Where outsourcing wins, without exception
Six situations make internal production the wrong answer regardless of how the arithmetic looks, and four of them are about capability rather than cost.
Low or irregular volume. Six weeks of work a year does not support a workshop. The fixed cost is being paid to produce nothing for most of the year, and the operator's judgement never accumulates because there is no repetition to accumulate from.
More than one or two decoration methods. Every additional method is another machine, another operator, another consumable chain and another set of maintenance obligations. A supplier has all of them installed and amortised, and this is usually the single strongest argument for buying outside.
Hard external deadlines with a spiky calendar. Events cluster. In-house capacity cannot absorb a cluster, and the rush that follows is where quality escapes. Book capacity outside instead, early, before the cluster arrives.
No local operator talent. Where the labour market is thin, recruiting is slow and the cost is high, and where the existing staff already have full responsibilities. This is common in organisations whose core business has nothing to do with manufacturing.
Certification and compliance requirements. Restricted-substance controls, documented testing and audited processes are expensive to build and trivial to buy from someone who holds them. Rebuilding them internally to satisfy a procurement questionnaire is a two-year project that nobody budgets for.
Capital and space are constrained. Floor space in a headquarters is expensive and a workshop needs power, extraction and a fire-safe storage arrangement. The opportunity cost of the room alone often exceeds the whole outsourcing cost for several years.
The common thread is that in-house production is a bet on continuity. Steady, repeated, narrow work justifies it. Everything else is a bet that does not pay, and the machine sitting idle is the visible cost of having taken it.