Question 2 of 7: Sand-Casting Terminology — Down-Runner Base, Expendable Core, and Pattern
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Examinations, May 2019 — 17-Ind-B2 Manufacturing Processes. 3-hour closed-book exam; candidates may use a Casio or Sharp approved calculator. Any five questions constitute a complete paper (only the first five as they appear are marked officially); all seven are answered below as a full study resource.
Reference texts. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. (primary text for this subject — material selection, casting, polymer processing, metal forming, powder metallurgy, and machining).
Check: every page’s printed footer reads “17-Ind-B2/May 2019”, so this is the May 2019 sitting. In Question 1(ii), option (3) is "Bending". Question 1 uses a multiple-choice (statement-selection) format.
The down-runner (sprue) is the near-vertical channel through which molten metal first enters the mold under gravity, and it accelerates continuously as it falls. The down-runner base (sprue base, or "sprue well") is an enlarged, rounded pocket at the bottom of the sprue, where the flow must turn from vertical to horizontal to enter the runner system.
It absorbs and redirects the kinetic energy of the fast-falling metal stream, converting the sharp 90° turn into a smoother change of direction.
By slowing and smoothing the flow at this transition, it reduces turbulence, splashing, and air entrapment that a sharp, un-cushioned corner would otherwise cause.
It reduces erosion of the mold/runner walls at the turn, limiting sand-inclusion defects that eroded mold material would introduce into the casting.
(ii) Function of an expendable core
A core is inserted into the mold cavity to produce an internal void, passage, or undercut feature in the finished casting — geometry the pattern itself cannot form, because the pattern only shapes the OUTER mold cavity.
An expendable core (typically bonded sand, sometimes with a binder cured separately from the mold) is sacrificial: after the casting solidifies, it is broken up and removed (shaken out, washed out, or dissolved) through whatever opening exists, rather than being withdrawn intact.
Because it does not need to be extracted as a solid piece, an expendable core can form complex, undercut, or fully enclosed internal cavities (e.g. a hollow engine block's water jacket) that no rigid, reusable core or mold half could ever be pulled out of without destroying the feature.
It is supported in the mold at core prints (seats formed by the pattern) so it holds its position and does not float or shift when the molten metal is poured around it.
(iii) What a pattern is for
A pattern is the master (usually reusable) tool that is packed into the sand (or otherwise used to shape the mold medium) to create the mold cavity — the negative-space impression that molten metal is later poured into.
It reproduces the casting's external geometry, but is not dimensionally identical to the final part: it is built oversized by a shrinkage allowance (to compensate for the metal's solid contraction on cooling) and, where the surface will be machined afterward, by extra machining-stock allowance.
It is built with draft (a slight taper on vertical faces) so it can be withdrawn cleanly from the packed sand without tearing the mold cavity.
It also carries the core prints that locate and seat any cores used to form internal features (linking directly to part (ii) above).
