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FEA Software Predicts Material Response to Repeated Snap Fits

By: Matthew H. Naitove 22. August 2014

Designing parts with snap fits present a particular challenge because of the repeated cyclic loading and unloading. During such cycles, the viscoelastic nature of thermoplastic materials determines how the bouncing back to “normal” occurs—and how it varies with time over multiple stress-strain cycles.

 

That’s part of the challenge that faced Novo Nordisk of Copenhagen, Denmark, a world leader in insulin self-injection pens. These pens are small, but they are precision instruments with a number of complex parts that must work in perfect coordination. Some pens are durable, containing a replaceable drug cartridge, while disposable ones come pre-filled with a drug. Injection typically involves twisting a short needle onto the pen, turning a dial to the required dose, and pushing a button to deliver the medication under the skin. After a given number of doses is injected, either the cartridge is replaced (for a durable device) or the pen is discarded (if a disposable version). In either case, audible clicks occur at key stages of the procedure, reassuring patients that they are engaging the device correctly at each step. But every one of those reassuring clicks represents a challenge to the design engineers. So do the clicks the patient never hears—those that occur during assembly of the pens in production.

 

To integrity of their designs, Novo Nordisk’s Device Simulation department rely on computer simulation with Abaqus finite-element analysis (FEA) software from Simulia, an application of Dassault Systèmes of France (U.S. headquarters in Providence, R.I.). “Over a decade ago, I and my colleagues explored a number of commercial software codes,” says Torben Strøm Hansen, principal scientist in the Device R&D division of Novo Nordisk. “We chose Abaqus because it was a well-integrated solution that could model the nonlinear behavior of the fine details our designs correctly, including the high number of interfaces in contact.”

CAD model of insulin pen components. Gray and red parts snap fit onto the green part.

 

Besides modeling viscoelastic response to repeated snap fits over time, the designers had to predict the behavior of the materials in different environments, including elevated temperatures, even though the pens are assembled from different materials. And, some materials may contain carbon or glass fibers that show anisotropic behavior, which can be hard to predict. Even just sitting on a pharmacy shelf or in a consumer’s medicine cabinet, the materials are prone to creep and relaxation over time at rates that can vary with temperature.

Injection molded ratchet component from a medical device used by Novo Nordisk for a snap-fit benchmark study.

 

Hansen’s team at Novo Nordisk is now using the “parallel rheological framework” methodology available in Abaqus to model polymers’ nonlinear viscoelasticity with greater accuracy than before. The framework makes use of an arbitrary number of viscoelastic networks and an elastic equilibrium network to create a nonlinear model to predict and track changes in the internal structural networks of a polymer as it responds to repeated cyclic snap-fit loads. “Having material models incorporating time-dependent viscous behavior is very important for our work,” says Hansen. “We’re now able to simulate both creep and relaxation with Abaqus.” Since every type of polymer shows a different response to temperature, load, etc., the team continues to explore ways to identify the characteristics of different polymer networks.

 

Not only are such advanced models useful to designers in fine-tuning the latest insulin pen configuration, the data can help in manufacturing processes at the factory. “We have a process-simulating capability, through Autodesk Moldflow, for which Abaqus has an interface. This allows us to input the stress fields that result from the injection molding process right into our models. As a result, we have greater insight into our manufacturing process and are more able to design parts that have very low levels of residual stress in critical regions.”

Exploring The Processing Of HDPE-Exfoliated Graphene Nanocomposites For Fuel Tanks

By: Lilli Manolis Sherman 21. August 2014

There were three best paper award winners just announced by the organizing committee of the SPE Automotive Composites Conference & Exhibition (ACCE) for its

fourteenth-annual show coming up Sept. 9-11, in Novi, Mich. I will be attending this event and, judging from the roster of papers to be presented, expect to further report on emerging developments, particularly in the thermoplastic composites arena.

 

One of the three winning papers, “Processing Methods of High Density Polyethylene-Exfoliated Graphene Nanoplatelet Nanocomposites for Automotive Fuel Tank Applications, is based on a study jointly conducted by the Composite Materials and Structures Center at Michigan State University and the Hyundai-Kia America Technical Center. Lead author Keith Honaker, a graduate student at Michigan State University and a 2013-2014 SPE ACCEE graduate scholarship award winner, will discuss how HDPE-exfoliated graphene (GnP) nanoplatelet composites were synthesized and tested.

 

In order to further enhance the mechanical and barrier properties of the composite, different processing techniques were explored including microlayer coextrusion and solution mixing with sonication followed by extrusion. The researchers investigated multiple modifications to the nanocomposite constituents including cryo-milling of the HDPE pellets and coating the platelets with a wax or polyolefin elastomer before extrusion.

 

Among the key results: simple melt mixing of HDPE and GnP resulted in an increase in stiffness, a decrease in Izod impact resistance, and a 50% decrease to both oxygen and fuel permeation with 5% wt. GnP. Meanwhile, microlayer coextrusion yielded a high alignement of the nanoplatelets in the direction of the flow and resulted in improved permeation resistance at low GnP concentrations, but did not result in improvement of barrier properties at concentrations above 5% wt.

 

In contrast, cryo-milling the HDPE pellets into a powder resulted in a minor decrease in mechanical properties and a 35% decrease in oxygen permeation. A wax coating on the platelets before melt extrusion resulted in an increase in both Izod impact resistance and barrier properties, but a decrease in flexural modulus. A polyolefin elastomeric coating on the GnP resulted in retaining the flexural modulus properties with only a slight improvement to Izod impact resistance and barrier properties.

 

Overall, the researchers concluded that obtaining outstanding barrier properties without affecting the mechanical properties of HDPE-GnP nanocomposites using melt mixing, extrusion and injection molding processes is very challenging. For this reason, the focus of their current research is on combining a few of these methods to create synergistic effects.

 

Want to find or compare materials data for different resins, grades, or suppliers? Check out Plastic Technology’s Plaspec Global materials database.

 

Polystyrene Recycling Partnership Spurred By Demand For Post-Consumer Content

By: Lilli Manolis Sherman 21. August 2014

Here are at least two companies making significant efforts to promote the recyclability of polystyrene and trying to halt the emergence of bans particularly for EPS foam in the food-service sector.

 

With a focus on the residential recycling stream and spurred by increasing demand for post-consumer content in food-service packaging and related applications, a new facility that will process both rigid polystyrene and EPS foam is being built  by Dart Container, Mason, Mich., and Plastics Recycling, Inc., (PRI) Indianapolis, Ind. The former is a manufacturer of a broad range of quality, single-use products for the food service, retail and food packaging industries and the world’s largest manufacturer of foam cups, while the latter has established itself as one of the nation’s largest and forward-thinking PS recyclers. The partnership is said to be ideal as it combines Dart’s washing technology for reclaimed PS and PRI’s recycling and compounding expertise.

 

Slated to be fully on stream within the first quarter of 2015, the facility will initially have a 25-million lb/yr capacity but is being designed to allow for growth. It will essentially boost PRI’s existing annual 60-million/lb EPS and PS recycling capacity at the Indianapolis site to 85 million/lbs. PRI’s owner Alan Shaw views the largely untapped residential recycling stream as a tremendous source for generating more of this valuable material. The facility is expected by the partners to enhance local governments’ ability to launch recycling efforts to remove rigid PS and EPS from the waste stream and generate revenue.

 

For its part, Dart Container has been strongly advocating the recyclability of PS in the last several months in an effort to convince municipalities across the country to bring the material into recycling steams instead of enacting bans restricting the use of foam in foodservice settings. By the end of 2013, the company had helped lobby legislators in New York who were considering a ban. The Big Apple ultimately decided to wait a year and test out the practicality of bringing foam products into the municipal recycling system. Although the tests are not as yet underway, Dart Container and PRI feel that the expanded operation in Indianapolis will prove a big boon as it will be the destination for recovered PS from New York City if the program gets underway.

 

Want to find or compare materials data for different resins, grades, or suppliers? Check out Plastic Technology’s "Plaspec Global materials database".

 

 

BMW Ups Its Stake in Composites

By: Matthew H. Naitove 20. August 2014

BMW continues its exploration of large composite parts for lightweight automotive structures. It just installed a 3600-metric-ton model of the new v-duo vertical v-duo two-platen injection press from Engel Austria (U.S. office in York, Pa.), a smaller version of which was shown at K 2013 in Dusseldorf (see our January show report). The machine was delivered to BMW’s plant in Landshut, Germany, where it will be used to mold large structural elements from fiber composites using the HP-RTM (high-pressure RTM) process. This process injects liquid reactive components for thermosets (epoxy or polyurethane) or thermoplastics (caprolactam for in-situ polymerization into nylon 6) over a layup of dry reinforcing mats or fabrics. BMW plans to use the Engel press for thermoset composites, but would not specify the resin system.

 

The press has two slide tables to remove finished parts and insert fiber reinforcements. Engel notes that the machine has a much smaller weight, footprint, and height than typical vertical compression presses. The clamp unit is accessible from all four sides instead of just two. The press uses ecodrive servo-driven hydraulic pumps for energy efficiency, without any hydraulic accumulators.

‘There are no unskilled jobs here anymore’

By: Tony Deligio 20. August 2014

Completed in conjunction with the Manufacturing Institute, the study, which was conducted between August 2013 and January 2014, surveyed more than 300 manufacturing executives hailing from companies with an average annual revenue of $100 million.

 

Cervinka’s quote highlights the predicament those manufacturers currently find themselves in. The jobs they need to fill are becoming more and more complex while the pool of people who can do them becomes more and more shallow.

 

The survey found that 80% of manufacturers reported a moderate to severe shortage in finding highly skilled workers, while at the same time, skilled and highly skilled roles make up 80% of their workforce.

 

As these companies face a labor shortfall, many are simultaneously attempting to boost production. According to the report, more than 50% of companies reported plans to increase U.S. based production by at least 5% in the next five years. In general, the U.S. manufacturing sector has enjoyed an annual growth rate of 7.7% between 2009 and 2011, reaching its highest level ever at more than $5.4 trillion in 2011, according to the U.S. Bureau of Economic Analysis – Manufacturing Industry Data. Productivity in the U.S. has jumped nearly 20% from 2001 to 2011, increasing is every year save two—2008 and 2009—according to U.S. Bureau of Labor Statistics cited by Accenture.

 

That’s despite the very real, and detrimental, impact of too few skilled workers. According to the report, 62% of respondents saw an increase in production downtime of 5% or more, while 66% reported cycle time increases of 5% or more.

 

Survey participants are actively working to address the gap, however, with almost 10% spending more than $5,000 annually per employee on skills training, with the average respondent spending about $1,000 annually per employee for skills training.

 

The report also identified some best practices companies are deploying to address the skills shortfall.

 

Digital learning experiences—“Embracing digital technologies to offer learning experiences anytime, anywhere.”

 

Combine formal and informal training—“All of the leading companies we interviewed had some form of established relationships with local community colleges or vocational technical programs.”

 

Use a certification approach to skills building—“Not only do employers gain confidence that their employees are able to perform at a given level of skill, employees gain confidence in their own abilities as they acquire new skills certifications.”

 

The bottom line is the gap is actually impacting companies’ bottom lines, with the report finding that earnings were reduced up to 11% annually due to increased production costs and revenue losses due to skills shortages. The problem will only get worse, with the U.S. Department of Labor stating the average age of manufacturing labor was 44.1 years in 2011.

 

In our September issue, Plastics Technology will see how some plastics processors are attempting to bridge the gap. Does your company have a contingency plan?

 

 




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