A Four-Part Mixing Pack, and the Three Decisions That Decided It
Body, upper cup, collar and base, with two sealing joints. The moulds were the easy part; the joints were the project.
| Item | Specification |
|---|---|
| Parts | 4 moulded + 2 silicone O-rings |
| Moulds | 4, single cavity |
| Body material | PETG, clear, 1.8–2.0 mm |
| Decorative parts | ABS, champagne-gold vacuum metallising |
| Sealing | Two O-rings in the collar, one per joint |
| Threads | Upper right-hand, lower left-hand |
| Tooling time | 38 days |
Decision 1 — the Collar Carries the Threads
The first sketch had threads on the body and on the cup. That forces two injection bodies with thick walls. Moving both threads onto a separate collar let the vessels stay blow-mouldable, which cut tooling and unit cost at the same time.
Decision 2 — Two O-Rings, Not One Gasket
A flat gasket on each joint needs a wide land and a flat face; the vessels had neither. Two O-rings in the collar, one above and one below, gave the seal without changing the body. The groove is 1.5–1.6 mm deep for a 2 mm cord, 20–25 % squeeze.
Decision 3 — the Lower Joint Is Left-Hand
With both joints right-handed, opening the cap loosened the body: a filled pack in a bathroom is enough to do it. The lower thread was reversed so the opening load tightens instead of releasing. It is a one-line change on a drawing and a different mould mechanism.
What the Sample Round Caught
The first samples sealed at the collar and wept at the cap: the plated ABS cap had a 0.2 mm step at the sealing face, invisible until the vacuum test. A tool correction fixed it, and the plating was moved off the sealing face entirely so the same failure cannot return on a reorder.
Building Something With More Than One Part?
Send the section and the closure — we return the stack and the seal plan.