Dimensional stability is a measure of a material’s ability to retain its fit, form, and functional properties throughout its lifecycle.
Engineering plastics are subject to a myriad of extreme conditions – from machining of parts to intense operating environments – that can impact how well both the material and the overall part function throughout their lifecycles. The material property that is most often affected by a lack of dimensional stability are the dimensions of the finished part, as excessive moisture or extreme temperature environments can cause materials to expand or contract.
Even if a part will not be exposed to particularly demanding operating conditions, it is still important to consider the dimensional stability of a material when machining the part to close tolerances – a process that subjects a material to high levels of mechanical stress.
In order to ensure performance and safe operation throughout a part’s lifetime, a plastic material’s dimensional stability is one of the key performance properties that must be taken into consideration during both material selection and part design. Mitsubishi Chemical group offers a comprehensive portfolio of engineering polymers covering an array of dimensional stability needs.
What conditions impact dimensional stability?
Dimensionally stable plastics generally exhibit the following performance properties:
There are many engineering applications in which the dimensional stability of the materials involved must be considered. Sealing components are an example of parts requiring dimensionally stable materials, as changes to the dimensions of the seal could result in leakage. Dimensional stability should also be a consideration in choosing a material for parts that will be subject to regular cleaning with steam, chemicals, or hot liquid.
Beyond the final application of the part, the dimensional stability of a material impacts machining and the final tolerances that can be acheived. With less dimensionally stable polymers, the build-up of heat during machining and internal stress in the underlying shape may make it difficult to consistently machine parts involving precise dimensions with tight tolerances. Sophisticated processing and state-of-the-art machining technology, along with polymers exhibiting superior dimensional stability, make it possible to machine intricate parts with small scales and tight tolerances, such as electrical connectors and test sockets.
Although metals are generally more dimensionally stable than plastics, many engineering plastics also exhibit excellent dimensional stability – as well as several clear advantages that make them preferrable to metals in many applications.
Our dimensionally stable thermoplastics can perform comparably, while also providing other key benefits that would be unattainable with metal parts. These advantages include:
- Lower raw material costs
- Lighter weight finished parts
- Specific electrical properties
- Resistance to chemical corrosion
- Improved friction and mating behavior
For applications requiring additional dimensional stability, thermoplastics can be reinforced with glass fibers or other fillers. Reinforced polymer grades typically exhibit greater thermal dimensional stability compared to unfilled grades, as well as other improvements in mechanical, chemical, and dielectric properties.
Dimensionally stable engineering plastics from Mitsubishi Chemical Group
Even when exposed to high temperatures, moisture, and mechanical stress, Mitsubishi Chemical Group offers a comprehensive range of dimensionally stable thermoplastics that exhibit consistent performance and demonstrate several benefits over metal components, including cost savings.
The following is a sample of some of our most dimensionally stable plastics. Contact us for information specific to your application and engineering needs, and to discuss our full portfolio of dimensionally stable polymer materials.
This cost-effective, general purpose copolymer acetal grade is often favored for its porosity free nature. Acetron® GP POM-C shapes offer low moisture and excellent machinability capabilities, making it a very versatile material that can excel in a multitude of environments. Due to these characteristics, Acetron® GP POM-C components are used frequently for Food Contact and medical applications, and meet FDA, USDA, NSF, Canada AG, and 3A-Dairy compliance requirements.
Ertalyte® TX is a polyethylene terephthalate polyester grade modified to offer superior dimensional stability, wear characteristics, and inertness over nylons, acetals, and unmodified polyesters. With a low moisture absorption rate, the electrical and mechanical properties of this PET-P material are unaffected by moisture. With such stable performance, Ertalyte® TX PET-P is ideal for use in applications involving high pressure and velocity conditions, as well as for noise abatement in environments with soft metal and plastic mating surfaces.
Possessing a unique combination of dimensional stability and wear resistance, Fluorosint® 500 PTFE is ideal for sealing applications requiring tight dimensional control. Compared with unfilled PTFE, this grade offers nine times greater resistance to deformation under load, as well as outstanding resistance to both chemicals and hydrolysis. With a linear thermal expansion rate only 25% that of unfilled virgin PTFE, Fluorosint® 500 rivals aluminum in its potential to resolve fit and clearance problems.
A polyphenylene sulfide reinforced with 40% glass fiber, Techtron® GF40 offers superior dimensional stability, stiffness, and creep resistance compared with unfilled PPS grades. This grade is favored for its exceptional performance in structural applications involving high static loads for extended periods of time, especially in high-temperature environments, as the material maintains its strength and stability in the range of 425°F (220°C).
Ketron® 1000 Polyetheretherketone PEEK is an EU 10/2011 and FDA Compliant, unfilled, general purpose grade that offers the highest elongation and toughness of all materials in the PEEK family. Ideal for instrument and seal components, where ductility, dimensional stability, and inertness are critical, Ketron® 1000 PEEK shapes are known for their ability to fit within a variety of applications and industries.
For applications with especially close tolerances and/or thermal stability needs, glass- and carbon fiber-reinforced Ketron® PEEK grades display even greater dimensional stability.
Semitron® MDS 100 modified PEEK was specially developed to provide a highly rigid, stable, and moisture-resistant platform for precision structural applications. With a unique proprietary polymer matrix, Semitron® MDS is ideal for use in parts requiring fine machining and precise tolerances. Between its very low moisture absorption and low CLTE, parts made from Semitron® MDS stay in specification.
This specialty, electrostatic dissipative PEI grade exhibits excellent dimensional stability characteristics, making it ideal for semiconductor and electronics handling environments as hot as 410o F. Offering low stress for tighter machining tolerances, a reduced CLTE, and minimal water absorption, Semitron® ESd 410C PEI components excel in handling tray, circuit board, hard disk drive, and sensitive processing equipment applications.
Duratron® U2300 PEI is a flame resistant polyetherimide that is 30% glass-reinforced, giving it the highest rigidity, strength, dimensional stability, and heat resistance of all Duratron® PEI shapes. Possessing the lowest Coefficient of Linear Thermal Expansion, a superior strength-to-weight ratio, and excellent tensile strength, this grade is favored as a solution for reusable medical devices, analytical instruments, electrical insulators and semiconductor process components, and structural components that require high strength and rigidity at elevated temperatures.
Duratron® T7530 PAI Polyamide-imide is a compression molded, 30% carbon fiber-reinforced grade that exhibits outstanding strength and stiffness in nonabrasive wear applications, and the lowest coefficient of thermal expansion of all PAI materials. With high-temperature resistance and great dimensional stability for extreme applications, Duratron® T7530 PAI has the ability to withstand high pressure environments in off-shore equipment applications, making this grade a reliable choice for long term performance requirements.
Explore how our dimensionally stable plastics perform in common engineering applications
High-pressure microfluidizer seal from Fluorosint ® 207 PTFE
The challenge: A mircofluidizer is a piece of medical equipment in which pharmaceutical liquids are subjected to 4,000-10,000 psi and high shear forces as a means of reducing their particle size. After each use, the machine is steam sterilized in an autoclaving cycle of at least 20 minutes. With the existing UHMW-PE seal, this sterilization process caused the intensifier pump to swell and shrink, leading to leakages and loss of pressure in the unit.
The solution: Our Fluorosint ® 207 PTFE material was chosen to replace the UHMW-PE component because of its exceptionally low moisture absorption, excellent dimensional stability, and unmatched non-permeability in steam.
The results: With improved resistance to creep and steam, Fluorosint ® 207 PTFE significantly extended the service life of the microfluidizer seal. The new part was able to withstand repeated autoclaving cycles and retained its mechanical strength far longer than conventional PTFE and PE materials.
Pistons and valve spools in liquid filling equipment from Ertalyte® PET-P
The challenge: Valve spool and piston components in liquid filling machines were historically made from stainless steel. Such parts are costly and require constant lubrication, prompting the OEM to look for FDA-compliant plastic materials to replace them. The polymer would need to have exceptionally low moisture absorption and withstand constant contact with liquids ranging from sauces and food to cleaning agents. Additionally, the parts would need to be machined to tight tolerances to perform as needed.
The solution: Not all polymers met the requirements – UHMW-PE could not withstand the pressure of the application, acetals wore too quickly, and nylons absorbed too much moisture. Ertalyte® PET-P was chosen for its excellent wear performance, minimal moisture absorption, and low thermal expansion, making it ideal for this application.
The results: The Ertalyte® PET-P rods delivered cost savings and performance advantages without sacrificing the accuracy of the filling systems. The machined PET-P parts hold +/- 0.0007- inch tolerances on the diameter of the piston and valves. Thick wall sections, very tight tolerances, and limited production volumes made machining the best manufacturing choice for the parts.
Testing and measuring the dimensional stability of engineering plastics
We perform a range of standardized tests on every thermoplastic in our portfolio, providing engineers with the detailed product data they need to evaluate candidate materials. The following tests are critical for measuring the dimensional stability of our materials.
The coefficient of linear thermal expansion (CLTE) is used to determine the rate at which a material expands as a function of temperature.
For each of our materials, Mitsubishi Chemical Group performs the ASTM E- 831 / ISO 11359 to calculate the rate of thermal expansion in three different temperature ranges: 73°F to 210°F (23°C to 100°C), 73°F to 300°F (23°C to 150°C), and over 300°F (150°C).
For each test, a material specimen is placed in a furnace. The temperature in the furnace is raised at a specified, controlled rate over the testing temperature range until the material fails at a maximum temperature. The changing dimensions of the specimen are recorded in detail.
In order to predict the rate at which a material will deform under constant mechanical stress in a given temperature range, we implement creep behavior testing across our portfolio.
We perform dynamic mechanical analysis (DMA) to understand the degree to which the dimensions of a plastic component will undergo creep when subject to varying degrees of compressive strain for elapsed periods of time.
The variables involved in DMA testing are temperature, time, and stress. In the following charts, the creep behavior of a selection of our materials was observed for 100,000 hours at room temperature (23°C) at the boiling point of water (100°C). During this time, specimens were subject to mechanical stress. As can be seen in the graphs, as time progresses, the load a material can withstand before deforming by 1% is significantly higher at lower temperatures than at more extreme temperatures.
Water absorption testing helps to evaluate how quickly a material will assimilate water when exposed to moisture, thus potentially altering its size, weight, and mechanical and electrical properties.
To calculate moisture absorption behavior, we perform the tests ISO 62 and ASTM D570, depending on the material specimens available.
These water absorption test methods involve submerging a thermoplastic specimen in water at room temperature – the standard temperature control for each of our products. After 24 hours submerged, we measure the percent weight gain of the plastic. Additionally, we also record the percent weight gain of the plastic at complete saturation when it cannot absorb any more water, regardless of time elapsed.
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