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PI (Polyimide / Vespel) CNC Machining Material Manual

Cnc plastic materials
Last updated: May 23, 2026

PI (Polyimide / Vespel) — CNC Machining Material Manual

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Rating legend — ★★★★★ best · ★☆☆☆☆ worst. For machinability/wear/heat resistance more stars = better; for cost, fewer stars = cheaper.

📋 Material Quick-Reference Card

┌──────────────────────────────────────────┐
│  Material Name: Polyimide (PI)            │
│  (Vespel-type high-performance plastic)   │
│  Category: Ultra-high-temperature         │
│            engineering plastic            │
│  Density: 1.34~1.43 g/cm³                 │
│  Filled Grades: up to ~1.6 g/cm³          │
│  Tensile Strength: 70~90 MPa              │
│  Flexural Strength: 110~140 MPa           │
│  Continuous Service Temp.: 260~300 ℃      │
│  Short-Term Temp.: up to ~480 ℃           │
│  Machinability: ★★★☆☆ (hard/brittle)      │
│  Wear Resistance: ★★★★★ (graphite grades) │
│  Heat Resistance: ★★★★★ (top-tier)        │
│  Chemical Resistance: ★★★★☆               │
│  Cost: ★☆☆☆☆ (ultra-premium material)    │
│  Keywords: Vespel, extreme heat, vacuum,  │
│            low outgassing, metal replace. │
└──────────────────────────────────────────┘

1. Material Overview

1.1 Introduction

PI (Polyimide) is a premium, ultra-high-temperature, dimensionally stable engineering plastic used where ordinary plastics fail. It is commonly associated with Vespel (DuPont), one of the best-known machinable polyimide shape families. PI is a high-end thermoset-like polyimide: it does not behave like a normal melt-processable thermoplastic and is one of the most heat-resistant machinable plastics available.

  • English Name: Polyimide / PI
  • Common Nicknames: Vespel-type material, high-temperature polyimide, imide polymer
  • Famous Brand Names: Vespel (DuPont), Meldin, P84-based PI shapes

1.2 Main Types / Grades ⭐ Important

Type Full Name Characteristics
Vespel SP-1 Unfilled polyimide Highest purity, excellent electrical insulation, low outgassing, very stable precision grade
Vespel SP-21 15% graphite-filled PI Lower friction, improved wear resistance, common bearing / bushing grade ⭐ Common for CNC wear parts
Vespel SP-3 MoS₂-filled PI Designed for vacuum / dry-running environments; excellent low-outgassing wear performance
Vespel SP-22 40% graphite-filled PI Lower CTE, better dimensional stability, higher stiffness, improved wear under load

💡 Graphite-filled PI grades are often chosen when the part is a high-temperature sliding component. For clean electrical insulation or low-contamination applications, start with unfilled PI / SP-1.

1.3 Raw Material Forms

Common forms for CNC machining:

  • PI Rod (round bar): bushings, seals, washers, small turned parts
  • PI Sheet/Plate: milled insulators, fixtures, spacers, precision plates
  • Isostatic molded / direct-formed shapes: near-net high-performance blanks, often sold in small sizes due to cost
  • Common colors: natural amber/brown for unfilled PI; black/dark gray for graphite-filled grades

2. Composition & Physical Properties

2.1 Material Composition

PI is an aromatic imide polymer built from highly heat-resistant imide ring structures. Machinable PI shapes are commonly produced by powder, compression, or direct-forming processes rather than ordinary injection molding. The material is often described as thermoset-like because it does not truly melt and flow during reheating; at excessive temperature it will degrade or char instead of behaving like POM, nylon, or ABS.

Type Molecular / Filler Structure
Unfilled PI Aromatic polyimide backbone, high purity, excellent insulation and low outgassing
Graphite-filled PI PI matrix with graphite for lower friction, better wear, lower CTE
MoS₂-filled PI PI matrix with molybdenum disulfide for dry-running / vacuum wear performance
High-graphite PI Higher graphite loading for stiffness, dimensional stability, and bearing performance

2.2 Physical Properties

Property Value
Density 1.34~1.43 g/cm³ (unfilled / typical)
Filled-Grade Density up to ~1.6 g/cm³
Melting Point No true melting point; does not melt like a thermoplastic
Glass Transition Very high; typically above the range of common engineering plastics
Long-term Service Temp. 260~300 ℃
Short-term Temperature Capability up to ~480 ℃
Thermal Conductivity Low to moderate; filled grades conduct heat slightly better
Water Absorption Moderate for a high-performance plastic; keep dry for precision work
Coefficient of Thermal Expansion Low; graphite-filled grades are especially stable

💡 PI combines extreme heat resistance + low CTE + low outgassing, making it a top-tier plastic for aerospace, vacuum, semiconductor, and high-temperature precision assemblies.

3. Mechanical & Chemical Properties

3.1 Mechanical Properties

Property Value
Tensile Strength 70~90 MPa
Flexural Strength 110~140 MPa
Elastic Modulus 2500~3500 MPa typical; filled grades can be higher
Elongation Low to moderate; relatively brittle compared with nylon / PC
Hardness High for a plastic; machines like a hard engineering material
Impact Strength Fair; avoid sharp internal corners and impact loading
Coefficient of Friction Low; graphite grades are self-lubricating

⚠️ PI is strong and extremely heat-resistant, but it is not a tough impact plastic. For shock, drop, or snap-fit parts, choose PC, nylon, or another tougher material.

3.2 Chemical Resistance

Medium Resistance
Fuels, oils, many solvents ✅ Good to excellent
Vacuum / space environment ✅ Excellent, low outgassing
Radiation exposure ✅ Excellent compared with most plastics
Weak acids / weak bases ✅ Generally good
Strong bases / alkalis ❌ Poor to fair; can attack PI
Concentrated acids ❌ Poor to fair, especially at elevated temperature
Hot caustic / high-temperature moisture ⚠️ Risk of chemical attack or hydrolysis

3.3 Notable Characteristics

  • Extreme heat resistance: continuous service around 260~300 ℃; short-term exposure can reach approximately 480 ℃
  • Does not truly melt: thermoset-like behavior; no normal melting point for remelt processing
  • Excellent wear and friction performance: graphite and MoS₂ grades are designed for dry sliding and bearing duty
  • Outstanding dimensional stability: low CTE, low creep, and excellent stability at elevated temperature
  • Low outgassing: suitable for vacuum, aerospace, and semiconductor environments
  • Excellent radiation resistance and inherently flame-retardant behavior
  • High material cost and brittleness: careful design and machining are required to avoid scrap

4. CNC Machining Process ⭐⭐ Core

4.1 Machinability Rating

★★★☆☆ Moderate machinability — PI can be machined accurately, but it behaves like a hard, somewhat brittle premium material:

  • Cuts cleanly when tools are sharp; no melting or gummy chip behavior like soft thermoplastics
  • Can hold tight tolerances due to excellent dimensional stability and low stress movement
  • Filled grades are abrasive and quickly wear ordinary cutting edges
  • Thin edges, sharp corners, and aggressive feeds may chip
  • Stock is extremely expensive, so process planning must minimize scrap
Item Recommendation
Tool Material Sharp carbide for general work; PCD / diamond tooling for graphite-filled or production machining
Cutting Edge Very sharp, polished edge; avoid worn tools that generate heat and dust
Rake Angle Positive rake angle, but avoid overly weak edges on interrupted cuts
Helix Angle Moderate to high helix for smooth cutting and dust/chip evacuation
Flutes 12 flutes for small tools and chip clearance; 23 flutes for rigid finishing setups
Operation Spindle Speed (RPM) Feed Rate (mm/min) Depth of Cut (mm)
Rough Milling 3000~8000 300~1000 0.5~2
Finish Milling 5000~12000 150~600 0.05~0.3
Turning 800~2500 0.05~0.20/rev 0.2~1
Drilling 800~2500 20~100 —

📌 Parameters are for reference only; adjust based on machine rigidity, tool diameter, grade/filler content, and part geometry.

4.4 Machining Challenges & Solutions

Challenge Cause Solution
Tool wear Graphite / MoS₂ filled grades are abrasive Use carbide, PCD, or diamond tooling; inspect edges frequently
Chipping / edge breakout PI is relatively brittle, especially at thin edges Use light cuts, sharp tools, support exit edges, add radii/chamfers
Very expensive scrap Premium PI stock is sold in small, high-cost shapes Verify program, leave machining allowance, use soft jaws/fixtures, machine samples first
Fine abrasive dust Dry machining produces small PI and filler particles Use vacuum extraction, air blast, enclosure cleaning, and PPE
Heat glazing / local burnishing Dull tools or rubbing instead of cutting Reduce rubbing, maintain feed per tooth, use sharp tools and air cooling
Hole breakout Drill exits unsupported material Peck drill, back up the part, use sharp drills, finish with boring/reaming if needed

4.5 Annealing Recommendation ⭐

PI is already extremely dimensionally stable, so generic annealing is usually not required for normal CNC parts. For ultra-precision parts, thick sections, or supplier-specific Vespel/PI shapes, follow the material supplier’s stress-relief guidance.

Reference Stress-Relief Approach:
• Use fully cured / fully processed PI stock whenever possible
• For precision work: rough machining → optional supplier-approved stress relief → finish machining
• Temperature/time/cooling: follow the exact grade datasheet; heat and cool slowly
• Do not apply generic nylon/POM annealing recipes to PI

💡 The main dimensional-control strategy is stable stock + symmetric roughing + light finishing cuts. PI normally moves far less than nylon, POM, or PTFE after machining.

4.6 Cooling Methods

  • Dry machining with air blast: common; keeps chips/dust clear and avoids fluid contamination
  • Vacuum extraction: strongly recommended for fine dust, especially graphite-filled grades
  • Water-soluble coolant: use only if compatible with the grade and downstream cleanliness requirements
  • ❌ Avoid aggressive chemical cutting fluids, dirty coolant, or processes that contaminate vacuum/semiconductor parts

5. Surface Treatment

PI is usually used for its functional performance, not decorative finishing. Surface treatment options are limited compared with metals, ABS, or PC:

Process Feasibility Notes
Polishing / fine sanding ✅ Feasible Can improve sealing faces and reduce tool marks; avoid overheating edges
Mechanical deburring / chamfering ✅ Recommended Important because brittle edges can chip during handling
Laser marking ✅ Feasible Common for traceability; test settings to avoid heat damage
Engraving ✅ Feasible Works well for part numbers and orientation marks
Bonding ⚠️ Requires pretreatment Surface activation, special adhesives, and process qualification are needed
Painting / coating ⚠️ Limited Adhesion is difficult; not common for functional PI parts
Electroplating ❌ Not recommended PI is normally selected instead of plated/decorated plastics
Dyeing ❌ Limited Color is generally determined by the raw grade: amber/brown or graphite black

💡 If rich cosmetic finishing is required, choose ABS, PC, or aluminum. PI is best reserved for extreme heat, vacuum, wear, and precision functional parts.

6. Applications & Material Selection

6.1 Typical Application Industries

Industry Application Parts
Aerospace / jet engines Bushings, seals, washers, wear pads, hot-zone insulating parts
Semiconductor equipment High-temperature wafer handling parts, etch/CVD components, test sockets, insulators
Vacuum / space systems Low-outgassing spacers, bearings, guides, structural insulating components
High-temperature machinery Bearings, bushings, thrust washers, wear rings, dry-running sliding parts
Electrical / electronics High-temperature electrical insulators, connectors, fixture components
Industrial metal replacement Lightweight precision parts where bronze, steel, or ceramic is too heavy or difficult to lubricate

6.2 Pros & Cons Summary

✅ Advantages ❌ Disadvantages
One of the highest heat-resistant machinable plastics Among the most expensive plastics; stock cost is very high
Excellent wear resistance and low friction in filled grades Relatively brittle; poor choice for impact or snap-fit parts
Outstanding dimensional stability and creep resistance at temperature Filled grades are abrasive and increase tool wear
Low outgassing for vacuum / aerospace / semiconductor use Limited stock sizes; often sold as small premium shapes
Excellent radiation resistance and inherent flame resistance Moderate moisture absorption; keep dry for precision work
Good resistance to many solvents, fuels, and oils Attacked by strong bases, concentrated acids, and hot caustic

6.3 Material Selection Guide

✔ Recommended for PI:

  • Extreme-temperature plastic parts where PEEK, PEI, POM, nylon, or PTFE cannot maintain performance
  • Ultra-high-end metal replacement parts requiring low weight, no lubrication, and high temperature stability
  • Vacuum, space, and semiconductor components requiring low outgassing
  • High-temperature bearings, bushings, thrust washers, seals, and wear parts
  • Precision insulators and fixtures that must hold dimension at elevated temperature

✘ Not recommended for:

  • Budget-sensitive parts → choose POM, nylon, PEI, or PEEK depending on requirements
  • High-impact or snap-fit parts → choose PC or nylon
  • Large prototypes with heavy material removal → redesign blank size or choose a cheaper material first
  • Strong alkali, concentrated acid, or hot caustic exposure → choose PTFE or other fluoropolymers
  • Cosmetic parts needing painting, dyeing, or electroplating → choose ABS, PC, or metal

⚠️ Safety & Handling Notes

Hazard Detail Precaution
Dust inhalation Fine PI machining dust may irritate the respiratory tract Use dust extraction/ventilation; wear a suitable mask or respirator for prolonged dry machining
Graphite-filled dust Graphite dust is messy and may be electrically conductive Keep electronics protected; clean machines and fixtures carefully after machining
Thermal decomposition PI is heat-resistant but extreme overheating can still generate irritating decomposition fumes Avoid dull tools, rubbing cuts, and local burning; ensure ventilation
Sharp brittle edges Thin PI edges can chip during machining, deburring, or assembly Add chamfers/radii; handle small precision parts carefully
High material cost PI/Vespel stock is premium and often sold in small shapes Verify drawings, setups, and toolpaths before cutting; minimize scrap allowance
Chemical exposure Generally stable/low-toxicity, but strong bases, concentrated acids, and hot caustic can attack it Confirm chemical compatibility before use; avoid unsafe cleaning chemicals

⚠️ Treat PI as an ultra-premium precision material. The biggest shop-floor risks are dust control, chipping, and scrapping expensive stock—not melting or gummy machining behavior.

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