Last updated: 2026-09-04 · 5 min read
Why Do Designers Core Out Thick Sections? — Uniform Walls Before Uniform Anything Else
Coring out removes material from the inside of thick sections — turning a solid boss, rib or handle into a hollow, uniformly walled structure — because injection molding punishes mass more than almost any other process property: cooling time scales roughly with the square of wall thickness, so doubling a wall quadruples the clock. The defect is not the thickness itself but the difference between thick and thin.
Key Facts
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Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.
Where Do Thick Sections Hide and What Do They Cost?
Five locations, each sharing one mechanism:
| Feature | Typical Failure | Coring Answer |
|---|---|---|
| Solid bosses | Sink opposite; long cooling | Hollow core or dished underside |
| Over-thick ribs | Sink shadow on show face | Rib thickness at 60% of wall |
| Solid handles and grips | Voids, warp, cycle blowout | Cored shell with inner ribbing |
| Multi-wall junctions | Local mass pile-up | Blend radii, staggered thinning |
| Thick flanges | Lost flatness at the seal | Uniform-wall frame with ribs |
Every row is where CAD reads strong but molding reads slow and sinking. Solid bosses are the most common offender — CAD makes a boss one click; the fix is coring the underside and letting a shell do its work. Over-thick ribs are the classic self-inflicted wound: a rib drawn at full wall to be stronger, producing a sink shadow on the cosmetic face it was meant to serve — and the 60%-of-wall guideline exists because that is where the junction stops misbehaving.
Solid handles and grips are the case buyers meet at quotation stage, when the molder prices the extended cycle and predicts the voids; coring turns them into shells with internal ribs that keep the hand-feel and lose the penalty. Junctions and flanges round out the list with subtler versions of the same law: where walls meet or mass accumulates in a strip, the geometry needs thinning and blending, not because the drawing is wrong but because frozen plastic does not read drawings.
How Do You Core Out Without Losing the Part?
Four steps, in order:
- Map the wall thickness everywhere before touching anything. Thick sections hide in plain sight — inside fillets where three surfaces meet, under cosmetic covers, in the lofts of organic shapes. Run a thickness analysis in your CAD tool, or section the model at regular intervals, and mark every region beyond about 25–30% over the nominal wall. The output is a heat map of future sink and a prioritized worklist; doing this before the mold design kickoff means every correction lands as a normal DFM revision instead of a mid-build change order.
- Apply the transition rules so the thinning does not create new problems. Uniform walls are the goal, and abrupt steps between walls are nearly as bad as thick sections — a sharp transition becomes its own stress raiser and flow disturbance. The standard moves: rib thickness near 60% of the adjoining wall; boss walls at half the nominal or cored with a through-hole; transitions blended over roughly three times the thickness difference; generous radii where cored interiors meet the outer wall. These proportions are the accumulated arithmetic of decades of molded parts.
- Choose the coring direction with the molder, not just the CAD. Every hollow interior must be formed by steel, and the direction of the coring decides what forms it: a cored boss that opens toward the ejection side is a simple core pin; one that opens sideways needs a lifter or slide; a fully enclosed void is impossible in straight injection molding. Bring the molder into the geometry decision early and the part usually gets redesigned by a few degrees of draft or a relocated opening into something the tool builds naturally; keep the molder out and the complexity lands in the quotation as a line item you could have deleted.
- Re-verify function after the material is gone. Coring removes mass, and mass was often doing mechanical work — stiffness in a handle, thread engagement in a boss, impact absorption in a grip. The recovery tool is geometry: ribs across the cored span restore bending stiffness at a fraction of the mass, gussets restore lateral strength, and a dished underside restores resistance to oil-canning. Re-run the structural checks — FEA where it exists, physical drop and load tests on first articles where it does not — and confirm the cored design meets the same requirements.
Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.
The parts that sink and warp for skipping coring fail more often than the parts that crack after coring — and the difference is usually a missing rib or an unreviewed load path, not a flaw in the concept of hollowing.
Frequently Asked Questions
How thick is “too thick” for injection molding?
The first answer is relative: any section meaningfully thicker than the part’s nominal wall — the working threshold is around 25–30% over — creates a shrink differential against its neighbors, and that differential, not the absolute number, is what produces sink and voids.
The second answer is material-specific: commodity shells commonly run near 2–3 mm, ABS comfortably below 3–4 mm, while filled or high-heat engineering grades may want less. The practical rule is two-sided: start wherever the thickness map lights up, and let the molder’s material guidance set the ceiling for the specific resin.
Does coring out weaken the part?
Coring done carelessly, yes; coring done as engineering, usually no — and sometimes it strengthens the part in the ways that matter. Stiffness comes from geometry far more than from mass: bending resistance scales with the cube of depth but only linearly with width, which is why an I-beam and a cored shell outperform their solid equivalents per gram.
The genuine risks are local — thread engagement in an over-cored boss, sealing width on a cored flange, creep under sustained load where the cross-section got thin — and those are exactly what the re-verification step exists to catch. The parts that fail after coring failed for a missing rib; the parts that sink for skipping coring fail more often.
Who pays for the mold complexity coring adds?
Run the comparison honestly and the answer is usually nobody — coring frequently simplifies the tool rather than complicating it. The thick sections it removes are the same ones that would otherwise demand oversized cooling, longer cycles, sink rework and the occasional steel-safe correction, and those costs arrive as production expenses forever, while a core pin or lifter is a one-time fabrication.
The cases where coring genuinely adds tooling cost are the geometry mistakes: a hollow that opens on the wrong face and demands an extra slide, or an enclosed pocket nobody can form. Settle the economics in the DFM review with the molder’s quotation — compare the delta in tooling price against the delta in cycle time, scrap allowance and cosmetic risk at your volume.
Key Takeaways
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The discipline runs in reverse as well: coring without re-checking strength trades a cosmetic defect for a structural one.
Done with the molder’s review and the thickness map in hand, it is the highest-leverage change most injection-molded designs can make — usually before any conversation about steel grades or gate locations is worth having.
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