Metal to Plastic Conversion Guide

What is metal-to-plastic conversion?

Metal-to-plastic conversion is just what it sounds like: It’s the process of taking parts originally made from metal and redesigning or reproducing them in plastic.

Plastic parts—and the injection molding methods that made them so versatile—were first developed in the late 19th century. These parts and processes became incredibly popular during World War II due to the massive, urgent need for affordable, reliable, and mass-produced products. As a result, injection molded plastic parts replaced many metal components, and the advantages those manufacturers gained are still increasing today as plastic technology continues to advance.

As engineering-grade resins and plastic materials improved in the 1950s, many parts previously made from metal began to be replaced by injection molded components.

Why are manufacturers hesitant to switch metal parts to plastic ones?

From perceived design challenges, to worries over reduced strength, to being resistant to change—there are plenty of reasons some manufacturers haven’t considered the metal-to-plastic conversion process. When manufacturers haven’t looked into replacing metal with plastic, it’s often because they’re not aware of the sophisticated qualities plastics have to offer. Others simply gravitate toward the reliability of using the same material used in the original design, rather than exploring plastic options that could match or outperform the current material’s properties. Fortunately, nearly all barriers can be overcome with expertise from the right partner, and the significant benefits of plastic conversion are available to nearly any OEM or producer of products containing metal parts.

Throughout this article, we’ll answer the most common questions and provide the crucial information you’ll need for a successful metal-to-plastic conversion, including:

• What are the benefits of metal-to-plastic conversion?
• What performance attributes can plastic provide?
• What are the steps involved in metal-to-plastic conversion?
• What questions should you ask before converting to plastic parts?
• How does design for manufacturing (DFM) factor into metal-to-plastic conversion?
• Which plastic materials should you use for converting metal to plastic components?
• Which industries benefit the most from metal-to-plastic conversion?
• What are the costs associated with converting parts to plastic?
• Examination of a metal-to-plastic conversion case study
• Why is Rosti the ideal partner for a metal-to-plastic conversion project?

From cutting costs to updating your product design to enhancing your manufacturing process, there are many reasons to switch metal components to plastic. Keep reading to find out more about plastic part benefits and explore a case study with real-life specifics.

What are the benefits of metal-to-plastic conversion?

If the metal parts you’re currently using are meeting your needs, converting might not seem like a pressing issue. Still, there are some significant reasons to look at your metal-to-plastic conversion options. Here are six ways converting from metal to plastic can touch nearly every area of your business.

1. Design Quality and Flexibility: Plastic can be stronger, lighter, and offer more aesthetic choices than metal. As a manufacturing material, plastic blends can be tailored to meet specific environmental, structural, and operational needs. With the right design and material selection, plastic components can outperform metal. Additionally, adding reinforcing features like ribs and gussets—molded right into the plastic—can boost strength and durability while reducing production time. When additives are blended directly into plastic, they improve targeted characteristics and create highly specialized materials. These additives enhance plastic performance for impact and tensile strength, friction, scratch and flame resistance. They also make plastics lighter, more suitable for food and medical use, and more environmentally friendly. The versatility and adaptability of plastic parts are also ideal for plumbing, appliances, filtration, and engine applications where avoiding leaks is crucial. Plus, today’s scientific plastic injection molding techniques produce parts with tight, precise tolerances that are ready to use with no extra machining. Lastly, aesthetics are key: plastic components offer design features like:
   • With the latest software and technology, injection molders can test different materials during the design phase—enabling predictive design adjustments and better performance results.
   • Capability to integrate and handle complex shapes and geometries, as well as combine parts.
   • Weight reduction and increased structural capabilities.
   • Since plastics come in a wide range of colors and can offer finishes and textures that are more appealing than metals, injection molded plastic parts tend to outshine metal counterparts. Additionally, plastic parts make it possible to enhance decoration or labeling within the mold itself, further boosting their visual appeal.
   • These are just a few features that let you unlock new design and performance possibilities for your products.
2. Cost Savings: Converting a metal part to plastic typically results in an overall cost savings of 25–50%, with benefits across manufacturing, lead time, and logistics.
3. A Faster Process: Transitioning from metal to plastic enables much faster production cycles and higher throughput. The accuracy of injection molding also removes the need for time-consuming multi-step processes like grinding and reworking final parts.
4. Greater Manufacturing Efficiency: Along with the aesthetic advantages mentioned earlier, final colors and finish options can be integrated into the molding process, eliminating the need for separate painting, lasering, and more. Additional features include:
   • Low maintenance – Unlike metal parts, plastic components don’t require coating or painting for material protection, making them virtually maintenance-free.
   • Longer tooling life – Although the tooling cost for metal and plastic parts is usually about the same, metal manufacturing is often slower. Also, the average tooling life for plastic parts is about ten times longer than that of a cast aluminum tool.
   • Easier changes – Switching to a less expensive metal is nearly impossible without a complete redesign. However, the cost of resins typically doesn’t impact the mold, providing greater flexibility in material quality and cost.
   • Plastic components can be produced faster than metal ones by using highly repeatable, cyclical processes that require fewer steps overall. Unlike metal production, plastic part manufacturing is often automated, mechanized, and needs minimal oversight.
   • Plastic injection molds can eliminate the need for secondary assembly steps by creating complex and geometrically varied components in a single operation. Metal fabrication often means welding together multiple pieces, attaching parts like bearings, and adding protective coatings after the fact. Plastic injection molds can produce a single, ready-to-use part, combining multiple pieces, incorporating supporting features, and mixing protective coatings directly into the material.
5. Improved Part Performance: Plastic parts are usually up to 50% lighter than metal ones and offer unique advantages metals simply can’t match, such as:
   • Reduced size, weight, and thickness – Especially important for small appliances, engines, medical equipment, and tech devices, injection molded parts can be designed thinner, lighter, and more compact than metal parts, providing valuable space savings.
   • Greater strength and durability – Today’s engineered plastics can handle significant stress, and parts made from them maintain tight tolerances, making them just as tough and reliable as metal components.
   • Resistance to impact, corrosion, and heat – With over 25,000 engineered plastics available, including hybrid and custom blends, injection molded parts can be tailored to meet very specific needs. Some blends are perfect for absorbing impacts and resisting corrosion and heat.
6. Material Costs: Plastic usually costs far less than sheet metal and has a much steadier price on the market. The injection molding process is also highly repeatable, creating less waste material. Plus, with lower melting temps and no further machining steps needed, less energy is required than for metal part production.
7. Packaging and Shipping Costs: The lightweight nature of plastic has clear benefits for engineering and performance, and can also dramatically cut your packaging, freight, and storage expenses.

What are the MAIN attributes that can be achieved with plastic?

In addition to the design flexibility and cost savings a metal-to-plastic conversion offers, plastic brings numerous benefits that metal can’t. There are over 25,000 engineered plastic materials to choose from, and they can be tailored for nearly any manufacturing application. Plus, custom blends can be developed to meet even the most specific performance requirements. Here are some ways these versatile plastics can help you get better product performance:

Weight: Plastic parts are typically at least 50% lighter than comparable metal parts. They can also be made smaller and thinner, which is a major advantage when manufacturing engines, appliances, auto parts, or anything where space is limited.

Strength: Modern injection-molded plastics are just as reliable as metal parts under stress, and they can have even higher tensile strength. Plus, using a single molded plastic part instead of several metal pieces gets rid of welds—and the weaknesses that welding can create.

Maintenance: Compared to metal, plastic parts need almost no maintenance since they don’t require initial or periodic coating or painting for protection. They also handle impacts, movement, heat, and corrosive environments better than many metal parts.

Innovation: Plastic part manufacturing gives your engineers flexibility to solve tough problems. Whether you’re dealing with harsh conditions, complex shapes and processes, weight or structural limits, or simply want a better look, switching to plastic can provide the creative solutions you need.

What are the steps in converting metal to plastic?

One of the most important first steps in a metal-to-plastic conversion project is to thoroughly understand the scope of the project. While manufacturers benefit from cost reductions by cutting manufacturing steps, consolidating parts, using less material, and lowering part weight, there’s also often a chance to add value by including text, finishes, or features that are tough to achieve with metal. Scheduling a meeting or online consultation with a knowledgeable injection molding expert is a great first step to pinpoint opportunities and any possible limits.

You can expect to follow a process similar to this:

1. Carefully review part design intent and identification: What makes a metal part a strong candidate for plastic replacement?
   • Parts produced in high volumes
   • Parts with complex geometries, assemblies, or extra operations that can be removed or reduced
   • Parts that need to be lighter and offer a wider range of design options
2. Define part requirements: Support the success of your production run by establishing the requirements ahead of time, such as:
   • Environmental exposures such as temperature, ultraviolet (UV) light, chemicals, and moisture
   • Structural performance needs such as strength, stiffness, and impact resistance
3. Review any special considerations: Are there any special needs or characteristics the part must have, such as:
   • Regulatory requirements
   • Unique characteristics (wear, conductivity, color, flame retardancy)
     These factors will help determine the type of plastic material used and how it is processed.
4. Conduct a cost analysis: Producing plastic components can significantly lower costs compared to metals when all factors are considered, including reductions in:
   • Raw material costs and density
   • Tooling and processing
   • Cycle times
   • Assembly steps and labor
   • Secondary operations
5. Review project design and support needs: It’s critical to walk through each step to fully understand the feasibility of a metal-to-plastic conversion. Rosti’s process includes reviewing 3D models of the target component(s) and systems. This allows Rosti engineers to participate in design for manufacturing (DFM) discussions covering molding, function, assembly, and sustainability. It’s an ongoing process as the design continues to evolve and improve.
6. Assess prototype molding needs and part validation: Rosti can help with material molding trials to ensure your part is ready for production. This process may include:
   • Prototyping
   • Reviews of parts, tooling, processes, and designs
   • SOLIDWORKS simulation and design validation
7. Transition to plastic component production: Production molding processes should be optimized to ensure a seamless move from part validation to production runs. Interested in learning more about the injection molding process from design to production? View and download our comprehensive

What questions should you consider before switching to plastic components?

As manufacturers explore switching from metal to plastic and review each part of the process, many questions can come up about evaluating different material properties, design changes, part validation, and more. Don’t hesitate to ask these questions as you move through the process, and make sure to keep your goals in mind throughout your project to ensure a successful outcome. Are you looking to reduce part weight? Improve quality or consistency? Do the function and look of your plastic component matter, and how will the new material affect these? Asking these questions early will help keep your project on the right track.

It’s recommended to carefully review the following questions, along with any others that come up during the process, with a trusted and experienced injection molder who can guide you through every aspect of your unique project.

What features or quality requirements make this part challenging for tooling or injection molding, and how easily can the design be transferred?

With optimized design, most parts can be injection molded. However, it’s important to strike a balance between what can actually be molded and the types of materials that will deliver better part performance. Quality requirements, the intended use of the component, and its possible exposure to different environmental elements should all be discussed with your molding partner.

Specifically, designing a plastic part for manufacturability (DFM) involves important considerations that span plastic part design, tooling, material selection, and production. When plastic part designers take a collaborative approach and bring in mold makers early in the design process, many benefits can be achieved.

In fact, studies have shown that 70–80 percent of the cost of a new product is determined by design, and manufacturers now rank plastics design consulting as a leading factor in lowering overall manufacturing costs. The clear division that used to exist between development and production has now blurred, as product manufacturers rely on injection molding design specialists to get involved in the process much earlier than before.

We dig a little deeper into this topic in the DFM section below, but some key design optimization elements and features to consider early in the design transfer or development process include:

1. • Draft angles
   • Wall thickness
   • Radii at edges
   • Ribs
   • Draft angle
   • Finishing elements
2. 
   

   Why is it so important to focus on wall thickness in a plastic part?

   
   

   Uniform wall thickness is vital in the design of injection molded parts. If the wall thickness is not consistent, it can result in issues with dimensional accuracy, warpage, and overall part integrity as the molten material cools.

   
   

   If areas of varying thickness are necessary, design engineers suggest making transitions as gradual as possible so that the material flows more evenly within the mold cavity. This helps ensure the entire mold is completely filled, reducing the risk of defects. Rounding or tapering thickness transitions minimizes molded-in stresses and stress concentrations that are caused by abrupt changes in thickness.

   
   

   Choosing the proper wall thickness for your part can have significant impacts on manufacturing cost and production speed. The minimum usable wall thickness depends on the part’s size and geometry, structural needs, and the resin’s flow characteristics. Wall thicknesses for injection molded parts are typically between 2mm and 4mm (0.080″ to 0.160″). Thin wall injection molding can produce walls as thin as 0.5mm (0.020″). Work with your injection molder and design engineer to ensure the right wall thickness is used for your part’s design and material choice.

   
   
3. 
   

   Where is the optimum location to gate this part?

   
   

   While high-quality injection molded plastic parts should have uniform wall thickness, sometimes the part’s intended use calls for variations between thicker and thinner sections. For metal-to-plastic conversion projects with such requirements, it’s recommended to gate at the thickest section and avoid gating into thin areas. Plastics Technology explains that failing to do so might cause voids or sink marks. Packing pressure compresses the plastic as it cools and shrinks in the mold. Adjusting packing pressure controls how much the melt is compressed along the flow path.

   
   

   The best packing takes place at the gate, and you need the best possible packing at thick sections to avoid defects. Gate location also influences polymer molecule orientation and part shrinkage. Shrinkage refers to the contraction of the molded part as it cools after injection. All materials have different shrink rates, depending on resin family (amorphous vs. crystalline materials), mold design, and processing conditions. We’ll discuss material types and their properties further below.

   
   

   In addition to selecting the right gate location, it’s recommended that appropriate angles or tapers be designed into features like walls, ribs, posts, and bosses that run parallel to the part’s ejection direction, making it easier to remove the part from the mold.

   
   

   Do you want to learn more about how shrinkage affects molded part contraction as it cools after injection? Download Rosti’s Plastic Injection Molding Material Shrink Rate Guide.

   
   
4. 
   

   Are there additional materials or processes that we could consider to bring additional value to the part?

   
   

   If a material’s performance can’t be achieved with available resins, custom material blends can be created to enhance properties by combining several resins. Adding reinforcements can strengthen parts and improve stiffness, which may help reduce warping and shrinkage. Additives like glass or carbon fiber can be used to boost part performance and improve flow, ejection, and dispersion.

   
   

   When additives are blended directly into plastic, they enhance specific attributes, creating highly customized materials. These additives improve plastic performance in areas like impact and tensile strength, friction resistance, scratch resistance, and flame resistance. Additives can make plastics lighter, safer for use in food handling and medical supplies, and more environmentally friendly.

   
   

   Sometimes, high-temperature or specialty resins may be considered for certain applications. These materials are more expensive up front than commodity resins, but they can provide exceptional durability and performance for complex parts, often reducing overall production costs.

   
   

   There are also options beyond injection molding that add more value and function to your part. Secondary manufacturing, assemblies, and post-molding processes can streamline your production and improve efficiency and cost savings. Services may include hot stamping, inserting, heat staking, and more.

   
   
5. 
   

   What testing can be facilitated to validate our decision and the performance of a part?

   
   

   Injection molders who take an innovative, scientific approach to manufacturing deliver better results overall.

   
   

   Plastic Prototyping: Rosti ensures injection mold tools are built to spec, so customers can be confident their prototype component is ready to be tested, marketed, or even used for short-term or low-volume production. This is achieved by partnering with plastic prototype molding experts to create custom mold tools and components designed with a production mindset. Fast turnaround and competitive pricing let customers order prototypes that closely match production part designs.

   
   

   SOLIDWORKS: Rosti continues to expand the resources available to help engineers with plastic part design, mold design, and material flow simulation. With the SOLIDWORKS^® Plastics Premium predictive II simulation package, Rosti’s engineering team can give further predictive insights during early plastic component design, which helps shorten the time to market by speeding up new product development cycles. SolidWorks Plastics Premium software is a design validation tool that offers insights into part geometry that would be difficult, costly, or impossible to predict with traditional methods.

   
   

   

   
   

   Plastic Residence Time: Calculating the plastic residence time for your manufacturing process will improve material performance and the quality of your finished product.

   
   

   Material is quickly affected by both temperature and the length of time it’s exposed. According to the Injection Molding Handbook, some polymers are “hydrolytically degradable and may suffer undesirable depolymerization effects due to chemical reaction of moisture with the polymer prior to devolatilization.” Knowing the residence time of your material can help you determine the ideal processing time and temperature for your needs.

   
   

   Part Validation: Part validation is crucial because designing parts for molding is often quite different from designing for metal forming. This usually means the part design will be new or significantly changed. What’s the right way to ensure the new design meets your requirements? Rosti engineers believe the best method is to build a single-cavity pilot or prototype tool in steel to validate all assumptions. This step not only confirms the part design, but also the tooling and processing, increasing the likelihood of a successful outcome.

   
   

What role does design for manufacturing (DFM) play in metal-to-plastic conversion?

The design for manufacturing process (DFM) is essential for any metal-to-plastic conversion. Once the project scope is defined, DFM provides a framework for injection molding engineers to collaborate on part design relating to tooling, molding, function, and sustainability. Rosti has deep experience recommending design changes to make parts lighter and more robust, focusing on key plastic design factors like nominal wall thicknesses, draft angles, structural rib intersections, assembly features, and more. Rosti’s engineers often mold tight-tolerance features, which allows for greater process control and reliability throughout production. The ultimate goal is to ensure the plastic part can be molded with the best possible features and quality for the life of the program.

Design for Manufacturing (DFM) means designing products to maximize manufacturing efficiency for the process and equipment used, so you can achieve the lowest unit cost at the highest quality. The main reason to use DFM when manufacturing a plastic injection molded product is that, as noted above, 70%–80% of its manufacturing costs are determined by design decisions.

DFM involves choosing the best manufacturing process for a part or product, investing in relevant technologies, applying advanced design principles, and selecting the right material properties to ensure the consistency and quality your customers want.

It’s also a key part of reducing manufacturing and assembly costs. While this overview explains the basics, a complete DFM plan should be a detailed engineering document focused on achieving the ideal balance between quality and cost.

13 DFM Guidelines for Reducing the Costs and Challenges Associated With Manufacturing a Plastic Component

1. Start DFM early in the design process before tooling begins
2. Bring engineers, designers, contract manufacturers, mold builders, and material suppliers together to review the design
3. Design to reduce the overall number of parts
4. Use a modular design
5. Minimize the number of machine operations required
6. Use standard components
7. Design parts to be multi-functional
8. Design parts for multiple uses
9. Design for easy fabrication
10. Avoid separate fasteners
11. Minimize assembly directions
12. Maximize compliance
13. Minimize handling and consider how your design affects packaging and shipping

What plastic materials are used for converting metal to plastic components?

Is plastic really as tough and durable as steel? Many manufacturers don’t realize that, when designed and engineered properly, plastics can achieve the same tight tolerances as metal parts—with even better performance in some cases. Plastic materials can be carefully formulated into high-performance blends to meet or exceed requirements for strength, flexibility, temperature and corrosion resistance, and more.

Choosing the right plastic for your application can take time, and it requires partnering with an experienced injection molder who works closely with resin experts to thoroughly evaluate your project and the part’s intended use. Rosti molds more than 150 resins every year and specializes in the following:

• PPS (Polyphenylene Sulfide)
• PPSU (Polyphenylsulfone)
• PSU (Polysulfone)
• Acetal
• PPO (Polyphenylene Oxide)
• TPV (Thermoplastic Vulcanizates)
• TPE (Thermoplastic Elastomer)
• PP (Polypropylene)
• HDPE (High-Density Polyethylene)
• LDPE (Low-Density Polyethylene)
• High-temperature resins
• Highly-filled resins (glass, mineral, talc)
• All engineering-grade Nylons

According to the American Society of Mechanical Engineers, one of the greatest advantages of plastics is the availability of more than 25,000 engineered materials for manufacturing applications. Key considerations for types of plastic are:

• Crystalline vs. amorphous: Resins used in the injection molding process consist of structures defined by their crystallinity – that is, how the molecules of the polymer are packed together. Crystalline structures are typically very ordered, which gives the material strength and rigidity. Amorphous polymers are the opposite.
• Additives: Additives affect strength, stiffness, temperature performance, appearance, packaging requirements, and cost.
• Carbon and stainless steel fillers improve conductive and/or shielding properties.
• Lubricant fillers improve wear and friction properties.
• Mineral fillers improve electrical performance, provide a weighted feel, sound dampening, higher specific gravity, and improve dimensional stability.
• Impact modifiers improve toughness.

When considering the intended end use of an injection molded part, understanding key characteristics is essential to selecting the best plastic material. Review these important differences in some of the more commonly used resins.

ABS (acrylonitrile butadiene styrene) is a common injection molding material that’s easy to source and mold and is available at a reasonable price point. It’s a strong, sturdy material with good impact resistance, is easy to machine, and meets a variety of aesthetic requirements. The resin is not ideal for parts that will be subjected to continuous intense heat, as it will warp or melt above or around 200 degrees Fahrenheit.

Polycarbonates are a high-grade class of thermoplastics that are easy to mold and often provide increased shatter resistance. Naturally transparent, polycarbonate is ideal for high-strength, glass-like applications (safety goggles, medical lab uses, and more). Polycarbonate isn’t suitable for applications where a higher degree of flexibility is needed or for colored aesthetic selections.

Glass-filled nylon is a reinforced thermoplastic where glass fibers are added to the base nylon resin to provide extra strength and heat resistance. This resin is also more non-conductive to electricity than many others, but despite its high melting point, it’s more susceptible to burning than other materials.

Polypropylene is a flexible thermoplastic suitable for both industrial and consumer applications. It’s known for being highly pliable and can be manipulated in many ways without losing its original shape. Polypropylene is also resistant to solvents, chemicals, and UV rays.

Acetal is known for its great friction resistance and is a very rigid material. It can be easily colored and dyed during molding; however, it doesn’t offer exceptional heat resistance.

Addressing specific needs early in the design process can help you avoid costly changes later. Balancing characteristics like stiffness, durability, and toughness is key to achieving optimal part performance. The importance of early design consultation in the injection molding process has been emphasized throughout this page. However, it’s especially critical when using high-temperature materials to enhance a part’s strength, stability, and other essential features unique to its application. In fact, standard molding techniques aren’t always effective with high-temperature and exotic resins.

What industries benefit most from metal-to-plastic conversion?

Many of the markets served by Rosti take advantage of the value that metal-to-plastic conversion offers. Applications typically focus on metal components that are heavy or require multiple manufacturing steps to achieve the desired features.

For example, the automotive and aviation industries have used metal-to-plastic conversion to reduce weight and improve fuel efficiency (replacing a metal component with a plastic one typically results in about a 50% weight reduction). In addition to reducing weight, components made from engineered plastics can be just as durable, heat- and chemical-resistant as metal components, making them suitable for harsh environmental or operating conditions.

The trend of converting metal to plastic is also increasing in the medical device industry. Tools and instruments that used to be made from metal are now being produced from plastic to reduce weight, while still maintaining high strength and stiffness. By incorporating high-temperature plastics, components can withstand increased disinfectant and sterilization requirements.

Other industries seeing a rise in metal to plastic conversions include agriculture, construction equipment, industrial pumps, fluid power applications, heavy equipment, appliances, and many consumer products.

What are the costs involved with metal to plastic conversion?

In most cases, creating a new plastic part requires tooling. Plastic injection mold tooling must be able to handle repeated temperature changes, cavity pressures over 30,000 psi, steel temperatures above 300 degrees Fahrenheit, and plastic temperatures above 700 degrees Fahrenheit. Highly-filled resins are also extremely abrasive on standard mold steels. Because of this, injection mold tooling is generally more expensive than tooling for many other manufacturing processes. While the per-unit cost savings from injection molding usually offset these tooling costs, tooling is still an important factor to include in your budget.

There’s no question that injection molding requires an initial investment in tooling. However, because of the speed of the injection molding process, it’s very likely that a plastic version of your part will end up costing significantly less.

Source: Design for Manufacturing and Assembly, D. Havel, https://www.slideshare.net/dhaval6693/dfma-57014998