Speed matters—there is pressure to add functionality to products with reduced costs, get products to market faster, and reduce cycle times. In addition to these expectations from customers, injection molders themselves are under extreme internal pressure to be lean and cost-efficient so they can compete effectively in the global marketplace. Therefore it’s tempting at times to rush decisions or make assumptions in order to meet production deadlines or save costs. This, however, can lead to mistakes that result in field failures. These of course bring on rework, recalls, disappointed customers, and higher costs. Many of these situations can usually be avoided by paying special attention to the following design-related failures:
Environmental stress cracking (ESC). This cracking phenomenon occurs when a particular chemical (at a threshold concentration) and a threshold stress are applied to plastic. Different plastic types have their own specific criteria for triggering ESC, which can be very dramatic and instantaneous. Some of these factors are molded-in stress, stress due to press fits, high stress in very sharp corners, or stress that results from how the part is used. Amorphous materials are more susceptible to ESC than crystalline ones.
UV degradation. Ultraviolet light can degrade the physical structure of plastic over time, weakening the electrochemical bonds through photochemical processes. This can also lead to oxidation, which further degrades plastic. Typical UV degradation starts as a surface defect because photons can’t penetrate too deeply into the plastic, creating a chalky surface appearance. Once a crack gets started on the surface, however, it will propagate rapidly into the interior, causing weakness and eventual failure.
Cycle fatigue. This is the failure that typically results from bending plastic back and forth. Consider a living hinge—when properly designed, the plastic molecules lay across the hinge area. In a poor design, the molecules are side-by-side across the joint—the motion is only resisted by the strength of the Van der Waal bonds between the molecules, which are much weaker than the inter-atomic bonds. Another example is high-frequency bending. The hysteretic heating created by the bending can’t be dissipated quickly enough by the thermally “insulating” plastic and heat builds up, eventually reaching a point where the plastic simply fails due to high temperature.
Long-term creep. Creep (and related stress relaxation) occur when plastic molecules are subjected to high sustained loads and slowly start to flow. Thermoplastics are viscoelastic, meaning they are both viscous and elastic. The elastic property allows the plastic to spring back to its original shape after deforming (short-term load), whereas the viscous portion allows the material to flow when a very long-term load is applied. This viscous-related creep can never be recovered.
The key to preventing these kinds of field failures is doing the basic homework up front. What is the real-world application—how and where will the product be used? Will there be long-term UV exposure? What chemicals will it come in contact with and how will they affect it? Typical compounds are household/hospital cleaners, oils, gasoline, automotive fluid, and ammonia.
What type of loads/forces will the part or product experience when it is being used? Will they be constant, impact, or intermittent forces? A good example of a constant load is a plastic part that is bolted to an assembly (without any steel inserts)—over time the part will creep and the original bolt torque will gradually diminish.An intermittent example is a valve body in clothes washer or faucet that must withstand intermittent loads over time. For impact situations, the plastic must be shatterproof (for example, handheld tool housings) in case they are dropped.
The material selected should be the best fit for the application of the part. Crystalline, amorphous, or blended plastics (which can also be modified with specific additives) all have best situations for use, depending on what the part does and the surrounding environmental conditions. Whenchemical resistance is required, crystalline materials are usually selected. For impact resistance, amorphous materials with glass fillers work well.
Sound plastic part design is the final piece to success. The part or product should be designed with even wall thickness whenever possible. Add radii and fillets to eliminate sharp corners; this way the stress that would normally be concentrated in the corners is now distributed between the two adjoining walls/surfaces, minimizing the stress load and reducing the risk of cracking.
Fortunately these problems don’t happen very often in well-established mature product lines. They are more common in new products or designs for which there is no precedent or body of production data. An experienced injection molder who has a deep knowledge of plastic behavior and follows scientific molding principles will be sure to eliminate these potential risks from your design and production processes.