| Problems | Possible Causes | Suggested Solutions |
| Lines and bands in the transverse direction of extrusion | 1. Excess molten material in polish roll nips | 1. Adjust sheet gauge and polish roll openings to just provide enough material to fill nips. |
| 2. Sheet sticks to polish rolls | 2. Improve cooling.
a. Reduce polish roll temperature.
b. Eliminate thin spots in sheet.
c. Improve sheet contact with polish rolls by increasing line tension.
d. Ensure nip gaps are the same on both sides of the sheet.
e. Reduce melt temperature. |
| Webbing | 1. Sheet too hot | 1. Reduce oven temperature or cycle time. |
| 2. Inadequate vacuum to evenly draw sheet | 2. Add vacuum holes in problem areas. |
| 3. Excessive draw | 3. Improve molding technique or part design.
a. Use plug or ring assist.
b. Use web blocks to minimize sag problems.
c. Redesign mold to improve cavity spacing or balance draw. |
| Excessive sheet sag | 1. Sheet too hot | 1. Reduce oven temperature or cycle time. |
| 2. Oven too wide | 2. Reduce melt flow of GPPS for blend or run on a more narrow thermoformer. |
| Uneven sag | 1. Sheet temperature not uniform | 1. Eliminate air drafts in oven and fix faulty heaters. |
| 1. Part temperature too hot for proper release | 1. Reduce part temperature.
a. Increase cooling cycle.
b. Reduce sheet temperature.
c. Lower mold temperature. |
| Stretch marks on part | 1. Plug assist sticks to sheet and causes freeze-off lines | 1. Eliminate sticking.
a. Change heated plug temperature to equal sheet temperature.
b. Apply release coating to plug assist.
c. Use lower stick material for basic plug construction. |
| 2. Cold mold causes curved chill lines around lip of part | 2. Increase mold temperature or increase air cushion as part is formed. |
| 3. Mold temperature varies between cavities | 3. Increase number of water channels or clean out plugged channels as necessary. |
| Nipples on mold side of thermoformed part | 1. Sheet too hot | 1. Reduce sheet temperature. |
| 2. Vacuum holes too large | 2. Plug holes and redrill with #80 drill bit. |
| Pock marks | 1. Air entrapment between part and mold | 1. Eliminate trapped air.
a. Slightly roughen large, flat mold surfaces with very fine grit blasting or glass beading of problem area.
b. Clean out plugged vacuum holes.
c. Add vacuum holes or vents as required. |
| Poor part detail | 1. Cold sheet | 1. Increase sheet temperature. |
| 2. Sheet temperature not uniform | 2. Eliminate air drafts in oven, screen oven heat and add clamp rail heaters as needed. |
| 3. Inadequate vacuum on part | 3. Improve contact with mold.
a. Fix vacuum leaks.
b. Clean plugged vacuum holes and vents.
c. Add vacuum holes or vents in problem areas.
d. Add moat or ring assist to ensure good seal around perimeter or part.
e. Make sure surge tank and vacuum pump are large enough to quickly evacuate the mold. |
| Poor wall distribution | 1. Sheet temperature not uniform | 1. Use screens, additional heaters or eliminate air drafts in oven. |
| 2. Poor mold design | 2. Reduce severe areas of draw, increase draft angles and reduce undercuts in mold. |
| 3. Sheet drags on mold lip during plug-assist forming | 3. Increase air cushion under sheet during plug assist travel.
a. Increase plug speed.
b. Reduce vacuum bleed rate.
c. Reduce vacuum hole size.
d. Raise mold surface around lip of part so sheet drag on lip is reduced. |
| Part warpage | 1. Mold too hot or cold | 1. Try setting mold temperature at 100°F (38°C). |
| 2. Uneven mold cooling | 2. Clean plugged water channels or add channels as needed. |
| 3. Poor part wall distribution | 3. Improve wall distribution as suggested in previous troubleshooting section above. |
| 4. Part not cooled adequately | 4. Increase cooling cycle time, reduce mold temperature or reduce sheet temperature. |
| 5. Poor mold contact | 5. Improve vacuum on part (see section under “Poor part detail”). |
| 6. Part design not rigid enough | 6. Add ribs and additional detail where possible. |
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