An Informational Brief on Polymer Machining

 

Polyethylene terephthalate (PET or PET-P) is a general-purpose thermoplastic polymer which belongs to the polyester family. Highlights of this material include: an excellent combination of mechanical, thermal, chemical resistance and dimensional stability. Another notable characteristic of this material is that it is one of the most recycled thermoplastics.

 

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

 

Properties of PET-P

 

It is beneficial to keep information on the properties of a thermoplastic before machining. This helps in selecting the right thermoplastic for a project. Furthermore, it assists in evaluating if the material is a candidate for the end-use requirement. Below are some of the key characteristics of Polyethylene Terephthalate Polyester:

 

Key Properties of PET-P

  • Chemical Resistance
  • Enhanced Electrical Properties
  • High Dimensional Stability
  • High Strength
  • Lightweight
  • Low Water Absorption

 

PET-P (Polyethylene Terephthalate Polyester) is a thermoplastic polyester, commonly referred to as just Polyester. With good dimensional stability, electrical properties, high strength, low water absorption, and good chemical resistance (with the exception of alkalis), PET-P offers a greater acidic resistance and stain resistance than Nylon or Acetal.

 

Applying PET-P reduces the need for heavy lubrication, making the material useful for manifolds, distribution valves and pistons. PET-P is also lightweight and is widely used for packaging material, electrical insulation and coatings.

 

While PET-P can be provided in FDA compliant grades, it should not be used for continuous use in hot water.

 

A wide range of extruded and compression molded shapes and sizes are available for machining. Some processing challenges may arise due to the uneven sizing of the rod diameter and plate thickness.

 

Material PropertyUnitsValue
Tensile Elongation at Break @73 F%20
Flexural Modulus of Elasticity @ 73 Fpsi490000
Tensile Modulus of Elasticity @ 73 Fpsi460000
Flexural Strength @ 73 Fpsi18000
Hardness Shore DD87
Tensile Strength @73 F, (ult)/(yld)psi12400/(ult)
Notched Izod Impact @73 Fft-lb/in of notch0.5
Heat Deflection Temperature @ 264 psiF240
Melting Point, (VS = Vicat Softening Temp)F491
Coefficient of Linear Thermal Expansion @73 Fin/in/F3.3E-05
Dielectric Strength, Short TermVolts/mil385
Water Absorption 24 hours% by weight0.07

 

Applications of PET-P

 

Due to its chemical resistance to solvents, acids and other liquids, PET-P is often used for packaging materials, such as soft drinks, flexible food packaging, even thermal insulation like space blankets. Among its other applications, you can find it as polyester yarn, spun fibers and microfiber towels and cleaning cloths.

 

Polyethylene Terephthalate helps make electrical devices, photovoltaic panels, switches and other critical energy and industrial components stronger and reliable. See the list below for other common PET-P applications.

 

Common Applications

  • Coatings
  • Distribution Valves
  • Electrical Insulation
  • Manifolds
  • Packaging Material
  • Pistons
  • Pharmaceutical Test Equipment

 

Grades of PET-P

 

At AIP, we machine various grades and brand name Polyethylene Terephthalate Polyester (PET-P), including the following:

 

  • ERTALYTE®
  • ERTALYTE® TX
  • RYNITE®
  • SUSUSTADUR® PET
  • TECAPET
  • VALOX™

 

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. Whatever your application, our machinists can help you in material selection, sizing and manufacturing techniques from concept to completion.

 

Machining PET-P

 

Annealing PET-P

As with any polymer CNC machined part, annealing and stress-relieving is crucial to the machining process. Without coolants, lubricants and trained procedures, cracking and crazing is inevitable in a machined part. We recommend slow heating and cooling during the annealing process of PET-P. Our process at AIP significantly reduces the chances of these stresses occurring from the heat generating during machining polymers like PET-P. Our machinists use computer controlled annealing ovens for the highest quality precision temperatures and time control.

 

Machining PET-P

PET-P is one of the most dimensionally stable plastic materials to machine, especially when trying to maintain tight tolerances or flatness. 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.

 

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.

 

Polyethylene Terephthalate Polyester Machining Guide: Supportive Information

 

Chemical Resistant Materials
Quality Assurance Certifications

 

Looking for more machining guides on performance thermoplastics?

 

Read Our HDPE Machining Guide
 

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An Informational Brief on Polymer Machining

 

ARDEL® Polyarylates are a family of aromatic polyesters.  It has inherent UV stability and superior retention of optical and mechanical properties.  These qualities combined make it an excellent choice for applications where weathering and wear are a concern.  To make a comparison, polyaryls have similar mechanical properties to polycarbonates.  Likewise, its impact strength can be compared to medium-impact ABS.

 

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

 

Properties of ARDEL® Polyarylate

 

It is helpful to keep information on the properties of a thermoplastic before machining. This helps in selecting the right material for your project.  Also, it assists in evaluating if the material is a candidate for the end-use requirement.  Below are some of the key characteristics of ARDEL® Polyarylate:

 

Key Properties

  • Chemical Resistance
  • Dimensional Stability
  • Excellent UV Resistance
  • Optical Characteristics
  • Inherent UV Stability
  • Dielectric Strength

 

ARDEL® Polyarylate is specifically formulated to endure extreme UV light. When exposed to UV, ARDEL® Polyarylate undergoes a molecular rearrangement resulting in the formation of a protective layer (UV stabilizer).

 

This high UV resistance makes ARDEL® Polyarylate the ideal material for any application where weathering effects could pose a problem.  ARDEL provides for long lasting components in automated UV curing equipment.

 

Further, this material also offers good dimensional stability, chemical resistance and optical characteristics.

 

Material PropertyUnitsValue
Tensile Elongation at Break @73 F%10
Flexural Modulus of Elasticity @ 73 Fpsi310000
Tensile Modulus of Elasticity @ 73 Fpsi300000
Flexural Strength @ 73 Fpsi11000
Densityg/cm3
lb/in3
1.21
0.044
Tensile Strength @73 F, (ult)/(yld)psi10000/(ult)
Notched Izod Impact @73 Fft-lb/in of notch3.8
Heat Deflection Temperature @ 264 psiF345
Flammability RatingUL94V-0
Coefficient of Linear Thermal Expansion @73 Fin/in/F3
Dielectric Strength, Short TermVolts/mil400
Water Absorption 24 hours% by weight0.26

 

Applications of ARDEL® Polyarylate

 

As mentioned previously, arylates are a good choice where weathering effects are an issue.  Since they have excellent mechanical and electrical properties and excellent chemical stability, arylates are used in a variety of applications including automobiles, precision and medical devices, electronic displays and other electrical parts.  Other parts include semiconductor molding compounds, decorative displays and protective coverings.  Finally, with excellent resistance to degradation from ultraviolet radiation, ARDEL® Polyarylate can be found in solar-energy panels.

 

Common Applications

  • Automotive Applications
  • Connectors
  • Electrical Parts
  • Protective Coverings

 

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.  Whatever your application, our machinists can help you in material selection, sizing and manufacturing techniques from concept to completion.

 

Machining ARDEL® Polyarylate

 

Annealing ARDEL® Polyarylate

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 part.  We recommend slow heating and cooling during the annealing process of thermoplastics.  Our process at AIP significantly reduces the chances of these stresses occurring from the heat generating during machining ARDEL® Polyarylate.  Our machinists use computer controlled annealing ovens for the highest quality precision temperatures and time control.

 

Machining ARDEL® Polyarylate

ARDEL® Polyarylate is one of the most dimensionally stable plastic materials to machine with a score of 3 (1 being easy and 10 being difficult), especially when trying to maintain tight tolerances or flatness.  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.

 

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.

 

ARDEL® Polyarylate Machining Guide: Supportive Information

 

Quality Assurance Certifications
 
 

Looking for more machining guides on performance thermoplastics?

 

Read Our ULTEM Machining Guide

 

Follow AIP Precision Machining on Linkedin

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What is Thermal Expansion (and Contraction)?

 

Thermal expansion or contraction occurs when a material is exposed to temperature change and thus leads to change in all dimensions, as well as other physical properties.  While this effect is most noticeable in gasses and liquids, it is also notable in solids.  Softer materials such as unreinforced polymers experience greater levels of dimensional change per each degree change in temperature.

 

When it comes to machining polymers, heat is always part of the equation.  Increasing the temperature on any polymer or composite can lead to significant changes in dimensions, to part warpage or to internal stress.  Therefore, it is crucial for machinists and designers to understand the amount of thermal expansion a material will undergo during machining operations.  Ideally, reducing any and all thermal contributors during machining would provide for the least machinist stress during production.

 

One of the tests that engineers and designers use to measure the dimensional stability of a material under the effects of heat is the coefficient of linear thermal expansion.  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 coefficient of linear thermal expansion (CLTE).

 

The CLTE Equation

 

The linear coefficient ‘CLTE or α for plastic and polymer materials is calculated as:

 

a = ΔL / (L0 * ΔT)

 

Where:

a is coefficient of linear thermal expansion per degree Celsius

 

ΔL is change in length of test specimen due to heating or to cooling

 

L0 is the original length of specimen at room temperature

 

ΔT is temperature change, °C, during test

 

Calculate a by dividing the linear expansion per unit length by the change in temperature. When reporting the mean coefficient of thermal expansion, the temperature ranges must be specified.  It is important to note that with many materials, a can change as temperature changes.  It is not always linear, but many times assumed to be linear for most less critical applications or quick estimates.

 

Recently we were brought in to evaluate a material cracking issue with a Polycarbonate Lens.  The cracks were noted around the fastener holes.  The fasteners mounted the lens to an aluminum frame.  It was discovered that there was basically no or minimal clearance between the fasteners and holes in the Polycarbonate.  As the thermal environment deviated over a 100 oF range, the lens obviously expanded and contracted resulting in the fracture.  Updating the design to allow for additional clearance solved the problem.

 

Applications of CLTE

 

Thermal expansion in materials causes premature cracks and stresses that can lead to part failure.  Understanding the CLTE is not only a fiscal concern, but also helpful in determining the type of material for quality and function.

 

  • It is required for design purposes.
  • It helps determine dimensional behavior of machined parts subject to temperature changes.
  • It also determines thermal stresses that can occur, and cause failure of a solid item composed of different materials when it is exposed to temperature (specially to predict efficient material bonding or while using plastics with metals).

 

 

How to Measure CLTE

 

The most widely used standards to measure coefficient of linear thermal expansion in plastics are ASTM D696, ASTM E831, ASTM E228 and ISO 11359.

 

Common methods for determining CLTE include:

  • Dilatometry
  • Interferometry
  • Thermomechanical analysis

 

Dilatometry

 

With this technique, the specimen is heated in a furnace and displacement of the ends of the specimen are transmitted to a sensor by means of push rod.  The push rods may be vitreous silica type, high-purity alumina type, or the isotropic graphite type.

 

ASTM D696 – This test covers determination of the coefficient of linear thermal expansion for plastic materials having coefficients of expansion greater than 1?µm/(m.°C) by use of a vitreous silica dilatometer. The nature of most plastics and the construction of the dilatometer make -30 to +30°C (-22°F to +86°F) a convenient temperature range for linear thermal expansion measurements of plastics, since most plastics are commonly used within this temperature range.

 

ASTM E228 – This test is used for temperatures other than -30°C to 30°C to determine linear thermal expansion of solid materials with a push-rod dilatometer.

 

Interferometry

 

Using optical interference methods, displacement of the material ends is measured in terms of the number of wavelengths of monochromatic light.  While precision is great that with dilatometry, interferometry is not used much above 700 °C (1290 °F).

 

ASTM E289 – provides a standard method for linear thermal expansion of rigid solids with interferometry that is applicable from –150 to 700 °C (–240 to 1290 °F). It is more applicable to materials having low or negative CLTE in the range of <5 × 10-6/K (2.8 × 10-6/°F) or where only limited lengths of thickness of other higher expansion coefficient materials are available.

 

Thermomechanical Analysis

 

Measurements are made with a thermomechanical analyzer that has a material holder and probe that transmits changes in length to a transducer.  The transducer converts the movements of the probe into an electrical signal.

 

ASTM E831 (and ISO 11359-2) – These tests are applicable to solid materials that exhibit sufficient rigidity over the test temperature range.  It is applicable to the temperature range from −120 to 900°C. The temperature range may be extended depending upon the instrumentation and calibration materials used.

 

 

Factors Affecting CLTE Measurements of Plastics

 

  1. Fibers and other fillers significantly reduce thermal expansion.
  2. The magnitude of CLTE increases with rising temperature.
  3. Molecular orientation also affects the thermal expansion of plastics. The thermal expansion is often affected by the cooling time during processing. This is especially true with semi-crystalline polymers whose crystallization process requires time.

 

List of CLTE Values for Precision Plastics

 

 

Polymer NameCLTE Value (10-6 / oC)
ABS – Acrylonitrile butadiene styrene72-108
PA – Nylon Polyamide, general purpose110
PAI – Polyamide-Imides (TORLON)3-4
PC – Polycarbonate65-70
PE – Polyethylene108-200
PEEK – Polyetheretherketone4.5-5.5
PEI – Polyetherimide (ULTEM)5-6
PP – Polypropylene72-90
PS – Polystyrene70
PSU – Polysulfone55-90
PTFE – Polytetrafluorethylene112-135
PVC – Polyvinyl chloride54-110
PVDF – Polyvinylidene fluoride (KYNAR)128-140

 

As the chart notes, some polymers such as Polyethylene (PE, HDPE, UHMWPE) and Nylon (PA) tend to move more with temperature change. However, polymers like PEEK and TORLON (PAI) have a resistance to heat that rivals those of metals like aluminum (21-24).

 

Carbon and glass reinforced polymers can result in metal like levels of CLTE.  This can be advantageous when mission critical polymer parts are to be mounted to metallic components without an allowance for expansion or contraction.

 

A chart can give a general idea of the CLTE, 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 CLTE of polymers and composites.  Your machinist will be able to identify how temperatures 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

 

What should you ask your machinist about the material pick for your project?

 

We’ve got 3 tips on choosing the right material for your design.

 

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