Frequently Asked Questions

Is it viable to convert my part material from metal to plastic?

When assessing plastic as a substitute for metal there are many factors to take into consideration. Not all are strictly design based, other factors can be desirability, finishing options, production cost, or sustainability to name a few.Metal to Plastic Parts

Motivation can also vary widely, from cost down exercises to weight saving and logistics.

Whatever the reason and regardless of whether this is a new part or a replacement, plastic is a very different material from metal and must be handled appropriately for the project to succeed.

 

How do metal and plastic differ from an engineering point of view?

  1. Plastics tend to have a non-linear time-dependent stress/strain behaviour. Below the yield point, metals typically have a nearly linear elastic behaviour.
  2. Instead of a single melting point, plastics typically have a ‘glass transition temperature’ whereas metals typically have a clearly defined melting point.
  3. Plastics tend to exhibit ‘creep’ whereas in metals, creep tends to be insignificant.
  4. Plastics can degrade in air whereas metals corrode.
  5. Plastics can be affected by bacterial infestation whereas metals will not.
  6. Thermal expansion of plastics can be up to 20x higher than in metal.
  7. Plastics are much slower at dissipating heat than metals. (although there are specific grades now targeted at manufacturing heat sinks)
  8. The softening and melting temperatures of plastics are much lower than in metals.
  9. Plastics are more elastic than metals.
  10. Plastics have a very different inner structure in terms of their chemical bonds as well as the structural arrangement of their macromolecules in comparison to the lattice structure of metals.

So why design in plastic rather than metal?

1) Cost

Generally plastic will be injection moulded although for some low volume/tight tolerance/high specification applications machining will be preferred. Metal on the other hand will be machined or cast/machined unless parts are small/complex in which case metal injection moulding (MIM) is an option.

Moulding brings with it tooling costs, which generally makes it more applicable to higher quantities, although this is something Plunkett Associates is driving down enormously!

Hence if quantity is present plastic can offer a cost advantage, so long as the material properties are adequate. This also gives rise to one of the reasons for transitioning to plastic, initial sales volumes only warrant machining, but sales take off and moulding is suddenly a competitive solution. The downside is the part will require redesigning to take full advantage of the material change.

2) Weight

Stringent weight constraints may also drive material selection to plastics. The aerospace sector being a good example of where weight is a primary driver and despite typically low quantities, moulded or cnc machined plastics are frequently found.

Plastic partThe addition of fillers such as glass or carbon fibres can also radically change plastic properties. This results in materials such as Grillamid, which is sold as a ‘metal replacement’ material.

3) Environmental

Environmental factors can make plastics very attractive as they do not corrode.

4) Surface Finish

Frequently part finish comes for free when moulding components as opposed to additional operations when machining metal. Colour and texture can be used to differentiate a product, and branding can be incorporated, all at minimal cost.

The decision whether or whether not to use plastic is reliant on an exact knowledge of the actual mechanical short and long term stresses to which the component will be exposed over its lifetime.  Additional information such as the end-use application along with any environmental conditions such as chemical exposure, extremes of heat or cold are all required before sensible designs can be considered and any decision about material made.

Summary

Vacuum cast plastic partCurrent metal-to-plastic trends focus on reducing weight, improving strength and corrosion resistance, and consolidating multiple metal parts into one plastic part. 

The ability for plastic parts to be just as tough as metal parts and achieve the same tight tolerances, with fewer secondary operations, plus an overall cost saving, often creates a win-win argument, in favour of using plastics.

Ultimately, the decision whether or whether not to use plastic is reliant on an exact knowledge of the actual mechanical short and long term stresses to which the component will be exposed over its lifetime.  Additional information such as the end-use application along with any environmental conditions such as chemical exposure, extremes of heat or cold are all required before sensible designs can be considered and any decision about material made.

The crux of it is that each part and each project is unique – what works for one may not work for another, as there are so many factors to take into account. For advice and the best solution for your part, please contact us. We will be happy to help.

  • “Plunkett Associates have excellent breadth of experience, work with high quality partners and have a flexibility that we have found enables challenges to be met successfully, cost effectively and in a timely manner.”
    Nick Skaer, CEO, Orthox Ltd
  • Plunkett associates always find a solution. Whatever we need to prototype, they have a great grasp on the technologies available, costs and lead-times so we get the right solution tailored to the project needs. Have always delivered and never let us down.
    Naim Audio
  • “I can’t recommend Tim and his team highly enough. We ended up with a design which can be produced and assembled quickly, efficiently and reliably, in volumes that make sense”
    Simon Leggett, Director, Olivewood Technology
  • "Tim's experience, expertise and supplier base have been fundamental to us getting this project off the ground."
    Sam Pearce, Jelly Products Ltd
  • “We were really surprised at how quickly they turned round such a high quality prototype and the parts certainly proved their worth. They were of high enough quality to provide a fantastic visual representation and to undergo rigorous testing."
    Stuart Wilson, Design Director, TRT Lighting
  • “Accuracy, finish and delivery all first class.”
    Andrew Peters, JMDA Ltd
  • “We use Plunkett Associates because we want someone we can trust, that knows what they’re talking about, has all the contacts, will provide reliable advice and feedback and will get the right result for us.”
    Jim Copestick. Design Manager, Thorlux Lighting
  • “We always know exactly what we’re getting and we know we won’t be let down on quality, price or lead times.”
    Mark Williams, Research and Development Director, Johnson Safety Products
  • “Plunkett Associates truly offer a complete turnkey solution with Speed, Effectiveness and Precision.”
    David Harden, Director, Enica Ltd
  • “Kirintec were thrilled with Plunketts. They made a complex process simple and we were just amazed at the lead times achieved. Plus, every single deadline was met without any drama.”
    Penny While, Marketing Director, Kirintec
  • “Thanks to Plunkett Associates flexible, bespoke and cost effective service I was able to produce the objects that I had originally had in mind.”
    Simon Ryder, Artist
  • To create such a consistent quality finish in chrome was an amazing achievement and the prototypes really gave our stand the wow factor.
    James Clark, Group Commercial Director, The Airdri Group
  • Interestingly, people who look at it in its own right with little knowledge of what it does, just like it! It’s tactile and it looks good…
    Tim Collings, Operations Director, Troika Systems Ltd
  • "With their simple to understand language, willingness to go the extra mile and approachability, Plunkett Associates were a great partner for our company. We couldn’t be happier with the results of our project. (A working prototype that they got right first time, by the way). Thank you!"
    Carolina Ballon-Forrester, Director, Kittyrama Ltd
  • Plunkett Associates worked on the design and production of titanium frames for an artificial heart valve where precision and consistency was paramount. Communications were efficient and productive with concepts readily incorporated into the final design, and suggestions for improvements all highly appropriate. The quality of the end product was ideal and I have no doubt that I will use the advice and expertise of Plunkett Associates again.
    David Wheatley, Wheatley Research Ltd
  • “Thanks for the present that arrived this morning, as promised. Parts look first class, as always.”
    Jon Todd, Design Manager, PHS Group
  • “Plunkett Associates have a fantastic supply chain, work with all sorts of production processes and will always do their utmost to solve a problem.”
    Naim Audio
  • “ Plunketts developed a solution that met our precision requirements at a cost significantly lower than alternative manufacturing approaches we had investigated”
    Nick Skaer, CEO, Orthox Ltd
  • “Doing business with you has been an absolute pleasure. You have made my goals a reality. You have been patient, helpful and above all, responsive. Best service that I have experienced in ages. I consider Plunkett Associates to be an integral part of my business plan moving forward.”
    Jet Cooper, Sculptor
Related Processes

CNC Machining

The benefits of CNC parts include substantial flexibility in material choice, high accuracy and repeatability.

more info

Injection Moulding

Produces production intent material properties. Injection Moulding is used for development as well as production parts.

more info

Read more expert articles in our FAQ section where we discuss topics on materials, methods and products.

Back to FAQs
  • “Plunkett Associates have excellent breadth of experience, work with high quality partners and have a flexibility that we have found enables challenges to be met successfully, cost effectively and in a timely manner.”
    Nick Skaer, CEO, Orthox Ltd
    Nick Skaer, CEO, Orthox Ltd
  • Plunkett associates always find a solution. Whatever we need to prototype, they have a great grasp on the technologies available, costs and lead-times so we get the right solution tailored to the project needs. Have always delivered and never let us down.
    Naim Audio
    Naim Audio
  • “I can’t recommend Tim and his team highly enough. We ended up with a design which can be produced and assembled quickly, efficiently and reliably, in volumes that make sense”
    Simon Leggett, Director, Olivewood Technology
    Simon Leggett, Director,  Olivewood Technology
  • "Tim's experience, expertise and supplier base have been fundamental to us getting this project off the ground."
    Sam Pearce, Jelly Products Ltd
    Sam Pearce, Jelly Products Ltd
  • “We were really surprised at how quickly they turned round such a high quality prototype and the parts certainly proved their worth. They were of high enough quality to provide a fantastic visual representation and to undergo rigorous testing."
    Stuart Wilson, Design Director, TRT Lighting
    Stuart Wilson, Design Director, TRT Lighting
  • “Accuracy, finish and delivery all first class.”
    Andrew Peters, JMDA Ltd
    Andrew Peters, JMDA Ltd
  • “We use Plunkett Associates because we want someone we can trust, that knows what they’re talking about, has all the contacts, will provide reliable advice and feedback and will get the right result for us.”
    Jim Copestick. Design Manager, Thorlux Lighting
    Jim Copestick. Design Manager, Thorlux Lighting
  • “We always know exactly what we’re getting and we know we won’t be let down on quality, price or lead times.”
    Mark Williams, Research and Development Director, Johnson Safety Products
    Mark Williams, Research and Development Director, Johnson Safety Products
  • “Plunkett Associates truly offer a complete turnkey solution with Speed, Effectiveness and Precision.”
    David Harden, Director, Enica Ltd
    David Harden, Director, Enica Ltd
  • “Kirintec were thrilled with Plunketts. They made a complex process simple and we were just amazed at the lead times achieved. Plus, every single deadline was met without any drama.”
    Penny While, Marketing Director, Kirintec
    Penny While, Marketing Director, Kirintec
  • “Thanks to Plunkett Associates flexible, bespoke and cost effective service I was able to produce the objects that I had originally had in mind.”
    Simon Ryder, Artist
    Simon Ryder, Artist
  • To create such a consistent quality finish in chrome was an amazing achievement and the prototypes really gave our stand the wow factor.
    James Clark, Group Commercial Director, The Airdri Group
    James Clark, Group Commercial Director, The Airdri Group
  • Interestingly, people who look at it in its own right with little knowledge of what it does, just like it! It’s tactile and it looks good…
    Tim Collings, Operations Director, Troika Systems Ltd
    Tim Collings, Operations Director, Troika Systems Ltd
  • "With their simple to understand language, willingness to go the extra mile and approachability, Plunkett Associates were a great partner for our company. We couldn’t be happier with the results of our project. (A working prototype that they got right first time, by the way). Thank you!"
    Carolina Ballon-Forrester, Director, Kittyrama Ltd
    Carolina Ballon-Forrester, Director, Kittyrama Ltd
  • Plunkett Associates worked on the design and production of titanium frames for an artificial heart valve where precision and consistency was paramount. Communications were efficient and productive with concepts readily incorporated into the final design, and suggestions for improvements all highly appropriate. The quality of the end product was ideal and I have no doubt that I will use the advice and expertise of Plunkett Associates again.
    David Wheatley, Wheatley Research Ltd
    David Wheatley, Wheatley Research Ltd
  • “Thanks for the present that arrived this morning, as promised. Parts look first class, as always.”
    Jon Todd, Design Manager, PHS Group
    Jon Todd, Design Manager, PHS Group
  • “Plunkett Associates have a fantastic supply chain, work with all sorts of production processes and will always do their utmost to solve a problem.”
    Naim Audio
    Naim Audio
  • “ Plunketts developed a solution that met our precision requirements at a cost significantly lower than alternative manufacturing approaches we had investigated”
    Nick Skaer, CEO, Orthox Ltd
    Nick Skaer, CEO, Orthox Ltd
  • “Doing business with you has been an absolute pleasure. You have made my goals a reality. You have been patient, helpful and above all, responsive. Best service that I have experienced in ages. I consider Plunkett Associates to be an integral part of my business plan moving forward.”
    Jet Cooper, Sculptor
    Jet Cooper, Sculptor