How to Prevent TPU Phone-Case Corners From Curling Off PEI

September 29, 2026 · by Priya Nair · 4 min read

Why TPU Phone Case Corners Curl Off Textured PEI (And How to Stop It)

TPU phone-case corners curl off a textured PEI sheet mainly because of uneven cooling and weak first-layer bonding at those thin, high-surface-area corner points. The fix: dial back part cooling near the bed, tighten up first-layer temperature and squish, and add a light adhesive boost right at the corners.

If you’ve printed flexible phone cases before, you know the drill. Everything looks great for the first 20 or 30 layers, then you glance over and one or two corners have popped up off the plate like a potato chip left in the sun. This is one of the most common TPU-specific failures out there, and honestly, it’s almost never a filament quality issue. It’s a thermal and adhesion problem, and it’s specific to how corners behave compared to the rest of the part.

Why Corners Curl First

Corners are the weak link in any print. Why? Because they cool from two directions at once, both edges losing heat simultaneously, and TPU’s flexibility means even a small amount of internal stress from cooling contraction is enough to physically peel the material up rather than crack it the way a rigid filament would.

• Faster heat loss: corners carry more exposed surface area relative to their mass.
• Stress concentration: shrinkage forces converge right at the corner point instead of spreading out along a straight edge.
• Weaker bond area: if the nozzle zips over the corner quickly, that spot gets less thermal dwell time to actually fuse into the PEI texture.

Step 1: Fix Your First-Layer Settings

Before you even think about adhesives, get the basics dialed in. Most curling problems die right here.

• First layer temperature: run it 5-10°C hotter than your usual TPU temp (commonly landing around 220-235°C depending on brand), just for that first layer.
• Bed temperature: 45-60°C works well for TPU on textured PEI. Go too cold and the layer never really bites into the texture.
• Squish: TPU wants a bit more squish than PLA does. Drop your Z-offset until you see a visible smear on the first layer rather than a neat rounded bead.
• First layer speed: slow it down to 15-20 mm/s. At higher speeds, corners simply don’t get enough time to bond.

Step 2: Control Cooling, the Real Root Cause

This is the step almost everyone skips, and it’s usually where the actual fix lives.

• Reduce fan speed for the first 5-10 layers. Blasting full cooling right away shocks the corners into contracting before they’ve had a chance to bond.
• Disable cooling entirely on layer 1, then ramp it up gradually, something like 0% to 30% to 60% over the first several layers.
• Check for drafts. An open window, an AC vent, or a gap in your enclosure blowing across one side of the bed will curl corners on that side only. It’s actually a useful diagnostic clue if you’re troubleshooting.
• Use an enclosure if your printer doesn’t already have one. Even a cardboard box taped together drops ambient airflow enough to make a real difference.

Step 3: Improve Adhesion at the Corners Specifically

If tweaking settings alone doesn’t fully solve it, go after the corners directly:

• Glue stick, corners only. A thin swipe of PVA-based glue stick at the four corner regions (skip the rest of the bed) adds tack without messing with the texture everywhere else.
• Clean the PEI sheet with IPA before every single print. Skin oils and dust cut PEI’s grip dramatically, and corners are always the first casualty.
• Add a brim. A 5-8mm brim anchors the corner geometry to the bed and gives you extra surface area to grab onto.
• Rotate the print 45° on the bed so the model’s corners aren’t lined up with any airflow direction or fan draft.

Step 4: Slicer-Level Tweaks

• Bump up first-layer line width to 120-140% of nozzle diameter for more contact area.
• Enable elephant foot compensation, but carefully. Overdo it and you’ll create a new adhesion problem with TPU.
• Slow down outer wall speed overall, not just on layer 1. TPU corners can still peel mid-print if walls cool too fast at any height, not just the base.

Quick Diagnostic Checklist

Conclusion

TPU corner curling on textured PEI comes down to two real culprits: weak first-layer bonding and uneven cooling stress. Warm up that first layer, slow your cooling ramp, and you’ll knock out most cases right there. Still lifting? Add a bit of targeted glue-stick tack and a small brim instead of coating the whole bed. Get these dialed in and textured PEI becomes just as dependable for flexible phone cases as it already is for PLA and PETG.