Deciding which plastics are right for a particular application is an elaborate, granular decision. The process involves an analysis of the material's mechanical, thermal, and chemical properties, processing ease, cost, and environment. The material is a big decision to the proper performance, durability, and reliability of the component. If you choose a bad material, it can lead to premature failure, increased maintenance, or extensive reworking. This manual gives you a roadmap for a systematic choice of plastic materials for engineers, designers, and material scientists in many areas of engineering (from manufacturing, electronics, and biomedical).
The very first step in material selection is determining exactly what kind of mechanical and physical stresses are put on the material. These are the criteria by which you usually decide on the right material.
The material should resist the mechanical loads and stresses such as tensile, bending, and compressive forces. We have to know the maximum load-bearing capacity, stress distribution, and life expectancy of the material under these conditions.
For the purpose of very precise tolerances, material should have little or no change in dimensions with mechanical stress. This is particularly true with precision components, where a single deviation can be critical for system performance.
Products that will be pushed or pushed against (e.g., bearings) will need to have excellent wear resistance. In these cases, materials such as reinforced nylon (PA) or polytetrafluoroethylene (PTFE) are typically used for better tribology.
Table 1: Mechanical Properties of Common Plastics.
There are some thermal differences with plastics that must be considered when choosing material for applications in which the temperature is sensitive.
Products like polycarbonate (PC), polysulfone (PSU) and polyetheretherketone (PEEK) are strong and rigid at high temperatures, so they are appropriate for environments that have high temperatures.
When dimensionally controlling applications, you need to keep an eye on a material's coefficient of thermal expansion (CTE). High-CTE materials may change substantially in volume and shape as temperature changes, which might lead to poor performance.
Table 2: Thermal Properties of Common Plastics.
This is the resistance of a material against degradation or reaction in a wide range of chemicals — particularly relevant in the pharmaceutical, food, and chemical manufacturing sectors.
Material like PTFE and PEEK is highly resistant to chemical attack and can be used in situations where the material will be exposed to corrosive solvents, acids, or bases.
Some polymers, like nylon and polycarbonate, absorb moisture that can have disastrous effects on their mechanical properties. The materials that have the lowest hygroscopicity or inherent moisture resistance are for humid applications.
The ease of processing becomes critical when deciding on a plastic, because this determines the complexity and cost of production.
Most thermoplastics, like polystyrene (PS) and polyethylene (PE), are suitable for injection molding and extrusion due to the ease of processing.
High-performance plastics like PEEK and polyimide (PI) require special processing machines and high temperatures. These are more difficult to work with, but the mechanical and thermal properties are very good and thus useful in the hard applications.
Another important aspect of material selection is environmental conditions for a material. Plastics that are used outdoors or under extreme conditions require special properties for long-term stability.
Polycarbonate, acrylic, and other materials can be treated with UV treatment for outdoor use and are typically preferred as they don't fade significantly after extended sunlight exposure.
Materials exposed to harsh weather conditions must be resistant to oxidation, cracking, and degradation. Weather-resistant polymers like polypropylene (PP) and high-density polyethylene (HDPE).
Some uses — such as in food, medical, and pharmaceutical applications — call for material that must pass stringent standards.
Certain applications, particularly in the food, medical, and pharmaceutical industries, require materials to meet strict regulatory standards.
In applications where fire resistance is critical, materials must meet UL94 flammability ratings. Depending on the requirements, materials meeting grades from HB to V-0 can be selected to ensure proper flame resistance.
As important as performance and ease of use are, the economics and ecological sustainability cannot be ignored.
Typical plastics like polyethylene and polypropylene offer economy and mechanical strength in general-purpose applications where costs are high.
Against a backdrop of sustainability pressure, biodegradable plastics like polylactic acid (PLA) are also getting more consideration, particularly for packaging use. They are an eco-friendly choice and also useful in some cases.
Selection of the right plastic material for any given task is an unavoidable multifactorial operation that incorporates mechanical properties, thermal behavior, chemical resistance, and environmental factors. A rigorous material selection makes sure all the parameters are considered to support the functional and regulatory specifications of the application. Engineers and material scientists can consider all these things and tweak them to optimize performance, minimize failure, and reduce the cost of production. Further, with a reputable partner like Alfa Chemistry, which stocks a wide variety of high-grade plastic materials, experts can be sure that the material of choice is in sync with their project's technical and commercial requirements.
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