Part Design: the Four Numbers That Decide Whether It Assembles

Wall Thickness Is a Compromise, Not a Target

A thick wall looks premium and costs twice: resin per part, and cycle time per shot. On a 200 ml PETG body, moving from 2.5 mm to 1.8 mm usually keeps the shelf look and cuts the cycle by a quarter — but the shoulder becomes the weak point under a pump stroke.

The rule we work to: 1.5–2.0 mm for a body under 300 ml, 0.55 mm minimum for a thin-wall bottle with a pump, and never thinner at the shoulder than at the base. If a design needs 2.5 mm for a reason we cannot see, we ask what the reason is before quoting.

Draft Is Not Optional on a Visible Part

Every vertical wall needs 0.5–1° of draft or the part drags on ejection and shows a scuff. On a clear bottle that scuff is visible in the shop light, which is why a "slight" draft angle is one of the most common causes of a rejected T1 sample.

Gate Position, on a Clear Part

The gate leaves a mark. On a transparent body it goes at the base centre or under the collar, never on the shoulder where the light hits. On a two-piece assembly the gate on the upper part is hidden by the collar, which is one reason a separate collar can be cheaper than a perfect single part.

The Tolerance Stack: Where Four Parts Go Wrong

Each part carries its own tolerance: ±0.1 mm on a thread, ±0.15 mm on a height, ±0.05 mm on a seal groove. On a two-part pack they add once. On a four-part assembly they add three times, and the joint that fails is usually the last one in the stack, not the one that looks weakest on a drawing.

We draw the stack before the tool, and we tell you which dimension to tighten. Tightening the wrong one adds cost and fixes nothing.

What to Send Us

A drawing with wall, heights and the thread callout; the closure you intend to use; and the volume each chamber holds. Five lines and a sketch is enough to run the stack.