Family mold economics: compare complete assemblies and excess parts

A family mold produces different components together, but their output ratio may not match the quantities required in the finished assembly. A low price per molded piece can therefore create extra components while another component still limits shipment. Compare the approved tooling proposals using complete saleable assemblies and any separately supported component demand.

Protolabs distinguishes multi-cavity molds making repeated versions of one part from family molds making different parts in a shared mold. It also identifies balancing and tooling complexity as considerations. Those are engineering questions for the qualified mold designer and supplier; the financial model should compare technically acceptable proposals rather than prescribe a cavity arrangement.

Start with the controlled bill of materials, the required material and quality specifications, and the planned production quantities. Record whether components also sell as spares or support other products. A part that is surplus for one assembly order may have another real use, but that use needs evidence and timing.

Assume a hypothetical assembly needs one component A and two components B. Proposal One is an approved family mold producing one A and one B in each shot. To supply 1,000 assemblies, it must produce 2,000 B components, requiring 2,000 shots. Those shots also produce 2,000 A components, leaving 1,000 A beyond this order's requirement.

Proposal Two is a separate technically approved design producing one A and two B per shot. It requires 1,000 shots to supply the same 1,000 assemblies, with no ratio-driven excess in this simplified example. Both calculations initially assume every produced component meets acceptance requirements; actual rejection exposure must be added separately.

Counting the first proposal's 4,000 pieces as 4,000 customer units would conceal the assembly relationship. The relevant deliverable is 1,000 completed component sets plus 1,000 additional A components. Keep those populations separate when discussing output, inventory and the cost of fulfilling the assembly order.

Suppose Proposal One has an approved observed cycle of sixteen seconds and Proposal Two has an approved observed cycle of twenty-four seconds. The first design has the faster individual shot, but its 2,000 required shots take approximately 8.89 machine-hours. The second design's 1,000 shots take approximately 6.67 machine-hours.

Proposal Two saves about 2.22 modeled running hours for the 1,000-set order despite its slower cycle. This comparison excludes setup, maintenance, inspection and other required work. Those stages belong in the full schedule, particularly where a technically different tool needs different preparation or qualification.

Do not treat every saved running hour as an immediate payroll or equipment-payment saving. Identify whether the time avoids an actual charge, releases a constrained machine for another confirmed job or simply creates idle capacity. The investment case should state which of those effects supports its claimed benefit.

Budget approved startup, trial and qualification output outside the ideal production-shot count. Those activities can require material and machine time before any components become eligible for the customer order. Show whether the supplier's quote includes them, and avoid counting qualification samples as available assembly stock without the appropriate release.

Assume the supplier quotes an illustrative $0.72 per shot for Proposal One and $0.95 per shot for Proposal Two, each covering the defined material and conversion scope for all components produced in that shot. The 1,000-set order costs $1,440 under the first proposal and $950 under the second, a $490 purchase difference.

If Proposal Two requires $9,800 more upfront tooling expenditure, the simple cumulative operating-cost crossover is 20,000 completed sets: $9,800 divided by $0.49 saved per set. At 12,000 sets, the modeled saving is $5,880, below the premium. At 24,000 sets, it is $11,760, exceeding the premium by $1,960 before other differences.

This is a transparent volume sensitivity, not a complete investment appraisal. Include the actual timing, maintenance, tool life, financing and quality costs where they differ. Preserve the forecast's uncertainty, especially if product revisions could reduce the volume produced on the original approved tooling.

The first proposal's shot price already includes producing the extra A components under the stated scope. Adding their full material cost again as a separate penalty would double count that cost. Instead, identify any additional storage, handling, obsolescence or disposal exposure and apply the appropriate inventory treatment with the accounting reviewer.

If confirmed spare-part demand can use the extra A components, revise the comparison to reflect that feasible use. Consider when those spares would otherwise have been produced and what expenditure is actually avoided. An unsupported resale assumption should not erase the difference between excess output and committed demand.

Component-specific rejection also matters. Losing B components can require further shots and create more A stock, while losing A may produce a different imbalance. Review accepted component counts and the approved replacement method instead of applying one overall piece-yield percentage that assumes every failure has the same assembly consequence.

A later change to one component can affect a shared tool differently from separate tooling. Ask the supplier how approved revisions, maintenance and unavailable cavities would be handled, and price the actual proposed arrangements. Do not assume a cavity can simply be disabled or modified without engineering review and an acceptable production result.

Compare the proposals against the current assembly mix and plausible documented changes. Separate tools may offer useful flexibility even where the initial per-set cost is higher. That tradeoff should be tied to real revision or demand scenarios rather than described as an unpriced benefit that always overrides the arithmetic.

Approve the investment with a record of accepted output ratios, shot counts, cycle assumptions, quote scope and the volume needed to recover the tooling difference. Update the model with production evidence after launch. The decision then remains traceable to the assemblies customers need and the technically approved means of producing them.