Why wall thickness rules the outcome
Of all the rules in prototyping, holding wall thickness within a proven range and uniform across the part prevents more failures than any other single habit. Warp, sink, cracking, and weak sections almost always trace back to thickness decisions made in the CAD model.
The good news is that it is the easiest rule to apply. It needs no special tool or machine, only discipline in the model before anyone cuts metal, pours resin, or closes a mold. A few minutes of thickness review saves days of rework.
What actually happens during solidification
Thick sections hold more heat, or cure more slowly, and shrink more as they cool; thin sections solidify quickly. When adjacent walls differ a lot, the thick one pulls the part into warp or leaves a sink, a shallow dimple, on its outer face.
In plastics the effect is worst because of high volumetric shrinkage; in metal CNC it appears as chatter and deflection of thin walls; in casting and molding it is the classic sink mark and hot-spot cracking that scrap whole batches.
Minimum wall thickness by process
For CNC, metal needs about 0.8 mm and plastic about 1.5 mm because thin walls deflect under the cutter. SLA resin needs about 1.0 mm and FDM about 1.2 mm. Sheet metal is limited to the stock gauge, so you cannot go thinner than the sheet you buy.
Injection molding and vacuum casting follow plastic-flow rules: typically 1.5 to 3.0 mm, and rarely below 0.5 mm, or the melt will not fill the cavity and the wall will flash, short-shot, or burn.
Maximum wall and the sink trap
Very thick solid walls, say above 4 to 6 mm in plastic, waste material, lengthen cycle time, and invite sink marks and internal voids. Thickness should follow function, not habit or a fear of thin sections.
When you need stiffness, do not simply thicken the wall. Add ribs, gussets, and bosses instead. A rib at about 60 percent of the wall thickness delivers most of the stiffness gain with a fraction of the mass and none of the sink risk.
Keep thickness uniform across the part
The single best rule is to aim for a uniform wall, ideally holding the thick-to-thin ratio across the whole part between 1:1 and 2:1. Uniform sections solidify together and hold their shape without internal pulling forces.
Where a feature must be thicker, such as a boss or a hinge barrel, taper it gradually toward the base and keep the jump small so stress concentrates less during cooling and the surface stays flat.
Transitions: taper, do not step
Never jump from 1 mm to 5 mm in a single step. A sudden step creates a hot spot and a stress concentrator that cracks or sinks as the part cools, and it is a common source of latent field failures.
Taper the transition over several millimeters using a draft-style slope of about 1:3 to 1:5. The gradual change lets heat and shrinkage distribute evenly so the part relaxes instead of fighting itself.
Corners, radii and draft
Sharp internal corners accumulate stress and are hard to fill or machine; add fillets of at least 0.5 mm so material flows and cutters do not gouge. External corners can stay crisp but still benefit from small radii that ease finishing.
For molded or cast parts, add draft of 1 to 3 degrees to walls so the part releases without dragging the surface. CNC and 3D printing need less draft, but both still appreciate relieved corners that avoid tool marks.
Process-specific thickness notes
In CNC, metal walls under 0.8 mm chatter and deflect, so keep at least that and add support ribs. In 3D printing, respect the per-process minimum or walls snap during support removal. In sheet metal, design to one gauge and add hems or flanges for stiffness.
For injection and vacuum casting, hold a uniform 1.5 to 3.0 mm with ribs. For SLA and SLS, watch over-cure and warpage on large flat thin areas and add subtle curvature to fight the tendency to bow.
How to check thickness before building
Section the model and measure the thinnest and thickest walls; if the ratio exceeds about 2:1, revise the offending feature. Many CAD packages display wall thickness or let you run a section analysis to catch problems early.
If the design is novel, machine or print a critical corner as a quick check part first. Finding a sink or warp on a twenty-dollar test is far cheaper than discovering it on a two-thousand-dollar batch.
Cost and lead-time impact
Over-thick walls add material and machine or cycle time directly to cost; under-thick walls cause scrap and rework that blow the schedule. Right-sized walls are the cheapest insurance a prototype can buy.
Uniform, in-range walls also let the shop run standard parameters without special fixturing, which shortens lead time and raises first-pass yield, so the whole program moves faster.
Decision checklist and FabProto support
Before release, list every wall and confirm each is above its process minimum, the thick-to-thin ratio is under 2:1, any necessary step is tapered, and ribs replace added thickness wherever stiffness is needed.
Send FabProto your model and we will run a wall-thickness and draft review for your chosen process, flag risky sections, and suggest ribs or tapers so the prototype holds up in real use rather than only on screen.