By Zan Smith, Frank Jaarsma, and David Sheridan (via pddnet.com)
Some applications work only in metal and others only in plastic. Many, however, occupy a large middle ground in which either will work. At such times, plastic is often the material of choice given its many benefits.
Compared to metal, plastic can reduce weight, dampen sound, create more complex parts, and eliminate secondary steps such as machining and coating. Some plastics resist corrosion and chemicals better than many metals even as they retain the strength of cast metals at elevated temperature.
The design of a plastic part to replace a metal one should proceed from the general to the specific. Begin by understanding the mechanical, environmental, thermal, electrical, and other conditions it will encounter over its life, as well as how it will be processed and assembled. Initial decisions on material, detailed design, processing, and fabrication evolve from this understanding and are essential for deriving preliminary costs.
In estimating costs, account for the direct material and manufacturing outlays, as well as those buried in overhead. A plastic part can often combine two or more metal parts, reducing inventory, labor, production floor space, assembly time, energy, and other elements. As the part component evolves from concept to production, the choice of resin, design, processing and assembly methods, and associated costs should be evaluated repeatedly.
Design in plastic, especially for metal replacement, should be guided by a team that combines production, design, engineering, research, sales, marketing, and purchasing expertise. For added depth, it can include members from the plastics supplier, molder, tool builder, and assembly equipment manufacturer. Plastics suppliers, for instance, help in understanding resin performance and assist with design, prototyping, molding, and pre-production evaluations.
Designers face many choices in replacing metal. Plastics, which range from commodity to high-performance resins, encompass thousands of options depending on their chemistry, chain length, and structure, and bonds between chains. Neat resins are modified by alloying different plastics together and by blending in additives, fillers, and reinforcements to alter impact, mechanical strength, stiffness, shrinkage, warpage, lubricity, and much more.
More demanding applications require engineering polymers in which price and performance tend to rise in unison. Higher performing resins generally can bear greater loads and thermal stress. They also tend to withstand acids, bases, solvents and other chemicals better, as well as resist the cracking, crazing, discoloring, softening, and melting such substances can cause.
The trick is to choose a resin that gives the best performance at the lowest price. To start, map the conditions a part will meet during processing, assembly, end use, and even recycling. In assembly, for example, the material must match the method used. Some resins are compatible with solvent bonding or ultrasonic welding and others work best with snapfit designs that require strength, flexibility, and dimensional stability.
Although a plastic may cost more than the metal it replaces – especially higher performance grades – they can make production far more efficient and cost less overall. A good example is the use of high-flow resins that allow for thinner, more complex parts that eliminate machining and other secondary operations needed with metals.
Design in plastic calls for a different mindset than design in metal. They are fundamentally different materials and so are processed and assembled differently. Most plastic parts are injection molded because this process is economical, efficient, precise, and produces finished parts in an automated operation. Compared with metal die-casting, injection molding is faster and offers longer mold life. Diecast molds usually last less than 100,000 cycles, while injection molding tools routinely withstand more than 4 million cycles before needing major maintenance.
In designing injection molded parts, their walls should be uniform (usually 0.8 to 5.0 mm thick) and as thin as possible to reduce weight and cycle time yet still meet mechanical and appearance specifications. The design should consider draft (a slight taper in the part) and avoid heavy masses of plastic that can extend cycle time and lead to moldedin stresses and distortions.
The part should have generous radii where elements intersect in order to reduce stress concentrations. Ribs and gussets can be used to buttress areas and add strength and stiffness without thickening walls. Multiple, evenly spaced ribs generally distribute the load better than large, isolated ones. Ribs should be as thin as possible when opposite visible areas so sink marks don’t form. Designers can also use sculpted surfaces to aid structural integrity.
Bosses are added to aid assembly. These helping components align with each other and act as mounting or fastening points. Openings, pockets, and blind holes can eliminate heavy sections or provide for mounting.
Mold and Tool Design
Many factors come into play that are often minimal when working with metal. The resin, part design and assembly method affect mold and tool design and are, in turn, affected by it. Wall thickness, for instance, influences a mold’s cooling system and the number of tool cavities, while surface finish determines if the mold must be polished or textured. In addition, the need to limit residual stress in the part impacts gating, the cooling system and runner design.
The mold should allow as broad a processing window as possible so resin properties and process variables, such as viscosity, hydraulic pressure, barrel temperature, and even longterm screw and barrel wear, can shift over time without harming part quality. Tool design encompasses such features as gating, cavities, draft, runners and slides, all of which can affect processing window and part quality.
Gate type, size, and position are chosen to aid part strength and appearance, resin alignment, mold filling time and resin flow length, among other factors. Tools usually have more cavities in high production situations and fewer cavities when finer tolerances are needed.
Draft usually involves tapers of 0.5 to 3 degrees. It allows parts to eject more easily from the mold and is affected by the position of the mold parting line, the length of a projection and part texture.
In terms of runners, a conventional system that operates without added heat can generate a large amount of scrap, which is often be reground and reused. Hot runners yield little or no scrap, but cost more and are more complex to operate. They are useful when runner volume is large relative to part size.
Some parts need complicated molds that call for expensive unscrewing or collapsing cores, slides, multiple plates, or intricate parting lines.
Computerized finite-element analyses can reduce mold development time and cost. These numerical tools include mold filling analysis, which evaluates gating position and size for optimum flow, the location of weld lines, and other factors, as well as cooling analysis, which looks at mold temperature distribution and cycle time and shrink analysis, which evaluates dimensional control, moldedin stresses and warpage.
The nature of the plastic used also affects part and mold design. Viscosity helps determine flow length in the cavity, gate placement, and mold cavity cooling. A resin’s ability to transfer heat affects the cooling system and its ability to prevent warpage and ensure optimum resin characteristics.
How much a resin shrinks as it cools is important because mold cavities are sized so part dimensions fall within the tolerances set. The filler present and its alignment also affect shrinkage, because it can cause more shrinkage in one direction than in another. Shrinkage also depends on part thickness and geometry.
Design in plastic should limit the number of secondary steps needed to complete a component. When a component is switched from metal to plastic, designers often find they consolidate two or more parts into one because molding allows for more complexity. Such consolidation is the most effective assembly method, since it joins parts in the mold. When combining two or more parts into one, the added mold complexity should not outweigh the savings due to the reduction in assembly steps.
Design should also make use of moldedin features that eliminate the welding, drilling, painting, thread tapping, and other secondary operations needed with metals. Efficiency can be improved by considering various assembly strategies, such as selecting an optimum joining method and minimizing the possibility of mating mistakes.
Molded-in joining systems, such as snapfits and pressfits, are fast, inexpensive, and avoid additional parts. Chemical bonding by solvent welding or adhesives creates air-tight gas seals and can be used when fasteners are impractical. Welding, via ultrasonic, vibration, spin, electromagnetic, or thermal methods, is fast and safe but requires similar materials. Mechanical fasteners, such as bolts, screws, rivets, and spring clips, are easy to use and allow nondestructive disassembly but require additional parts and can be labor intensive.
Plastics components can be made to exacting tolerances as they emerge from the mold, secondary sizing operations are not needed as if often the case with metal. Tolerances determine the precision needed in mold tooling, assembly and finishing. Special care is needed with if tolerances are tight, because the variations in each processing step, such as mold filling and cooling, are additive and can cause a loss of critical dimensions if they are not controlled carefully.
The initial design of a plastic part creates a theoretical part. Subsequent analyses, simulations, and prototyping evaluate how well the part works in the intended use and provides essential feedback to refine the design. Computerized design tools, including computeraided design, engineering and manufacturing (CAD, CAE and CAM) software, help finalize the part.
CAE, for instance, determines optimum geometry by evaluating direct loads or deflections. It often proceeds from classical equations to nonlinear effects to give increasingly accurate estimates of how the stresses encountered during processing, assembly, shipping and end use will affect a part. These evaluations aid the final decisions made on materials, processing and such design elements as walls, ribs, draft and radii.
Prototypes are often used to test essential properties, verify design and highlight assembly issues. They also enable customers to respond to the part concept. Prototypes can use the same material and manufacturing process as will be used for the finished part or the same material and a different process. The latter has different properties than a molded model. The least exact method replicates part geometry from a different material and a different process, such as stereolithography.
Plastics have grown increasingly sophisticated in recent decades, and so have been adopted for a widening range of demanding applications traditionally met by metals. Plastics have been adopted in many markets because they reduce weight, take more complex shapes, withstand environments many metals cannot tolerate, reduce total costs and much more. This has led to great diversity in how plastics are used and created a solid track record for the use of plastics instead of metal in many markets, including autos, trucks, airplanes and other areas of transportation, medical devices and instruments, appliances, electrical and electronic components, and office and industrial equipment.
Product Components Corporation is proud to offer plastic fasteners of all types, special orders, and low minimum orders. For more information, please call (800) 336-0406 and ask for Christina or Mary.