The single most important rule in injection molding is that wall thickness should be as uniform as possible, and as thin as the part's function allows. Most molded thermoplastic parts land somewhere between roughly 0.04 and 0.12 inches (about 1 to 3 mm), with the exact figure driven by material, part size, and load. Almost every classic molding defect — sink marks, warpage, voids, short shots, long cycle times — traces back to walls that are uneven, too thick, or too thin. Get this one decision right and the rest of the design gets easier. Get it wrong and no amount of process tuning at the molder will save the part.

Why uniform walls matter so much

Molten plastic shrinks as it cools, and thicker sections cool more slowly. When one region is still shrinking while its neighbor has already frozen, the part pulls itself out of shape. That is warpage. If the thick section is trapped behind a frozen skin, the shrinkage happens internally and you get a void, or the surface dimples inward and you get a sink mark on a cosmetic face.

A part with genuinely uniform walls cools evenly, shrinks predictably, comes out flatter, and cycles faster because cooling time scales roughly with the square of the thickest section. Thick walls do not just look heavy on the drawing — they lengthen the cycle, and cycle time is the largest lever on your per-part price.

Typical ranges by material

Treat these as starting points for a conversation with your molder, not as absolutes. Flow length, gate location, and glass fill all shift them.

  • ABS: roughly 0.045–0.14 in — forgiving, good surface finish, the default for consumer housings.
  • Polypropylene: roughly 0.03–0.15 in — flows well, high shrink, living hinges are its signature capability.
  • Polycarbonate: roughly 0.04–0.15 in — strong and tough but viscous, so thin long walls are harder to fill. The ABS versus polycarbonate tradeoff often comes down to exactly this.
  • Nylon: roughly 0.03–0.12 in — thin walls fill easily, but it absorbs moisture and moves dimensionally.
  • Acetal (POM): roughly 0.03–0.12 in — excellent for gears and bearings, but high crystalline shrink punishes thickness variation harshly.
  • Glass-filled grades: stiffer and lower-shrink, but more prone to warping along fiber orientation, so uniformity matters even more.

If you have not locked the material yet, do that alongside the wall thickness decision rather than after it — our guide to choosing a plastic for your product covers the tradeoffs.

Need strength? Use ribs, not thickness

Stiffness scales with the cube of thickness in bending, so the instinct to thicken a flexing wall is understandable — and almost always wrong. Doubling a wall roughly quadruples the cooling time, adds material cost, and creates sink marks on the opposite face. Ribs deliver the same stiffness for a fraction of the material.

Practical rib guidelines:

  • Rib thickness: 50–60 percent of the wall it grows from. Thicker ribs telegraph as sink marks on the show surface.
  • Rib height: up to about three times the nominal wall. Taller ribs are hard to fill and hard to eject.
  • Draft: at least half a degree per side, more for textured surfaces.
  • Spacing: keep ribs at least twice the wall thickness apart so the steel between them can be cooled.
  • Many small ribs beat one large one for both stiffness and cosmetics.

The same logic governs bosses. A boss should be a hollow cylinder tied to nearby walls with gussets, not a solid post — a solid boss is a thick section by definition and will sink.

Transitions: when you truly cannot be uniform

Sometimes a part must change thickness. When it does, blend it: taper the transition over at least three times the thickness difference, and never step abruptly. Coring is the other half of the answer — hollow out thick regions from the non-cosmetic side so the remaining wall is nominal thickness. A part that looks chunky from outside can be entirely nominal-wall once cored properly.

Also mind the corners. Sharp internal corners concentrate stress and restrict flow; a fillet of roughly half the wall thickness on the inside and one and a half times on the outside keeps the wall constant around the bend. Uneven corners are one of the most common causes of the problems described in our guide to injection molding defects.

The cost impact is bigger than it looks

Wall thickness affects your molded part cost through three channels at once. Material volume is the obvious one. Cycle time is the larger one, because thicker walls need disproportionately longer cooling. And tooling is the hidden one, because parts that warp need mold rework, additional cooling circuits, or extra gates — each of which is a change to cut steel. Trimming a heavy wall down to nominal early is one of the cheapest cost reductions available in a hardware program.

How to spot a wall thickness problem in an existing part

  • Sink marks or gloss variation directly opposite a rib, boss, or internal feature.
  • Parts that will not sit flat, or lids that fit at one corner and gap at another.
  • Snap fits that engage on one side of the part and not the other.
  • A CAD thickness analysis showing a wide spread rather than a tight band around nominal.
  • Mold flow or structural FEA simulation predicting warp that matches what you see in hand.

Before the tool is cut

Run a thickness analysis on every molded part in the assembly, ask the molder for a flow analysis on anything large or cosmetic, and hold a proper design for manufacturing review. Note that this rule does not transfer from prototypes: parts built by 3D printing tolerate thickness variation that molding will not, so a printed part that works tells you nothing about whether the molded version will come out flat. Verify thickness against molding rules, not against your prototype.

Want a second pair of eyes on the wall thickness and rib strategy of a part before you commit to steel? Send us the model through our contact form and we will review it for moldability.