A thermoformed part designs differently than an injection-molded or sheet-metal part. The rules that matter most: give every vertical wall draft, keep the draw ratio shallow enough to preserve wall thickness, use generous radii, and put tight tolerances only on CNC-trimmed features. A part drawn to those rules quotes faster, tools cheaper, and forms more consistently.
Most of the cost and molding challenges PCI sees in incoming models trace back to one of those four rules. Here is each one in enough depth to apply.
Thermoforming design guides published across the industry converge on a minimum draft of 2 to 3 degrees on walls formed over male tooling, and 5 degrees or more where a molded-in texture runs down a wall. More draft is always cheaper than less.
Draft exists because thermoplastic shrinks onto the tool as it cools. A vertical wall with no draft grips the mold and fights removal, which slows the cycle and can distort or stress the part. Female tools and female features on male tools are more forgiving, since shrink pulls the part away from the cavity walls rather than onto them, and can run with less draft where geometry demands it.
Texture compounds the requirement. A textured surface dragged across a tool shows stretching more, so textured walls need extra draft to minimize the change in pattern. When a design cannot give up the draft, the fix is usually a tooling one, and it is worth a conversation with the toolmaker before the geometry freezes. PCI's in-house tooling department handles that review as part of design-for-manufacturing feedback.
A thermoformed part starts as a flat sheet, and every inch of depth the sheet stretches into thins the wall. The deeper the draw relative to the part's opening, the thinner the final wall. Designers control finished thickness by specifying starting gauge, keeping draws shallow where possible, and letting the thermoformer apply techniques such as plug assist on deep sections.
The working concept is a real draw ratio: the surface area of the finished part divided by the footprint of the sheet that formed it. A shallow tray barely stretches the material, so the finished wall stays close to the starting gauge. A deep enclosure might stretch the sheet to double its original area, which cuts average wall thickness roughly in half and thins corners the most, since material stretches last and least into corner geometry.
This is why heavy-gauge drawings should specify starting gauge and minimum wall at critical locations rather than a uniform finished thickness, which sheet forming physically cannot deliver. An experienced thermoformer manages the thinning with tool design, plug assist, and heating profile, and will tell you at quote time whether the minimum wall you need is reachable from a sensible gauge.
Formed features on heavy-gauge parts commonly carry tolerances of roughly plus or minus 0.060 to 0.125 inch or more depending on part size and material, while CNC-trimmed features hold considerably tighter, with plus or minus 0.030 inch typical in industry design guidance. The exact numbers for a given part depend on geometry and material shrink, and are confirmed during design review.
The split matters more than the numbers. Formed dimensions ride on material shrink, which varies with resin, gauge, and geometry: HDPE shrinks substantially more than ABS, and shrink is what the tool must be compensated for. Trimmed dimensions ride on a 5-axis CNC router working against a fixed fixture, which is a machining operation with machining repeatability.
The design implication is direct. Put mounting holes, mating edges, and interface features where the router creates them, and let styled or formed surfaces carry open tolerances. A drawing that blankets the part in tight tolerances forces inspection and fixturing cost onto features that never touch anything. Flag the dimensions that control fit; PCI's engineering review will confirm what each one can hold in the chosen material.
Corner radii should be as generous as the design allows, with a common industry rule of thumb setting the minimum inside radius at or above the starting sheet thickness. Undercuts are formable, but each one adds tooling complexity in the form of moving cores or split tools, so they should earn their place.
Radii and wall thickness are linked. Sheet stretching into a sharp corner thins aggressively at exactly the point most likely to take an impact, so a tight radius trades away strength where the part needs it most. Opening the radius lets material flow into the corner and keeps the wall closer to nominal.
Undercuts deserve the same scrutiny as tight tolerances. Snap features, return flanges, and wrapped edges are all achievable, and sometimes they eliminate fasteners or secondary parts, which is a good trade. The point is to make the trade knowingly. A ten-minute call with the toolmaker often finds a way to keep the function and drop the tooling motion.
Material choice moves every number above. Shrink rate sets tool compensation and achievable formed tolerances, stiffness sets how much wall a panel needs to resist flex, and temperature capability sets whether the finished thickness survives the service environment.
A few examples show the range. HDPE brings outstanding chemical resistance and toughness with a melting point around 266 degrees Fahrenheit per material supplier specifications, but its high shrink demands more tooling compensation and looser formed tolerances than ABS. PETG offers roughly four times the impact resistance of acrylic per material manufacturer spec comparisons, which lets a designer thin a clear guard without giving up durability. Polycarbonate's stiffness supports large flat panels that would oil-can in a softer resin.
The practical takeaway: pick the material and set the design rules together, not in sequence. PCI forms more than a dozen material families and will flag when a geometry and a resin are working against each other.
Before the design freezes. A design-for-manufacturing review at the model stage costs nothing and routinely removes tooling motions, relaxes unneeded tolerances, and consolidates assemblies, savings that are locked out once tooling is cut.
Consolidation is where early reviews pay off most. A thermoformed part can absorb brackets, covers, and fastened panels into one formed geometry, the same logic that turned a 718-pound welded steel belt guard into a single 38-pound ABS part in a PCI case study. Those opportunities are visible in a CAD model and invisible in a finished drawing package.
PCI has run heavy-gauge design reviews since 1972, with tooling, forming, and trimming under one roof, which means the feedback comes from the people who will build the tool.
Ribs can be formed into the sheet geometry itself, and stiffness is often designed in through crowns, steps, and corrugations. Bosses and other back-side features that injection molding molds in are typically added to a thermoformed part as bonded or fastened components.
Machine bed size is the limit, and heavy-gauge equipment commonly forms parts several feet on a side from a single sheet. Large single-piece geometry is one of the process's main advantages over injection molding, where mold and press size make big parts prohibitively expensive.
No, and a drawing should not specify one. Wall thickness varies with draw depth by the physics of sheet forming. Specify starting gauge plus minimum wall at the locations that carry load, and let the process determine the rest.
Have a model in progress? Send it to PCI for a design review before you freeze the geometry. The feedback covers draft, draw, radii, tolerance placement, and material fit, and it comes back with the quote.