 Some engineering firms specialize in metals. Others “specialize” in everything. The Madison Group pours all its resources and energies into understanding the intricacies and subtleties of plastics. We learn how they interact with designs, materials, manufacturing processes and service environments, and then put that knowledge to work for you.
Our experience shows that plastic failures are typically due to a combination of factors rather than a change in one, unique parameter. Not only do we understand how a small change in temperature, for example, may affect a plastic's strength and stiffness over time, we understand how to analyze and modify one or more elements of the chain of events that occur when producing plastics - from manufacture to the point of failure, whether it's during mold design or during in-service use.
Plastic is a relatively new material that has enriched our lives by giving us lighter-weight vehicles and planes, medical devices that have helped saved lives, or improved everyday life with implants. It is used in construction to make our homes and offices more energy efficient. Our children's toys are nearly exclusively made from it. Plastic is part of our every day life - it would be very difficult to function without it. Plastic offers many advantages over metal - high strength-to-weight ratio, chemical resistance, low cost, and ease of processing. Designing a part with plastic gives the engineer greater flexibility over that of metals.
Similar to metal, plastic parts can and will fail for various reasons. There are many disadvantages to plastics that can lead to unexpected or premature failure if not taken into account. The engineer well conversed in designing with metal is very susceptible to the common failures of plastics.
 The failure of a plastic part can be very much different than that of metal. One of the most common modes of failure has to do with the change of properties of the plastic over time and/or temperature - something that is rarely considered when designing or working with metals. The strength and stiffness of many common plastics change dramatically over a relatively small change in temperature. Some plastics, i.e. polypropylene (PP) and PVC, can become quite brittle and will shatter when exposed to a very cold winter day and put under load. The stiffness of many plastics will change drastically when exposed to hot conditions. Likewise, the amount of load or stress that some plastics can withstand change over time, a phenomenon called creep. Plastics can also fail when combining stress and chemicals, ESC. In the presence of stress, plastics can fail when in contact with chemicals to which they are otherwise resistant. This combination, often overseen, is responsible for about one third of all plastic failures.
Plastic failures typically include one or more of the following:
- Material selection
- Additive selection
- Design problems
- Manufacturing conditions
- Processing equipment
- Breakdown of polymer molecules by processing, UV rays, moisture, contaminants, chemical attack or the environment
- ESC - Environmental Stress Cracking (combination of stress and chemicals)
- Time/Temperature issues (creep)
Quality assurance is important to avoid failure of your plastic part on the field. The quality of the material used for processing as well as the quality of the part after processing must be controlled. Problems can occur when molders pick the material to be used. Molders tend to focus on faster processing times (cycle times) to reduce costs. A material that can be molded faster has a lower molecular weight (higher melt flow rate-MFR), however a material with higher melt flow rate has lower mechanical properties, especially, impact resistant. Impact resistance is also highly dependant on density in materials such as Polyethylene (i.e. HDPE, LDPE). Wide spec materials are also a common source of problems. These materials are less expensive, but at the same time their properties vary greatly from batch to batch. This can be detrimental for some applications causing the final part to fail or function depending on the material batch.
The Madison Group can help you design a quality assurance program to reduce the risks of parts failing due to fluctuations on the material or processing conditions. We are also able to perform design verification analysis and suggest improvements to reduce the likelihood of failure.
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