Designing machined plastics components follow the approach used for metals
but differ and require special considerations.
When machining Quadrant stock shapes remember, the importance of adjusting
your machining methods to whatever plastics or polymer materials you are
working with requires special considerations that include:
Elastic Behavior
The stress/strain behavior of a plastics differs from that of a metal
in several respects.
. The yield stress is lower
. The yield strain is higher
. The slope of the stress/strain curve may not be constant below the yield
point
The modulus as determined using standard tests is generally reported as
the ratio of stress to strain at the origin of loading up to 0.2% strain.
The effects of time, temperature and strain rate generally require consideration
due to the viscoelasticity of plastics. Strains below 1% remain within
the elastic limits of most engineering plastics and therefore allow analysis
based upon the assumption the material is linearly elastic, homogeneous,
and isotropic. Another common practice is to design components so that
the maximum working stress is 25% of the material’s strength. This also
minimizes plastics’ time-dependent stress/strain behavior.
Impact Strength
Although a number of plastics are well suited for high impact applications,
most parts made from rigid engineering plastics require minor design modifications.
Sharp interior corners, thread roots and grooves should be broadly radiused
(0.040” min.) to minimize the notch sensitivity of these materials. The
relative notch sensitivity or impact resistance of plastics is commonly
reported using Izod impact strength. Materials with higher Izod impact
strengths are more impact resistant.
Thermal Properties
Two important thermal properties for designing plastic components are:
Continuous Service Temperature - the temperature above which significant
and permanent degradation of the plastics occurs with long exposure.
Heat Deflection Temperature - the softening temperature of a plastic as
defined by the ASTM test method (D 648). It is commonly referred to as
the maximum service temperature for a highly stressed, unconstrained component.
Note: The strength and stiffness of plastics can be significantly affected
by relatively small changes in temperature. Dynamic Modulus Analysis (DMA)
curves can be used to predict the effects of temperature change on a given
material.
Dimensional Stability
Plastics expand and contract 10 times more than many metals. A material’s
dimensional stability is affected by temperature, moisture absorption
and load. Assemblies, press fits, adhesive joints and machined tolerances
must reflect these differences. Certain plastics such as nylons are hygroscopic
– absorbing up to 8% water (by weight, when submerged). This can result
in a dimensional change of up to 3%. Plastics’ inherently lower modulus
of elasticity can also contribute to dimensional change including part
distortion during and after machining.
As a rule of thumb ... keep cool when machining plastics, most importantly
if we are milling. Even plastics want to be cool so two coolants that
suggest using are Trim 9106 (Master Chemical Corp) and Polycut (Tullco)
to minimize localized part heat-up and prolong tool life.
Quadrant Engineering Plastic Products, a global leader in engineering
plastics for machining, provides an introduction to common physical properties
and terms used to characterize materials and design plastic components.
Please visit their website for more information. Physical property values
for specific materials can be found in their Products and Applications
Guide (LIT.Quadrant).
Thermoplastics and Thermosets
Plastics are commonly described as being either a thermoplastic (meltable)
or a thermoset (non meltable). Thermoset materials such as phenolic and
epoxy were developed as early as 1900 and were some of the earliest “high
volume” plastics. Both thermoplastic and thermoset stock shapes are available
for machined parts, although thermoplastic stock shapes are much more
commonly used today. Their ease of fabrication, selflubricating characteristics,
and broad size and shape availability make thermoplastics ideal for bearing
and wear parts as well as structural components.
All statements, technical information and recommendations contained in
this release are presented in good faith, based upon tests believed to
be reliable and practical field experience. The reader is cautioned, however,
that Quadrant Engineering Plastic Products does not guarantee the accuracy
or completeness of this information and it is the customer’s responsibility
to determine the suitability of Quadrant’s products in any given application.
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