From prototype to pilot production: managing the transition for medical devices

The transition from prototype to pilot production is one of the most consequential – and most commonly underestimated – stages of medical device development. It is the point at which development decisions become manufacturing decisions, and where problems that were manageable at the prototype stage can become expensive at volume.

Getting the transition right means thinking about it earlier than feels necessary. The process, material and tolerance decisions made during late-stage prototyping have a direct bearing on what pilot production looks like, how long tooling takes, and whether the first moulded parts actually match the design intent.

This article is for engineers and product development leads approaching that transition – covering the key decisions involved, the risks that most commonly cause delays, and how to structure the process to avoid them.

What pilot production actually means

Pilot production sits between the end of the prototype phase and the start of volume manufacture. Its purpose is to produce a small number of parts – typically tens to low hundreds – in production-intent materials and processes, for use in design validation, regulatory submission, clinical evaluation or initial market release.

The distinction matters because pilot production parts are not prototypes. They need to be made in the same process and from the same material as the eventual production run, or the validation data they generate does not transfer. A clinical evaluation carried out on SLA parts does not validate a device that will be injection moulded in a medical-grade polymer. A design validation build using CNC machined components does not demonstrate that the injection moulded version will perform the same way.

This is the point at which the transition from prototyping processes to production processes needs to happen – and it is earlier than many development programmes plan for.

The injection moulding transition: when and how to make it

For plastic components, injection moulding is the production process for the vast majority of medical devices. The transition from prototype to moulded parts involves two steps: tooling and first-off validation.

When to commit to tooling

Committing to injection mould tooling before the design is sufficiently stable is one of the most common – and most costly – mistakes in medical device development. Tool modifications are expensive, slow, and sometimes not possible without scrapping and remaking the tool entirely. The general principle is to commit to tooling only when the design has been validated to the point where only minor adjustments are anticipated.

In practice, that means:

• Geometry has been confirmed through functional prototype testing
• Material selection is finalised – the production polymer has been selected, and its properties are understood
• Tolerances on critical features have been confirmed as achievable in the moulding process
• Assembly interfaces have been verified against mating components

Vacuum casting is often the right bridge at this stage. It produces small batches of polyurethane parts with properties that approximate a moulded polymer – close enough to carry out meaningful evaluation – without the cost and lead time of hard tooling. Running a vacuum cast evaluation batch before committing to a mould tool is a low-risk way to catch design issues while they are still cheap to fix.

What to tell your toolmaker

The quality of a mould tool depends heavily on the information provided at the design stage. Before tooling commences, it is worth confirming:

• The production material, including grade and any additives – different polymers shrink differently, and the tool is cut to account for the specific shrinkage of the specified material
• Critical tolerances – which dimensions need to be held tightly, and which have more latitude
• Surface finish requirements – tool steel finish translates directly to part surface finish
• Parting line position – which affects both the aesthetics of the part and the complexity of the tool
• Any features that require side actions, lifters or collapsible cores – these affect tool cost, lead time and long-term maintenance

Having the same supplier manage prototyping and toolmaking is an advantage here. The toolmaker already has the part history, understands how the design has evolved, and has seen the issues that came up during prototype testing. That context does not have to be transferred across a supplier handover.

Material continuity: closing the gap between prototype and production

One of the most common sources of problems at the pilot production stage is a gap between the material used in prototype testing and the material used in the moulded part.

This can happen for straightforward reasons – the SLA resin used for prototype evaluation does not have the same properties as the production polymer, so the functional test data does not fully transfer. It can also happen when the production material is changed late in development without re-running the relevant tests.

The way to manage this is to finalise production material selection earlier in the development cycle than feels urgent, and to run at least some of the functional evaluation in a material that closely represents production intent. Vacuum casting in a polyurethane grade that approximates the production polymer is one approach. CNC machining in the actual production material – where that material is available in billet form – is another.

For programmes following a regulated pathway, material continuity also has a documentation dimension. The biological evaluation for the device needs to cover the materials used in the final product, not just the materials used in prototype testing. If the production material changes after the biological evaluation has been carried out, the evaluation may need to be updated.

Tolerances: what changes when you move to moulding

Prototype processes – CNC machining in particular – can hold tolerances that injection moulding cannot reliably replicate. This is not a problem if the design accounts for it. It becomes a problem if tight tolerances specified during prototype development are carried forward into the moulded part without review.

Injection moulding tolerances depend on the material, the part geometry, the tool design and the process parameters. As a general guide, ±0.1 to ±0.2 mm is achievable across most features in a well-designed tool with a stable process. Tighter tolerances are possible on specific features with careful tool design, but they add cost and require more process control.

The review to carry out before tooling is: which tolerances on this part actually matter for function and assembly, and which were inherited from the prototype drawing without being challenged? Tolerances that were easy to hold in CNC machining and are not critical to function can often be relaxed for moulding, which simplifies the tool and reduces the risk of out-of-tolerance parts in production.

This is also the point at which GD&T becomes particularly useful. Defining tolerances in terms of functional requirements – position of a bore relative to an assembly datum, flatness of a sealing surface – rather than as absolute dimensions makes it easier to translate the design intent from prototype to moulded part without inadvertently tightening or relaxing requirements that matter.

Inspection at the pilot production stage

First-off inspection is standard practice at the start of a moulding run – measuring a sample of parts from the first shots against the drawing to confirm that the tool is producing parts within tolerance before the full run proceeds.

For medical device programmes, inspection at the pilot production stage typically goes further than a simple dimensional check. It may include:

• Full dimensional report against the drawing, including all GD&T callouts
• Material verification – confirming the correct grade was used
• Visual inspection for surface defects, sink marks, weld lines and other moulding artefacts
• Functional checks – confirming that assembled interfaces work as intended

Having inspection carried out by the same supplier who made the tool and ran the parts means that any issues identified are addressed immediately, without the delay of parts being shipped to a third party for measurement and then returned for adjustment.

A note on regulatory submissions

For devices following a regulated pathway to market – EU MDR, FDA 510(k), ISO 13485 certification or similar – the pilot production stage generates some of the documentation that will form part of the technical file or design history file.

This includes dimensional inspection records, material certifications, and process records for the moulding run. It is worth establishing what documentation is required for the regulatory submission before pilot production begins, so that the right records are captured at the time rather than reconstructed afterwards.

If your programme has specific documentation requirements, raise these at the tooling stage. It is straightforward to build the right recording practices into the process from the start; it is considerably harder to produce compliant records retrospectively.

Managing the transition without losing programme knowledge

The practical risk in any supplier handover – from prototype supplier to toolmaker, or from toolmaker to moulder – is that programme knowledge gets lost in the transfer. The history of design decisions, the issues that came up in prototype testing, the tolerances that proved difficult to hold, the assembly interfaces that needed adjustment – all of this is valuable context that should inform the tooling and production process.

The simplest way to manage this risk is to minimise the number of handovers. A supplier who can take a programme from prototype through to pilot production – covering vacuum casting bridge builds, injection mould tooling, first-off inspection and low-volume moulding – retains that knowledge throughout, without it having to be transferred and potentially lost at each stage.

At Prototype Projects, we offer this full journey under one roof. Our team has managed the prototype-to-production transition for medical device programmes across a range of applications, and we are used to the documentation and quality requirements that regulated development brings.

Ready to start your programme?

Whether you are approaching the end of prototype development and planning the tooling stage, or earlier in development and thinking ahead to production, our team is happy to discuss the transition at any point.

Call us on 01763 249760, email office@prototypeprojects.com, or request a quote.