by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Startup and Running Friction in Polymer Bearings

Startup and running friction can vary based on several different factors, and in this article we focus on what they are, why their values differ, and typical coefficients, as well as a detailed look at why these differences occur. It ends with a discussion of what impact startup and running friction have on bearing design.

Startup and Running Friction

Polymer bearings tend to exhibit higher breakaway (startup) friction than steady-state running friction due to factors such as static adhesion, microasperity interlocking, and transfer-film formation dynamics. 

The startup friction coefficient µₛ is measured at the onset of motion and represents static friction. The term “startup” does not refer solely to time zero, however. It represents the peak friction force or torque required to break the bearing free after a period of rest. Startup friction is actually the regime of dry contact when the polymer surface is still unconditioned. Unconditioned  means that the transfer film on the counterface is incomplete or patchy

The running friction coefficient µₖ represents a kinetic or dynamic measure of friction. It takes place when steady sliding motion is established and is represented by. Running friction relates to the frictional resistance that exists when two surfaces are in motion, steadily sliding against each other. 

Running friction applies after the initial breakaway event has occurred and the system has moved past issues such as static adhesion and micro-locking. As a result, the coefficient of running friction is typically lower and more stable than startup friction, especially for materials such as PTFE and UHMW-PE.

Why Startup and Running Friction Can Differ in a Polymer

There are some key factors that differentiate startup friction from running friction in polymers. For example, at rest, there is adhesion and junction growth. Polymer chains can increase the real contact load at under load rest (creep/relaxation), thereby increasing µₛ.In addition, at startup, there will be surface roughness and plowing. The roughness increases issues with mechanical interlocking and plowing. These two effects also raise the starting friction value.

In running friction conditions, materials like PTFE form a transfer film that reduces the effect of asperities and surface roughness, which reduces running friction. There is, however, a risk of stick-slip. This phenomenon is more likely to occur when the stiffness of the system is low, the speed is low, and the µₛ / µₖ ratio is high.

Typical Coefficients of Friction

The values below represent commonly used engineering polymers and are typical dry sliding vs steel values. These values can vary with pressure, speed, temperature, finish, fillers, and test method.

  • PTFE (virgin)
    • Startup friction (µₛ): ~0.05–0.10, often nearly identical to running friction
    • Running friction (µₖ): ~0.05–0.10
    • Minimal difference between startup and running friction
  • PEEK (unfilled)
    • Startup friction (µₛ): ~0.20
    • Running friction (µₖ): ~0.25
    • Exhibits a noticeable increase from startup to running friction
  • UHMW-PE
    • Startup friction (µₛ): ~0.15–0.20
    • Running friction (µₖ): ~0.10–0.20
    • Running friction can be equal to or lower than startup friction
  • Nylon 66 (PA66)
    • Startup friction (µₛ): ~0.20 (against steel)
    • Running friction (µₖ): ~0.15–0.25 (typical)
    • Moderate variability depending on surface finish and condition

What Is Behind the Difference Between Startup and Running Friction

Several factors account for the difference between startup and running friction. Pressure and dwell time, for example, mean that higher loads and long dwell times increase the real contact area and have the potential to raise µₛ. For speed, higher speeds can actually reduce friction after the polymer transfer film stabilizes, but can also raise heat generation. 

Temperatures are known to impact polymer modulus and creep, which can shift both µₛ and µₖ and alter the risk of stick-slip. In addition, the counterface material and hardness will affect the adhesion and transfer film, which is why it is important that the frictional coefficient used in design calculations represents the friction against the counterface material (e.g., PTFE vs steel, PEEK vs aluminum).

Note that PTFE-filled PEEK, MoS₂-filled nylon, and glass/bronze-filled PTFE shift friction and wear differently, often lowering friction but sometimes increasing counterface wear.

Surface finish also has a significant impact. If the surface finish is too rough, plowing will occur, increasing both friction and wear. On the other hand, if the surface finish is too smooth it can increase adhesion issues.

Impact on Bearing Design

Startup and running friction impact material selection, clearance, and surface finish in bearing design. Startup friction is dominated by static friction and adhesion at rest. This fact significantly impacts breakaway torque and can be a limiting factor in low-speed, intermittent, or precision motion systems. In such systems, stick-slip, noise, and control instability are unacceptable. 

Running friction, on the other hand, is governed by dynamic friction. Once motion is established, it controls steady-state heat generation, wear rate, and long-term dimensional stability. It directly influences PV limits and service life. 

Because many polymers exhibit higher startup friction than running friction, engineers need to balance low breakaway forces with acceptable operating temperatures and wear. This is usually accomplished through the use of self-lubricating materials, fillers, or surface texturing to manage both regimes. A successful polymer bearing design accounts for the full friction lifecycle, ensuring reliable motion at startup without sacrificing durability during continuous operation.

Conclusion

Startup and running friction have a significant impact on bearing design, as well as factors such as material fillers, pressure, temperature, and counterface material. If you are looking for a polymer bearing solution, contact the experts at Advanced EMC. Our team of bearing specialists can help you find the best bearing material for your design and can help you select the optimal material from our range of bearing-grade polymers.

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).