Specifying plastics is infinitely more complicated than picking the right metal to do a job.
But there’s no question plastics may be worth the bother for the right application because of their ability to dramatically simplify design, reduce weight and offer properties that metals can’t, such as corrosion resistance or lubricity.
But failure to understand the complexities of plastics can lead to poor performance, or worse, catastrophe.
“In metals, safety factors are mathematical and are very predictable,” says Josh McIlvaine, design and development leader at DuPont Automotive in Troy, MI. “Composite materials by nature are made up of multiple components, which adds to the difficulty in predicting long-term properties such as fatigue. That’s particularly troublesome because there isn’t any fatigue limit for engineering thermoplastics. And really what we found is that the only way to validate a component is to do actual testing in the environment at the actual end temperature.”
Add to that the fact engineers are putting plastics into much tougher applications than they were 10 years ago.
“The easy metal-to-plastic applications are long gone,” says Karl Hoppe, senior product development engineer at RTP, a specialty compounder based in Winona, MN. “Now you’re seeing higher loads, higher temperatures and more demanding chemical atmospheres. Plastics are being asked to do more today than they’ve ever been asked to do before.”
Interviews with technical experts at leading engineering resin companies indicate these critical points to consider:
Viscoelasticity. Performance of plastics depends on strain rate, temperature and time, not just stress and strain. Plastics tend to creep at room temperatures, unlike metals.
Special design considerations. Issues such as draft angle, nominal wall thickness, corners and ribs require special attention.
Impact of reinforcements and other additives. Glass-fiber orientation in a real part is likely to be far different than a test part, creating the possibility of far different strength patterns than expected.
Effects of manufacturing. Mold design, as well as the molding process itself can induce stress and other failure points into a finished plastic product.
Field conditions. Make sure you know the chemical atmospheres your products will encounter. In one infamous example, when polycarbonate was first selected for car bumpers, designers failed to consider the effect of spilled gasoline.
Short-term modulus information on data sheets fail to predict strain on a part when stress is applied over a long period, particularly when thermal cycling is involved. That phenomenon is known as creep and one of the effects can be stress relaxation. The result can be part failure. Example: A bolt may fail to hold clamped sections over the life of a part.
“If you don’t use compression limiters, connections can weaken over time,” says DuPont’s McIlvaine. “The best way to deal with that is to have inserts with structural attachment points.” Inserts could be molded in, heat-staked in or pressed in after the molding process.
Ask your materials’ suppliers for isochronous stress versus strain curves for specific materials. The graphs plot percentage of strain versus specific stresses at different periods of time. You can check the apparent modulus for a given point for a given section of a part. Add strength only at the points where needed.
“There are a lot of basic errors I still see in design,” says John Bruner, CAE manager at DSM Engineering Plastics. “The number one and two rules of plastics design are still violated, leading to molding errors or performance problems.” Number one is nominal wall thickness. It’s important to keep wall thickness uniform throughout a part. Heavy sections can cause voids and nonuniform shrinkage. “Materials with improved flow characteristics may help get around some of the problems,” adds Bruner.
Other significant design issues when converting from metals to plastics include sharp corners and overuse of ribbing.
Sharp internal corners are a major reason plastic parts fail because they concentrate stress on a single point. “For metal designs, a radius of 0.005 or 0.010 inch is generous, but for plastics we like to see that radius at 0.020 to 0.030 so that a corner doesn’t act as a stress riser that leads to crack propagation,” says Bob Lamb, technical program leader at Victrex, a UK-based producer of high-performance polymers, such as polyet. This is especially true if that part may see loads applied in cycles.
Ribs are an important way to add strength and rigidity to a molded part. The appropriate use of ribs can cut weight of the part and reduce molding time, as well. Some engineers, however, overuse ribs as a safety factor. “I see an overuse of ribbing,” says DSM’s Bruner. “Parts become more difficult to fill and you end up with very little steel in the mold.” Excessive ribbing can lead to warpage and stress concentration. It’s best to leave any questionable ribs off the drawing, according to the DuPont Design Guide. They can be added after prototype testing if needed.
The Right Orientation
Plastics used to replace metal for structural applications often use glass reinforcements for strength and rigidity.
“The biggest issue I see is kind of like that old saying in real estate: location, location, location. In reinforced plastics, it’s orientation, orientation, orientation,” says Bob Sherman, a senior CEA analyst at RTP. Properties of glass-reinforced plastics depend heavily on how the fibers are oriented in the part. The orientation of fibers are determined by part design, tool design and molding conditions. Performance data from a test bar may be meaningless because the fibres are lined up. Properties in the transverse direction (perpendicular to flow) can be significantly lower.
It’s important to look at the strength of the part where flow lines meet, called the “knit line.” Experts such as K.C. Desai of Solvay Advanced Polymers in Alpharetta, GA recommend collaborative use of mold flow analysis, as well as structural analysis when designing parts with glass reinforcements. “When they take place in a silo manner, the orientation effect can be ignored,” says Desai. “New programs integrate code from the two programs, significantly enhancing their effectiveness.”
Other Issues to Consider with Glass Reinforcement
“One example of a metal mind-set is that a part gets stronger when you add more material,” says Sherman. “That’s not necessarily the case with reinforced plastics. One thing you can do wrong is just to beef the part up. The orientation that produces the higher strength of a glass-reinforced part is produced by the shearing of the material in the filling process. So when you start thickening up a part and make it thick, you lose the reduced orientation you get in that shearing process. As a result, the properties in that cross section can actually go down even though the cross section is bigger.”
There are new (and expensive) grades that provide rigidity without glass reinforcements. One example is PrimoSpire self-reinforced polyphenylene (SRP), a transparent amorphous plastic introduced by . One target is metal implants. “You don’t want to use reinforcements in areas where there are health and safety issues,” says Jamal el-Hibri, a senior research associate at Solvay Advanced Polymers. Electronic parts that require isotropic properties are also a target.
It’s important that plastics are molded under the thermal and other conditions recommended by the resin producer. “You can specify the right material and have the right design, but if the material is not processed properly, you won’t obtain the full material properties,” says Victrex’s Lamb.
The high-heat, advanced thermoplastics often used to replace metals are a good example.
“PEEK has a melt temperature of 649F and first-time processors of PEEK might not use the right mold temperature,” says Lamb. It’s important that cooling of PEEK parts is carefully controlled. “A water-cooled tool would cool too fast and the part would become amorphous PEEK,” says Lamb. Victrex recommends oil-cooling at 350 to 400F.
Tool design issues also have a significant effect on part performance. Location of gates, for example, determines where weld lines will be located. “Some people erroneously think that the more gates you have the better,” says Solvay’s Desai. “When you have more gates you have more weld lines.”
The gate location originally specified by a design engineer often gets changed. “Ninety percent of the time, the gate location will change from the prototype phase to the production process,” says DuPont’s McIlvaine. “This might happen because of cost issues in the prototype process or because of tooling limitations.”
Another broad area to consider for product performance is the chemical atmosphere the part will encounter when in use. Chemicals attack plastics and can diminish their properties. “Occasional splashing can be more aggressive for certain chemicals than complete immersion,” says Solvay’s el-Hibri. It’s important to screen plastics with supplier information and then simulate actual conditions with part testing.
Thirty years ago, this was as simple as testing polycarbonate for stress cracking when exposed to gasoline. Today, plastics are going into extreme applications. “We’re seeing a lot of activity for down-hole applications in the oil and gas industry,” says John Walling, a regional business manager for Victrex. “As the price of oil increases, producers are drilling deeper and deeper.”
PEEK is now used for energy-absorbing springs for seals and backup rings used in the oil and gas industry. Parts are tested extensively in conditions that mimic atmospheres in the field. And for a good reason. A part failure in an offshore well could trigger repairs costing millions of dollars.
A newly developed blend of engineering plastics is replacing steel used to protect toes in safety shoes. The blend had to undergo significant drop testing after exposure to fuel, diluted sodium hydroxide solution and diluted sulfuric acid. The polycarbonate/PBT blend from Bayer MaterialScience also was subjected to a static load of 1.5 metric tons.
“Today we can do things with advanced engineering thermoplastics we didn’t think could have been done even 10 years ago,” says DuPont’s McIlvaine. As pressures grow to light-weight, miniaturize, reduce costs and expand capabilities, engineers will push the boundaries of plastic even further, he adds.
A polycarbonate/PBT blend is replacing steel as toe caps in safety shoes, meeting impact and chemical requirements.
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