Material Selection

Getting the optimum balance of performance, quality and cost requires a careful combination of material and plastics part design Once end use requirements have been specified, the designer can begin searching for plastic materials that are suitable for the application

Material Selection Material selection/screening process is accomplished by comparing material properties with a property profile derived from the end use requirements Best to select 3-6 materials during the initial process based on the property profile and costs

Material Selection Material Selection should be based primarily on the functional requirements of the parts –Those items without which the part will not work Then go to design considerations –Those items involving manufacturing factors without which the part cannot be made profitably

Material Selection Functional requirements are physical properties –The ability to operate over a wide range of temperatures –Resistance to chemicals and environments –Any other condition that can be described by testable parameters Some functional are more important than others –These should be given a top priority

Material Selection Design considerations are the –Raw material costs –Processing cost –Tooling costs –Recyclability –Familiarity of material –Acceptance of material in the market place

Material Selection Final choice of material almost always involves compromises between functional requirements and design considerations

Property Profile

Step 1 Develop property profile The polymer required is classified and ranked according to the following representative grouping of functional requirements The list can be expanded or filtered to fit particular needs

Step 1 Rigid Flexible Opaque Translucent Transparent High strength to weight Ordinary strength to weight High Temperature resistance Sterlizable Ductile at low temperatures Resistant to chemicals Dielectric constant Arc resistance Nonflammable Slow burning Very low density Ordinary density Wear resistance Friction coefficient Acceptable for food Post-consumer recyclable Weather resistant

Step 2 It is now possible to refer material property data sheets and databases to arrive at a short list of candidates that match with the property profile developed in Step 1 If similar application exist, begin with the current materials

Step 3 Material properties are used to select likely candidates with corresponding property value or ranking listed Best to begin with properties that cannot be enhance by design –Coefficient of thermal expansion –Transparency –Chemical resistance –Softening temperature

Step 3 Then go to mechanical properties, which can generally be enhanced by design –Wall thickness –Ribs –Reinforcements

Step 4 The next step in the elimination process is to consider items such as –Creep modulus –Dielectric strength –Mold shrinkage –Costs The designer may want to consider statistically designed experiments to narrow the list of candidates

Material Selection Example There are a wide range of properties within polymer families and between families The problem here is to select a injection moldable polymer based on mechanical properties as a first priority The property profile is developed first

Material Selection Example Property Profile

Material Selection Example

In the preceding table all the resins checked yes meet the preliminary specification The next step is to consider secondary matters such as creep, dielectric strength, shrinkage and cost.

Material Selection Example

From the data in the preceding table, the higher cost resins can be eliminated first The filled resins can be eliminated next because they are more difficult to process This leaves 6/6 nylon and PPO The HDTUL of nylon is must lower than PPO The shrinkage values for PPO are much less than nylon PPO is the best choice