Metal Replacement: When to Switch Metal for Plastic

Replacing a metal component with an injection moulded one is neither a passing trend nor a simple cost-cutting exercise. It is an engineering decision, and it is assessed against measurable criteria: loads, temperatures, the chemical aggressiveness of the operating environment, tolerances and annual volumes. When these five parameters align, metal replacement reduces weight, cost and lead time. When they do not, metal remains the correct answer.

In this article we look at when conversion makes sense, which polymer corresponds to which metal, how the economic break-even point is calculated, and what happened in two real projects we managed in-house, from redesign through to moulding.

What metal replacement means, and why it is gaining ground

Metal replacement is the substitution of a metal component with a technopolymer component, generally produced by injection moulding. It is not a one-to-one swap: the part has to be redesigned, because a geometry conceived for die casting, milling or turning is not the geometry that makes the best use of a polymer.

The momentum comes from three directions that add up.

The first is the cost of machining operations. A die-cast component almost always requires CNC secondary machining, deburring and surface treatments. Every step adds cost, lead time and one more opportunity for scrap. An injection moulded part, if the mould is properly designed, leaves the press already finished and already within tolerance.

The second is weight reduction. In automotive, domestic appliances and machinery components, every gram removed has a value, whether in fuel consumption, ergonomics or transport cost. Technopolymers filled with glass or carbon fibre reach densities of around 1.4 – 1.6 g/cm³, against 2.7 for aluminium and 7.8 for steel.

The third is functional integration. A well-designed mould makes it possible to obtain in a single part what previously required three metal components and two screws: snap-fit clips, seal seats, ribs, overmoulded inserts. Fewer part numbers in stock, less assembly, fewer points of failure.

The five questions to ask before converting a component

Before discussing materials, a design engineer should answer five questions. If even one of them is left without a numerical answer, the assessment is not yet mature.

What are the real static and dynamic loads? Not the nominal catalogue load, but the peak in service and the expected number of cycles. Polymers behave viscoelastically: under a constant load over time they deform through creep, a phenomenon that in metal at room temperature is negligible. A component that performs perfectly in a static test may fail after a thousand hours under the same load applied continuously.

At what temperature does it operate, and for how long? Occasional peak temperature must be distinguished from continuous service temperature. A glass-filled PA66 withstands peaks well above 200 °C, but it should not be held there permanently; a PPS or a PEEK can be. This is the parameter that most often determines the material on its own.

What does it come into contact with? Oils, detergents, fuels, steam, food liquids, weathering, UV radiation. Some polymers with excellent mechanical performance are vulnerable to hydrolysis or to stress cracking in the presence of specific solvents. Here the choice must be based on technical datasheets and, where necessary, on dedicated testing.

What tolerances are genuinely required? Injection moulding achieves very tight tolerances, but the polymer shrinks as it cools, and shrinkage is not isotropic in filled materials. A tolerance within hundredths of a millimetre is achievable, but it has to be won in mould design, not recovered afterwards. It is essential to know which dimensions are critical and which are not: demanding hundredths on every dimension inflates the cost of the mould for no reason.

How many parts per year, and for how many years? This is the question that determines economic feasibility, and we address it in detail below. The principle in short: the mould is an initial investment amortised over volume. Below a certain threshold, machined metal remains more cost-effective even when it is technically inferior.

Which polymer for which metal

There is no automatic correspondence, but there are recurring paths. The table below is a starting point for the technical discussion, not a prescription: the final selection always depends on the combination of the five parameters set out above.

The practical criterion to keep in mind is this: the filler determines stiffness, the matrix determines thermal and chemical resistance. Increasing the glass fibre content raises modulus and strength, but reduces impact resilience and amplifies shrinkage anisotropy. It is a trade-off to be calibrated part by part, and this is where knowledge of how the material actually behaves in the mould comes into play, not only the supplier’s datasheet. In our moulding and production section you will find the range of technopolymers we process, including PEEK, LCP, PPS/PPSU and carbon-filled PA.

One final note: conversion is not always complete. In many projects the best solution is a hybrid one, with plastic-metal overmoulding that retains the metal insert where localised strength is required and encloses it in polymer everywhere else.

The economic break-even: mould cost against savings per part

This is where most projects are decided. The logic is straightforward, and it is worth writing it out as a formula.

The minimum break-even volume is:

V_min = Mould cost ÷ (Current unit cost in metal − Estimated unit cost in polymer)

Here is an example with realistic order-of-magnitude figures, to be adapted to each specific case. A die-cast aluminium component with two CNC secondary operations costs €4.20 per part. The version moulded in glass-filled PA66, with a two-cavity mould, is estimated at €1.10 per part. The differential is €3.10 per part. If the mould costs €38,000, break-even is reached at approximately 12,300 parts. With an annual requirement of 40,000 parts, the mould pays for itself in less than four months, and from the fifth month onwards the project generates a net saving of more than €120,000 per year.

Three elements should be included in the calculation and are often forgotten:

Downstream cost avoided. If conversion eliminates the assembly of three components, the saving is not only on the part but on line minutes, part numbers managed and assembly scrap. In our experience, this is the item that most frequently moves break-even forward in a favourable direction.

The number of cavities. A four-cavity mould costs more than a two-cavity one, but it lowers the cost per part and shortens lead time. The optimal choice depends on the demand profile over time, not on the peak.

The service life of the mould. A well-built and well-maintained mould produces millions of cycles. Amortising it over three years when it will last ten artificially penalises the business case.

Akron case study: replacing metal in an air distribution system

In the forced air distribution sector, a customer had a component that was complex to machine and to assemble, penalised by weight, size and surface finish, and burdened by secondary operations on metal parts.

The intervention was not a material substitution but a complete re-engineering. The original component was rethought to be mouldable in technopolymer, optimising the geometry for functional integration: fewer parts, fewer process steps, a more compact layout. In some areas of the part, two-material moulding was adopted in order to combine different properties within the same element, with inserts and dedicated cycles managed directly in our mould.

The result is a lighter component, smaller in size, better in appearance and significantly lower in production cost, with unchanged functional performance. The full project is documented in the Lightweight Ventilation case study.

Akron case study: PEEK instead of metal for high-pressure coffee

The second case concerns components for a coffee machine intended for vending applications, therefore with direct contact with hot liquids, repeated pressure and temperature cycles, and food compatibility requirements.

The existing parts were made of metal by die casting followed by CNC machining: costly, with uncompetitive lead times. The analysis led to the selection of PEEK, validated for mechanical strength, thermal stability, behaviour towards chemical agents and suitability for food contact. The design redefined the geometry to optimise injection flow, and the mould was built in-house to high-precision standards.

The most significant technical point is this: tolerances within hundredths of a millimetre were achieved directly at the moulding stage, eliminating subsequent machining entirely. The other results follow from that: lower overall costs, a substantial reduction in lead time, lighter components and stable performance even after repeated cycles. The details are in the High-Pressure Coffee case study.

When metal remains the right choice

A serious supplier also says when conversion is not worthwhile. There are situations in which it should be advised against, and recognising them in advance avoids spending time and budget on prototypes destined to fail.

Low volumes. Below a few thousand parts per year, with a short life cycle, the mould is never amortised. In these cases machining from solid or metal 3D printing remain the more rational options.

High loads concentrated on small surfaces. Where surface hardness, resistance to metal-on-metal abrasive wear or the ability to withstand very high clamping forces is required, polymer does not compete, except with overmoulded metal inserts.

Continuous temperatures above the limits of the material. If the part operates permanently above the limits of the candidate polymer, the degradation of properties over time makes conversion unsustainable, even when initial testing appears positive.

Requirements for conductivity, EMC shielding or heat dissipation. Conductive compounds exist, but their performance remains far from that of metal. When the primary function of the part is to conduct current or heat, metal is the answer.

Safety components governed by standards that prescribe the material. In some fields, compliance explicitly requires metal, and the technical discussion ends there.

One further consideration concerns creep: a component subjected to a permanent load for years, such as a preloaded structural element, is almost always a poor candidate for conversion, however encouraging short-term testing may be.

How to start the assessment with DFM

The fastest way to establish whether a component can be converted is to start from the actual part, not from a generic hypothesis. The path we follow is structured in four steps.

We begin with an analysis of the existing component, in order to identify critical functions, stress areas and interface constraints with the rest of the system. This is followed by redesign for moulding, which is the real core of metal replacement: uniform wall thickness, draft angles, ribs in place of solid masses, positioning of gates and weld lines. Material selection comes next, cross-referencing the five initial parameters, and finally validation, with mould flow analysis and prototyping before the mould is frozen.

This is precisely the content of our technical feasibility analysis: we assess the component and indicate whether conversion makes sense, with which material and with what estimated economic return, before a single euro is spent on tooling.

If you have a metal component that costs too much or that you would like to make lighter, the fastest way to get an answer is to send us the drawing and the operating data. Contact us and we will discuss it with our engineers.