Part 2: Heat-Related Factors in Material Degradation and Deformation

The Heat Challenge in Machining Sensitive Materials

In part one, we discussed the heat related factors as a primary limiting factor in nearly every machining application. In most applications, heat is a primary driver of tool failure. There are applications that this isn’t the case. Take for example machining engineered plastics like PEEK.

When machining materials like PEEK, heat is a limiting factor in the speed at which we machine, but for a different reason. Unlike most applications where excess heat causes rapid and premature tool failure, when machining plastics heat causes rapid degradation of the material itself.

If you’ve spent anytime machining plastic, you’ve most likely encountered slight deformations during machining, edges and corners that aren’t defined well, features or surfaces with tearing, burrs etc. These are all a result of heat transferring into the workpiece causing deformation to the workpiece. This deformation is a direct result of heat transferring into the workpiece pushing the material into a “rubbery state”.

To counter act this we have limited options, sharper tools, slower speeds, better chip evacuation etc. Generally speaking, the first response is to slow down our speed. Slowing down when machining plastic is counterintuitive to many because it would seem you could machine something like plastic, which is easy to machine, as fast as you want, as opposed to something like titanium would would seem to be difficult to machine.

Understanding Heat-Related Material/Part Deformation

If you machine plastic with any frequency, it would be wise to spend time understanding the material behaviour during machining to produce an effective machining strategy. To do this, we need to understand an important concept for the material we plan to machine.

Tg: The glass transition temperature (Tg) is the point at which a material changes from a hard and brittle glassy state to a more flexible, rubbery state as temperature increases. This transition is significant for plastics because the Tg temperature is often relatively close to ambient temperature, and in some cases below ambient temperature. Below Tg, amorphous polymers are harder and more brittle, while above Tg, they become softer and more flexible. Tg affects a material’s machinabilty and properties such as tensile strength, modulus elasticity, and transparency.

The typical glass transition temperature (Tg) of PEEK (Polyetheretherketone) is around 143°C (289°F). However, the exact Tg can vary depending on factors such as molecular weight, grade, and formulation. For example, unfilled PEEK generally has a Tg in the range of 143°C to 150°C, while glass-filled PEEK can have a Tg as high as 160°C or more.

When plastic temperature exceeds its glass transition temperature (Tg) it leads to softening, deformation, gumming, or melting, which compromises dimensional accuracy and surface finish. Here are the main causes of exceeding Tg during machining:

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🔥 Primary Causes

• Excessive Cutting Heat
  
  • Friction between the cutting tool and plastic generates heat.
  • Poor chip evacuation traps heat near the workpiece.
  • High cutting speeds, especially without cooling, amplify heat buildup.

• High Spindle Speeds and Feed Rates
  
  • High spindle speeds increase friction and localized heat.
  • Too slow a feed rate means the tool spends more time in contact, generating more heat per unit of material removed.

• Inappropriate Tool Geometry
  
  • Dull tools or incorrect rake angles increase friction and heat.
  • Tools not designed for plastic (e.g., too little clearance or poor chip evacuation) worsen the problem.

• Lack of Coolant or Air Blast
  

   

  • Machining plastics dry can cause rapid temperature rise do to chip build up in the cutting zone.
  • Plastics often benefit from air blasts or mist coolants to dissipate heat without chemical interaction.

• Poor Material Selection
  
  
  • Some plastics have a low Tg (e.g., polystyrene ~100°C, ABS ~105°C).
  • Machining operations that generate even modest heat can exceed Tg in such materials.

🛠️ Traditional Solutions to Prevent Exceeding Tg

• Use sharp, properly ground tools with positive rake angles.
• Reduce spindle speeds and optimize feed rates to balance heat generation and cutting efficiency.
• Improve chip evacuation to carry heat away from the cutting zone.
• Use air blasts or appropriate coolants designed for plastics (avoid water or petroleum-based coolants unless verified compatible).
• Pre-cool the plastic workpiece, especially for high-speed operations or precision cuts.
• Choose plastics with higher Tg for demanding machining processes (e.g., PEEK, Ultem).

How ICT’s CryoCut™ Enables Higher Cutting Speeds

Industrial Cryotech’s cryogenic machining system offers a revolutionary solution to the heat problem. By cooling the cutting zone using liquid nitrogen at sub zero temperatures we dramatically reduce the heat generated during machining, and the negative effects of the heat.

Key Advantages of Industrial Cryotech’s Vista Cryogenic Solution

1. Enhanced Process Consistency: Consistent temperature ensures predictable cutting forces and dimensional stability—critical for qualification in aerospace and medical manufacturing.
2. Increased Material Removal Rates: Lower cutting temperatures at the point of cut keeps the material below it’s Tg temperature allowing for higher speeds and deeper cuts while preserving material integrity and dimensional accuracy.
3. Compliance with Cleanroom and Sterile Requirements: Medical device and some aerospace components must be manufactured in cleanroom environments or be free of particulates and residues. LN₂ vaporizes cleanly—no particulate, residue, or fluid disposal. Helps meet ISO Class 7 or higher cleanroom standards.
4. Preserves Material Integrity: Heat from machining can cause molecular changes in some plastics, especially when the material is exposed to temperatures near or above its glass transition temperature (Tg) or melting point (Tm). These changes can degrade the material’s mechanical, thermal, and chemical properties.
5. Superior Heat Dissipation: CryoCut™ absorbs and removes heat far more effectively than traditional coolants, keeping tools and workpieces significantly cooler even at high speeds.
6. Cleaner and More Sustainable: Cryogenic machining often eliminates the need for oil- or water-based coolants, reducing environmental impact while boosting shop safety and cleanliness.
7. Inert and Leaves no Residue: Medical and aerospace regulations (e.g., FDA, ISO 13485, AS9100) prohibit material contamination. Lubricants, oils, or coolants can absorb into polymers or chemically react with the material.

The Solution:

CryoCut™ Vista: Industrial Cryotech’s CryoCut™ Vista system was engineered specifically to be used with sensitive materials or in precision machining applications and harnesses the cooling power of liquid nitrogen to deliver substantial gains in cutting speeds and feed rates—while increasing process stability and part quality. The results speak for themselves:

The Result: Faster, Cleaner, More Cost-Effective Machining

For more information and selecting the correct system for you application please send us an email @ [email protected].