Decoding plastics machining

While considerable time is often spent understanding the properties and machining characteristics of metals, equal attention should be devoted to thermoplastic polymers. As material science advances, thermoplastic polymers are becoming an ever present part of CNC machining.

To simplify your education journey in machining thermoplastic polymers, we have done much of the groundwork and condensed the information into key concepts that are easy to grasp. But first, let’s review some fundamental principles.

Material Variables Affecting Polymer Cutting Conditions:

• Glass Transition Temperature (Tg)
• Melt Temperature (Tm)
• Viscosity
• Molecular Weight

Thermoplastic polymers are generally more sensitive to heat and force compared to metals. Understanding the molecular behavior, especially Tg (glass transition temperature) and Tm (melt temperature), is crucial for effective machining.

Glass Transition Temperature (Tg)

The glass transition temperature is a critical property of amorphous polymers. It marks the point at which the material transitions from a hard, glassy state to a softer, rubber-like state. Below Tg, the polymer behaves like a rigid solid with limited molecular mobility. Above Tg, it becomes more flexible as polymer chains gain mobility, impacting the material’s machinability.

Melt Temperature (Tm)

The melt temperature refers to the point at which a polymer transitions from solid to liquid. This varies depending on the polymer’s chemical structure.

Examples of Polymer Melt Temperatures:

  • Polyetheretherketone (PEEK): Withstands up to 250°C (482°F). Commonly used in medical and aerospace applications, it is 70% lighter than steel and half the weight of aluminum.
  • Polytetrafluoroethylene (PTFE/Teflon): Handles temperatures up to 280°C (500°F) and is ideal for low-friction, high-heat applications.
  • Polyphenylene Sulfide (PPS): Operates up to 220°C (428°F) with excellent mechanical strength, often used in injection molding.
  • Polyamide (Nylon): Withstands up to 200°C (392°F), though water exposure can weaken it.

Comparison with Metals:

  • Steel: The melting point of steel ranges from 1370°C to 1540°C (2500°F to 2800°F), depending on the alloy.
  • Aluminum: The melting point of aluminum is 660°C (1220°F).

As polymers heat during machining and approach their Tm, they enter a rubbery phase, losing stiffness. This loss of stiffness results in increased deflection/distortion and can result in surface tearing, waviness, and thermal distortion, making precision machining challenging. Conversely, operating temperatures below Tg cause the material to exhibit brittle behavior, leading to chipping or fracturing.

Key to Machining Thermoplastic Polymer: Preventing the Rubbery Zone

Preventing polymers from entering the rubbery phase during machining is essential for clean, precise results. Heat generated during machining depends on several variables:

Machining Variables that affect polymer cutting conditions:

  • Cutting Speed (time)
  • Feedrate (time)
  • Depth of cut (DOC) (time)
  • Cutting Edge Radius
  • Cutting Edge Condition
  • Tool Angles (rake/clearance)
  • Tool surface tribological properties (anti-friction)

Tool Selection and Setup

Determining the optimal tooling for machining polymers can be challenging for newcomers. As a general rule, tools with high positive rake and clearance angles, sharp cutting edges, and geometries that minimize force and aid chip evacuation are recommended. Speeds, feeds and DOC are application specific and are limited by our ability to manage heat in the cutting zone. No matter how much water soluble coolant you apply to the cutting zone, removal of heat is limited.

Managing Heat in the Cutting Zone

Effective heat management is crucial for maintaining polymer materials in their optimal “cold flow” phase. Traditional cryogenic methods, such as liquid nitrogen, often lead to unpredictable temperature swings, causing materials to enter the brittle glassy phase.

The CryoCut Vista Advantage

CryoCut Vista introduces cutting-edge cryogenic technology that allows precise control of the cutting zone temperature. This system ensures materials remain in the cold flow phase, resulting in superior surface finishes, improved chip formation, and consistent quality.

With CryoCut Vista, users can set and maintain a desired temperature at the nozzle, preventing materials from entering the rubbery zone even under aggressive machining conditions. For repeatability, users can document the optimal temperature for each setup and program it into subsequent operations.

By leveraging the CryoCut Vista system, you can achieve unparalleled precision and quality in machining polymers, regardless of the material or application.