Industrial Design Blog

Accounting for long term Structural Integrity in Rotationally Molded Parts

Posted by Michael Paloian on 25 June 2010 | 0 Comments

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Designers are always challenged by the limited physical properties of polyethylene resins when designing rotationally molded parts. The chemical structure of polyethylene is one of the reasons it behaves the way it does. Polyethylene’s structure is comprised of long linear chains of repeating carbon and hydrogen atoms that resemble strings of pearls.

These long molecular strings tend to slip over each other with relative ease when compared to other engineering resins which have bulkier chemical structures. This stringy composition of molecules accounts for the resin’s relatively low heat distortion temperature, tensile strength and tensile modulus. When these properties are examined together over an extended period of time the resulting change is referred to as creep. Creep is the physical distortion and change in tensile modulus over time. The rate of change is exponential, occurring extremely fast at the beginning and gradually decreasing with time. The long time required to measure creep is the reason polyethylene resin manufacturers don’t offer it in their properties data sheets. Testing is expensive and time consuming. Despite the lack of published data, creep cannot be ignored during the design process.  

 

 

By now you may be asking yourself, “How does creep affect product design?”. Let’s cite a few examples where creep can have a major affect on a product performance resulting in long term failure. The first example is fastening two parts together with a bolt. When a bolt is tightened to a specific torque, a compressive load is transferred to the plastic under the head. The plastic is thus subjected to stresses at a specific temperature for an indefinite period of time. Now let’s assume the part was assembled at room temperature (20 degrees C, 68 degrees F) and later subjected to a continuous operating temperature of 45 degrees C ( 113 degrees F). The modulus at the elevated temperature will be lower and the plastic will tend to cold flow until it reaches a point of equilibrium. As the material flows, the force under the bolt head will decrease, lowering torque and causing the assembly to loosen. If the application is critical, parts could eventually disassemble and fail. Unfortunately these problems are not encountered until many units are sold. 

Let’s now consider a pallet which must stack one upon another under a static load in a warehouse. If each pallet is fully loaded, and stacked 8 high, the bottom pallet will experience the total load of all the pallets above it. If the phenomenon of creep is ignored, the bottom pallet could permanently distort over an extended period of time causing premature failure. Factors of temperature and stress within the pallet under these conditions must be accounted for. Again these problems will not be observed immediately and are typically realized after hundreds or thousands of units are sold. 

 

 

How does one predict the long term behavior of a rotationally molded part if the data does not exist? The first method is to perform a finite element analysis on the part based on the loads and flexural modulus at the operating temperature. The second is to conduct long term testing. Obviously the former method is less costly, faster and more risky. Typically a combination of both methods would provide the most cost affective and lowest risk method of verification.  

 

In closing, creep is a complex phenomenon which is difficult to predict and often ignored. However, as technological advances in computer modeling and analysis improve; long term performance can be estimated with greater certainty. There is no substitute for good testing however. Every product should be adequately tested under simulated operating conditions to verify long term performance.