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● Supply Note: PRES PPSU Sheets Are Mill-Annealed
● Why PPSU Plates Distort During CNC Milling
● Step 1: Pre-Machine Annealing (Eliminate Initial Stress)
● Step 2: Symmetrical Roughing with Allowance
● Step 3: Interim Annealing (Remove Roughing Stress)
● Step 4: Finish Machining — Sharp Edge, Low Feed
● Coolant Rules: No Chlorinated EP Additives
● Thin-Wall & Pocket Machining Strategy
● Step 5: Post-Machine Annealing (Final Stress Relief)
● Complete 3-Cycle Annealing Summary
>> Need PPSU Sheet Stock for Your CNC Project?
To prevent PPSU sheet warpage during CNC milling, run a 3-cycle annealing process: pre-anneal the blank at 140–160°C for 2 hours and furnace-cool below 60°C; rough both faces alternately leaving 0.3–0.5mm allowance, then re-anneal; finish with sharp uncoated carbide at low feed using compressed-air only (never chlorine-containing EP coolant); final post-machine anneal locks dimensional stability. Here's the complete cycle:
| Step | Process | Temp & Time | Cooling |
|---|---|---|---|
| 1. Pre-anneal | Stress relief before first cut | 140–160°C × 2h | Furnace cool to <60°C |
| 2. Rough A/B | Alternate faces, leave 0.3–0.5mm | — | Air blow only |
| 3. Interim anneal | Remove roughing stress | 140–160°C × 2h | Furnace cool to <60°C |
| 4. Finish | Sharp carbide, low feed, climb mill | — | Compressed air (no chlorine EP) |
| 5. Post-anneal | Final stress relief | 140–160°C × 2h | Furnace cool to <60°C |
If you source PPSU sheets from other suppliers without documented mill annealing: Always follow the full 3-cycle process (Steps 1–5) exactly as outlined below. Unannealed stock carries hidden planar stress that will cause severe warpage once material is removed.
The PPSU sheet is formed by extrusion. When the PPSU material is extruded from the mold, its surface cools rapidly, but the interior of the sheet remains at a high temperature, locking the residual stress in the sheet - thicker PPSU sheets will have greater residual stress (this is why our sheets require heat treatment before processing even after annealing ). When you clamp the PPSU material and remove the material only from one side, this stress balance is disrupted:
Single-sided milling removes the "surface layer" that maintained the stress balance → The opposite side will generate a stress reaction towards the tool;
Clamping fake-flat (forcing a bowed blank flat with pressure) stores elastic energy → released the moment the part is unclamped
Cutting forces introduce new micro-stress along tool paths → manifests as corner cracks and edge lifting 12–48 hours post-machining
The only way to hold flatness is a full stress-relief cycle before, between, and after machining — never a single anneal, never only before, never only after.
Before starting the processing, you can perform a preheating treatment according to the following parameters
| Parameter | Setting | Why It Matters |
|---|---|---|
| Temperature | 140–160°C (284–320°F) | Above Tg (220°C? No — PPSU Tg ≈ 220°C, so 140–160°C is sub-Tg stress relief, safe for dimensional stability) |
| Hold Time | 2 hours minimum (add 1h per 25mm thickness above 20mm) | Ensures thermal equilibrium through the full cross-section |
| Cooling | Furnace cool — close oven door, cool to <60°C before removing | Prevents new thermal stress gradient |
| Oven Type | Circulating air (convection), not radiant | Radiant hot spots cause localized over-softening |
Machine both faces alternately — never remove all material from one side in a single pass. The goal is to keep stress balanced while approaching final dimensions.
| Parameter | Recommended Value | Notes |
|---|---|---|
| Allowance per face | 0.3–0.5 mm | Total thickness loss ≈ 0.6–1.0mm from roughing |
| Cutting direction | Climb mill (down-milling) | Pushes workpiece into the table/support, reducing lift and tear-out |
| Tool | Uncoated fine-grain carbide, sharp edge | No TiN/TiAlN coatings — they flake on PPSU and embed abrasive particles |
| Cutting speed (Vc) | 300–500 m/min | Higher than rods turning (100–200 m/min) — sheet milling runs faster |
| Tooth feed (fz) | 0.05–0.15 mm/z (finish), 0.1–0.3 mm/z (rough) | Keep axial depth shallow: 0.1–0.3mm per pass |
| Corner radius on end mill | R ≥ 0.5mm (prefer R0.8–1.0mm) | Sharp square corners concentrate stress → crack after stress relief |
Pre-anneal 150°C × 2h → furnace cool
Rough Face A: mill to 19.4mm (remove 0.6mm, leave 0.5mm allowance on A)
Flip → Rough Face B: mill to 18.8mm (remove 0.6mm, leave 0.5mm allowance on B)
Interim anneal 150°C × 2h → furnace cool
Finish Face A: mill to 18.35mm (remove 0.45mm)
Flip → Finish Face B: mill to 18.0mm final (remove 0.35mm)
Post-anneal 150°C × 2h → furnace cool
Measure and ship
After roughing, the cutting forces have introduced new internal stress along every tool path. If you skip this step and go straight to finishing, the finish pass will still warp — the roughing stress is now locked into the thinner blank.
| Parameter | Setting |
|---|---|
| Temperature | 140–160°C (284–320°F) |
| Hold Time | 2 hours (or 1h per 25mm of current thickness) |
| Cooling | Furnace cool to <60°C |
Now machine to final dimensions. The part is stress-relieved and dimensionally stable — your job is to not introduce new stress.
| Do ✅ | Don't ❌ |
|---|---|
| Use razor-sharp uncoated carbide end mills | Use dull tools (increase cutting force → heat → stress) |
| Low feed: tooth feed 0.05–0.10 mm/z | High feed (work-hardens the surface layer) |
| Climb mill (down-milling) throughout | Conventional milling (lifts thin walls, tears edges) |
| Shallow axial depth: 0.1–0.3 mm per pass | Deep axial cuts (>1mm) on thin sections |
| Corner radius R ≥ 0.5mm on all internal corners | Sharp 90° internal corners (stress concentration → crack) |
| Compressed air blow-off for chip evacuation | Liquid coolant with unknown EP package |
PPSU is an amorphous polymer — its molecular chains are particularly vulnerable to stress cracking when exposed to certain chemicals under tensile stress. Chlorinated extreme-pressure (EP) additives in soluble oils are the #1 cause of post-machining cracking.
| ✅ Allowed | ❌ Forbidden |
|---|---|
| Compressed air blow-off (preferred) | Liquid coolants with chlorinated EP additives |
| Chlorine-free soluble oil (with certificate of analysis) | Water-emulsion with unknown EP package |
| Deionized water mist (verified pH neutral) | Tap water (dissolved chlorides & minerals) |
| Dry cutting (if tooling allows) | IPA, acetone, or solvent-based fluids |
Deep pockets and thin walls (<1.5mm) are where most PPSU sheet machining fails — even with perfect annealing, poor pocket strategy causes spring-back and corner cracking.
Leave support ribs in thin-wall pockets until the final pass. Machine the perimeter walls first at 1.0–1.5mm thickness, then go back and thin to final 0.8–1.0mm after the post-anneal.
Jump-sequence pocket milling: don't machine pockets in order 1→2→3→4. Jump between distant pockets to distribute heat and stress evenly across the plate.
Internal corner radius: minimum R0.5mm, preferably R0.8–1.0mm. Stress concentration at sharp corners is the #1 cause of delayed cracking.
Through-holes near edges: drill/bore before pocketing. The edge material needs maximum support during pocket milling.
After the finish pass, the part still carries micro-stress from the final cutter path — especially along thin walls and in corners. One last cycle locks dimensional stability:
| Parameter | Setting |
|---|---|
| Temperature | 140–160°C (284–320°F) |
| Hold Time | 2 hours (or 1 hour per 25mm of final wall thickness) |
| Cooling | Furnace cool below 60°C |
Raw PPSU plate
↓
【Anneal 1】150°C × 2h → furnace cool to <60°C
↓
Rough Face A (leave 0.3–0.5mm) → Flip → Rough Face B (leave 0.3–0.5mm)
↓
【Anneal 2】150°C × 2h → furnace cool to <60°C
↓
Finish Face A (sharp carbide, low feed, climb mill)
↓
Flip → Finish Face B → final dimensions
↓
【Anneal 3】150°C × 2h → furnace cool to <60°C
↓
Measure (after cooling) → QC → Ship
We stock natural amber PPSU sheets from 1mm to 120mm thickness, FDA and USP Class VI compliant, ready for immediate CNC machining.
