A Future with Plastic Parts
A polymer is a compound consisting of long-chain molecules with each molecule made up of repeating units connected together. Polymers can be separated into plastics and rubbers. The accelerating transition from traditional metal and ceramic parts to plastic parts has been on the rise.
The first synthetic plastic was developed in the early 1900s by the Belgian born American chemist L.H. Baekeland. This plastic involved the reaction and polymerization of phenol and formaldehyde to form what is called bakelite. It was not until the 1920s and 30s that polyvinylchloride (1912), polystyrene (1927), and polyethylene (1932) were developed. These are the plastics that many of us are the most familiar with.
Fast forward almost a century and plastics are everywhere. Currently the annual usage of polymers exceeds that of metal. Some of the main reasons for the rise of plastic includes:
Plastic can be formed into intricate part geometries with no further processing required.
Plastic is low density relative to metal and ceramics.
Plastic has good strength- to-weight ratios (certain polymers).
Plastic is highly resistant to corrosion.
Plastic has low electrical and thermal conductivity.
Plastic requires less energy to produce than metals and is cost competitive.
Certain plastics are transparent.
Although plastic parts have all of these great benefits, they also suffer from a number of distinct drawbacks when compared to metals and ceramics. Some of these drawbacks include: plastic has relatively low strength, plastic has a low service temperature, some plastics degrade when exposed to sunlight, and plastic exhibits viscoelastic properties. These drawbacks have manifested in significantly shorter service life for plastic parts and have resulted in significant losses in both residential and commercial settings. With the production of plastic parts on the continual rise and the introduction of 3D printing, plastic failures are our imminent future. Some common failure modes of plastic include:
Plastic Creep – one of the most common failure modes of plastic is creep. One way to conceptualize creep is to think of solid plastic on a molecular level as long strands of spaghetti all tangled together. When a load below the yield strength is applied to plastic for a prolonged period of time, the spaghetti strands have a tendency to untangle. This can result in either excessive deformation of the part or cracking of the part in the area where disentanglement occurred. Proper part design and proper installation are necessary to mitigate creep failures.
Contamination – Occasionally contaminants enter the plastic formulation and can have a negative impact on the properties of the part. This is a manufacturing deficiency. Fourier Transform Infrared Spectroscopy (FTIR) can be used to determine if a contaminant is in the plastic formulation.
Degradation – Plastic parts can be susceptible to degradation when exposed to sunlight or radiation, they can degrade due to oxidation or hydrolysis, and ultimately will degrade over time. This is one of the many reasons why plastic parts typically have a shorter service life than traditional metal or ceramic parts.
Chemical Incompatibility – Some plastics are susceptible to degradation from exposure to various chemicals such as certain cleaning chemicals that can be found around your home. These chemicals react with plastics and damage it’s molecular structure. One reason this can happen is due to human error.
Part Design – Poor part design can result in plastic parts being manufactured with molded-in stress or expected regions of stress concentration. This is commonly observed in plastic parts with sharp transition geometries and is a design deficiency. One example of this can be observed in the Figure 1 which shows a plastic water filter manifold that has failed at an abrupt transition.
With plastic part failures expected to dramatically increase, let us help you determine if subrogation is a potential avenue of recovery for the plastic part failures that you will encounter. Our staff at Caskanette Udall Consulting Engineers specialize in all types of material failures.
