by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Avoiding Costly Mistakes in PTFE Machining: A Guide for Design Engineers

Mistakes in PTFE machining can be surprisingly costly, despite the material’s reputation as a high-performance solution. PTFE (polytetrafluoroethylene) is valued for its exceptional chemical resistance, low coefficient of friction, and wide thermal operating range.

PTFE (polytetrafluoroethylene) is a standout material in high-performance engineering applications thanks to its remarkable chemical resistance, low coefficient of friction, and wide thermal operating range. These properties make PTFE ideal for use in seals, insulators, gaskets, and precision components found in various sectors, including medical, aerospace, semiconductor, and chemical processing. Its low mechanical strength, high thermal expansion, and tendency to deform under load create a unique set of challenges that can result in costly failures if not properly accounted for.

This article examines the most common and costly mistakes made during PTFE machining and provides guidance on how to avoid them.

Ignoring PTFE’s Thermal Expansion 

PTFE has a high coefficient of thermal expansion (CTE), ranging from 100 to 200 x 10^-6/°C. This is significantly higher than metals or rigid polymers. And a part machined at 22°C may expand or contract enough in service to cause critical dimensional shifts.

Failing to account for this thermal behavior during design and machining can result in components that seize, leak, or fail due to interference or clearance issues. Engineers must anticipate thermal conditions during part operation and adjust dimensions accordingly. In some cases, mating materials should also be chosen based on matched coefficients of thermal expansion (CTEs) to mitigate differential movement.

Not Accounting for Cold Flow (Creep) 

Cold flow, or creep, is the tendency of PTFE to slowly deform under constant mechanical stress. Unlike elastic deformation, creep is time-dependent and permanent. This behavior is especially problematic in applications involving sealing, clamping, or structural support.

When machining PTFE components, failure to design for creep can lead to reduced sealing force, shifting of components, or dimensional instability over time. For this reason, engineers should avoid thin unsupported walls and instead incorporate mechanical features that distribute load or accommodate long-term movement. Additionally, engineers should also minimize the presence of stress concentrations and increase the contact area between the PTFE part and the mating surface.

CNC Machine Operator
Caucasian CNC Machine Operator Wearing Blue Hard Hat and Safety Glasses. Industrial Concept.

Using Inappropriate Tooling and Feed Rates 

PTFE is soft and stringy, with a tendency to deform under tool pressure rather than shear cleanly. Standard tools may produce poor results, including smearing, tearing, or excessive burrs. Inappropriate tooling can also lead to tool chatter and vibration, which can affect surface finish and dimensional accuracy.

For optimal results, sharp carbide or HSS tools with positive rake angles around 0° to 15° should be used. Moderate feed rates help achieve clean cuts without excessive heat buildup. Tool paths should also be optimized to avoid chip welding or surface drag.

Over-Specifying Tolerances 

While PTFE can be machined to tight tolerances, its dimensional stability is limited by its thermal expansion and mechanical softness. Applying unnecessarily tight tolerances can increase machining time, tool wear, inspection failures, and scrap rate.

A tolerance of ±0.001″ may be achievable, but only under tightly controlled conditions. Design engineers should assess whether looser tolerances are acceptable based on function, especially for components that will operate in fluctuating thermal or pressure environments. When tight tolerances are required, consider post-machining inspection at operational temperature or even in the assembled state.

Skipping the Annealing Process 

Machining introduces residual stresses into PTFE, especially in deep cuts or during high-speed operations. These stresses can cause parts to warp or shrink unpredictably after machining. Annealing, which involves heating the material below its melting point and slowly cooling it, helps relieve these internal stresses.

Omitting this step can result in dimensional drift, especially for precision components or parts with asymmetrical features. For best results, anneal PTFE rods or billets before and sometimes after machining to ensure dimensional stability, especially in mission-critical applications.

Contaminating Parts with Coolants or Lubricants 

PTFE is chemically inert and non-absorbent, but its surface can trap oils, coolants, or metal debris. In industries like medical, food, or semiconductor manufacturing, even trace contaminants can cause product failure, rejection, or regulatory issues.

Machining PTFE is best done dry or with inert coolants (e.g., clean compressed air or specialty fluids). After machining, components should be thoroughly cleaned to remove surface residues. For highly sensitive applications, specify cleanroom-compatible handling and post-process inspection.

Inadequate Fixturing and Workholding 

Because PTFE is soft and deformable, improper clamping can lead to distortion during machining. Parts may spring back when unclamped, resulting in dimensional errors that are difficult to trace. Clamping too tightly can also leave marks or indentations that compromise function.

To prevent this, use soft jaws, vacuum fixtures, or contour-matched supports. Spread clamping forces over a larger area and avoid over-tightening. Designing the machining setup with minimal mechanical stress in mind is crucial for preserving the final part geometry.

The Answer to Mistakes in PTFE Machining

PTFE offers outstanding chemical resistance, temperature tolerance, and friction performance, but these benefits come with a price: complexity in machining. Failing to account for PTFE’s thermal expansion, creep, dimensional instability, and sensitivity to contamination can lead to part rejection, system failure, or compliance issues.

By understanding and mitigating these machining challenges, engineers and manufacturers can fully leverage the performance benefits of PTFE while maintaining cost-effective, reliable production. Whether you’re working with high-purity medical components, aerospace seals, or precision insulators, success lies in mastering the details of PTFE machining.

If you require expert support for precision-machined PTFE components or wish to explore custom-machined solutions, Advanced EMC Technologies offers the engineering expertise and production capabilities to ensure the success of your project.

by Denise Sullivan Denise Sullivan No Comments

Understanding Injection Molding Tolerances: Exploring Standard Requirements and Polypropylene Shrink Rate

injection molding tolerances

Injection molding is a widely used manufacturing process for producing plastic parts with great precision and efficiency. However, achieving the desired dimensional accuracy of the final product can be challenging due to various factors, including material properties and process parameters. One crucial aspect to consider in injection molding is tolerances, which determine the acceptable deviations from the intended dimensions. In this article, we will delve into the world of injection molding tolerances, focusing on standard requirements and the specific shrink rate of polypropylene.

What are Injection Molding Tolerances?

Injection molding tolerances refer to the allowable range of dimensional variations in a molded part compared to its intended design specifications. These variations can occur due to several factors during injection molding, such as material shrinkage, thermal expansion/contraction, tooling wear, and machine repeatability.

Why are Tolerances Important in Injection Molding?

Tolerances play a vital role in ensuring injection-molded parts meet their functional requirements and fit together correctly with other components or assemblies. Achieving tight tolerances helps prevent issues like part misalignment or interference that could compromise product performance or assembly quality.

Moreover, understanding tolerancing requirements enables manufacturers to optimize production processes by minimizing costs associated with rework or scrap caused by out-of-tolerance parts.

Standard Requirements for Injection Molding Tolerances

The International Organization for Standardization (ISO) has established standards for determining tolerance limits in various manufacturing processes. In particular, ISO 20457-1:2018 specifies general principles for dimensioning and tolerancing applicable to plastic moldings produced by injection molding.

Accordingly, these standards define three categories of tolerance classes based on increasing levels of precision:

  1. Standard Class: This class represents typical commercial tolerance levels suitable for most applications. Parts manufactured within normal tolerance limits are generally acceptable for functional purposes.
  2. Medium Class: Parts within the medium tolerance class possess tighter dimensional requirements than the standard class. These tolerances are typically employed when higher precision is needed, such as in applications with stricter fit or alignment requirements.
  3. High Class: The high tolerance class defines the most stringent dimensional requirements and is usually reserved for specialized applications that demand exceptional precision, such as optical components or medical devices.

Polypropylene Shrink Rate in Injection Molding

Polypropylene (PP) is a commonly used thermoplastic material known for its excellent chemical resistance, low density, and high impact strength. However, like most plastics, it undergoes a certain degree of shrinkage during cooling after being injected into the mold cavity.

Understanding and accounting for the shrink rate of polypropylene is crucial to ensure accurate part dimensions in injection molding processes involving this material.

The shrink rate of polypropylene can vary depending on factors such as:

  • Crystallinity: Polypropylene exists in different crystalline forms with varying degrees of shrinkage. Generally, amorphous regions exhibit higher shrinkage compared to crystalline areas.
  • Molecular Weight: Higher molecular weight grades of polypropylene tend to have lower shrink rates due to increased chain entanglement.
  • Mold Temperature: Controlling mold temperature can influence the cooling rate of polypropylene and consequently affect its overall shrinkage behavior.
  • Part Geometry: Variations in wall thicknesses or part design features can lead to differential cooling rates and non-uniform shrinkage across different sections of the molded part.

It is important to note that manufacturers should consult material suppliers’ data sheets or conduct their trials to determine specific shrink rates for their chosen grade of polypropylene under relevant processing conditions.

Understanding injection molding tolerances is essential for ensuring that molded parts meet their intended design specifications while considering the inherent limitations of the manufacturing process. By adhering to standard requirements and accounting for specific material properties like polypropylene’s shrink rate, manufacturers can produce high-quality plastic components that meet functional requirements reliably.

FAQs about Injection Molding Tolerances

  1. What factors influence tolerances in injection molding?

Several factors can influence tolerances in injection molding, including material shrinkage, tool wear, machine repeatability, and thermal expansion/contraction. Considering these factors during the design and manufacturing is essential to achieve the desired dimensional accuracy.

  1. How are tolerances specified in injection molding?

Tolerances are typically specified through a combination of plus-minus dimensions of geometric dimensioning and tolerancing (GD&T) symbols on engineering drawings. These specifications outline each feature or component’s acceptable range of dimensional variations.

  1. What is the role of mold design in achieving tight tolerances?

Mold design is crucial in achieving tight tolerances by ensuring consistent part filling, cooling, and ejection. Proper gating systems, cooling channel placement, and venting strategies help minimize variations caused by uneven cooling or improper material flow.

  1. Can injection molding machines achieve high precision tolerances consistently?

Modern injection molding machines equipped with advanced control systems can achieve high precision tolerances consistently when operated within their specified process windows. However, machine condition and maintenance can affect its ability to maintain tight tolerance levels over extended production runs.

  1. Are there industry-specific standards for injection molding tolerancing?

While ISO 20457-1:2018 provides general guidelines for dimensioning and tolerance requirements for plastic moldings produced by injection molding, some industries may have specific standards tailored to their unique needs (e.g., automotive or aerospace).