In a high-volume packaging operation, adding just one gram of material to a plastic bottle to prevent buckling can add hundreds of thousands of dollars to the cost. Adding a little more science can help packagers design and manufacture failure-free plastic containers at the lowest possible cost.
We help people who are having problems or trying to avoid problems, Robert States of Stress Engineering Services told the March meeting of the Petroleum Packaging Council. States, a senior associate with the Mason, Ohio, firm, described how to prevent failure of plastics in packaging applications.
Where can something go wrong? The list is long, States said. The goal is to identify potential problems early in the process to eliminate trial and error development. Product failures happen all the time – fortunately, some aspects of failure in plastic parts are unique.
There are three primary causes of structural failure, States continued. A product can be insufficient as designed; defects can be introduced during manufacture; and products can be abused.
Failures are usually predictable, preventable, repeat occurrences and gross errors. But expected failures never happen, and actual failures are never expected.
When you look at the anatomy of a product failure, there are usually multiple causes, and there is usually no single root cause. If you want to identify a single cause, invariably it is human error. Somebody missed something. The result is material damage or degradation, or structural failure.
The challenges, said States, are identifying the errors that are significant and knowing what to look for in the design phase. Material failures must follow a logic ruled by physics, and prior experience is the starting point.
States introduced the material failure logic model, logical expressions based on fundamental material behavior that identify the risks for failure in the product life cycle. For example, if the challenge is leakage from a typical blow molded plastic motor oil bottle, States explained to LubesnGreases in a followup telephone interview, you would look for:
1. Susceptible materials in the material selection phase; this could involve bad selection or unauthorized substitution.
2. In the design phase, look for stress concentrations, forced fit, crevices on surfaces, etc.
3. In manufacturing, potential problems include plastic deformation, inadequate drying, lack of fusion, contamination and material substitution.
4. Finally, once the bottle is in service, it may experience excess heat or cold or wetness or loads that were not considered in the design phase.
A typical outcome of this logical process, said States, could be identification of cracking of the bottle, leading to leakage, caused by residual stresses from the manufacturing process and the chemicals in the bottle attacking the plastic. The logic model might be: Susceptible material + sustained stress + internal pressure = failure risk.
How do plastics fail?
The obvious failures are excessive deformation and distortion, melting or dissolving due to temperature or environment, and brittle fractures or cracking due to impact. But not-so-obvious delayed cracking or static cracking, said States, accounts for a very large percent of service failures.
Loss of or low molecular weight is the single most important problem, he continued. With plastics, a longer molecular chain means higher molecular weight, and the longer the molecular chain, the more carbon atoms, and therefore the better the properties. But if I break the chain, I lose desirable properties. The manufacturing process can change these properties, he added, resulting in delayed cracking, increased brittleness and reduced resistance to stresses. Other mechanisms that can cause loss of molecular weight in the plastic include too-high shear rates during injection molding, too-high mold temperatures, and excessive use of regrind.
Finding the product design sweet spot early is the key to reducing costs, development time and reducing or eliminating failures, States went on. That sweet spot, he said, is a combination of three measurable factors: materials, mechanics and manufacturing.
What is unique to plastics is that the properties are directional, States explained. Unlike working with metals, when you are designing a plastic container, you must know how the material is oriented. With the correct orientation, the bonds between the polymer chains can be strong and resilient. Long chains and high molecular weight result in strong, ductile, tough plastic packages.
Mechanics, Manufacturing and…Me?
When a package fails, said States, the materials expert says its all the materials fault. The manufacturing expert says its not made right. But the mechanical engineer says its not designed right.
To take the guesswork out of package failure analysis, you need to understand first principles of mechanics, the principles that describe the problem, States said. For example, if you have a screw cap on a bottle that doesnt seal well, you need to understand the principles of torque. This drives the design and helps you identify the problem.
Or, he continued, if you have stacked bottles that are buckling, you know that factors in bottle strength include thickness and geometry. Before you start adding material to expand thickness, you can look at tweaking the bottles geometry. When you understand whats driving the buckling, when you understand the math and whats important, you can solve the problem.
Several manufacturing process are used in plastic packaging, including injection molding, blow molding, compression molding, thermoforming, extrusion and heat sealing, States said, and each has strengths and weaknesses. Be smart about selecting your plastic package supplier, States cautioned. Everyone has established suppliers, but sometimes theyll supply you what they do, rather than whats right for your problem.
Blow molding is a low-pressure process, widely used for lubricant bottles. You can generally only control the outside geometry, States noted, and material degradation can occur due to the processing parameters. There is a high probability of localized thin spots. With extrusion blow molding, you can hold tolerances of about plus-or-minus 0.5 percent, and the process is much less precise than injection molding.
There are lots of kinds of blow molding, States went on. You have to get the platform right. Is it wheel, shuttle or injection blow molding? Each has its unique attributes. Wheel and shuttle are most common for motor oil bottles, he noted. There is no one answer.
In addition to materials, mechanics and manufacturing, there is an additional critical variable, States added: Man. The people who plan and implement packaging projects commonly have three mandates – get to market fast, reduce capital expenditures and avoid all glitches. Its typical to want to spend as little as possible on product development, but the result is often unsatisfactory. You will pay now or pay more later, said States. As you move from concept to engineering to tooling to production, changes cost more over time.
The key to reducing costs and development time and minimizing failures is to do all the homework up front. The early stages of the project are where you want to find out about the things that you do not know about.
Buckling is one of the most common forms of rigid package failure, States said, and there are many buckling modes for rigid packaging. They can lead to failure in the structural stability of unit loads.
He emphasized, Buckling is a result of many factors. Its driven by the geometry – displacement – and material stiffness – modulus. Not by the stress.
The overall shape of the bottle is very important, States continued. In top-load tests, changing the shape or weight very slightly makes a huge difference on top load. Shape and thickness distribution matter.
Creep buckling is another challenge, particularly for longer-term warehousing of stacked containers. Plastic will keep moving over time, said States. It continues to deform, so a leaning stack can fail. Vacuum or pressure buckling can also be a concern for lube packagers. If you fill at high elevation and go to sea level, you could have a vacuum buckling problem, States said.
The logic model in a buckling case might be: High temperature exposure + head space (the air gap makes a big difference in buckling panels) + blow-molded capped bottle = potential issue. Go back to first principles, said States. If your filling equipment is erratic and out-of-control, fix that first.
Environmental stress cracking is another common form of failure. Its driven by the interaction of the environment, the sustained loads on the package and often by the process by which the package was manufactured. Cracks can form on the inner surface of a bottle that do not penetrate to the outer surface. The logic statement for this cracking might be: Internal pressure + stress concentration + susceptible material = potential issue.
Using the material failure logic model, States concluded, you can shift much downstream learning regarding critical performance-limiting attributes to the concept development phase, eliminating costly trial and error development.
Invariably, he said, several events cause failure. If you can identify one, the failure will usually go away.