Polymer Support Services Frequency Asked Questions - Avient

How do I identify key application requirements?

Taking the time to really understand things such as performance requirements, industry nuances, and regulatory demands provides vital insight into which material or design will bring your end-product vision to life.

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Performance Requirements
Defining the expectations of the final product and the parameters around development is important to ensure success. Product use cases are critical to ensure such things as:

• Colors will last through long-term outdoor exposure
• The product doesn’t break from temperature fluctuations, exposure to moisture, or impact/drops
• If the product does break, it breaks in a preferred way (i.e., brittle vs ductile failure) to minimize user harm or further product damage
• Scratch & mar performance, wear resistance, and abrasion resistance minimize the appearance of wear and tear through normal user interaction
• Materials do not degrade when exposed to chemicals

Taking this a step further and considering end-of-life (reclaiming) goals may also influence design and material selection to align with your sustainability goals.

Industry Nuances
Every industry has a history and expectation of how things are done. Understanding the materials typically used in a particular industry or application provides context to then explore material alternatives and manufacturing methods. It also provides context on development timelines, some industries have tight timelines based on annual launch windows while others have a long specification process.

Regulatory / Compliance Demands 
Ensuring you have approved material for your application use case is critical. Navigating approvals and, let’s be honest, paperwork can be overwhelming. Demands and requirements may vary by region and/or industry, so be sure to lean on the material expertise of your supplier to support you in meeting stringent requirements, such as those found in healthcare and food packaging. Some common requirements for which documentation or additional expertise during development can help include: 

• UL
• USP Class VI
• ISO
• REACH
• RoHS

How do I choose the right plastic for my application?

Ensuring your material holds up in all use cases of your end application and meets performance expectations is complex. Sorting through countless material options can be even more daunting. Working with an experienced material supplier provides the combined benefit of in-depth material science knowledge and plastics processing insight to help you make an informed decision about which plastic is right for your application.

For example, specialized and sustainable polymer formulations may also be available, as well as colorants and additives to enhance performance or aesthetics, to enable proper material selection for each application’s unique requirements.

There are a number of things to consider when selecting the right material. Below are just a few considerations.

Thermoset vs Thermoplastic (What’s the difference?)
Thermoset materials are chemically cross-linked and cannot be reprocessed or recycled. Once the material is cured it is permanently rigid. Common examples are epoxy, silicone, and polyurethane (PU). Thermoplastic materials are not molecularly cross-linked and can be reprocessed or recycled. Reheating softens the material allowing it to be reshaped before becoming rigid once cooled. Common examples are polypropylene (PP), nylon (PA), polyketone (PK), and thermoplastic elastomers (TPE). Thermoset materials perform better in applications that require a harder material with higher temperature resistance. Thermoplastics are ideal for applications with less extreme temperature requirements and support sustainability initiatives since it can be reprocessed after molding.

Amorphous vs Semi-Crystalline Thermoplastics
An amorphous thermoplastic material has no specific molecular structure while a semi-crystalline polymer has some order. 
 

Examples of amorphous polymers are acrylonitrile butadiene styrene/acrylonitrile styrene acrylate (ABS/ASA), polycarbonate (PC), polymethyl methacrylate (PMMA), and copolyesters. These materials are typically transparent, with good impact strength, a glossy surface finish, and less warpage than semi-crystalline materials. The fatigue resistance, wear performance, chemical resistance, and sensitivity to stress cracking are considered poor. However, properties can be modified with fillers and/or additives.

Examples of semi-crystalline polymers are PP, polyethylene (PE), PA6, PA66, polyphthalamide (PPA), polybutylene/polyethylene terephthalate (PBT/PET), polyoxymethylene (POM) or acetal, polyphenylene sulfide (PPS), and polyether ether ketone (PEEK). These materials are typically opaque, with a sharp melting point, have good strength, excellent wear performance, and good chemical resistance. The impact strength, surface finish and warpage are not inherently good. However, properties can be modified easily with fillers and/or additives. 

Polymer Pyramid - Base Resins

There are a number of polymers available to use as is or to be used to develop specialty materials. Your materials supplier can consult with your product development team to select the right material to meet your performance requirements and end product goals.

Ensure material consistency with production globally
Due to raw material availability and localized technical support, ensuring the product manufactured in one region is the same as the product manufactured in another can be a challenge. Working with a material supplier that has R&D and technical support globally can help overcome that challenge.

Getting consistency and performance globally encompasses more than material availability. It’s important that the material formulation chosen for a product can be produced accurately across regions. This regional collaboration allows a local source to be identified to manufacture a similar compound with the same quality standards anywhere in the world. Materials suppliers with a robust global footprint can help ensure materials are also backed by localized technical support to help with molding trials or to work through manufacturing challenges anywhere in the world. 

Global Locations for Specialty Engineered Materials

What is Computer-Aided Engineering (CAE)?

CAE is the use of computer software to simulate real-world application performance in order to evaluate and optimize part design and material selection before investing in equipment.

Mold Filling Simulation
Simulates how melted plastic flows during the injection molding process to validate part design & material solution. Evaluates component design viability with preferred material solution. For fiber reinforced plastics, it will analyze the fiber for orientation and length. It also enables prediction of part quality and manufacturability by determining shrink / warp and weld line location. 

Finite Element Analysis (FEA) 
Virtually simulates how an application will perform in real-world conditions with a high level of precision. Capable of representing a variety of problems, test methods, models and outputs to evaluate the design, process and material combination for validation or optimization. 

Material Datasets Available
A number of material characterization data sets are available in .udb, moldex 3D, and SIMPOE format.

Using CAE to optimize part, tool or process design
Leveraging mold filling simulation, data software, and physical simulation capabilities, can help inform your decision-making process. A material supplier combines its materials, design, and technical expertise to select from a range of solutions to more accurately predict performance and confirm that requirements are being met. In addition to common environmental simulation or functional performance testing services, it can be helpful to conduct individualized testing and simulations in order to optimize material formulations, product performance, or design. By incorporating all the parameters (part design, tool layout, processing requirements, and material selection) there is an opportunity to proactively adjust the gate and runner location and size to ensure quality parts are consistently produced. 

How can onsite technical support improve product outcomes?

Most material suppliers offer technical support, but when working with your supplier, be sure to get clarity on local technical support availability. Onsite expertise adds tremendous value because they can consult on process development, mold design/construction, material training, troubleshooting, and continuous improvement opportunities to ensure product development success.

Equipment Requirements
Based on the material selected, there may be tweaks that can be incorporated to optimize manufacturability. Rely on your supplier to get expert guidance on material handling, drying equipment and sizing needs, as well as primary processing and secondary operations equipment improvement opportunities.    

Process optimization
The product development process does not end with design and testing. Take advantage of local technical support and consultation on process development, mold design/construction, material training, troubleshooting, and continuous improvement. This includes material preparation, primary process development, preparation requirements for secondary operations, and tooling debug / optimization.   

Contact us to discuss your requirements of thermoplastic compounds. Our experienced sales team can help you identify the options that best suit your needs.

Polymer Material Handling
Depending on your formulation’s base resin there may be different requirements for storage, preparation, and processing to minimize manufacturing challenges. Ask your material supplier for more information about startup, processing temperatures, shutdown, drying, gate size, gate types, flow rates, purging, regrind, etc. For Avient materials, this information is found in some of our online processing guides.

Troubleshooting
Material processing guides are helpful tools that offer recommendations for common troubleshooting topics. However, Avient’s experienced application development technical service (ADTS) teams can be onsite for first shots off the tool to help develop a process and be available to troubleshoot any defects that may arise.

This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. If life safety is involved, or reliably or efficacy are absolutely required, every part should be engineered and materials selected accordingly. If you look through this paper and see the many factors involved and how environment and application influences material selection, you can understand why an engineer will be very reluctant to recommend a specific material for someone else’s part.

However, many of Protolabs’ customers who design parts are not engineers, and many applications of Protolabs manufactured parts are quite benign and are expected to stay well within the performance envelope of common plastics. If your application lives at room temperature, it doesn’t have appreciable loads, and you’re willing to make a few parts and whack them with a hammer to see if they’re strong enough for your use, look for the simplified suggestions for selecting materials at the bottom, called “Don’t Make Me Do the Math.”

Material selection can be a guessing game. First, there is a general gap in understanding the fundamental relationship between the internal structure of the material and its properties. Second, accurately defining application requirements is usually given insufficient time and attention. Finally, even when these first two hurdles are overcome it can be hard to find accurate property data for materials.

Comparing Injection Molding Materials

The Standard Material Data Sheet

The standard material data sheet consists almost entirely of performance characteristics measured at room temperature. In addition, the performance characteristics are associated with catastrophic events that are not considered to be an acceptable outcome for engineered plastic products. Tensile strength at yield and elongation at break represent the standard metrics of material performance, but yield and break are not the desired responses of plastic parts when they are placed under load.

Determining the appropriate material for your application involves synthesizing information from a variety of incomplete sources. The data-sheet is the primary source of information, and you should learn to extract as much information as possible from this source. Appendix A shows a data sheet for 30% glass fiber-reinforced PBT polyester. This is a good example of a reasonably detailed data sheet.

More than 85,000 commercial options for plastic materials are listed in materials databases, and the real number is probably over 90,000. This extensive set of options can be sorted into approximately 45 polymer families or blends, and these 45 families can be further divided into two broad categories: thermosets and thermoplastics. While thermosets were the first commercial polymers, their use has diminished to the point where they constitute only about 15% of all the material processed in a given year. Therefore, this paper focuses on thermoplastics.

More detailed information can sometimes be obtained from design manuals and application notes published by individual material suppliers and can fill in the gaps in the data sheet. Supplemental information is usually more available for higher performance engineering and specialty materials than it is for commodity materials. If you really want to understand a material you need to be prepared to do a little detective work.

Understanding the Maximum Short-Use Temperature

Maximum short-term use temperature is possibly the most important data sheet parameter. Traditionally, this is the deflection temperature under load (DTUL), also called the heat deflection temperature (HDT). Another related parameter is the Vicat softening temperature. Because DTUL measures mechanical deflection and the Vicat point is closer to the actual melting or softening point of the polymer, the Vicat number will typically be higher. For a material such as the glass reinforced PBT of Appendix A, which is a semi-crystalline material, all of these values will be very close to the crystalline melting point of the polymer, 223°C (435°F). Any application that involves even momentary excursions above this temperature will eliminate this polymer from consideration.

The upper-temperature limit for filled or unfilled amorphous polymers can also be found by looking at HDT or DTUL. For example, for unfilled polycarbonate the HDT values range between 130–140°C depending upon the grade. Vicat softening points, where provided, are a few degrees higher. Amorphous polymers do not show a significant crystalline structure when they solidify, therefore they do not have a melting point. However, they do exhibit something called “glass transition.” From a practical standpoint, this is the temperature at which amorphous polymers lose their load-bearing properties.

For polycarbonate this value, when measured by dynamic mechanical methods, is approximately 153°C, just a few degrees above the Vicat softening point and 10–20°C above the DTUL, depending upon the specimen geometry and how the DTUL is measured. Vicat softening temperatures and DTUL values should never be used as long-term performance characteristics. However, they can be used to gauge short-term heat resistance when short-term is defined in minutes. Any application environment that involves excursions to temperatures above these properties will eliminate that particular material from consideration regardless of any other attributes it may possess.

Yield and Tensile Strength

Long-term performance when a material is under constant stress involves a property called “creep resistance;” if the stress is periodic then fatigue resistance becomes the dominant consideration. The relationship between stress, time, and temperature is complicated and frequently the data needed to make good decisions about long-term behavior of a material under load are not available. Here again the data sheet can provide an upper limit. The upper limit for ductile materials is the yield strength of the material and for brittle materials it is the stress at break. Both values define the point at which the material fails catastrophically. Any environment that involves stresses and strains higher than these values eliminates the material from even short-term consideration. Beyond this simple filter, you will next need to look at long-term temperature effects.

Understanding the Relationship between Temperature and Aging

All polymers have a long-term sensitivity to oxygen and this sensitivity increases at higher temperatures. Degradation associated with aging is captured by a property called the “relative thermal index,” or RTI. This value comes from a test mandated and administered by Underwriters Laboratories. It is currently the best gauge for measuring the long-term effects of aging on the mechanical and electrical properties of polymers. RTI testing begins by measuring key baseline properties such as tensile strength, notched Izod impact resistance, and arc resistance. Test specimens are then aged at multiple temperatures and the baseline properties are monitored until they decline to 50% of the original values. The time required to reach 50% performance is called the “time to failure.” If three or four aging temperatures are used and the logarithm of the time to failure is plotted as a function of reciprocal temperature, the data points can be fitted to a straight line. This line is then extrapolated to a standard time (normally, about eight years) and the temperature predicted to cause failure at the standard time is the relative thermal index. For most thermoplastics the RTI values are lower than DTUL and Vicat softening values. This is the case for the glass-filled PBT in our Appendix A sample data sheet where the DTUL and Vicat values are all above 200°C (392°F) while the RTI values are 140°C (284°F). However, it is possible for soft, flexible materials with good oxidative stability, such as PTFE, to have RTI values that are higher than their DTUL numbers. RTI values can be used to predict long-term performance where aging is the primary concern.

The aging process follows an empirical rule that relates degradation to temperature. The rate of degradation doubles with each 10°C increase in temperature. This is an exponential relationship so a change of 20°C will increase the degradation rate by a factor of 2^2 or 4 while an increase of 30°C increases the rate by a factor of 2^3 or 8. Since RTI is indexed to a time frame of approximately eight years, you can estimate that a material could survive for four years at a temperature 10°C above the RTI, two years at a point 20°C above the RTI, and one year at a point 30°C above the RTI. Safety factors should be built into this calculation since studies show that the actual acceleration factor, while nominally 2, can vary from as low as 1.8 to as high as 2.5.

Modulus

Modulus is provided on virtually every data sheet. Most often this is provided as tensile modulus or flexural modulus. The modulus relates stress to strain and can be thought of as a measure of stiffness. In most cases the modulus is calculated in the linear region of the stress-strain curve. Linearity often is lost at very low strains. Figure 1 shows a magnified view of the early portion of a stress-strain curve for a highly-glass-fiber-reinforced nylon 6/6. While the modulus of this material at room temperature is given as MPa (1,537,000 psi), the graph shows that the stress-strain plot departs from linearity at approximately 0.4%. Beyond this point each incremental increase in stress produces a progressively larger corresponding strain. Figure 2 shows that while the slope of the modulus line reflects the value provided on the data sheet, the effective slope of the line connecting the origin to the yield point has a slope that is only 40% of this reported value. Therefore, when using the modulus as a selection property it is important to understand the position of the application stress on the stress-strain curve. As application stresses approach the yield point the expected lifetime of the product declines. Table 2 shows the maximum operating stress for a polycarbonate material as a function of time at two different temperatures. At very short times, less than an hour, the stress limit is nearly the same as the yield stress for the given temperature. As the time frame of the application increases under load, the maximum allowable working stress declines.

Stress Cracking—The Most Common Cause of Field Failure in Plastic Parts

If a chemical is present in the application environment that is capable of inducing a phenomenon called “stress cracking,” the maximum operating stress declines. Table 3 shows the maximum working stress for the same polycarbonate profiled in Table 2 where the constant stress is coupled with the presence of a fluid that acts as a stress crack agent. It shows that the mechanical capability if the declines compared with the properties when this chemical is absent. Failure of a plastic under the combined influence of stress and a chemical agent is called environmental stress cracking (ESC) and it is the most common cause of field failure in plastic parts.

These two materials represent behavior typical of their respective structures. Both possess a glass transition that represents the onset of molecular motion in the amorphous regions of the structure. In the amorphous polycarbonate this results in the complete loss of all useful mechanical properties over a relatively narrow temperature range. However, in the nylon the decline in modulus, while significant, is not catastrophic and about 20% of the room temperature performance remains. This is a measure of the contribution of the crystal structure in the polymer. All amorphous polymers exhibit temperature-dependent behavior that is similar to that of polycarbonate, and all semi-crystalline materials display a property-temperature profile that is similar to that of nylon. The essential differences are in the exact transition temperatures for each polymer.

Impact Resistance

Evaluating impact performance from typical data sheet values is challenging because the industry employs many different methods for testing impact resistance and reporting the results. The most common test method for evaluating impact resistance is the notched Izod test. The test employs a specimen with a sharp notch machined into the part and a swinging pendulum is used to impart the energy needed to produce failure.

The minimal radius in the Izod notch often exaggerates differences in ductility because of differences in notch sensitivity among materials. For example, polycarbonate and amorphous PET polyester both possess good practical toughness. PET polyester is more notch-sensitive that polycarbonate. Consequently, the notched Izod impact values for polycarbonate at room temperature may be much higher than those for some grades of PET polyester, giving the impression that polycarbonate is a much tougher material. A more complete picture of impact performance can be obtained if impact results can be obtained from different types of impact tests.

Other Material Properties

Properties other than thermal and mechanical can be important in specific applications. These include electrical properties such as dielectric constant and strength, surface and volume resistivity, and coefficient of thermal expansion. Standard measurements of the coefficient of thermal expansion are made between -30°C and +30°C. However, some suppliers will provide values across multiple temperature ranges such as those represented in Table 5. When this more complete picture is available it shows that these types of properties are also dependent upon temperature and the values tend to increase with increasing temperature.

Plastics are generally considered to be excellent electrical insulators unless a compound is made specifically to dissipate static or be somewhat conductive through the addition of ingredients such as carbon or stainless steel. Therefore, values for resistivity are very high for most materials, between 10^10 and 10^16 ohms or ohm-cm for surface and volume resistivity, respectively. Sustained electrical stress can result in the dielectric breakdown of a material over time. This behavior will depend upon the magnitude of the voltage being applied and is captured most effectively by properties that may be part of the standard data sheet but may be more readily found in the Underwriters Laboratories database as part of their Yellow Card system. This method applies numerical values to properties such as high amperage ignition, arc track resistance, and continuous tracking index with the lowest values for each metric indicating superior performance and higher values associated with lower levels of performance.

Don’t Make Me Do The Math!

A consultant usually won’t make a material recommendation without understanding the complete application requirements for the part, and without running a design analysis on the 3D model of the part. Sometimes, it’s not cost-effective to fully engineer the part to come up with a material selection. If you want to short-circuit the materials engineering and take an educated chance on material selection, here are a few rules of thumb you can use:

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1. Try ABS. ABS works for many, many applications. It’s reasonably priced, strong, relatively tough, has a decent appearance and is forgiving even if you don’t follow all the standard design rules for plastic parts. It does have a relatively low melting point. All the parts in our demo mold are ABS.
2. If it needs to be cheap and rigidity and cosmetics aren’t really important, try polypropylene (PP). Our design cube is made of PP.
3. If you need something a little tougher than ABS or able to withstand a little higher temperature, try polycarbonate (PC). PC is less forgiving than ABS if you don’t follow the standard design rules for plastic parts.
4. If it needs to be nice-looking and transparent, try acrylic (PMMA). PMMA can be a little brittle. A transparent PC will be tougher than PMMA but a little less cosmetically nice.
5. If rules 1–4 don’t point you where you need to go, then you need to start doing the math.