February 9, 1999
In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.
All injection molded parts have a nominal wall. Many product designers ignore a part?s nominal wall and consider it little more than a place for mounting functional details, such as louvers, hollow bosses, textures, and stiffening ribs. I?ll admit these walls are rather mundane things in comparison to other, more intellectually stimulating features such as snapfit latches, molded-in hinges, or threads. A molded part?s nominal wall is, however, its single most important feature. A part?s nominal wall defines its size, shape, and appearance. It is the structural element, or frame, that establishes a part?s overall strength. The nominal wall provides a mounting place for all other features that can be incorporated into a multifunctional part. None of these other design details can be at their best unless they are attached to a properly designed nominal wall, or frame.Wall ThicknessA new part?s basic size and shape are established during the concept design phase. During this part of the project, the designer?s mind is occupied with many things, and little, if any, thought is given to what the actual wall thickness will be. As the product progresses on into the part design phase, someone has to make this critical decision.This first criteria for selecting a wall thickness has to be based on the functional requirements of the product in the hands of the end-user. Obviously, the part has to be strong enough to perform its intended function. Tensile strength and stiffness relate to the thickness of a part. The thicker the part, the stronger it will be. Determining the ideal thickness required to provide a given strength and function has now been reduced to a science. From a functional perspective, the correct interpretation of a finite element analysis can be relied upon to optimize the selection of a suitable wall thickness.Molding ConsiderationsFunction considerations have the priority, but it must be remembered that the part also has to be molded. Injection molding is a melt flow process that imposes limitations on how far the molten plastic material will flow for a given wall thickness. Theoretically, there is no limit to the maximum wall thickness that can be produced by injection molding. There is, however, a definite limit to how thin a part of a given size can be.I have molded small nylon parts with a nominal wall thickness of less than .01 inch. The thickest injection molded parts I have personally been involved with were filter plates with thicknesses of as much as 4.5 inches. Based on my own experience, then, the possible range of thickness for injection molding can be a minimum of .01 inch with a maximum of 4.5 inches. These thicknesses are possible, but in my opinion, they are not practical.My .01-inch medical component required very high mold and melt temperatures with maximum injection speed and pressure. At this wall thickness, the size of the gate and the overall size of the part are severely limited.The 4.5-inch thick polypropylene parts were produced using a specialized injection molding process called ?flow molding.? The molding cycle time was more than three hours. It is a very unusual molder and customer who can live with a three-hour cycle time.Single-use, disposable, and portable electric parts are now pushing the state-of-the-art in thin-wall molding. Some sections of larger parts are being successfully molded with wall thicknesses of less than .01 inch. Each individual plastic material has its own wall thickness limitations for good quality, efficient injection molding. For conventional injection molding, a practical minimum wall thickness is in the range of .03 to .04 inch. In those instances where thinner walls are required, the designer should be guided by prior experience with similar parts and materials. In the absence of such actual experience, the minimum possible wall thickness can be scientifically determined by computer-aided mold filling analysis.Many large, injection molded industrial parts and some smaller parts, such as optical lenses, are produced with very thick walls. These parts are produced on abnormally long molding cycles, using special and sometimes proprietary molding procedures. In my opinion, the injection molding process is at its best with a maximum wall thickness of .25 inch. Thicker walls have a propensity for excessive mold shrinkage, internal voids, molded-in stress, and post-molding warpage.A designer, faced with a need for a thick wall that cannot be cored out, should recognize there are other plastic molding processes better suited to producing really thick-walled parts. Injection molded structural foam, gas-assist injection molding, compression molding, and reaction injection molding are all processes that excel at producing thick-walled parts.If the thick sections are being used for strength, the designer could consider minimizing the wall thickness by upgrading to a stronger or fiber-reinforced plastic material or providing additional strength with ribs or gussets. For a given thickness, domed walls are stiffer than flat walls.The use of fiber-reinforced materials is increasing. The presence of the fibers reduces a material?s mold shrinkage, and this allows the molding of thicker walls. Reinforcing fibers have a length-to-thickness ratio. During a melt flow process, such as injection molding, these fibers have a tendency to align themselves parallel to the direction of flow. At a wall thickness of .05 inch, approximately 90 percent of the fibers will be aligned in the flow direction. Such a part would have low shrinkage in the direction of flow. Shrinkage perpendicular to flow would be higher. With different shrinkages in different directions, there is an increase in molded-in stress and a propensity for warpage.At a wall thickness of .25 inch, fiber alignment will be in the range of 5 percent. Such a part would have more uniform shrinkage and physical properties in every direction. Fiber-reinforced parts will have reasonably uniform fiber orientations, shrinkage, and physical properties at wall thicknesses of not less than .125 inch.Cost ConsiderationsThe cost of a molded part is dictated by its size, shape, and the plastic material being specified. For a given plastic material, a part?s cost relates directly to its wall thickness. Thicker parts require more plastic material. Thick-walled parts result in longer filling and cooling cycle times. These two cost factors combine to produce a large increase in cost with only a small increase in thickness.Selecting the optimum wall thickness normally turns out to be a compromise between costs, function, and molding requirements. For example, a wall thickness of only .005 inch may be all that is required for a polyester part to perform its function as an electrical insulator. Regrettably, that wall would have to be thickened in order to be able to mold the part. In some instances, a thicker than practical wall is required for structural strength. In other cases, thick sections can only be cored out with expensive collapsing cores or by redesigning a one-piece structure as two separate parts. All of these alternatives are undesirable but necessary compromises.Practical vs. PossibleThe successful plastic part designer is one who knows the difference between what is possible and what is practical. The good designer always strives to balance the alternatives in order to achieve the required objective within the range of what is practical. These are the designers who become well known for consistently creating high quality, low cost parts that work right the first time.The less creative, uninformed, and lazy designers simply take the path of least resistance. They design whatever they want and pass the consequences on to their molders. Establishing benchmarks for the industry to strive for is often cited as a justification for these impractical designers. This sounds good, but parts designed in the possible range, beyond what is known to be practical, turn out to be the kind of parts that require multiple mold revisions. These parts have a narrow processing window that results in only marginally acceptable parts, which are more costly than necessary.All things considered, the ideal nominal wall thickness is the thinnest wall that will meet all of a part?s functional, manufacturing, and cost requirements. Here in the late 1990s, that ideal wall thickness can be determined in a scientific manner. Strength requirements can be determined by FEA with a high probability of success. Moldability can be predicted by mold filling analysis software. That leaves cost as the one remaining variable. In the final analysis, the part?s cost will be determined by the skill and knowledge of the chosen injection molder.Determining the ideal wall thickness is an important detail that has to be carefully resolved during the part design phase of the project. There are additional nominal wall considerations which have to be addressed, but they are topics for future articles.
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