What is HDT?

 

The Heat Deflection Temperature (HDT), or Heat Distortion Temperature, is a measure of a polymer’s resistance to alteration under a given load at an elevated temperature.  It is also known as the ‘deflection temperature under load’ (DTUL) or ‘heat deflection temperature under load (HDTUL)’.  Basically, it tests the stiffness of a material as the temperature increases.

 

It is the temperature at which a polymer test bar will be bent at 0.25 mm under a given weight.  It is one of the two basic test methods for assigning a value to the performance of a plastic at high temperature.  The 0.25mm value is arbitrary and does not have any significant meaning.

 

Why is HDT significant?

 

As with any machined part, during the design phase, it is critical for a machinist to know how a material will react to heat produced while machining occurs.  Tools produce heat when they come in contact with materials and plastics have a tendency to move with heat.  In order to get a finished product with the right dimensions and tolerances, it is important to understand the heat deflection temperature of a given polymer.

 

Other reasons include:

  • HDT represents a value which can be used to compare different materials with each other
  • It is applied in product design, engineering and manufacture of products using thermoplastic components
  • A higher HDT temperature means a faster molding process in injection molding processes

 

Tests to Measure Heat Deflection Temperatures of Plastics

 

The American Society for Testing and Materials, or ASTM, standard for measuring HDT is called ASTM D 648; this standard is equivalent to the ISO 75.

 

The two common loads used in heat deflection testing are:

  • 46 MPa (67 psi) – this load is usually for softer grades of plastic like polyethylene (PE) or LDPE.
  • 8 MPa (264 psi) – this load is used for more durable grades of plastic like PEEK or polycarbonate (PC).

 

There are tests performed at higher loads such as 5.0 MPa (725 psi) or 8.0 MPa (1160 psi), but we won’t discuss them in this brief.

 

Limitations that are associated with the determination of the HDT is that the sample is not thermally isotropic and, in thick samples in particular, will contain a temperature gradient.

 

During the ASTM D 648 test, a testing rod made of the selected polymer is placed on an apparatus like the one in the diagram below.

 

 

 

Source: SEKISUI Polymer Innovation
 

The bar is molded a specific thickness and width.  The sample is then submerged in oil while the temperature incrementally increases (usually about 2 oC per minute).  The constant applied force, or load, is pressed to the midpoint of the test bar.  The temperature at which a bar of material is deformed 0.25mm is recorded as the HDT.

 

HDT at 1.8 Mpa (264 psi) Values for Common Polymers

 

 

Polymer NameMin Value (o C)Max Value (o C)
ABS – Acrylonitrile butadiene styrene88100
PA – Nylon Polyamide, 66 30% Glass Fiber230255
PAI – Polyamide-Imides (TORLON)275280
PBI – Polybenzimidazole (CELAZOLE)426.6
PC – Polycarbonate, high heat140180
PE – Polyethylene, 30% glass fiber121121
PEEK – Polyetheretherketone150160
PEI – Polyetherimide (ULTEM)190200
PP – Polypropylene (30-40% Glass fiber-reinforced)125140
PP – Polypropylene Homopolymer/Copolymer5060
PS – Polystyrene, high heat85100
PSU – Polysulfone160174
PTFE – Polytetrafluorethylene4550
PVC – Polyvinyl chloride, rigid5475
PVDF – Polyvinylidene fluoride (KYNAR)50125

 

Factors That Influence HDT

 

The HDT gives a short-term performance under load at elevated temperatures for a polymer by measuring the effect of temperature on stiffness.  Yet, this is only an estimate and should not be used to predict how the final part or component will perform.

 

Other factors will significantly influence the final thermal performance of an application.

 

These factors include:

  • The time of exposure to elevated temperature
  • The rate of temperature increase
  • The part geometry

 

The HDT measure for a specific polymer grade also depends on the base resin and the presence of reinforcing agents, fillers or plasticizers.

 

For instance, in the chart above, the homopolymer or copolymer of polypropylene has a HDT value range of 50-60 oC. Compare that value to the 30-40% glass-fiber reinforced grade of polypropylene, which is more than double the temperature (125-140 oC).  A factor like this would influence the material choice for a designer wanting to use polypropylene for the end use product.

 

A combination of additives will always have a different effect on the HDT and the performance of a polymer overall.

 

  • Reinforced and filled grades have a higher HDT (harder and stiffer under the heat)
  • Plasticizers decrease HDT by making the polymer softer and more flexible

 

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Data charts can give you the heat deflection 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 HDT and other values of polymers and composites.  Your machinist will be able to give you a detailed response on how the heat deflection 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.

 

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Learn more about the material properties we consider when
working on a precision plastics machining project.

 

Read our blog on the CUT of Polymers
 

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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.

 

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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.

 

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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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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.

 

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The last step in finishing a part is to apply an appropriate finish. Finishing a machined part can be as simple as smoothing off the burrs and other sharp edges or painting and coating the material to improve aesthetics and functionality. In the initial design phase of your project, talk to your machinist about the finishing processes to get a highly precise and extremely resilient piece.

 

AIP has over 37 years of experience machining complex components from thermoplastics. In this technical blog, we discuss putting the finishing touches on your CNC machined part, including: finishing, deburring, painting and polishing.

 

Common Finishing Techniques for CNC Machining

 

As Machined

The “as machined” part is the standard finish for the material. Many times, it has visible tools marks, but it has no additional cost to the machining process. Also, this finish has the tightest dimensional tolerances. Our standard AIP machined finish has almost no tool marks – we go the extra mile for our customers to produce unparalleled results. Taking pride in our craftsmanship and attention to detail is what makes us stand out from other CNC machine shops in the industry.

 

Deburring

Deburring involves the removal of burrs and sharp edges. A variety of tools may be used including die grinders, deburring scrapers, files and various stones.

 

Painting

Painting a finished part fulfills two requirements: 1) improves appearance (aesthetics) and 2) enhances the function of the piece.

 

Various coatings and treatments provide protection and add color to the surface of machined parts:

 

  • Plating: Chrome plating, nickel plating and other kinds of metal can be applied via plating processes.
  • Painting: Resins generally come in many different colors and can be painted to fit the exact specifications of a project.
  • Powder Coating: Powder coating adds a wear and corrosion finish to the surface of a part. It has a higher impact resistance compared to anodizing and a large range of colors are available.
  • Silk Screening: This is an inexpensive way to print text or logos on the surface of a CNC machined part. The print can be applied only to external surfaces on a part.

 

Polishing

There are several different types of polishing to finish off a machined plastic part. Here are a few of the most common methods:

 

  • Vibratory Polishing: This method uses rotating or vibrating tumblers along with a variety of media to deburr, remove tooling marks and polish parts. It is convenient for large bulk items that need polishing. Put them in a tumbler and go do something else.
  • Bead Blasting: This process uses compressed air to blast an abrasive media at the material. This method is done inside of a blast cabinet. It adds a uniform matte or satin surface finish on a machined part and removes all tool marks.
  • Filing: Filing down the edges or burrs on a small machined part is a craft, however, a good file offers efficiency. This technique is often taught to apprentice machinists.
  • Stoning: Machinists use stones and oil to deburr and knock off sharp edges that tear and snag.

 

Case Study: Making a Splash with Machined PPS

 

The finishing plays a major role in the quality, durability and utility of a machined plastic part. For our client in the theme park business, it meant reducing a water ride’s overhauls by 25 times.

 

When a popular ride experiences downtime, the negative impact on guest satisfaction is immediate. Lost interest and value in a park experience can mean loss of customers and in effect revenue. For our Florida theme park client, this was the case with their log flume coaster ride. While this ride was thrilling for its daily customers, the ride required nightly repair and part replacement. They specifically needed new wheel bushings from a more innovative material.

 

Since we had worked with this client previously, we were able to assess the project. The log flume’s passenger carts originally used bronze bushings due to their nice, soft wear, however, the speed and load of the carts generated a great deal of heat when the ride would plunge into its steep vertical drop. The moment each cart hit the cool water below, the wheel bushings would suddenly experience “shock cooling” damage.

 

Between this and the constant exposure to chemicals in the water (chlorine), the bronze bushings had a very short life cycle.

 

Our team selected Quadrant’s BG1326, a bearing-grade high-performance thermoplastic PPS.

 

PPS CNC Machining ExamplePPS has a low moisture absorption rate and can be machined to the exact tolerances necessary for clearing and shaft. With low-wear, high temperature stability and a low coefficient of friction, the chosen PPS grade proved to be an excellent fit for the log flume’s wheel bushings.

 

Our machinists at AIP worked diligently with the ride engineers to ensure the PPS bushings were built to exact specifications. The chosen method for machining the parts was precision machining. This way the components could meet the precise tolerances and finishes demanded by the speed and load of the log flume.

 

The Benefits

 

The main benefit of the machined PPS bushings was the reduction in ride downtime. The previous bronze bushings required around 25 times the overhauls of the new PPS bushings. The change in materials not only saved on maintenance and inventory costs, but improved guest satisfaction with the increase in uptime.

 

The PPS bushings also removed the potential for grease to affect seals or chemicals in the water. As a self-lubricating plastic, PPS removed the need for nightly greasing the wheel bushings. The lower energy cost of the PPS material also made for a more environmentally friendly and efficient design.

 

Preventing Contamination

 

Some companies machine both metals and plastics, which can open the door to contamination of a product. Although some sources state that most CNC machining tools can be used for both metal machining and plastic machining, this is not recommended. Past experiences have shown parts going to customer without cracks, only to develop surface warpage and cracks over time due to exposure to metal machine shop fluids.

 

Mitigate contamination by working with a facility like AIP that works solely with polymers. We ensure the highest level of sanitation down to the sub-molecular level by designing, heat-treating and machining only plastics. This allows us to eliminate the risk of metallic cross contamination.

 

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Learn more about our capabilities and reach out to our machinists for a consultation on your precision machined project.

 

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