Design for 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.

• Short-term loading
• Long-term loading
• Repeated loading
• Enhance heat dissipation
• Loading at extreme temperatures

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 proportional limit, 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:

• Press-fit and snap-fit assemblies
• Tapered fit between plastic and metal components
• Over-stressed joints between mating parts
• Thread-forming screws
• Counter-bored screw heads

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.

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

Cyclic loadings  
The cyclical loading application is relatively infrequent and there is a long time between applications. If the loading is cyclical, use the Proportional limit for design calculations.

Repeated loadings  
If the loading is repeated at short intervals and for long periods, use the S-N (stress vs. number of cycles) curves as the design criterion.

Avoid micro cracks  
Smooth surfaces, as produced by highly polished mold surfaces, reduce the tendency for micro cracks to form.

Avoid stress concentration  
To avoid stress concentration, use a smooth, generous radius in areas like corners where the width and thickness changes.

Enhance heat dissipation  
At higher frequency or amplitudes with repeated loads, plastic parts tend to run hotter and fail sooner. Designing with thin walls and fatigue-resistant conductive materials is generally recommended to maximize heat transfer.

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:

• Hot liquid containers
• Hot water plumbing components
• Devices containing heating elements
• Shipped in vehicles sitting in direct sunlight
• Stored in buildings without air conditioning

Below room temperature  

• Refrigeration components
• Shipped in the hold of an airplane

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.