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Case study · Ooni Dough Proofing Trays

A deceptively simple tray, from a contradictory brief to the production line.

Ooni's dough proofing trays are a modular, injection-moulded polypropylene system for proofing and storing dough balls. I was design lead and the sole design engineer from consumer research and the brief right through to production release, covering the concept and industrial design, the tolerancing, DFM with the tooling supplier, and production support.

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4.8★
across 460+ customer reviews
6
dough balls in a tray that still fits a domestic fridge
4 of 5
conflicting requirements met in full, the fifth by a quantified compromise
4
stacking combinations, each analysed to a tolerance stack
The design process

How the project ran, end to end

A tray sounds like the simplest product in the range, and that's exactly why it's a good example. With no electronics and no moving parts, everything comes down to the brief, the geometry, the tolerances and how it's moulded. As the only engineer on it, I took it through every stage.

  1. Consumer research and the brief

    I started with how people actually proof and store dough, and what customers said about the previous tray, then wrote the brief: fits in a fridge, holds six dough balls, nests for storage, and releases dough cleanly.

    User insights · brief creation

  2. Turn the brief into a specification

    Putting numbers on every requirement showed that five of them couldn't all be met at once. I proved that with hand calculations so the trade-off could be decided on evidence.

    Requirements engineering · hand calculation

  3. Explore concepts and the industrial design

    Concepts were sketched and modelled against the conflicting requirements, aiming for a minimalist look that sits well in a modern kitchen.

    Concept design · industrial design · CMF

  4. Prototype and test

    Printed prototypes let me check nesting, stacking, lid fit and dough release by hand before any steel was cut.

    Rapid prototyping · physical testing

  5. Tolerance and DFM with the toolmaker

    Statistical tolerance stacks for every way the parts fit together, plus draft, wall thickness and flow length worked through with the tooling supplier.

    Tolerance stack analysis · DFM · draft analysis

  6. Production CAD and the drawing pack

    Manufacturing-ready CAD and a full technical drawing pack, with all the project documentation managed alongside it.

    Production CAD · GD&T · technical documentation

  7. Tooling and production support

    I supported the supplier through tooling, checked the first articles against the analysis with CMM data, and released the product to production.

    Supplier collaboration · first-article inspection
3.1 · The specification Geometry, tolerance & DFM Results
Skills used

What this project called on

Engineering
Requirements engineeringHand calculationTolerance stack analysisStatistical tolerancingDFMConcept designIndustrial designCMFUser insightsTechnical documentation
Tools
SolidWorksDraft analysisManufacturing drawings & GD&TCMM inspection data
Manufacturing
Injection mouldingPolypropyleneLarge thin-walled mouldingsWarp controlFirst-article inspectionHigh-volume production
Case study 3.1 · Nesting, tolerance & DFM

Making a usable specification from an impossible brief

Problem

Each requirement in the brief made sense on its own. The trays had to fit in a fridge for cold proofing, giving a maximum outer envelope of 420 × 320 × 100 mm. They had to hold six 250 g dough balls, like the old tray, so the inside had to be at least 365 × 265 × 70 mm. They had to nest to save space in shipping and storage, they mustn't wedge together, and there could be no internal features, because those make it hard to get the dough out.

Together, they couldn't all be satisfied. Nesting needs draft, and draft plus fixed inner dimensions eats into the fixed outer envelope. Stopping the trays wedging needs a feature to bottom out on, but "no internal features" rules one out on the inside, and a clean inner face means a clean outer face too. The brief contradicted itself, so my first engineering task was to prove that in numbers rather than argue it in a meeting.

Requirements as received: fits in a fridge · holds six 250 g dough balls · trays nest · no wedging · no internal features.
Fig. 59, Required draft against nesting depth, for three wall thicknesses
Fig. 60, Concepts explored against the conflicting requirements
Process

Quantify the conflict, then hide the solution in plain sight

  1. 01Write the nesting condition. For one tray to enter another, the inside at the entry plane has to clear the outer wall of the tray above. Over a nesting depth h, at draft angle α and wall thickness t, that's 2h·tan α ≥ 2t + c, with c the running clearance.
  2. 02Put the trade-off in front of the stakeholders. With the outer and inner dimensions both fixed, h and t were fixed too, leaving draft as the only free variable. I mapped the draft required against nesting height and wall thickness with hand calculations and took the chart to the team, so the decision was made on the numbers.
  3. 03Make the anti-wedge feature something users want anyway. Draft alone means trays jam on the taper. I added external ribs on the tray wall, designed as handles, so the tray bottoms out on the ribs instead. They stop the wedge, stiffen a large thin-walled moulding against warp, and leave a gap to get your fingers under.
  4. 04Tolerance every way the parts meet. Lid on tray (seal and rattle), lid in tray (drop-in clearance), tray in tray (nest depth and wedge) and tray on lid (stack stability) are four different loops through the same features, so each got its own tolerance stack. Contributors included cavity dimensions, mould shrinkage across a large PP part, warp from uneven cooling, base flatness and the position of the rib seats.
  5. 05Turn pass/fail into a scrappage rate. Treating each contributor as normal and independent, σgap = √Σσi² and Z = (gmin − μgap) / σgap, so Φ(Z) predicts the fraction of assemblies that wedge or rattle. That let me tighten the rib seat tolerance where it was worth paying for and open everything else up.
  6. 06Work through the DFM with the toolmaker. Draft analysis on the production part, and a wall thickness, flow and stiffness trade: a 250:1 flow-length ratio for the chosen PP grade over a 310 mm longest flow path meant a wall of at least 1.25 mm to fill in theory. Tapered internal walls were engineered for clean, damage-free dough release.
Fig. 62, Nesting and draft hand calculations, notebook
Fig. 63, Ribs as handles, bottoming out in the nest
Fig. 65, The four stacking combinations
Fig. 64, Gap distribution against the wedge and rattle limits
Fig. 66, Draft analysis on the production part
Fig. 67, Wall thickness, flow and stiffness trade in the DFM review
Results

Requirements met, a compromise found for the conflict, and into production

4.8★460+ reviews
6dough balls per tray
Lowscrappage, as predicted

The trays nest without wedging, bottom out on a defined feature, have clean internal faces and fit in a domestic fridge. They hold six dough balls and are tall enough to bulk prove them too, and they work with the Pizza Prep Lid to extend the range.

The scrappage the stack predicted was borne out in production, with low scrappage and low failure, and CMM inspection of the first articles confirmed the rib seats and base flatness sat where the analysis said they would. The trays now sit at 4.8 stars across 460+ reviews.

Fig. 68, Trays stack and nest, lids stack and nest
Fig. 69, Stacked trays
Fig. 70, Hero shot
How I could help you

The same process, on your product

Turning a wish list into a specification you can actually build, then proving the tolerances before the tool is cut, is the core of Manufacturing Preparation. If your brief has requirements pulling against each other, a Feasibility Study is a good place to find out early.

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