POLYMERS
This section is intended to provide a cursory understanding of polymers from the mechanical engineering perspective. We will remain rooted in practical engineering fundamentals without going too in depth. While scientists may be concerned with the chemical process of polymerization, in most cases, this is of little interest to the mechanical engineer looking to utilize polymers in their design. The focus of the engineer is to take what science has discovered and use it to undertake projects, product developments and creations that never existed for the overall benefit of human kind or improvement in the quality of life.
As a Maker, the below represents some key fundamentals that should be understood about polymers:
PERFORMANCE
The Maker Engineer is primarily concerned about a polymers performance. Polymer performance can be split into five major categories which are: thermal, mechanical, chemical, flammability, and wear resistance. When the Maker Engineer is considering a polymer material, the dominant consideration is temperature stability. Temperature stability of several polymers is well represented in what is called the "polymer performance pyramid". Below is an example of a typical polymer performance pyramid. Polymers at the bottom have the lowest temperature stability while polymers at the top have the highest. Polymers grouped on the left side are amorphous polymers, with semi-crystalline polymers on the right. While not a classification of properties, it can be generally assumed polymers higher up on the pyramid have a higher cost per pound.
Commodity Polymers
Commodity polymers have continuous service temperatures at or below 212 F (100 C). They represent the base of the polymer pyramid and are produced in high volumes around the world. They have the lowest cost per pound, but also have the lowest temperature stability and relatively weak mechanical properties. For this reason, these polymers are limited to use for general purpose, low temperature applications with low mechanical property requirements. Applications include packaging, bottling, and price competitive consumer products and durable goods.
Engineering Polymers
Engineering Polymers have a continuous service temperature approximately between 212 - 300 F (100 - 149 C) and represent a large portion of products that need a bit more thermal and mechanical performance then commodity polymers can offer. These include polymer work horses such as Nylon, Poly Carbonate (PC), Acetal (POM), and PVC. Nylon and PVC are by far the most used of the engineering grade polymers and is suitable for many applications particularly when fiber / particle reinforcements are used (more on that later).
High Performance Polymers
High Performance Polymers have a continuous service temperature of 302 - 482 F (150 - 250 C) and include materials such as PTFE (Teflon), Polysulfone (PSU), Polyethersulfone (PES), PPS, PPSU, and PPA (High Temp Nylon). When is comes to annual production, high performance polymers represent a small percentage of the market and are relatively expensive. Of the high performance polymers, PTFE is the most widely used, but has the disadvantage of not being melt processable. However, due to it's very low friction coefficient PTFE is widely used. It is also typical to blend 5-15% PTFE with a base polymer to reduce friction and wear. In a more recent trend, High Temp Nylon (PPA) is seeing increased use in the automotive industry in under the hood applications replacing what would have traditionally been made of cast aluminum to reduce vehicle weight and improve fuel economy. High performance polymers are used in demanding engineering applications and industries including Medical, Oil & Gas, Automotive, Chemical, Aerospace and Nuclear industries.
Ultra-High Performance Polymers
Ultra-High Performance Polymers have a continuous service temperature greater then 482 F (250 C). These include materials such as PEEK, PEK, PI, PAI, and PBI. PBI is currently the highest performing polymer having a heat deflection temperature of 815 F and a compressive yield strength of 57 ksi. Ultra-Performance polymers are very expensive but still see use in Aerospace, Military, Medical and Oil & Gas applications. In recent years, PEEK is rapidly becoming the "go-to" ultra-high performance polymer as it offers excellent properties in nearly all performance categories.
TEMPERATURE PERFORMANCE
The Maker Engineer should consider the temperature requirement of there design as the first property to narrow down optimal material selection. The Maker Engineer should be concerned with the Heat Deflection Temperature, the Glass Transition Temperature and the Melting Temperature.
Heat Deflection Temperature
The Heat Deflection Temperature is the point where deflection begins to occur in the polymer at relatively low stress (264 psi). Conversely, it can be considered the temperature at which the polymer can no longer bear load. The test method is standardized under ASTM D648. For the Maker Engineer, it is best to select a polymer with a heat deflection temperature higher then the operating / design point.
Glass Transition Temperature
The Glass Transition temperature (Tg) is the approximate temperature at which the amorphous portion of the polymer changes from a solid (glass) state to more of a rubbery and viscous state. At low temperatures, the amorphous region is in a glass state and the polymers is generally hard, and rigid. If the polymer is heated to the glass transition temperature is will enter a rubbery state and will be soft and flexible. In a rubbery state, the mechanical properties begin to degrade rapidly. The Maker Engineer should select materials such that the operating temperature of their design is below the glass transition temperature by a good margin. Fortunately, for the Maker Engineer empirical data for each type of polymer material is readily available. Below is an example data showing the glass transition temperature of PEEK with a glass transition temperature around 289 F where a steep drop in strength occurs.
Melting Temperature
The melting temperature is the point at which the polymer has completely transitioned to the liquid state. For the Maker Engineer this is minimum temperature at which the material can be processed as a thermoplastic for manufacturing. Whether processing a polymer for injection molding or fused filament fabrication the melting temperature must be exceeded to be successful.
MECHANICAL PERFORMANCE
Next, the Maker Engineer should consider the MECHANICAL PERFORMANCE requirement of the polymer. There are many mechanical properties to consider but the major ones tend to be:
Tensile Strength or Ultimate Tensile Strength
Tensile strength is the maximum stress a material can withstand in tension (pulled) before fracture. The tensile strength of a material is found experimentally by conducting a tensile test and recording engineering stress vs strain. The highest point on the stress strain curve is the tensile strength and is measured in force per unit area (psi or Mpa). The tensile strength of a material is an intensive property and does not vary with material size. However it generally is dependent on outside factors including surface defects, temperature, and environment. For this reason, tensile test methods are standardized under ASTM D638 for polymer materials. Below is an example engineering stress vs strain curve for various unfilled PEEK grades.
Yield Strength
The yield strength is the stress at which the material begins to permanently deform and will no longer return to its original shape once the stress is removed. This permanent deformity is non-reversible and is called plastic deformation. When the stress of a component exceeds the yield strength is has failed. The yield strength is often used as the maximum allowable static load limit of a mechanical component.
Modulous of Elasticity or Young's Modulous
The modulus of elasticity is a measure of the materials stiffness. Graphically, it is the slope of the engineering stress strain curve in the elastic region.
Specific Stiffness (Stiffness to Weight Ratio)
The Specific Stiffness is the materials elastic modulous divided by the materials density. Increasing specific stiffness equates to higher performance. Applications were total weight is a major consideration see wide use of high specific modulous materials. If the Maker Engineer is designing a drone for example, materials with higher specific stiffness will lead to superior in flight performance and maneuverability.
