Injection Molded Part Cost Estimation: A Guide

Two suppliers quote the same part at 0.42 and 0.71 per piece. Both are telling the truth. One has a 16-cavity tool running a two-second cycle, the other has an 8-cavity tool with a hot runner problem, and the quote reflects that. Understanding how a molded part price is constructed is what turns a quote into a negotiation and a design decision.

Figure 1. Injection Molded Part Cost Estimation: A Guide.

This guide breaks the price into its components, gives the formulas, and shows where design changes actually move the number.

The cost structure of a molded part

A molded part price has six components. Four are recurring per-part costs, two are one-time.

The first thing to notice: material dominates. Any design change that adds 1 g to the part adds far more to the price than a process improvement that shaves 0.2 s off the cycle. This is why wall thickness and part weight are the highest-leverage cost levers in the whole design.

Step 1 — Material cost

`` Material cost per part = part weight × material price per kg + runner weight × material price per kg × (1 − regrind credit) ``

Practical points:

• Part weight, not part volume. Small features, ribs and bosses matter more than they look. Weigh a prototype or estimate from CAD mass properties.
• Runner weight is often forgotten. In an 8-cavity cold runner tool, the runner can be 30–60% of the shot weight. A cold runner is not "free".
• Regrind of the runner is normal in-house practice for non-critical parts, but it is not free: it costs the regrinding operation and it is usually valued at 50–70% of virgin material. For medical, optical or highly regulated parts, assume runner scrap at full material price.
• Material price varies with resin grade, color and volume. A glass-filled PA66 with a UV package can cost three times a commodity PP.

Step 2 — Machine time

`` Machine cost per part = cycle time (s) × machine hourly rate ÷ 3600 ÷ cavitation ``

The two inputs that matter:

Cycle time is the sum of injection, hold, cooling and mold-open time. Cooling usually dominates:

• Cooling time grows with the square of wall thickness. Doubling the wall thickness roughly quadruples the cooling time. This is the single most powerful cost fact in plastic design.
• Hot runner systems, conformal cooling and higher-efficiency cooling layouts attack the cooling share directly.
• Mold-open and ejection time is small but not zero in high-cavity tools.

Machine hourly rate scales with clamp tonnage. A 1,500-ton machine may cost four to six times a 150-ton machine per hour. So the part design that needs a smaller machine is a cheaper part — clamp tonnage is driven by projected area, and thin walls with high injection pressure push it up.

Cavitation divides the machine cost. Doubling cavities halves the machine cost per part, provided the cycle time and the machine size do not rise. High-cavity tools pay off when the annual volume justifies the tool cost.

Step 3 — Labor and secondary operations

• Degating. Manual degating is a real per-part cost. A tunnel or pin-point gate with automatic degating removes it.
• Inspection. The inspection plan is part of the cost. A part with a tight cosmetic spec and a dimensional report per lot costs more than a part with a functional spec.
• Secondary operations — pad printing, laser marking, ultrasonic welding, assembly, leak test — can each add more than the molding cost of a small part. Design for assembly to keep these out.

Step 4 — Scrap and yield

Apply a reject allowance based on realistic yield for the process: 1–3% for a mature well-designed part, 5–10% for a tight-tolerance or cosmetic-critical part, higher during ramp-up. Design details that quietly raise the reject rate:

• Tolerances tighter than the process can hold (see the tolerance standards guide)
• Cosmetic requirements on a location the gate creates a weld line on
• Undercuts that need a lifter or slide and produce occasional drag marks
• Deep, thin ribs that short-shot intermittently

Step 5 — Tooling amortization

Tool cost is mostly a function of cavitation, size, surface finish and mechanism complexity:

Amortize across the expected production volume, and check the breakeven when comparing a slow cheap tool against a fast expensive one:

`` Breakeven quantity = (tool cost B − tool cost A) ÷ (part cost A − part cost B) ``

If the answer is below the expected lifetime volume, the more expensive tool is the cheaper choice — and this is a calculation a supplier can be asked to show.

Worked example

A PP enclosure lid, 45 g part, 20 g runner, 2.0 mm nominal wall, 200,000 parts per year, 3-year life.

Now change one thing: reduce the nominal wall from 2.0 to 1.6 mm.

• Part weight falls to about 36 g, saving 0.0117 per part in material.
• Cooling time falls roughly 36%, taking the cycle from 28 s to about 21 s, saving 0.0110 per part in machine time.
• Combined saving ≈ 0.023 per part, about 11% of the recurring cost — plus a possible drop to a smaller machine.

That is the shape of the leverage in plastic part costing: weight and wall thickness move the price, everything else trims it.

What to check on a supplier quote

• Is the part weight quoted or estimated? Ask for grams, and for the runner weight.
• Is the cycle time credible for the wall thickness, or is it optimistic to win the job?
• How many cavities, and does the machine tonnage match the part?
• Is regrind assumed, and at what percentage? Regrind affects both price and part properties.
• What is the reject allowance and who pays for scrap above it?
• Is tooling amortized into the price or paid separately, and who owns the tool?
• Is the quoted material the specified material, or a substitute grade?
• Are secondary operations and packaging included or billed separately?
• What is the price at different volumes? A single price for 10,000 and 1,000,000 parts hides the cavitation assumption.

Design decisions that lower part cost, ranked

1. Reduce wall thickness where stiffness allows — cuts material and cycle time together.
2. Design for a cold-runner-free or automatic-degating gate — removes labor and scrap.
3. Reduce the number of parts in the assembly (DFMA) — removes tools, assembly labor and fasteners.
4. Loosen tolerances to the process capability — reduces rejects and inspection.
5. Keep the part inside a smaller machine's clamp tonnage — cuts the hourly rate.
6. Avoid undercuts and mechanisms — cuts tool cost and cycle time.
7. Increase cavitation once volume justifies it — cuts machine cost per part.
8. Conformal cooling on thick or critical sections — cuts cycle time where cooling dominates.

Cost estimation is not an accounting exercise performed after the design; it is a design review tool. The design freeze is the moment when the price is fixed — the process can only protect it, not change it.

Figure 2. Quick reference: the key checks and reference values from this guide.