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Change-up Pitch: From Metal to Plastic

By James D. Sawyer Editor in Chief, SME Media

A scientific analysis of a part’s characteristics could lead to a switch in materials and that could help lower cost, increase production speed and decrease weight.

Even though it’s been around since the 1950s, when engineering-grade resins were first introduced, many manufacturers still are not familiar with the many benefits that metal-to-plastic conversion provides. As this process continues to evolve, especially with the rapid development of new advanced plastics that can outperform metal at lower cost, fewer injection molders are capable of providing the necessary engineering expertise to develop a product from metal into plastic.

Automotive manufacturers are especially familiar with converting existing metal products or parts to plastic. This knowledge is driven by pressure from a federal mandate calling for automobiles to average 54.5 mpg by 2025. One way to increase fuel efficiency is by reducingvehicle weight. Metal is far heavier than plastic. With the proper design, engineered plastics can be just as strong as metal. Their chemical resistance and heat resistance, combined with their lighter weight, make them especially useful for fuel systems, fluid handling systems, and other under-the-hood, high-temperature applications.

For example, heat-resistant PEEK thermoplastic (polyether ether ether ketone) has been used to replace piston rings in automotive automatic transmissions. Engineered plastics can also absorb vibration better because of their damping qualities. Plastics have also been used in gear systems, such as wear plates. Glass-fiber and carbon-fiber additives in engineered plastics can make them strong enough to serve as brackets and exterior panels. Plastics that are engineered to be thermally and electrically conductive can be used as EMI/RFI shields, or in automotive electronics.

Other suitable applications include pumps and fluid/chemical power applications (valves and fittings).

  • Benefits that metal-to-plastic conversion can provide include:
  • Tensile strength comparable to metal
  • Reduced part weight
  • Highly repeatable process (less scrap)
  • Lower manufacturing costs
  • Enhanced regulatory compliance
  • Greater design flexibility
  • Increased market stability for material cost
  • Lower packaging and shipping costs
  • Up to six times longer tool life

Current metal-to-plastic trends focus on reducing weight, improving strength and corrosion resistance, and consolidating multiple metal parts into one plastic part. Plastic parts can be just as tough as metal parts and achieve the same tight tolerances, with fewer secondary operations. With the proper design, engineered resins with long-glass filler can actually exceed the physical properties of metal. With appropriate part design, plastic parts can be designed to perform just as well as the metal parts that are being converted to plastic. In fact, plastics can be engineered to have specific physical and chemical characteristics that are better than metal.

In general, companies can expect to achieve an overall cost savings of 25–50% by converting to plastic parts. There are several ways this cost reduction is achieved. Multiple metal parts can be replaced by one injection-molded part made of tough, engineered plastic, eliminating the need for fasteners and assembly. Colors can be added to the plastic melt, eliminating secondary operations for painting or laser marking. Plastic has a nominal impact on part cost; sheet metal, however, has a much bigger impact thanks to the need to weld, grind, and add dent and scratch resistance and noise dampening.

Many engineers who typically work with metal parts are not familiar with the history or success of reinforced plastics, their strength-to-weight performance, or how they can actually improve product performance and longevity.

Let’s Start with Cost

Cost is usually the top topic for every project. Competition is tough in a global marketplace. Operations are lean. Let’s say you are using a metal part. If a plastic part can be just as good as your metal part or even just a little bit better, why change to plastic if it doesn’t create a real performance advantage? The answer: it will save you a lot of money.

The single most important benefit in metal-to-plastic conversion is cost savings with improved product quality. Below are seven reasons why replacing metal casting with injection- or insert-molded plastic saves money and actually improves part quality.

Decreased piece part prices. After initial tooling costs are paid, the piece part pricing is usually much less than the same part produced in metal, whether it is a stamping, casting or a die-cast part. This cost savings is realized because the injection molding process has faster cycle times (more parts made per machine hour) and these parts are identical, which eliminates secondary machining.

Elimination of time-consuming and costly secondary operations. For example, plastic material can be colored with color concentrates before molding, eliminating secondary painting operations. Injection molds can be textured or given various levels of polished surfaces before molding. The costly assembly of several metal stampings or castings fastened together can often be replaced by a single injection molded part, incorporating the features of the total assembly. This can eliminate the need for welded joints between metal parts, which also eliminates risks for leakage. If multiple assemblies are required, the plastic parts can be engineered to have snap-together features to eliminate any fasteners.

Reduce product weight and improve ease of use. One of the biggest advantages of using plastics instead of metals is weight reduction. Reducing product weight with plastics gives you more parts per pound of material, significantly reduces shipping costs, and improves the end-user’s physical ease in utilizing the product.

A comparison of the specific gravity values of metals to plastics shows how dramatic the difference in weight can be:

Metals Plastics
Aluminum 2.5–2.8 Polycarbonate 1.2–1.4
Brass 8.4–8.7 Nylon (most types) 1.2–1.7
Copper 8.8 Polyethylene 0.92–0.95
Zinc 6..9–7.2 Polypropylene 0.90–1.04
Steels 7.7 ABS 1.02–1.4

 

Gain greater product structural strength. Plastic parts can actually be stronger than metal parts by using engineering-grade materials. In addition, the ability to mold in features for structural strength, such as ribs, bosses, and gussets when the part is originally produced, (instead of fastening, welding, and gluing operations afterwards) can affect the total strength of the assembled part, as well as reduce additional costs.

Increased product design options. It is much easier to produce finished level complex part designs out of plastic than metal because of its dimensional stability. Tight tolerances can be achieved as molded in conjunction with complex shapes due to advanced injection mold/tool design capabilities. In addition, plastic allows for thin-walled parts with uniform wall dimensions due to high injection pressure capabilities, replacing the more costly thicker-walled features of die-cast metal parts.

Plastics also have advantages over metals in the prototype stage. Cheap, soft tools can be used to try out different materials and finalize design. Prototype parts can also be easily machined out of plastic slabs, sheets, rod, etc.

Plastic materials can be reused. Reusing materials by adding regrind (ground up runners and scrap parts) to virgin materials generates significant cost savings (up to 40%). Most injection molding plastics are thermoplastics and can be easily reprocessed (thermoset materials, however, cannot). Regrind can be used immediately, rather than needing to be re-smelted like steel.

Increased product life. The environmental vulnerability of metals can be replaced with the durability and longevity of plastics. Most plastic materials have greater chemical resistance compared to most metals. Plastics do not rust or oxidize as metals do and most are not affected by the acids or base compounds that corrode metal.

The first step is finding out if your project is suitable for metal-to-plastic conversion. This requires considerable analysis and is not necessarily determined quickly. It is essential to fully understand the end-use application, environmental conditions, material evaluations and analysis, manufacturability, and economic feasibility.

It is critical to know how the product will be used. Design engineers must be able to accurately evaluate the real-world environment that will be impacting the product, including chemical exposure or contact solutions, temperature ranges, shielding, and forces (including worst-case scenarios). All this information must be analyzed together to make the best material choice. Perhaps the most important rule at this stage is to never make any assumptions. The lack of a complete and thorough application review could result in selecting the wrong material, which could derail the entire development process.

Selecting the plastic. Polymer science has made tremendous strides over the years, advancing the ability to compound a variety of plastic materials with fillers and reinforcements that provide a tremendous amount of structural integrity. Plastic can be as strong as metal with proper material selection paired with design optimization, and weigh far less. Here are some characteristics to consider:

Metal advantages:

  • Thermal conductivity
  • Electrical conductivity
  • High stiffness
  • Low tendency to creep
  • Low coefficient of thermal expansion
  • High strengths
  • Less warp problems
  • Narrow tolerances can be realized

Plastic advantages:

  • No corrosion
  • Low density
  • Design freedom
  • Extended tool life
  • Chemical resistance
  • Recycling
  • Less assembly necessary
  • Reduction of part numbers

Choosing the right plastic can take some time, depending on the project. One of the greatest advantages of plastics is the availability of more than 25,000 engineered materials for manufacturing applications. New, higher-performance blends and hybrids can also be custom-designed to meet very specific performance requirements. Key considerations for type of plastic are:

  • Crystalline vs. amorphous: evaluate requirements such as chemical resistance, impact, flow, processing, etc.
  • Additives: how do filled and unfilled plastics compare? Additives will affect strength, rigidity, cost, FR package requirements, heat, and cost.
  • Long-glass fiber additives improve stiffness and strength, increase temperature performance up to 150° C, and create a moderate surface appearance.
  • Short-glass fiber additives improve stiffness, increase temperature performance, and improve appearance over long glass (glass content of 30% or less allows parts to look as good as unreinforced plastic parts).
  • Carbon and stainless-steel fillers improve conductive and/or shielding properties.
  • Lubricant fillers improve wear and friction properties.
  • Mineral fillers improve electrical performance, weighted feel, sound dampening, dimensional stability, and increase specific gravity.
  • Impact modifiers improve toughness.
  • Flame retardants increase resistance to burning.

Testing the plastic. Plastics respond differently to injection molding depending on their physical and chemical characteristics—these include strength and flexibility, melting and cooling behavior over a range of temperatures, polymer structures, and chemical bonds. These can be enhanced by adding filler materials or by creating blends with very specific properties, creating hybrid plastics with superior mechanical, thermal, chemical, electrical, and environmental properties, such as:

  • Mechanical: tensile strength, flexural modulus (stiffness), impact resistance, creep, and dimensional stability.
  • Thermal properties: thermal expansion, heat deflective temperature, relative thermal index, coefficient of thermal expansion, mechanical response at temperature, plastic stability.
  • Chemical properties: chemical resistance (semi-crystalline vs. amorphous), molecular weight, and part stress (combination of design, assembly, process, and environment can reduce chemical resistance).
  • Environmental: resistance to weather, humidity and ultraviolet light—how does the combination of these factors affect color shift, gloss retention, and loss of material properties?
  • Electrical: Conductivity, shielding, dielectric strength, dielectric constant loss factor, and electrostatic requirements.

Other Design Considerations

Injection molding is faster, more efficient, and can achieve higher tolerances compared to making die-cast metal parts. In fact, it is easier to make complex, high-performance parts or products with the highly controlled injection molding process, which requires fewer steps. Tools last 500,000–1,000,000 cycles or more with injection molding compared to die casting and require less maintenance and downtime. To optimize part quality and make the molding system last as long as possible:

  • Proper mold steel should be selected for the plastic of choice.
  • Molds need to be properly vented to minimize erosion.
  • Processing is properly designed to eliminate flash, which can damage the mold parting line.
  • Hot runner systems are compatible with the selected material.
  • Screw style and material are well-suited for the selected plastic.

One of the greatest benefits of metal-to-plastic conversion is the design freedom it creates. For example, multiple metal parts that need to be fastened together can be designed into one part that can be injection-molded. This also expands the geometrical possibilities for new designs. Part consolidation lowers inventory and increases productivity. Labor savings are gained by reducing assembly and secondary operations (for example, by integrating color into the melt no painting is required).

Plastic mechanical properties are far below those of most metals. For example, heavily glass-reinforced plastics might reach 2M psi modulus whereas aluminum is 10M psi and steel is 30M psi. These properties can be designed around, but “shape” changes (moment of inertia) are required to get to the same stiffness as a metal part. To make a structure “equivalent” in stiffness requires the product of Moment of Inertia (I) and Young’s Modulus (E) to be equal. (This is, however, only really true for short-term loading. Long-term loading, at elevated temperatures, could cause plastic to creep; in this case, a material’s creep modulus would need to be considered.)

Support features (ribs, gussets, radii) can also be added easily after first sampling. The advantage here is you can start with lightweight part and then add features as needed.

A designer switching from a metal-to-plastic design frame of mind needs to understand the mechanical/structural differences between metal and plastic. Simply substituting plastic for metal in a design rarely works because plastics have different mechanical properties, which will affect how the product performs in the end-user environment. These variations, however, can be corrected by adding design features such as greater wall thickness or ribs for strength.

Determining how a plastic performs in the end-user environment often depends on prototype testing (for example, analyzing post-mold shrinkage and warpage). It is also relatively easy to sample several materials using the same mold at prototyping stage to make a final decision. It is especially important to evaluate strength, hardness, flexibility, corrosion resistance, fatigue, and long-term creep. For example, one issue with plastic bending (at high frequency) is that plastic can’t transfer the heat created by the inherent hysteresis. Metals, being much more thermally conductive (without as much hysteretic loss), don’t have the same problem and can be fatigue-tested much more quickly. Testing needs to be appropriate to capture true fatigue resistance and creep. Metals also can be made to creep, but the temperature and loading requirements are much higher. In general, metal creep does not exist at typical, everyday usage temperatures and loads. Plastics can easily be caused to creep/stress relax if parts are not designed correctly.

Scientific Injection Molding

Metal-to-plastic conversion requires the best possible manufacturing design and controls, especially for high-performance, critical-tolerance parts. This can be delivered through the use of scientific molding, which uses detailed material science and precise measurement to completely understand, down to the molecular level, what’s happening during each stage of the injection molding process. This will eliminate annoying issues associated with:

  • Resin optimization
  • Evaluating and blending color concentrates
  • Molding and tooling design
  • Process and material variations
  • Not meeting specs
  • Re-validation
  • Meeting regulatory requirements

Standard molding procedures are simply not accurate enough for metal-to-plastic conversion and the tolerance it requires.

By fully understanding how all the many material and process parameters interact together at all times during production, our scientific molding engineers maintain high precision across multiple production runs. Sophisticated software and sensors monitor each phase of the manufacturing process, allowing our team to know what’s happening with the material inside the mold at all times; they know how small changes in pressure, temperature, viscosity, flow rate, material moisture rate, fill time, and cooling rate impact quality of the final product and can correct any process variations within seconds.

Because all the data is recorded, the manufacturing process can be easily replicated as needed—even when production is transferred from one machine to another—saving a tremendous amount of setup time. This is also essential for validating the process and meeting any regulatory requirements.

By understanding every phase of the metal-to-plastic conversion process, and all aspects of material behavior, scientific molding engineers can design the most efficient process possible for your product—saving money on material costs and speeding up throughput because “peak production” is maintained through the entire process.

For example, a client approached Kaysun about manufacturing one of its air-operated double diaphragm pumps in plastic, instead of metal. The goal was to increase corrosion resistance and chemical resistance to achieve longer pump life, at a lower cost.

Using a scientific molding approach, Kaysun worked closely with the client design team to select resins that possessed the specific mechanical and chemical properties that were needed to withstand the various environmental conditions that would be affecting the pumps. Several general resin families were considered, including unfilled polypropylene, PVDF (a derivative of Teflon with very high chemical resistance), and some carbon-filled static dissipative resins for the mining industry (spark resistant).

After PVDF was selected as the material, the part design was adjusted to better fit the injection molding process. Structural ribs and material core-outs were added to minimize thick sections and balanced material flow. Cooling circuits were designed to keep the cycles as low as possible, yet still achieve dimensional repeatability. This was challenging because the pump required walls up to 1.5″ (38 mm) in thickness, which is very difficult to achieve with injection molding and requires extremely precise control. The final plastic product met all the performance goals set by the client, including lower production costs, and continues to be manufactured today.

Parting Thoughts

Perhaps the most exciting advantage of metal-to-plastic conversion is the design freedom it provides engineers. They can think in more creative ways about complex geometry, performance in harsh environments, shielding considerations, weight and structural limits, thermal management, and product differentiation—both for performance and how the product looks on the shelf.

As material suppliers continue to develop high-strength thermoplastics that are increasingly impact-resistant, corrosion-resistant, and heat-resistant, more companies are converting from metal components to plastic. Advantages include lighter weight, consolidation of parts, net shape components, aesthetic improvements, and better durability.

It is highly likely that the most impressive breakthroughs with metal-to-plastic conversion have yet to come—especially with advances in engineered plastics. In fact, the process could revolutionize the way manufacturing is done across a range of industries as more R&D teams recognize the tremendous potential that metal-to-plastic conversion has for improved performance and enhanced manufacturability of complex, mission-critical parts.

Edited by Yearbook Editor James D. Sawyer from information provided by Kaysun Corp.

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