Understanding the Cost Factors Behind Injection Molding Projects
Injection molding costs can jump from quote to quote—even when the part looks the exact same on the drawing. That’s because cost isn’t tied to one magic number. It’s a chain reaction where part design affects cycle time, material choice affects scrap rate, and production volume determines how tooling is spread out. Then, what happens after molding—trimming, inspection, packaging—quietly adds more to the project budget than people expect. When you miss even one variable, the math starts to fall apart.
To best prepare you for your next plastic components project, our injection molding company is breaking down how to estimate costs in a way that makes sense.
Upfront Costs That Shape Your Budget Early
Upfront costs arrive before the first plastic part ever exists. They’re often quoted separately within project scopes, but they still belong in the full cost picture for your project.
This includes:
- Injection Mold Tooling Cost: Tooling design is the highest upfront cost and varies based on part complexity, expected production volume, and tool material.
- Part Design and Engineering Costs: Engineering work, like CAD refinement and DFM reviews, reduces costly mold changes later down the line. This will often lower cycle time and scrap in the production phase.
- Prototyping: Product prototyping helps validate the form, fit, and function of a plastic component before full tooling, but multiple revisions can add to your budget during early-stage planning.
- Qualification, Inspection, and Documentation: Quality documentation and inspection support consistent production and help prevent rejected shipments, delays, and disputes.
How to Calculate Upfront Project Costs
To see the true cost of injection molding parts for your project, spread upfront expenses across volume.
Let’s say your project has these upfront costs:
- Tooling (mold build): $75,000
- Engineering + DFM review: $10,000
- Prototyping + sampling: $5,000
Total upfront costs = $90,000
Now, let’s assume your projected production volume is 300,000 parts.
Use this formula:
Amortized cost per part = Upfront costs ÷ Total projected part volume
Calculation:
$90,000 ÷ 300,000 = $0.30 per part
Even though tooling and development are paid upfront, they add $0.30 per part when you calculate the true injection molding cost for your project.
Material Costs
Common material categories for injection molded plastics include:
- Polypropylene, Polystyrene, and Polyethylene: Offer lower costs and are easier to process, but can result in limited performance.
- ABS, Polycarbonate, and Nylon: Come at a higher cost but offer improved strength and temperature resistance.
- PEEK and PPSU: Premium pricing with demanding processing requirements.
How to Calculate Material Cost Per Part
To calculate material cost, you’ll start with the total amount of resin used for each part—including runner waste (if applicable)—then add a small scrap factor to keep your estimate realistic.
For example:
- Part weight: 80 grams
- Runner weight: 20 grams (cold runner system)
- Scrap rate: 2%
- Resin price: $2.60 per lb
Step 1: Calculate total resin weight per cycle
Part + runner = 80g + 20g = 100g
Step 2: Add scrap factor
100g × 1.02 = 102g total resin per part
Step 3: Convert grams to pounds
102g ÷ 453.6 = 0.225 lb of resin per part
Step 4: Calculate material cost per part
Material cost per part = 0.225 lb × $2.60/lb = $0.59 per part
Final material cost per part: $0.59
This method gives you a realistic injection molding cost estimate because it accounts for the resin you actually consume—not just what ends up in the finished plastic component.
Cycle Time and Machine Hour Rate
Cycle time directly affects the number of parts you produce per hour. A mold designed for mass-produced injection-molded parts is built very differently from one intended for short runs, prototyping, or limited production volumes. Longer cycles mean fewer parts and a higher cost per unit. Cooling time usually dominates the cycle, especially for thicker components.
Cycle time is often influenced by:
- Cooling Time: The cooling system process is often the largest cost contributor.
- Wall Thickness and Part Volume: Thicker sections take longer to cool.
- Mold Temperature Requirements: Higher temps increase cycle length.
- Material Type: Some resins cool and release faster than others.
- Ejection Design: Poor ejection adds time and risk to your budget.
Machine size matters, as larger machines cost more to operate. If a plastic part requires a 500-ton press rather than a 200-ton press, the hourly rate will increase quickly.
How to Calculate Machine Cost Per Part
Machine cost depends on the hourly rate and cycle time:
- Machine Hourly Rate: Based on press size and process demands.
- Cycle Time: Convert seconds to hours for calculation.
- Cavitation: Divide output by parts produced per cycle.
Let’s assume these numbers for your project:
- Machine hourly rate: $85/hour
- Cycle time: 32 seconds
- Mold cavitation: 4 parts per cycle (4-cavity mold)
Step 1: Convert cycle time to hours
32 seconds ÷ 3600 = 0.0089 hours per cycle
Step 2: Calculate machine cost per cycle
$85 × 0.0089 = $0.76 per cycle
Step 3: Divide by parts per cycle
$0.76 ÷ 4 = $0.19 machine cost per part
Final machine cost per part: $0.19
Saving even a few seconds per cycle, or increasing cavity count, can noticeably reduce injection molding costs in high-volume production.
Labor Costs
Labor cost varies based on your project’s automation level. Fully automated cycles reduce hands-on time, while manual processes increase labor and introduce variability among your finished products.
Labor may include:
- Machine Operation: Monitoring and adjustments during runs.
- Quality Checks: Visual inspection or measurement.
- Manual Trimming or Deflashing: Removing excess material.
- Assembly or Insert Loading: Adding components during or after molding.
- Packaging: Preparing parts for shipment.
How to Calculate Manual Labor Costs
Let’s say your parts need manual handling and packing after molding.
Using this example, let’s calculate how you can budget for injection molding labor costs:
- Labor rate: $28/hour
- Hands-on time: 0.75 minutes per part (45 seconds for trim + quick inspection + bagging)
Formula:
Labor cost per part = (Labor rate per hour ÷ 60) × labor minutes per part
Calculation:
($28 ÷ 60) × 0.75 = $0.466 × 0.75 = $0.35 per part
Labor cost per part = $0.35
How to Calculate Labor Cost Per Part
Now, let’s say labor isn’t happening on every part, but the run requires setup time.
This can help calculate per plastic part:
- Setup time: 2.5 labor hours per run
- Labor rate: $28/hour
- Run quantity: 12,000 parts
Formula:
Labor cost per part = Total labor hours per run × labor rate ÷ parts produced per run
Calculation:
(2.5 × $28) ÷ 12,000 = $70 ÷ 12,000 = $0.0058 per part
Set up labor cost per part = $0.006 per part (about half a cent)
This is why short runs feel more expensive—if you only molded 1,000 parts, the same setup would cost $0.07 per part.
How to Calculate Post-Production Costs
Post-production costs often include trimming, labeling, and packaging materials.
Assume these costs account for:
- Trimming time: 0.30 minutes per part
- Labor rate: $28/hour
- Packaging materials: $0.08 per part (bag + label + carton share)
Trimming labor cost:
($28 ÷ 60) × 0.30 = $0.466 × 0.30 = $0.14 per part
Total post-production cost per part:
$0.14 + $0.08 = $0.22 per part
Post-production cost per part = $0.22
These calculations help you build a more realistic injection molding cost estimate—because labor and post-production are often where a “cheap” part production quote quietly creeps upward, exceeding your project budget.
Price Your Next Plastic Project with Clarity—Contact Advanced Plastiform Inc. Today
When you’re quoting a plastic part, the last thing you want to rely on is guesswork. At Advanced Plastiform Inc., our custom plastics engineering company helps manufacturers price injection-molded projects with clear, practical cost breakdowns, so you can make decisions with confidence.
Get real numbers for your next plastic manufacturing project by calling 919-404-2080 or filling out our contact form to get started. We work with clients across industries throughout Georgia, Maryland, North Carolina, Pennsylvania, South Carolina, Tennessee, and Virginia.
