Someone asked whether a 2mm-thick, 30cm-wide circular plate could even be injection molded. The honest answer is that thickness alone does not make this part hard: 2mm is a completely normal wall for injection molding. What makes it worth a second look is flow length against wall thickness — how far the melt has to travel through that thin a channel before it reaches the farthest point on the part — and, closely related, where the gate sits. Get those two right and a thin, wide plate is routine. Get them wrong and the same plate can short-shot or warp on the first shot off the tool.
Why does "thin and wide" sound impossible, but usually isn't?
Because the two numbers that actually decide moldability are not thickness and width on their own — they are flow length and wall thickness, taken together. A thin wall is not a problem by itself; injection molding runs plenty of parts well under 2mm. A long, unsupported flow distance is not a problem by itself either, if the wall is thick enough to carry it. The plate in question combines a thin wall with a wide diameter, which is exactly the combination that pushes flow length past what that thickness can comfortably carry — not because either number is extreme on its own, but because of the ratio between them.
What actually breaks first: flow length against wall thickness
A thinner wall drains heat from the melt faster, which shortens how far it can flow before the front freezes and stops. Molten plastic loses heat to the steel mold the instant it touches it, and a thin channel has proportionally more cold steel surface for the same volume of plastic than a thick one does. As the melt travels, a frozen skin forms at the wall and narrows the still-molten core the material is actually flowing through. Push the melt far enough through a thin enough channel and that core narrows to nothing before it reaches the far side — an incomplete fill, commonly called a short-shot.
DFM references quantify this as the flow-length-to-thickness ratio: the distance from the gate to the farthest fill point, divided by the wall thickness. Ordinary thin-wall guidance treats a ratio above roughly 75 as the point where you need higher fill pressure and faster injection speed just to finish filling before the front freezes. A 2mm wall filled from one edge of a 30cm plate has to travel close to the full diameter to reach the opposite side — a ratio well past that line for an unremarkable-looking part.
Why can't you just make the whole thing thicker?
Because thickening the part trades a fill problem for a shrinkage and warp problem, and only fixes the first one if you do it everywhere at once. Plastic is a poor conductor of heat, so a thick section cools and solidifies more slowly than a thin one sitting right next to it. In a part with uneven thickness, the thin areas freeze and shrink first while the thick areas are still cooling and shrinking later, and that mismatch in timing pulls on the already-solid material around it — the mechanism behind sink marks, internal voids, and warp. Published mold-design guidance treats uniform wall thickness as the primary lever for consistent fill, pressure, and cooling, and where a thickness change is genuinely unavoidable, it calls for a gradual taper — roughly a 3-to-1 length-to-thickness ramp — rather than a sudden step, so the transition does not become its own defect site.
Where should the gate go on a wide, symmetric part?
At the center, for a part shaped like this one. Gate location sets both the fill pattern and the maximum flow length the melt has to travel, and for a round, symmetric plate, a center gate gives a balanced radial front that reaches every edge at close to the same time and the same pressure. It also roughly halves the worst-case flow length: the melt only travels to the radius, not across the full diameter, so a 30cm-diameter plate gated from the center faces something closer to a 15cm flow length instead of 30cm from one edge. Gating the same plate from one edge instead forces a long, one-directional fill, with the pressure and temperature both dropping by the time the melt reaches the far side — the classic setup for uneven packing and a warped edge on the side that filled last.
What did we check this against?
The mechanics above are standard, citable injection-molding DFM fundamentals, not a house opinion: flow-length limits and the ratio threshold appear in general part-and-mold design references and thin-wall processing guidance; uniform wall thickness as the primary lever against sink and warp, and the gradual-taper rule for unavoidable transitions, appear across resin-supplier design guides; and center-gating for a symmetric part's balanced radial fill is standard mold-design guidance for round or square parts. None of this is exotic — it is the same reasoning a moldmaker would walk through looking at this exact plate.
How do you check flow length and wall thickness on your own part?
By measuring the wall thickness across the actual geometry, not eyeballing "2mm" as a single number for the whole part. A part rarely has one uniform thickness in practice — ribs, bosses, and transitions all shift the local wall — and the flow-length question only makes sense once you know where the thin and thick sections actually are.
Fabdose reads your STEP or STP file on your own computer and reports minimum, maximum, and average wall thickness, estimated from the part's shell geometry, along with where the thickness varies most across the part. That is a useful first read for a plate like this one — it tells you where the thin sections sit and how much they vary from the thick ones. It is worth being precise about what it is not, though: this measurement is not yet pinned to an individual face the way fabdose locates undercuts and draft-angle violations (we cover how those two are found and fixed separately). If your part needs a face-by-face wall-thickness call-out, or an actual fill simulation to confirm flow length against a specific gate location, treat this measurement as a starting point and follow up with a manufacturer's DFM review or a moldflow analysis before you commit to tooling. For the deterministic checks it does perform today — undercuts and draft, the kind of finding we walked through face-by-face on a real open-source part — it runs locally, before the file goes to anyone who profits from the answer.
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FAQ
Is a 2mm-thick, 30cm-wide circular plate too thin to injection mold?
Not on thickness alone. 2mm is a normal, moldable wall for a huge range of resins. What actually limits the part is flow length against that thickness — how far the melt has to travel from the gate before it fills the farthest point, versus how thin a channel it is traveling through. The plate is not impossible; it needs the flow-length question answered before it needs a thickness answer.
What is the flow-length-to-thickness ratio?
It is the distance the melt travels from the gate to the farthest fill point, divided by the wall thickness. A ratio above roughly 75 is the point where an ordinary molding setup needs higher pressure and faster fill speed to finish before the melt front freezes. Gating from an edge instead of the center roughly doubles that ratio for the same wide, symmetric part.
Why is uniform wall thickness the fix, rather than just making the plate thicker?
Because the risk is a long flow length through a thin wall, not thinness on its own. Thickening the whole part fixes the flow-length math but slows cooling unevenly if the thickness is not even everywhere, which is its own source of sink and warp. Uniform thickness, with any unavoidable change ramped through a gradual taper, is the reliable lever.
Where should the gate go on a wide, symmetric part?
At the center. A center gate gives a balanced radial fill that reaches every edge at close to the same time and pressure, and it roughly halves the worst-case flow length compared with gating from one edge.
How do I check flow length and wall thickness on my own part?
Fabdose reports minimum, maximum, and average wall thickness on your STEP or STP file, estimated from the part's shell geometry, on your own computer. It is a useful first read, but it does not yet pin the measurement to an individual face the way it locates undercuts and draft violations — for a face-by-face wall-thickness call-out or a fill simulation, follow up with a manufacturer's DFM review.
Fabdose is a desktop tool for checking STEP and STP files against injection molding design rules. It locates undercuts and draft-angle violations face by face, and reports minimum, maximum, and average wall thickness estimated from the part's shell geometry, before the file goes out for a quote. It does not quote jobs or sell manufacturing, and it does not run a flow simulation. Your CAD geometry is processed on your own computer; defect findings and descriptions are processed by AI. The flow-length and wall-thickness fundamentals above are drawn from established plastics and mold-design references; treat the ratio and taper figures as practical guidelines, not guarantees for your specific resin and geometry.