Other Effects

Plastic materials have many other properties that, while they do not appear in the calculations, can influence analysis because of their effect on stress and strain behavior. Some will also affect the dimensional stability of the parts.

Additives are chemicals added to enhance certain functional or processing capabilities of a plastic. Because additives may adversely affect mechanical properties, they can affect

Table 6.3 Published Coefficients of Friction

Material m Source Notes

Polyetherimide PEI 0.20–0.25 A *

Polycarbonate PC 0.25–0.30 A *

Acetal 0.20–0.35 A *

Nylon 6 0.17–0.26 A *

Polybutylene terephthalate PBT 0.35–0.40 A *

Polycarbonate=Polyethylene terephthalate PC=PET 0.40–0.50 A *

Acrylonitrile-butadiene-styrene ABS 0.50–0.60 A *

Polyethylene terephthalate PET 0.18–0.25 A *

Polytetrafluoroethylene PTFE 0.12–0.22 B **

Polyethylene PE rigid 0.20–0.25 (2.0) B **

Polypropylene PP 0.25–0.30 (1.5) B **

Polyaxymethelene; Polyformaldehyde POM 0.20–0.35 (1.5) B **

Polyamide PA 0.30–0.40 (1.5) B **

Polybutylene terephthalate PBT 0.35–0.40 B **

Polystyrene PS 0.40–0.50 (1.2) B **

Styrene acrylonitrile SAN 0.45–0.55 B **

Polycarbonate PC 0.45–0.55 (1.2) B **

Polymethyl methacrylate PMMA 0.50–0.60 (1.2) B **

Acrylonitrile-butadiene-styrene ABS 0.50–0.65 (1.2) B ** Polyethylene PE flexible 0.55–0.60 (1.2) B **

Polyvinyl chloride PVC 0.55–0.60 (1.0) B **

Slider specimen vs. Plate specimen At 10.6 mm=sec. T ***

Polypropylene (as molded) vs. Polypropylene (as molded) 0.71 T ***

Nylon (as molded) vs. Nylon (as molded) 0.65 T ***

Polypropylene (abraded) vs. Polypropylene (abraded) 0.27 T ***

Nylon (machined) vs. Nylon (machined) 0.47 T ***

Mild steel vs. Polypropylene (abraded) 0.31 T ***

Mild steel vs. Nylon (machined) 0.30 T ***

Polypropylene (abraded) vs. Mild steel 0.38 T ***

Nylon (machined) vs. Mild steel 0.40 T ***

A—Modulus Snap-Fit Design Manual, Allied Signal Plastics, 1997.

B—Snap-fit Joints for Plastics a Design Guide, Polymers Division, Bayer Corporation, 1998. T—Plastic Process Engineering, James L. Throne, Marcel Dekker, Inc., 1979.

* The values are for the given material tested against itself.

** Values are for the material tested against steel. Friction between different plastics will be equal to or slightly lower than these values. Friction between the same materials will generally be higher; a multiplier is shown in parenthesis if it is known.

snap-fit feature performance. Examples of additives include impact modifiers, UV stabilizers, coloring agents, and flame-retardants.

Plastics will exhibit accelerated aging at elevated temperatures. All plastics will experience degradation of mechanical properties at elevated temperatures over the long term. A comparison of thermal stability values will indicate the severity of the degradation. Sometimes stress-strain curves are generated to show performance at elevated temperatures. Creep is a relatively long-term increase in strain (i.e., deflection) under a sustained load. The rate of creep for a material depends on the applied stress, temperature, and time. Stress- strain curves showing the effects of long-term creep are required for long-term performance analysis. From these curves, a creep modulus can be determined and used in the calculations. Plastic properties are sensitive to temperature effects. In general, materials become softer and more ductile and the modulus decreases with increasing temperature. The deflection temperature under load (DTUL), also called the heat deflection temperature or HDT, is a single point measurement that may be useful for quality control or for initial screening of materials for short-term heat resistance. However, the DTUL value should not be used as design data.

Fatigue endurance. For applications subjected to cyclic loads, SN curves can be generated. Cyclic loading, particularly reversing loads, can significantly reduce the life of a plastic part. Notch sensitivity is the ease with which a crack propagates through a material from a notch, initial crack, or a corner. A stress concentration factor related to the effect of sharp corners on local stress should be included in all calculations.

Chemical and ultra-violet effects may degrade mechanical properties. In general, as tem- perature and=or stress level increases, the plastic’s resistance to these other effects will decrease.

Mold design and part processing can affect feature performance. Thick sections and improper cooling can cause voids or internal stresses. Mold flow patterns, knit lines, and placement of gates can adversely affect feature strength. Identical features in different areas of a part may have different strength and strain capabilities.

Plastic behavior is rate dependent. This means it is affected by the speed of the applied load. Stress-strain tests are conducted at a standard speed and may not represent actual load rate in an application. For a given plastic, a high load rate will typically result in behavior similar to that at a low temperature: more rigid and brittle. A slow load rate results in behaviors similar to high temperature behavior (more ductile and flexible), Fig. 6.9.

Figure 6.9 Effects of temperature and strain rate on stress-strain behavior (courtesy of Ticona LLC, Designing With Plastic—the Fundamentals)

The amount of recycled content or regrind as well as the effectiveness of the material mixing process (for uniformity) prior to molding can affect mechanical properties and part-to-part consistency.

Stress relaxation is a relatively long-term decrease in stress under a constant strain. (Creep involves constant stress; stress relaxation involves constant strain.) Data similar to creep data can be generated and a relaxation modulus determined, but relaxation data are not as available as creep data. The creep modulus can be used as an approximation of the relaxation modulus. Toughness is the ability to absorb mechanical energy (impact) through elastic or plastic deformation without fracturing. Material toughness is measured by the area under the stress- strain curve. Tests for impact resistance under specific conditions include the Izod and Charpy tests of notched specimens, the tensile impact test, and the falling dart impact test. Water absorption. Some plastics, nylons for example, are very susceptible to moisture and humidity levels. Moisture content can affect mechanical properties as well as dimensional stability. Materials with low water absorption have better dimensional stability. Mechanical properties are often given at two humidity conditions: Dry as molded (DAM) and 50% relative humidity. Moisture content can affect mechanical properties (especially stiffness), electrical conductivity, and dimensional stability. Nylon is particularly susceptible, use impact modified nylon to minimize moisture sensitivity.

Coefficient of Linear Thermal Expansion (CLTE) is a measure of the material’s linear dimensional change under temperature changes. The lower the CLTE, the greater the dimensional stability. The mating and base parts should have similar values of CLTE if possible. Careful consideration of constraint and compliance during feature selection will minimize the effects of CLTE differentials. Table 6.4 shows CLTE values for some plastics and, for comparison, some common metals.

Use CLTE to estimate compliance requirements in the interface, particularly when parts are large or differences between the expansion rates of the joined materials are significant. When these conditions exist, it is also more important to avoid over-constraint due to opposing features in the interface.

Mold shrinkage. Percentage of part shrinkage as it cools from the actual mold shape will affect final dimensions. In general, amorphous plastics have lower shrinkage than crystalline and glass-filled are lower than unfilled (neat) plastics. An excellent source of tolerance data for a wide variety of polymers is reference [5].