Three Details on a Drawing That Decide What the Tool Costs

Most tool problems we are asked to fix did not start in the tool shop. They started on a drawing nobody questioned.

Three details decide whether a mold runs for years or fights you from T1. None of them cost anything to change on paper. All three cost weeks once steel is cut.

1. Wall thickness that varies more than about 1.5 to 1

Thin sections cool first and shrink first. The part warps toward the thick side, the tool gets blamed, and no amount of steel work fixes a geometry problem.

Two things follow from this. First, uniform wall is cheaper than variable wall, in every material, at every cavitation. Second, where a thickness change is unavoidable, the transition matters more than the ratio: a gradual taper distributes shrinkage, a step concentrates it.

If the drawing has a boss on a thin wall, look at the boss wall before you look at anything else.

2. A gate location decided after the tool design is finished

The gate sets fill pattern, weld line position, shrinkage direction and where the part is likely to sink. Move it late and you are not editing a file, you are re-cutting a cavity.

The practical question is not "where is the best gate" but "who decides". If the gate is chosen by the toolmaker alone, it will be chosen for toolmaking convenience. If it is chosen with the molder and the part designer in the room, it will be chosen for the part.

Ask for a fill analysis before the steel is ordered. On a simple part it confirms what everyone assumed. On a complex one it pays for itself the first time it moves a gate.

3. A cosmetic spec that says "no visible defects"

Every tool has a surface standard whether or not it is written down. If yours is not written, you get the toolmaker's default — and it may not be the one your customer inspects against.

Write it as a class: which surfaces are A-surface, which are B, which are hidden. Name the texture, the grain number if there is one, and where weld lines and gate marks are permitted. It takes ten minutes and it removes the most common argument at sampling.

What it costs to get these wrong

A wall thickness change after steel exists means re-machining or a new insert. A late gate move means re-cutting. A missing cosmetic standard means a re-polish nobody budgeted for, or a rejected batch nobody expected.

The cheapest sentence in any RFQ is the one that says which surfaces are cosmetic and which are structural. Add it. Then add the wall thickness rule and the gate decision owner. Three lines, and half of the surprises at T1 disappear.

We ask for the 3D plus a 2D with tolerances before quoting, not because we enjoy paperwork, but because a quote built on half a drawing is a guess with a number attached.