Jul 1st, 2007
Design Focus: Simulation—a primary part of the design process
Be it structural or flow analysis, simulation is becoming an integral tool in development of injection-molded components.
|Gear failure attributed to higher operating temperature than accounted for by the FEA.Redesigned gear (left) has tooth width of 5.4 mm, compared with a tooth width of 4.4 mm (right) in the old design.
Accurate simulation of flow in a runner is important when employing technologies such as MeltFlipper (right simulation). Simulation on left is of a standard runner.
“Integrative simulation” assists in development of a pedestrian-safe, lower-bumper stiffener (shown in blue) that yields to a defined extent.
Abaqus FEA simulation result shows the stress in a plastic component that results from the fitting of a metal insert.
FEA was used to validate EPDM-foamed faucet deckplate design.
Market drivers, including shorter development cycles and stricter product safety requirements, are driving adoption of flow-simulation software as well as finite element analysis (FEA) tools for testing the structural integrity of plastic components.
Says Allen Peng, Moldex3D product manager at simulation software vendor CoreTech System Co. (Taipei, Taiwan), “Processors realize it’s not just a good tool, but an essential one, and we are seeing global market growth for flow-simulation software of 20% per annum, and 30% per annum in Asia where more and more of the world’s design work is being carried out.”
James Yeh, general manager of Greater China for Moldflow Taiwan, Inc. concurs, noting the relative importance of flow simulation in the design process is rising. “Several years ago, CAE was merely one part of the workflow. Designers would use it once the CAD phase was complete, or even after unsuccessful trial shots where it was used as a troubleshooter. Nowadays, there is no time to take several trial shots and no budget for costly re-engineering of a part, so CAE is coming in when the part is only 80% designed.”
Notwithstanding the importance of flow analysis, software vendors stress that a proper understanding of materials and processing fundamentals are prerequisite in order for designers to apply it successfully. “You don’t need to be an expert but you do need a certain level of common sense regarding molding, plastic materials, and mechanical design,” says Moldflow’s Yeh. Adds CoreTech managing director Vito Tsai, “Proper training is also paramount. Software users need to know the concepts of flow simulation and have the ability to make judgments on product modifications. The software can give you basic advice on how to improve a part, such as gate relocation, but you do need an understanding of plastics to deploy it effectively.”
Flow simulation must also always be applied to good CAD models. “You can’t do too much if the CAD design is poor. It’s not a problem-solver in that respect,” says Moldflow’s Yeh. Consequently, the company offers a package called CAD Doctor that will help fill any gaps in a substandard CAD model.
The designer also has the option of employing 2.5D simulations such as Moldflow’s Fusion product, or full 3D packages available from both Moldflow and CoreTech for simulation. Moldflow says thin-wall parts such as cellphones can readily be simulated with 2.5D, but thicker parts need the full 3D treatment.
CoreTech’s Tsai, however, believes that even with thin-wall parts, full 3D simulation can be important because accurate simulation of flow in the runners is also of significance. “We treat the runner with equal importance as the part itself,” he says. “Once you modify the runner, you also modify the cavity flow, and if you can control shear heat in the runner, then you can control residual stress in the part as well as weld lines,” Tsai notes. CoreTech has used its software to accurately simulate flow when using MeltFlipper technology from Beaumont Technologies, Inc. (Erie, PA).
When flow-simulation analysis is performed, part designers in general perceive a primary mission to be the elimination of the forming of weld lines in order to ensure structural integrity, which, in principle, is a good thing. However, notes Paul Tres, senior technical consultant at product development consultant ETS Inc. (Bloomfield Hills, MI), “When a weld line is present in a plastic part it acts as a control failure mechanism, allowing the analyst to accurately predict its location, as well as the loss of mechanical properties, which can be significant—from about 40% for crystalline and amorphous neat polymers to almost 60% for neat liquid crystal polymers,” Tres says.
Designers can use diaphragm or ring-type gates to completely eliminate the weld lines present in the part, according to Tres, but most of the time separating the gate and runner from the part requires rupturing the gate from the rest of the part, which creates a very large number of microcracks. “If the micro-cracks areas of the part are exposed to any type of load, in time any microcrack can become a major crack, thus producing an unpredictable failure of the part,” says Tres. Hans-Peter Beringer, senior manager, engineering plastics, BASF (Singapore), says weld lines can be strengthened by designing-in thicker sections or additional ribbing, for example.
BASF (Ludwigshafen, Germany) is also at the forefront of FEA software development for quantifying the modulus of fiber-reinforced structural parts on a localized basis. A lower-bumper stiffener (LBS) molded from polyamide and developed jointly with Adam Opel GmbH (Rüsselsheim, Germany) utilized the fiber software from BASF in its design.
The LBS weighs about one kilogram, is one-meter long, and is installed behind the front bumper so as to diminish the risk of serious knee injury in the event of a collision with a pedestrian. The BASF software, based on a process called integrative simulation, is fed with the results of a Moldflow simulation as well as experimental data that correlates strain rate with fiber orientation in a resin in order to accurately map the Young’s modulus distribution across the part. “When we simulate, the component has to be treated as part of the entire car chassis and not as a discrete component,” notes Beringer.
Besides pedestrian protection, Beringer says that simulation is very important when engineering failure modes into parts so that they protect vehicle occupants and limit damage to other components. “We have to ensure the part fails in a manner where it does not result in collateral damage to other components, such as engine components, or danger to passengers.”
FEA also needs to be properly applied to be successful. Andrew Hulme, business manager for design and simulation at Rapra Technology (Shrewsbury, UK), says, “The problem is that approximately 70% of plastics fail before their design lifetime. The challenge with all polymers is that they are nonlinear, visco-elastic materials and that their properties depend on the time under load, temperature, environment, and the stress or strain level applied.” Hulme says entering the tensile modulus into the FEA program at 20ºC, for example, will guarantee your product is under-designed. “What’s needed is the creep modulus at the temperature in the usage environment.”
ExxonMobil Chemical (Akron, OH) is one company addressing this data issue with the release of a new FEA database for Santoprene elastomers for semi-dynamic applications such as when an elastomeric weather strip slides against glass. Other applications, such as pipeseals, pump diaphragms, rack-and-pinion boots, and an EPDM-foamed faucet deckplate, have benefited through the use of FEA.
“The accuracy of FEAs are normally within 10%,” says Ward Narhi, senior design engineer, ExxonMobil Chemical’s Santoprene-brand TPVs. “Using genuine real-world test data is the basis on which accurate simulation can be achieved. When we undertake modeling exercises, we draw upon static and semi-dynamic FEA databases of data covering 300 grades of Santoprene-brand TPVs.”
Knowing which database to use is very important and completely application-dependent, according to Narhi. “The static database, for example, is used for applications with a constant load; if the application is actuated numerous times over its life, then the semi-dynamic data is applicable.” Using the wrong dataset could result in over- or under-predicting the mechanical behavior of the application by 25% or more.
Rapra’s Hulme says molding effects must also be taken into account when conducting FEA. “Modeling must be carried out on a [simulated] finished part and not a perfect CAD model.” In one recent case of part failure, a component of a fluid control system comprised of a plastic tubular bottle with a stainless steel insert was failing prematurely, even though it had been designed to endure the 20 MPa of stress over a 20-year lifetime. Flow analysis using 3DSigma software from Rapra showed that the processing conditions that prioritized productivity through use of a low tool temperature were resulting in additional molded-in stress of 7–10 MPa. Using this simulation data in FEA analysis indicated peak stress of 30 MPa, and failure after around 170 days.
To aid the link between flow simulation and FEA, Moldflow has announced the Moldflow Structural Alliance with ANSYS Inc. (Canonsburg, PA) and SIMULIA (Providence, RI), vendors of Abaqus and ANSYS FEA software, respectively. This makes analytical data from flow simulation available to these packages without adding time to the analysis. “Structural-analysis software programs assume plastic parts have consistent material properties throughout the entire part. However, due to the nature of the injection molding process, the flow pattern of molten plastic introduces variations in the structural performance of the part,” notes Moldflow.
The importance of FEA modeling under the correct environment was also highlighted in a recent analysis by ETS. Acetal homopolymer planetary gears used in an auto-seat lumbar-support mechanism were found to fail during validation for a new automotive program at around 9000 cycles instead of the requirement of almost 19,000 durability cycles without failure. ETS attributed the failure mainly to a higher operational temperature of 35°C due to friction between actuator components, whereas FEA was carried out at 23°C. Mechanical properties of the acetal at this temperature were insufficient to bear the tooth bending stresses.
As with flow simulation, there are various options available when it comes to FEA. Linear and nonlinear models can be applied for both materials and geometry. Linear material models work fine for highly reinforced and filled polymers (exceeding 40% by weight), says ETS’ Tres. “When nonlinear material and nonlinear geometry are combined in any analysis and the convergence factors are small, it will take a rather large amount of time to get proper and meaningful results,” cautions Tres.