Meaning
Basic cost accounting in plastic manufacturing relies on calculations that combine material, machine, labor, and tooling expenses to determine the cost of a single finished part. This formulation is called unit piece price arithmetic, and it is the foundation of quoting and profitability analysis in the molding industry. It takes into account the resin weight, cycle time, scrap rate, and machine hourly rate.
A small decrease in cycle time can significantly reduce the overhead portion of the calculation, lowering the price per part. Conversely, a high scrap rate increases the effective material cost of each accepted piece.
Cost Composition
Manufacturing quotes are built on the distribution of fixed and variable costs across the total production volume. In unit piece price arithmetic, the material cost is a variable expense that depends on the part weight and current resin prices. Machine time and labor are semi-variable expenses that are distributed over the number of parts produced per hour.
Tooling amortization is a fixed cost that decrease per unit as volume increases.
Cycle Optimization
Productivity improvements directly influence the calculated cost per unit by reducing the machine time required for each cycle. A faster cycle means more parts are produced per hour, which dilutes the fixed overhead costs. Moulders use this calculation to justify investments in better cooling systems or automation that can trim seconds off the cycle.
It also helps in negotiating prices with customers.
Scrap Contribution
Defect rates must be factored into the pricing structure to ensure the run remains profitable. A higher reject rate means that the cost of wasted material and machine time must be absorbed by the passing parts.
