This article was originally published in October 2024 and has been updated in March 2026 to reflect recent insights and developments.

At OPD, we see surface continuity play a key role not just in how products look, but in how they are perceived, used, and realised in manufacture.

As products become more refined - particularly in medical devices, premium consumer products and high-spec industrial equipment - expectations for seamless, high-quality surfaces continue to rise.

We 3D model a huge variety of parts in many sectors ranging from: jigs and fixtures, wearables, PPE, smartwatch charging docks, kitchenware and more. For me, one of the most challenging but rewarding ways to model is using surface modelling.

Surface modelling refers to creating simple planar or complex 3D surfaces from curves or splines. These surfaces have zero thickness but are usually joined together to form a solid body at some point during the modelling process.

When modelling with surfaces the aim is to produce beautifully smooth, purposeful surfaces and surface transitions that exactly capture the design intent and styling set out by the industrial design team which are usually not achievable with more regularly used extrudes, revolves, and sweeps. Many product examples of this can be seen on our website. These include: the three wearables that make up the offering from gaitQ or the various safety helmets and face masks.

Achieving this level of surface quality isn’t just a visual exercise. It requires careful control of geometry, tolerances and transitions between surfaces.

Within our New Product Science™ approach, these decisions are defined by clear requirements and validated through iteration, helping ensure that what looks right in CAD performs as expected in real-world use and manufacture. And that is before the surface transition or continuity is considered. Do we want to model using G1, G2 or G3 surface connections? The answer is not always G3.

Surface continuity and manufacture

Manufacturing processes, material behaviour, and tooling constraints all influence how surfaces are realised in production. Small deviations in curvature or transitions can become highly visible in the final product.

By considering these factors early in development, we reduce late-stage iteration and help ensure that the final product reflects the intended design - both visually and functionally.

Our in-house prototyping and pilot production capabilities also allow us to physically validate surface quality early - reducing risk before tooling.

Types of surface, connections

Here is a brief summary of the possible surface connections we can model.

To help us understand what is going on, we can place “Curvature Combs” on top of curves and surfaces to exaggerate the minute variations of curvature that we cannot see by eye:

G0: Point continuity

Two surfaces or curves touching one another in any way but not tangentially.

Look how the curvature combs are at different heights and do not touch each other.

G1: Tangential

Two surfaces or curves touching tangentially meaning they share the same angle at that junction. Think of the rad feature in SolidWorks.

The curvature combs are touching but are at drastically different heights.

G2: Curvature Continuous

Two surfaces or curves touching tangentially. The curvature of both surfaces at the intersection point will also be the same.

The curvature combs are touching and at the same height but there is not a smooth transition between them.

G3: Curvature Continuous

Two surfaces or curves touching tangentially. The curvature of both surfaces at the intersection point will also be the same. The rate of change or “acceleration” of the curve at that point will also be the same for each curve.

The curvature combs are now touching, at the same height and there is a super smooth transition between them.

Deciding what level surface continuity to use

When moving from G0 up to G3 surface connections, the level of control required, design time and model finessing heavily increases with each step. Therefore, the desired surface connections are a huge consideration before commencing any surface model. Depending on the geometry we are trying to model, this decision could either lengthen development time for a barely noticeable gain or be the deciding factor that successfully visually communicates the brands values of style and quality. To be clear - we are not talking about the specific styling of a product but how each surface interacts with each other. Here at OPD we think the product can and should always have a well-considered sympathetic aesthetic. Once that has been refined but before commencing final modelling, like any design challenge, we do a lot of upfront thinking first and consider the requirements and context of the product such as:

Sector

What sector does the product sit within? Is it a high-end consumer product where the surface quality needs to align with the company's values or an industrial product that will be physically put through its paces and where aesthetics is not the key driver of the product?

Manufacture

How will the part be made? Injection moulding? CNC?

What will the surface texture likely be? Will the part be high gloss, possibly showing up every tiny imperfection? Or will the part be textured, meaning G3 continuity may be obscured?

Where will the split-lines sit relative to the surface connections? If the split lines sit where surfaces connect, a high level of continuity may not be needed and the design time could be reduced.

Software

What CAD package should be used?

Does the output need to be native editable CAD or is a non-native output okay?

What software will be used to interpolate the surface continuity, considering both manufacture and production?

Project logistics

What design time do we have available to allocate to this stage in the project?

What is important to the client, and what are their expectations?

Can “Major” surfaces have a higher level of continuity compared to the smaller “Minor” surfaces?

Example: wireless in-ear headphones and charging case

We can surface model using any of these continuities as we did recently when we modelled a set of wireless in-ear headphones and charging case. The case is made up of multiple surfaces and the smooth transitions can be seen by the smoothness of the zebra stripes that have been projected onto the surface. As discussed above, the main question in our minds was, are the surface connections critical to the success of the product? Here a high surface continuity was critical to what was required so we started sculpting and kept going until we were happy with even the smallest details of the design.

As CAD tools and surface analysis methods continue to evolve, designers are better able to evaluate continuity earlier in development - reducing risk and improving confidence in the final result.

Whether developing medical devices, industrial equipment or consumer products, surface continuity plays a key role in delivering products that feel considered, high-quality and fit for purpose.

Getting it right requires more than visual judgement - it depends on understanding geometry, manufacturing and real-world performance.

If you’re developing a product where form, usability and manufacturability need to come together, we’d be happy to share our experience.