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Predicting part strength
The success of failure of the plastics part design is often determined by how accurately
the part strength (stiffness) can be predicted. The types of strength correspond to the
load and restraint conditions to which the part is subjected, such as tensile, compressive,
torsional, flexural, and shear. The strength of a plastics part will depend on the material,
the geometry of the part, constraint conditions on the part, and the residual stresses and
orientations that result from the molding process.
Loading/operating conditions
The strength values that must be used for designing viable, long-lived plastics parts depend on the
nature of the expected load:
l   Short-term loading
l   Long-term loading
l   Repeated loading
l   Enhance heat dissipation
l   Loading at extreme temperatures
Relevant material properties associated with the various loading conditions are discussed in Material
properties for part design.

Short-term loading
Short-term loads are those imposed during handling and assembly, and during usage where the load
is applied occasionally with short durations. The following suggestions apply to parts that will be
subject to short-term loading conditions.
Use proportional limit in stress-strain curve
Designers should consider the stress-strain behavior of the plastic material when designing parts for
bearing short-term loads. The proportional limit should be used as the maximum allowable stress in
the design calculations to avoid permanent deformation of the part and possible loss of function.
Use stiffeners and fiber reinforcements
Stiffeners, such as ribs and gussets, are often used to increase the part strength. Fiber
reinforcements, oriented in a favorable direction, can also increase the part strength. You should
consider using ribs for parts with large spans. Increasing the rib height and/or decreasing the spacing
(span) between the ribs also improves part strength.
Long-term loading
Long-term loading occurs when parts are placed under high external loads, within the proportionallimit, for extended periods of time. This term also refers to parts that must withstand high internal or
residual stresses that result from either the molding process or from the following assembly
processes:
l   Press-fit and snap-fit assemblies
l   Tapered fit between plastic and metal components
l   Over-stressed joints between mating parts
l   Thread-forming screws
l   Counter-bored screw heads
The design rules given below apply to parts that will be subject to long-term loading conditions.
Use Creep modulus
Creep modulus should be used in the design calculations to avoid stress-cracking failure, to
maintain the tightness of joints, and to maintain part functionality.
Designing for press-fit and snap-fit assemblies
For Press-fit joints and Snap-fit joints, design snap-fit and press-fit components so that the strain is
reduced to the as-molded dimensions after assembly.
Using fasteners
There are several design alternatives you can use for incorporating fasteners into a plastics part.
These strategies a discussed in Fasteners.

Design features to avoid over-tightening
Plastic-to-plastic surfaces should be designed to limit the distance that the joint can be closed.
Providing stop surfaces can prevent a screw from being over-tightened beyond the design intent or
limit the depth of engagement of two matching taper surfaces.
Repeated loading
When parts are subject to conditions of repeated loading, you need to consider the number of loads
that part will be expected to withstand over its life span. The table below gives examples of types of
repeated loads. The corresponding numbers are the expected number of times the loading may
occur.

Type of load                                                                                  Number of loads
Repeated assembly and disassembly                                           Less than 1,000
Gear teeth with rapidly repeated loading of each tooth            Greater than 10,000
Spring components                                                                         Greater than 10,000

Read through the following suggestions if the part you’re designing will need to withstand repeated
loadings, like the ones given above.

High velocity and impact loading
High velocity loading refers to velocities greater than one meter per second, while impact loading
refers to velocities greater than 50 meters per second. Avoid high velocity and impact loading on
areas that are highly stressed from residual and/or assembly stresses. When designing a part that
must withstand these types of loading conditions, keep the following suggestions in mind.
Use proportional limit
Use the Proportional limit in the design calculation for the expected loading rate range.
Avoid stress concentration
To avoid stress concentration, use a smooth, generous radius in areas like corners where the width
and thickness change.
Avoid material degradation
High melt temperatures over a prolonged period of time can cause the resin to become brittle. The
amount of time the resin is at high temperatures should be minimized by selecting a proper melt
temperature and by sizing a proper injection barrel to fit the job.

Loading at extreme temperatures
Storage, shipping, and usage temperatures can easily exceed or go below the normal room
temperature range of 20º to 30ºC. Following are examples of conditions under which a part will
need to withstand temperatures above or below the ambient room temperature.

Above room temperature
Plastics parts stored or operated in these conditions will need to accommodate very high
temperatures:
l   Hot liquid containers
l   Hot water plumbing components
l   Devices containing heating elements
l   Shipped in vehicles sitting in direct sunlight
l   Stored in buildings without air conditioning
Below room temperature
Plastics parts stored or operated in these conditions will need to accommodate very low
temperatures:.
l   Refrigeration components
l   Shipped in the hold of an airplane
Designing for extreme temperatures
You’ll need to design parts to accommodate the changes in temperature they’ll be exposed to. The
following suggestions should help.
Use the proportional limit
Use the proportional limit for the expected exposure temperature in design calculations to avoid
permanent distortion of the part.
Allow differential expansion and contraction
Do not rigidly fasten materials with large differences in coefficient of thermal expansion. Use
fastening methods that allow for the greater expansion and contraction of the plastics parts. Design
for assembly gives recommendations for designing this type of plastic part. Alternatives include
slots that allow the free end to expand on one axis while maintaining the location in the other two
axes.