrinkled area.
Use recessed pocket in web area.
Increase draft and/or radii if possible.
Use mechanical assist.
If there is more than one tool, increase the distance between tools.

Blister, voids, pits, bubbles

Possible Causes and what to do:

Overheating
Cooler heater temperature.
Shorten the heating cycle.

Moisture in sheet
Pre-dry sheet.
Heat sheet from both sides
Cooler heater temperature
Use material immediately after pre-dry

Uneven heating
Use screening method to even the head distribution.
Check for heaters that are out.
Adjust the heater zones.

Excessive sheet sag

Possible Causes and what to do:

Sheet too hot
Cooler heater temperature.
Shorten the heating cycle.

Sheet too large

Use screens or zone heaters to cool center of the sheet.

Shiny streak, spots on formed parts.

Possible Causes and what to do:

Overheated sheet in area
Use screens to cool shiny area of sheet.
Cooler heater temperature.
Shorter heating cycle.

Part warpage.

Possible Causes and what to do:

Uneven cooling
Cool entire part consistently.
If water cooled tube, add more channels or tubing, Check for plugged water flow.
Creat moat below trim line of tool.

Wall thinning
Use plug assist pre-stretch material before tool contact.
Increase sheet guage.
Check for uniform heating of sheet.

Sheet tears when forming.

Possible Causes and what to do:

Tool cold
Raise tool temperature just below sheet forming window.

Part removed from tool too fast
Cool part adequately.

Large flat surface
Use ribs, or make part concave, convex.

Sheet too cold
Increase heating cycle or heater temperature.
Check for uniform heating of sheet.

Sheet too hot
Decrease heating cycle of heater temperature.
Check for uniform heating of sheet.

Tool design
Increase radius of corner(s).

Feed rate of tool
Slow down rate of speed when tool is pushed into sheet

Poor definition, detail of forming.

Possible Causes and what to do:

Sheet too cold
Increase heating cycle.
Increase heater temperature.

Poor vacuum
Check for heat uniformity of sheet.
Check for clogged vacuum holes.
Add more vacuum holes, increase size of current vacuum holes.
Check for vacuum leaks.

Cold clamp frame
Preheat clamp frame

Slow vacuum, sheet not drawn fast enough
Check system: 25° of Hg vacuum pressure.
Check for vacuum leaks.
Check for plugged vacuum hose, increase size of vacuum hose, increase size of vacuum holes or add more holes.
Add vacuum pump capacity

Additional Pressure

Use plug or frame assist
Use 20-50 psi air pressure on opposite side of tool surface.

Mark off lines, chill marks

Possible Causes and what to do:

Tool temperature too low
Increase tool temperature, do not exceed forming window of material.
Relieve mold corners.
Check temperature control of tool, if using heated tool

Cool plug assist temperature
 Increase plug assist temperature.

Sheet too hot
Reduce heater setting.
Lower surface temperature of sheet with forced air before tool contact.
Heat sheet slower.

Surface marking on formed part

Possible Causes and what to do:

Poor vacuum

Add vacuum holes.
Check for vacuum leaks.
 Check for vacuum pressure.

Air entrapment over smooth tool surface
Lightly sand surface of tool to create scratch lines for trapped air to escape.

 Tool too hot
Lower tool temperature.

 Dirt on tool
 Clean tool surface.

Dust in atmosphere
Use ionized air gun.
Isolate forming area.

Dirt on tool
Use ionized forced air, clean sheet.

Improper tool composite
Use aluminum when possible.
Avoid heat sink tool materials.

Tool surface too rough
Sand tool surface with fine grit sandpaper.
Change tool material compound.

Sheet whitening

Possible Causes and what to do:

Sheet stretching beyond yield point because of under-heating
Increase sheet temperature, vacuum, speed of drape.

Nipples on tool side of formed part.

Possible Causes and what to do:

Sheet too hot.
Reduce heater temperature.
▫ Reduce heat cycle time.
▫ Reduce sheet temperature before tool contact.

Sag variation between blanks.

Possible Causes and what to do:

Sheet variation temperature.
Check for air drafts during heat cycle.
Vacuum holes too large.
Plug holes and re-drill.

Excessive shrinkage or part distortion after tool removal.

Possible Causes and what to do:

Removed part too soon from tool.
Increase cooling cycle
Use cooling fixture.

Excessive wall thinning.

Possible Causes and what to do:

Improper sheet sag.
Mount mold on top plate.
Use snap back method.

Hot cold spots in sheet
Check heat system for consistency.
Use screens/heater zones for consistent heat temperature

Stray drafts
Restrict draft low in forming area.

Cool tool
Increase tool temperature.

Too much sheet sag
Use screens/zone heaters to control center sheet temperature

Thin corners in deep draw.

Possible Causes and what to do:

Sheet too thin.
Use heavier gauge material
Check heating system for consistency.

Sheet temperature variation.
Use heating zones or screens for sheet consistency.
Check heat system for consistency.

Tool temperature
Heat tool temperature just below forming window.

Forming technique.
Use plug assist to pre-stretch material.

Shrink marks in corners (inside radii of tool).

Possible Causes and what to do:

Poor vacuum.
Check for plugged vacuum holes
Check for vacuum leaks.
Add vacuum holes.
Add vacuum pump capacity.

Tool surface too smooth.
Sand smooth tool surface.

Part sticking to tool.

Possible Causes and what to do:

Wooden tool.
Lower tool surface temperature.
Use tool release agent.

Tool too hot.
Lower tool surface temperature.

Sheet too hot.
Lower sheet temperature before contact of tool.

Tool undercut.
Increase air eject pressure,
Remove tool ASAP and use cooling fixture for formed part,
Redesign tool to breakaway,

Poor draft angle.
Use female tool.
Increase draft angle.
Remove tool ASAP and use cooling fixture on formed part.

Improper plug assist temperature.
Cool plug temperature.
Use tool release compound.

Sheet sticking to plug assist.

Possible Causes and what to do:

Heating of sheet uneven.
Use screens for uniform sheet heat.
Check heating system for heaters out.

Pre-stretch bubble non-uniform.

Possible Causes and what to do:

Air draft.
Check for air leaks in clamp frame.
Enclose area around forming machine from draft.

Non-consistent air flow.
In pre-stretch box, use baffle.

Under designed tool
Re-evaluate design of tool.

Cold formed.
Core of sheet is not at forming temperature when drawn down on tool.

During service corners cracking.

Possible Causes and what to do:

Stress concentration
Increase fillets.
Increase sheet temperature.
For deep draw down on tool before cooling cycle befits.

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Hop Industries Corporation

1251 Valley Brook Avenue
Lyndhurst, NJ 07071- 3509
T: 201.438.6200  -  F: 201.438.5444

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