How to Build an Accurate Plastic Part Costing Sheet?

Launching a plastic manufacturing project without a precise Plastic Part Costing Sheet is like navigating without a map—you might arrive, but the journey will be inefficient and costly. A detailed costing sheet is the foundational financial blueprint, transforming a 3D model into a predictable business case. For procurement managers, product designers, and business owners, mastering this document is essential for budgeting, vendor negotiation, and achieving target margins. This guide moves beyond generic formulas to provide a systematic, real-world methodology. We will dissect every cost category, from raw polymer granules to landed cost, offering actionable steps and industry insights to ensure your Plastic Part Costing Sheet reflects the true economics of production.

What Is Plastic Part Costing Sheet?

A Plastic Part Costing Sheet is a structured financial model that itemizes all expenses associated with producing a molded plastic component. It is not a single number but a breakdown of fixed costs (like tooling) and variable costs (like material per part). This dynamic document serves three critical purposes: 1) Budgeting and Quotation, providing a transparent basis for price negotiations; 2) Cost Optimization, identifying the largest cost drivers for targeted engineering changes (e.g., wall thickness reduction); and 3) "Should-Cost" Analysis, enabling an internal benchmark to evaluate supplier quotes. A professional costing sheet brings objectivity to a process often shrouded in estimation.

Which Cost Categories Must Be Captured First?

A comprehensive cost model is built on five pillars. Missing any one compromises the total.

1. Material Cost: The expense of the plastic resin used in the part and the scrap (runner, sprue).
2. Machine/Processing Cost: The cost of operating the injection molding press, based on cycle time and an hourly machine rate.
3. Tooling (Mold) Cost: The one-time, non-recurring engineering (NRE) cost of the mold, amortized over the project's lifetime volume.
4. Labor & Overhead Cost: The cost of machine operators, quality inspectors, and factory overhead (utilities, maintenance, administration).
5. Secondary & Ancillary Costs: All post-molding expenses: assembly, decoration, packaging, shipping, and tariffs.

How to Calculate Material Cost per Part?

This is more than part weight times price per kg. It accounts for the total shot weight and material utilization efficiency.

• Step 1: Determine Total Shot Weight. This is the weight of the part(s) plus the entire runner and sprue system. Use CAD software to calculate part volume and multiply by material density. For the runner, rely on mold design or estimate based on standard layouts.
• Step 2: Apply Material Price. Use current market prices from suppliers (e.g., $3.50/kg for Polypropylene). For engineering resins, prices can be $8-$15/kg or far higher for specialties like PEEK.
• Step 3: Factor in Regrind (Recycled Sprue/Runner). If clean, in-house regrind is reused, it offsets virgin material cost. A common model: Material Cost per Part = (Part Weight + (Runner Weight * (1 - Regrind Usage %))) * Material Price per kg. For example, with a 20% regrind blend: Cost = (15g + (10g * 0.8)) * $0.0035/g = $0.0805.

Table: Material Cost Calculation Example

How to Estimate Cycle Time and Machine Rate?

Cycle time drives both machine and labor costs. A scientific estimate is key.

• Estimating Cycle Time: The cycle has three main segments: Injection Time (usually short), Cooling Time (the major component, governed by the square of the thickest wall section), and Mold Opening/Ejection Time (fixed). Use rule-of-thumb calculations or simulation software (e.g., Moldflow). For a 2mm wall PP part, cooling time might be ~20 seconds, with a total cycle of ~30 seconds.
• Determining Machine Hourly Rate: This is the fully burdened cost to run a specific press per hour. It includes:
  • Machine Depreciation (Purchase Price / Lifespan in hours)
  • Factory Floor Space
  • Energy Consumption (A 300-ton press may draw 30-50 kW/hr)
  • Routine Maintenance
    A typical rate for a mid-size press might be $60 - $120 per hour. Divide this by 3600 seconds/hour to get a cost-per-second.
• Calculation: Machine Cost per Part = (Cycle Time in seconds / 3600) * Hourly Machine Rate.

Mold Cost: How to Amortize Across Volume?

Tooling is a capital investment spread over the part's production life.

• Estimating Mold Cost: Factors include mold size, steel grade (soft aluminum vs. hardened steel), complexity (number of cavities, side-actions, hot runner system), and surface finish. A simple single-cavity aluminum mold may cost $5,000, while a multi-cavity production steel mold can exceed $100,000.
• Amortization Formula: Amortized Tooling Cost per Part = Total Mold Cost / Total Projected Production Volume. This highlights the economy of scale: a $50,000 mold adds $5 to the cost of the first 10,000 parts but only $0.50 to the cost of the 100,000th part.
• Critical Consideration: Always clarify mold ownership and maintenance. Who pays for repairs or storage? These terms must be in the costing sheet notes.

What Labor and Overhead Rates Apply?

Labor is not just the operator standing at one machine. In modern, automated cells, one operator may oversee multiple presses.

• Fully Burdened Labor Rate: This is the operator's wage plus benefits, taxes, and training. A $25/hr wage can become a $40-$50/hr burdened rate.
• Overhead Rate: Often applied as a percentage of direct labor or machine cost. It covers factory management, utilities (beyond machine power), QA department, and R&D. A common range is 50% to 150% of the direct labor cost. For a highly automated facility with high machine rates, overhead may be baked into the machine hour rate.
• Calculation: If an operator manages 4 machines with a 30-second cycle, their labor is attributed across many parts: Labor Cost per Part = (Labor Time per Part in hours) * Burdened Labor Rate.

Secondary Operations: What Hidden Expenses Arise?

These are often the "gotcha" costs that inflate the final price.

• Post-Processing: Deflashing, degating (if not automated), trimming.
• Assembly: Ultrasonic welding, press fits, adhesive bonding, adding inserts or fasteners.
• Decoration & Finishing: Painting, pad printing, laser etching, texture application (which is part of the mold cost).
• Quality Inspection: 100% visual inspection, routine CMM checks, or functional testing. For a medical device, this can be a significant, mandatory cost center.

Packaging, Shipping, and Tariff Considerations

The cost isn't complete until the part is at your door.

• Packaging: Custom clamshells, blister packs, or simple poly bags. Consider cost and volume.
• Shipping: Freight charges (sea vs. air), insurance, and customs brokerage fees.
• Tariffs & Duties: Based on country of origin, material type, and part classification (Harmonized Tariff Schedule code). This can add 0% to 25%+ to the product cost. For example, sourcing finished plastic components from certain regions may incur specific anti-dumping duties.

Conclusion

Building an accurate Plastic Part Costing Sheet is an exercise in forensic accounting and manufacturing insight. It requires peering beyond the obvious to capture the full ecosystem of expenses, from the energy consumed during cooling to the amortized mill of a mold cavity. By meticulously quantifying each category outlined here—material, machine time, tooling, labor, overhead, and secondary costs—you gain more than a price. You gain a powerful tool for design for manufacturability (DFM) decisions, supplier negotiation, and strategic planning. This transparent, data-driven approach demystifies costing, transforming it from a black box into a lever for profitability and competitive advantage.

FAQ on Plastic Part Costing Sheets

What is the most common mistake in plastic part costing?
Underestimating the cycle time and its domino effect. An optimistic cycle time error of just 5 seconds cascades into higher machine cost, higher labor cost, and lower factory capacity, drastically impacting unit cost at volume.

How does part design affect the costing sheet most?
Design dictates the three biggest cost drivers: Material Weight (wall thickness, size), Cycle Time (wall thickness for cooling, complexity for ejection), and Mold Cost (part size, undercuts requiring side-actions, surface finish). A good DFM review targets these areas.

Should I request a detailed costing sheet from my supplier?
Absolutely. A reputable supplier should provide a transparent breakdown. If they refuse or give only a lump sum, it's a red flag. A detailed sheet allows you to verify assumptions and collaborate on cost reduction.

How often should a costing sheet be updated?
With every major design revision and at least annually for stable projects. Raw material prices fluctuate, labor rates change, and currency exchange rates vary. A static cost model quickly becomes inaccurate.

Can I use a standard Excel template for this?
Yes, a well-structured Excel template is the industry standard. It should have separate, linked sections for each major cost category, allowing you to play with "what-if" scenarios (e.g., changing volume, material, or number of cavities).

Contact Yigu technology for custom manufacturing.

At Yigu Technology, we believe transparency builds the strongest partnerships. We don't just provide quotes; we provide detailed, line-item Plastic Part Costing Sheets that educate and empower our clients. Our engineers collaborate with you during the DFM phase to optimize the design for cost-effectiveness, targeting the levers that matter most. We combine real-world production data, current material pricing, and transparent overhead structures to deliver accurate, defensible, and competitive cost models.

If you seek a manufacturing partner who treats your budget with the same care as your part quality, contact us.

Contact Yigu Technology today for a transparent, no-obligation cost analysis and DFM review of your project.