When it comes to designing plastic components, questions must be asked and addressed in order to effectively select materials and design techniques for the selected materials. Let us begin to dive into this discussion and effective ideas to consider.
Step 1. Determine Whether the Component is a: Bearing and Wear Application (Frictional Forces) or Structural (Static or Dynamic) Application:
Determine the primary function of the component, this will direct you to a group of plastic materials to choose from. For instance, crystalline materials (Acetal or Nylon) outperform amorphous materials like PEI, Polycarbonate, Polysulfone and Ultem in a bearing and wear applications. Once you choose a material group, you can further reduce your choices by knowing what additives are best suited for your application.
1. Wear Properties: Are enhanced by MoS2, Graphite, Carbon Fiber and Polymeric
Lubricants (PTFE, Waxes).
Once you have determined these 2 questions, you can further reduce your material group choice by determining the application's mechanical property requirements. For a Bearing and Wear application, consider the wear performance expressed in (PV and "k"-factor). Calculate the PV (Pressure (psi) x velocity (fpm)) required. Select materials whose limiting PV's are above the PV you have calculated for the application. Next is to note the "k" wear factor of your material choices, the lower the "k" factor the longer the material is expected to last.
If your application is more of a structural component: These are commonly designed for maximum continuous operating stresses equal to 25% of their ultimate strength at a specific temperature. Also take into account the viscoelastic behavior of plastics that result in creep.
2. Structural Properties: Are enhanced by Glass Fiber and Carbon Fiber.
Step 2. Determine the Thermal Requirements of the Application for the Component. Study both Typical and Extreme Conditions:
The material's heat resistance is typically the Heat Deflection Temperature and Continuous Service Temperature.
1. Heat Deflection Temperature: Is an indication of a material's softening temperature and is typically accepted as a Maximum Temperature Limit for Moderate to highly stressed and unstrained components.
2. Continuous Service Temperature: Is an indication of a material's permanent
physical property degradation occurs after long term exposure to the reported
temperature.
Note: The melting point of crystalline materials and glass transition temperature of amorphous materials are the short-term temperature extremes to which form stability is maintained. For most engineering plastic materials, using them at or above these temperatures should be avoided.
Step 3. Determine the Chemical Exposure during the Use and Cleaning for the Component:
Chemical Exposure is difficult to predict, there are chemical exposure guidelines out there for plastics but they do not take into account the Concentration, Temperature, Time and Stress of the component. Each point here has a role in defining suitability for use of the component's environment.
1. Acetal, Ertalyte, Nylon and PET-P are typically suitable for industrial environments.
2. High performance Crystalline materials such as Flourosint filled PTFE, Ketron,
PEEK and PPS are more suitable in aggressive chemical environments.
It is strongly recommended that you test under end-use conditions, specific chemical resistances can be found on the material's property comparison charts on the internet or from plastic material distributors.
Step 4. Determine the Material Characteristics for the Component: Dimensional Stability, Regulatory or Agency Compliance, Relative Impact Resistance and Toughness:
Materials with higher Izod impact, Tensile Impact Strength and Tensile Elongation are typically less Notch Sensitive and Tougher for applications involving Shock Loading. Refer to a material's Mechanical Property Comparisons Chart.
Plastics can expand and contract with temperature changes 10 to 15 times more than metals and steel. Coefficient of linear thermal expansion is used to estimate the expansion rate for plastic materials. This is typically both as a function of temperature and as an average value, refer to a material's Coefficients of Linear Thermal Expansion Chart for these values. Modulus of Elasticity and Water Absorption also contribute to the dimensional stability of a material. Be sure to include the effects of humidity and steam. Refer to a material's Dynamic Modulus Chart for comparing the stiffness as temperature increases. This chart should graphically display a Dynamic Modulus curve of a material softening temperature.
1. Dynamic Modulus, what is it? The concept of a material's elastic behavior, when a
force is applied (Stress) to an elastic material there is an amount of stretch. The
amount of stretch is typically described as strain, the amount of stretch per unit
length. There are formulas for calculating this behavior. When a force is applied to an elastic material, it stretches a set amount until the force is removed. The material will almost return to its original length, some materials and ceramics come close if the strain was not too great. Plastics are viscoelastic and although the formulas can be used to get an approximation of the materials response to a load (The strain must be low, typically 1% or less), material stiffness is also depends on the amount of time under load.
Viscoelastic material will have a higher modulus, there for it will be stiffer when a load is applied for a short amount time versus a longer period of time. This behavior is better known as creep (a load applied in which a minor deflection is
caused in a few minutes will increase its deflection if left for several days) modulus is also temperature dependent. Plastic materials typically soften when heated and stiffen if cooled, Dynamic Curves are the elastic response of materials (Stiffness) to a short duration of force at various temperatures. Use Creep data to aid in predicting a materials behavior under continuous load for long period of time.
2. How to use the Dynamic Modulus Curves? Here is one of our cases: Application
required components that are exposed to 120 - 145°F, components are static, under load and will remain sealed and in a dry environment. Static and dry eliminate the need for a highly chemical and wear resistant plastic material choice. So main criteria and focus is heat ( Thermal) First initial thoughts were Acetal, Nylon and PET-P, stiffness (module) at room temperature are similar. All of these choices have a heat deflection temperature well above our requirement.
Which one is best and which one would you choose?
Again heat deflection temperature tells you how hot the material has to get before its stiffness drops to a certain value. Reviewing material comparison charts we know Nylon at 200°F is as stiff as Acetal at 220°F and as stiff as PET- P at 240°F. At these temperatures the material choices all have a modulus of about 148,000 psi, what we didn't know is: do these materials retain their room temperature stiffness then suddenly soften at the heat deflection temperature, or gradually soften as temperature is increased? For this we needed to study a Dynamic Modulus Curves of each of the material choices. We used a base temperature of 160°F which is above the application requirement and observed the dynamic modulus of Acetal is 386,000 psi, Nylon is 391,000 psi and PET-P is 471,000 psi. At the application temperature PET-P is over 20% stiffer than either Acetal and Nylon, since it was important to limit deflection under load at this temperature. We decided to go with PET-P as a better choice. Do you agree?
Agencies such as the Food and Drug Administration (FDA), U.S. Department of Agriculture (USDA), Underwriters Laboratory (UL), 3A-Dairy Association and American Bureau of Shipping (ABS) commonly approve or set specific guidelines for material usage within their respective industries.
Step 5. Determine the Material's Cost-Effective Shape of the component, like Castings, Compression Molding, Extrusion and Injection Molding:
Investigate all of the possible shapes for your component, a reduction in fabrication costs can be obtained with the most economical shape.
Note: From process to process, many material choices remain the same. Their are however physical property differences based upon the processing technique used to create the shape.
1. Compression Molded products are isotropic and exhibit equal properties in all directions.
2. Extruded products exhibit slight anisotropic behavior.
3. Injection Molded products exhibit the greatest anisotropic properties and are
directionally dependent.
Choose the Casting Process: Large Stock Shapes, Near Net Shapes, Rod, Plate, Tubular Bar, Custom Cast Parts.
Choose the Compression Molded Process: Various Shapes in Advanced Engineered Plastic Materials, Rod, Disc, Plate, Tubular Bar.
Choose the Extruded Process: Long Lengths, Small Diameters, Rod, Plate, Tubular Bar and Bushing Stock.
Choose the Injection Molded Process: Small Shapes in Advanced Engineered Plastic Materials, Rod, Disc, Tubular Bar.
Step 6. Determine the Machinability of the Material: This is also a material selection criteria, some materials should be stress relieved to enhance machinability.
Typically glass and carbon reinforced plastic grades are considerably more abrasive on tooling and the moving parts of the machinery that cut them. These grades are also more Notch Sensitive during machining than unfilled grades. Reinforced plastic grades are typically more stable during machining.
Imidized materials like PAI, PBI, Celazole and Torlon can be challenging to fabricate due to their extreme hardness. Carbide and Polycrystalline Diamond tools are typically used during machining of these plastic materials. There are Machinability Charts to aid in the assessing and rating for each plastic material, this is typically found on a Material Property Comparison Chart.
Step 7. If required, request the material's Certifications when you order. Be sure you are not purchasing an inferior plastic material, state this requirement before placing an order. This is not always possible to get after a purchase is made and received.
All trademarks and service marks are property of their respective manufactures. All statements, technical information, and recommendations contained herein are presented based upon tests believed to be reliable and practical field experience. The reader is cautioned, that Diversified Deigns cannot guarantee accuracy or completeness of this information. It is the customer's or user's responsibility to determine the suitability of specific products in any given application.
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