The continuous service temperature, or continuous use temperature (CUT), is the maximum ambient service temperature in air that a material can withstand and maintain 50% of its initial physical properties after long-term service.

 

What is long-term service?  It’s defined as 100,000 hours of service – well over 11 years, if used 24 hours a day, 365 days a year.  The continuous use temperature property tells machinists and users what will happen to a part over the course of roughly 11 years of continuous use.  It’s the temperature at which the polymer will degrade, embrittle and start to break down.

 

It is important for the designers, engineers and users to take this measurement into consideration for CNC machining.  During the design phase, this not only helps with initial material selection, but plays a role in predicting the life span of a part.  At AIP, we take great care in providing unrivaled results to ensure the optimal dimensions and properties for machined polymers and composites.  Join us in this technical brief as we give an in-depth explanation of the continuous use temperature for machined polymers.

 

What Affects Continuous Use Temperature?

 

The base material polymer structure affects the continuous use temperature of a machined part.  The time that is involved and the loading levels that are used in the testing can affect the CUT value.  Also, additives and reinforcements should be taken into consideration.  They may have an effect on the maximum continuous use temperature value.

 

Tests to Measure Continuous Use Temperature of Plastics

 

The continuous use temperature is measured in degrees Celsius (o C) or Fahrenheit (o F).  One of the common tests used to compare different materials in terms of continuous use temperature is the Underwriter Laboratory (UL) Relative Thermal Index or RTI.

 

UL 746B

This test method is used to determine RTI values.  The RTI is based on a loss of properties of the plastic versus time. In general, when the plastic is exposed to this maximum continuous use temperature – good, long-term performance is observed. However, it does not consider short-term thermal spikes.

 

RTI gives an indication of the aging temperature that a material can endure for 100,000 hours and still retain at least half of the initial property being measured. However, different properties for materials decay at dissimilar rates. This is the primary reason why often RTI values are associated with a particular property and the related CUTs are given as a range of values rather than as a single value.

 

Determination of RTI Value

  1. Groups of test pieces are placed in ovens at four different pre-set temperatures.
  2. At specified time intervals, the test pieces are taken out of the ovens and tested for mechanical and electrical properties of interest.
  3. The results are plotted on a property versus time graph until the property that is being tested declines to 50 percent or less of its initial value.

 

In this analysis, the 50 percent value of the property is referred to as the half-life of that particular property. The half-life values are then, plotted against the reciprocal of the absolute aging temperature. This plot results in a straight line that can be extrapolated, if needed, to indicate the half-life of the property at other temperatures.

 

The results that are obtained in this testing procedure can also be compared to a material with a known aging performance.

 

Types of RTI

There are three general classes of properties that are associated with the RTI.  The three values for a particular polymer are often different from each other.  They are the following:

  • The RTI Electrical that is associated with insulating properties.
  • The RTI Mechanical Impact which is related to the impact resistance, toughness, elongation and flexibility.
  • And, the RTI Mechanical Strength that is associated with the mechanical properties or the structural integrity of the plastics.

 

Continuous Use Temperature Values for Common Polymers

 

Polymer NameMin Value (o C)Max Value (o C)
ABS – Acrylonitrile butadiene styrene-2080
PA – Nylon Polyamide, 66 30% Glass Fiber100150
PAI – Polyamide-Imides (TORLON)-196220-280
PBI – Polybenzimidazole (CELAZOLE)204540
PC – Polycarbonate, high heat100140
PE – Polyethylene, 30% glass fiber100130
PEEK – Polyetheretherketone154260
PEI – Polyetherimide (ULTEM)170170
PP – Polypropylene100130
PS – Polystyrene, high heat7590
PSU – Polysulfone150180
PTFE – Polytetrafluorethylene260
PVC – Polyvinyl chloride, rigid5080
PVDF – Polyvinylidene fluoride (KYNAR)149

 

Continuous use Temperature Does Not Define Polymer Strength

 

It is important to note that the continuous use temperature does not define a part’s ability to handle a load under a specific temperature. One material that proves this is PTFE. PTFE is an advanced thermoplastic that can handle 500 o F continuous service without breakdown. Yet, it is a soft material, which bends easily at room temperature. This property is called the heat deflection temperature (HDT), which is another important property to consider.

 

AIP: Unrivaled Precision Machining

 

Data charts can give you the Continuous Use Temperature, glass transition and other values. However, a chart can give a general idea of these values, but an entire data set with the curve of a material is the best way to determine the right material for your project.

 

Be sure to work with a plastics machining company that can provide you a wide range of data on the CUT of polymers and composites. Your sales engineer will be able to give you a detailed response on how the continuous use temperature will affect your project’s design and functionality. Talk to one our engineers at AIP about your project design, and we will work with you to provide unrivaled expertise from your project’s initial concept to completion.

 

Supportive Information

 

Certifications and Regulatory Resources

 

Our team is dedicated to providing unparalleled, quality machined polymers and composites. Learn more about the material properties we consider when working on a project.

 

Read our blog on Moisture Absorption
 

 

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An Informational Brief on Polymer Machining

 

Copolyester elastomers (COPE) are high-performance, high-temperature elastomers that have several properties of thermoset rubber with the processing ease of engineering plastics. One of these thermoplastic elastomers (TPC-ET) is DuPont’s HYTREL®. It is known for toughness, tear resistance and good flex fatigue along with good chemical and temperature resistance. For these reasons, it is an excellent general engineering thermoplastic for everyday to mission critical applications.

 

AIP has over 37 years of experience machining complex components from thermoplastics like HYTREL® thermoplastic elastomer. In this insightful technical brief, we will discuss what goes into machining HYTREL® and how it differs from other manufacturing options such as metal machining, injection molding, and 3D printing.

 

Properties of HYTREL®

 

Machinists should keep data on the properties of the thermoplastics they use. This aids in selecting the right material for a project. Also, it helps determine if the material is a good candidate for the end-use. Below are some of the key characteristics of HYTREL® TPC-ET:

 

Key Properties

  • Excellent Flex Fatigue
  • Excellent Strength
  • Flame Retardant
  • Fluid Resistance
  • Hydrocarbon Resistance
  • Hydrolysis Resistance
  • UV Resistance

 

Description

HYTREL is a brand name for TPC-ET (thermoplastic polyester elastomer). HYTREL comes in several different grades, such as heat stabilized, UV protected, hydrolysis-resistant and flame retardant.

 

Characterized by excellent strength and flex fatigue while being resistant to hydrocarbons and most fluids, HYTREL can be utilized within a wide range of engineering applications.

 

The table below displays an overview of the material properties, units and values for machining HYTREL® 5556:

 

Material PropertyUnitsValue
Tensile Elongation at Break @73 F%560
Flexural Modulus of Elasticity @ 73 Fpsi30000
Tensile Modulus of Elasticity @ 73 Fpsi
Flexural Strength @ 73 Fpsi
Specific Gravity @73 FASTM D7921.20
Tensile Strength @73 F, (ult)/(yld)psi1000 (5%)
Notched Izod Impact @73 Fft-lb/in of notchNo Break
Heat Deflection Temperature @ 264 psiF120
Flammability RatingUL94HB(3.05mm)
Coefficient of Linear Thermal Expansion @73 Fin/in/F6.5E-05
Dielectric Strength, Short TermVolts/mil410
Water Absorption, Immersion, 24
Water Absorption, Saturation
% by weight
% by weight
0.30

 

Applications of HYTREL®

 

HYTREL’s properties make it a great general purpose engineering thermoplastic. It has qualities similar to thermoset rubber with the machining ease of engineering plastics. For this reason, it can be found on medical devices, electrical cabling insulation, mechanical gears, automotive components. As a performance thermoplastic, it can flex in multiple directions, cycle after cycle, long after rubber would break.

 

Common Applications

  • Cable insulation and jacketing
  • Chassis suspension Systems
  • Food Contact Materials
  • Innovative Furniture Design
  • Mechanical Gears
  • Medical Device Materials
  • Polymers for Oil and Gas
  • Railway Technology for the Long Haul
  • Seals and Gaskets
  • Sustainability in Airbag Systems
  • Thermoplastic Tubing and Elastomeric Hose
  • Mobile Phone Housing & Components
  • Plastics For Sporting Goods

 

AIP Machining Capabilities: Unrivaled Expertise

 

Our close ties with the industry’s leading plastics manufacturers give us even further insight and access to technical help in material selection, sizing and manufacturing procedures. If you are looking for a trademarked material for your project, we have a host of material bases available for expert machining. Whatever your application, our machinists can help you in material selection, sizing and manufacturing techniques from concept to completion.

 

Our Suppliers

We machine DuPont’s HYTREL® and HYTREL® resin at AIP Precision Machining. If you have a question about the grade we use or machining specifications for this brand, our machinists are happy to help you.

 

 

Machining HYTREL®

 

Annealing HYTREL®

As with any CNC machined part, annealing and stress-relieving is crucial to the machining process. Coolants, lubricants and trained procedures prevent cracking and crazing in a precision machined component. We recommend slow heating and cooling during the annealing process of thermoplastics. This reduces the chances of these stresses occurring from the heat generating during machining polymers like HYTREL®. Our AIP machinists use computer controlled annealing ovens for the highest quality precision temperatures and time control. If you have a specific question about the annealing process for HYTREL® or other thermoplastics, our machinists at AIP can provide an in-depth consultation.

 

Machining HYTREL®

HYTREL® can be injection molded, extruded or thermoformed. At AIP, we CNC machine HYTREL® and HYTREL® resin. For the best results, use sharp tools, avoid excessive clamping and cutting forces and use coolants to prevent overheating. We recommend non-aromatic, water-soluble coolants because they are most suitable for ideal surface finishes and close tolerances. These include pressurized air and spray mists. Coolants have the additional benefit of extending tool life as well. These are general machining guidelines. For further information, speak to a CNC machinist at AIP to get specific machining information on HYTREL® and other performance thermoplastics.

 

Some companies machine both metals and plastics, which has detrimental outcomes for machined polymer products. Many past experiences have shown parts going to customer without cracks, only to develop surface cracks and warping over time due to exposure to metal machine shop fluids. Be sure to use a facility like AIP that only machines polymers.

 

Preventing Contamination

Contamination is a serious concern when machining polymer components for technically demanding industries such as aerospace and medical sciences. To ensure the highest level of sanitation down to the sub-molecular level, AIP Precision Machining designs, heat-treats, and machines only plastics with any sub-manufactured metalwork processed outside our facility. This allows us to de-risk the process from metallic cross contamination.

 

HYTREL® Machining Guide: Supportive Information

 

Quality Assurance Certifications
Miscellaneous Materials

 

Looking for more machining guides on thermoplastics from supplier DuPont?

 

Read Our Delrin Machining Guide
 

 

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Most polymers have a natural tendency to absorb water. In fact, some superabsorbent polymers are highly sought after in advanced applications for medical, construction and more. Yet, moisture absorption of thermoplastics leads to changes with regard to processing and properties.

 

It is crucial for machinists and designers to understand the moisture absorption of thermoplastics for CNC machining. During the design phase, this not only helps with initial material selection, but plays a role in predicting the life span of a part. At AIP, we take great care in providing unrivaled results to ensure the optimal dimensions and properties for machined polymers and composites. Join us in this technical brief as we give an in-depth explanation of the effects of moisture absorption for machined polymers.

 

Plastics Machining and the Importance of Water Absorption

 

Moisture / water absorption is the capacity of a plastic or a polymer to absorb moisture from its environment. Absorbed moisture sometimes acts as a plasticizer, reducing the glass transition temperature and strength of plastic (this is a reversible side effect). However, absorbed water also can lead to irreversible degradation of the polymer structure.

 

Some effects include:

  • Dimensional and mass changes (swelling) caused by water absorption
  • Extraction of water-soluble components
  • Changes in mechanical (elasticity, tensile strength, impact strength) and electrical performance

 

Water absorption is expressed as increase in weight percent or % weight gain of a plastic specimen under the following testing procedures:

  • Water Absorption 24 hrs at 23°C – Immersion of a plastic specimen in distilled water during 24 hours at 23°C
  • Water Absorption 24 hrs at 100°C– Immersion of a plastic specimen in distilled boiling water during 24 hours Water Absorption at saturation – Immersion of a plastic specimen in distilled water at 23°C.  Measurement occurs when the polymer does not absorb water anymore
  • Water Absorption at Equilibrium– Plastic specimen is exposed to a humid environment — generally at 50% relative humidity — at a specified temperature — 23°C or 73.4°F — for 24 hours

 

(Source: Omnexus)

 

Exposure to humidity, immersion and boiling water can result in different material responses. The equilibrium moisture content can be used to compare the amount of water absorbed by different types of plastics when they are exposed to moisture.

 

Plastics Processing and Properties

 

Polymers are affected by moisture from their environment or from simply sitting on a shelf waiting to be used.  This is not a huge concern, but when the moisture absorption reaches more than 1% or 2%+, this can result in enough dimensional movement to create concerns.

 

Moisture Absorption Affects:

  • Mechanical properties
  • Wear properties
  • Dimensions

 

For example, parts made from TORLON (PAI) require special attention due to a 1.7% moisture at saturation value.  While this number may not sound like much, it is enough to cause a precision machined TORLON part to exceed tolerance; in this scenario, the part cannot be used.

 

Therefore, it is important to properly package these mission critical polymers for lasting shelf-life and function.  This can be achieved in two ways:  1) Vacuum-sealing them in a moisture-impermeable layer or 2) packaging them with bags of desiccant.  This prevents moisture uptake in humid environments.

 

Tests to Measure Water Absorption of Plastics

 

Source

 

ASTM D570 – Standard Test Method for Water Absorption of Plastics

 

This test method for rate of water absorption has two main functions:

  1. A guide to the proportion of water absorbed by a material and consequently, in those cases where the relationships between moisture and electrical or mechanical properties, dimensions, or appearance have been determined, as a guide to the effects of exposure to water or humid conditions on such properties.
  2. A control test on the uniformity of a product. It is particularly applicable to sheet, rod, and tube arms when the test is made on the finished product.

 

Procedure:  Parts are dried in an oven for a specified time and temperature and then placed in a desiccator to cool.  Upon cooling, the specimens are weighed to establish a point of reference.  The material is then submerged in water at standardized conditions (usually 23°C for 24 hours or until equilibrium).  Specimens are removed from the liquid, dried and weighed.

 

What affects water absorption?

  • Type of plastic
  • Morphology (crystalline, amorphous…)
  • Type and proportion of additives, fillers and reinforcements used
  • Fiber fraction and orientation (in composites)
  • Relative humidity and temperature
  • Length of exposure

 

Water Absorption Values for Common Polymers

 

Polymer NameMin Value (% weight)Max Value (% weight)
ABS – Acrylonitrile butadiene styrene0.051.80
PA – Nylon Polyamide, 66 30% Glass Fiber0.801.10
PAI – Polyamide-Imides (TORLON)0.100.30
PBI – Polybenzimidazole (CELAZOLE)0.45
PC – Polycarbonate, high heat0.100.20
PE – Polyethylene, 30% glass fiber0.020.06
PEEK – Polyetheretherketone0.100.50
PEI – Polyetherimide (ULTEM)0.200.30
PP – Polypropylene0.010.10
PS – Polystyrene, high heat0.010.07
PSU – Polysulfone0.200.80
PTFE – Polytetrafluorethylene0.0050.015
PVC – Polyvinyl chloride, rigid0.040.40
PVDF – Polyvinylidene fluoride (KYNAR)0.030.05

 

As the chart notes, some polymers such as Nylon (PA) have a higher rate of % gain from moisture absorption.  However, polymers like PVDF and PTFE have a very low % gain in weight after the ASTM D570 test – which makes them excellent candidates for applications where moisture is a factor.

 

Performance thermoplastics are often exposed to high temperature applications (aircraft engines) which also absorb high levels of moisture.  This is common in materials such as PBI (Celazole) and PAI (Torlon), since these polymers absorb moisture at high rates but are also specified in high temperature applications.

 

Basically, what can happen is that these materials absorb the moisture if not properly stored and packaged.  Then if subject to high levels of heat without time for the moisture to dissipate, the internal moisture boils and turns to steam causing the parts to crack and blister.

 

Managing Moisture Absorption

For predictable machined part fit and performance, stock shapes and finished parts should be stored in a dry environment.  Both finished parts and stock shapes should be packed in moisture barrier packaging.  Only open packaging just prior to use.  In the event that a part may have adsorbed so much moisture as to risk shocking it when placing it in high temperature or vacuum service, consider drying the material prior to use or re-use.

 

Your machining facility will have specifications on temperature and storage procedures for all polymers, stock shapes and components.  When it comes to critical applications, work with a machine shop that has high standards for storing products.  After all, machining a polymer is only part of the entire process; wasted machining, revenue and parts is not worth risking poor storage conditions.  The table below shows some common packaging for polymers to increase and preserve shelf-life.

 

 

Generally, you can find a polymer’s 24 Hour and Saturation Moisture Absorption Values on a data chart.  A chart can give a general idea of the moisture absorption, but an entire data set with the curve of a material is the best way to determine the right material for your project.  Be sure to work with a plastics machining company that can provide you a wide range of data on the moisture absorption of polymers and composites.  Your machinist will be able to identify how moisture and humidity will affect your project’s design and functionality.  Talk to one our engineers at AIP about your project design, and we will work with you to provide unrivaled expertise from your project’s initial concept to completion.

 

Supporting Materials

Certifications and Regulatory Resources

 

Want to learn more about factors that contribute to effective CNC machining?

Read Our Blog on CLTE
 

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