Every plastic part design reaches a fork in the road: machine it or mold it. CNC machining delivers parts in days with no tooling investment and plus or minus 0.05 mm precision. Injection molding requires a $5,000-80,000 mold and 2-8 weeks of lead time, but produces parts at $0.50-5.00 each at volumes where CNC costs $15-50 each. The decision is not about which process is better — it is about which process matches your volume, timeline, tolerance, and material requirements at the lowest total cost.
This guide lays out the process comparison data, volume breakpoints, and hybrid strategies that Nylon Plastic uses with customers every day. The goal is not to steer you toward molding (which is our largest business) or machining — but to help you choose the right process for where you are in the product lifecycle.
Process Comparison at a Glance
| Yếu tố | Chế tạo CNC | Ép phun | Winner |
|---|---|---|---|
| Tooling cost | $0 (no mold required) | $5,000-80,000+ | CNC for under 500 pcs |
| Per-part cost (100 pcs) | $15-50 | $20-60 (tooling dominates) | CNC |
| Per-part cost (10,000 pcs) | $15-50 | $0.80-4.00 | Ép phun |
| Lead time (first parts) | 3-10 days | 15-30 days (mold) + 1-5 days (parts) | CNC |
| Tolerance | plus or minus 0.05-0.10 mm | plus or minus 0.10-0.30 mm | CNC |
| Surface finish | As-machined Ra 0.8-3.2 um | SPI A3-D3 (0.01-8.0 um Ra) | Injection (cosmetic) |
| Material options | Any rigid plastic (sheet/rod/block) | Any injection-grade thermoplastic | Injection (broader) |
| Design changes | Free (revise CAM program) | $1,000-10,000+ (steel-safe mods only) | CNC |
| Minimum wall | 1.0 mm (2.0 mm preferred) | 0.5 mm (1.0 mm preferred for structure) | Ép phun |
| Scalability | Linear cost with volume | Amortized tooling, low marginal cost | Injection (10,000+) |
Volume Break-Even Analysis
The break-even point where injection molding becomes cheaper than CNC machining depends on part complexity and size. Rule of thumb for a palm-sized part (50-100g): Below 250 pcs: CNC is cheaper. 250-1,000 pcs: costs are roughly equal; choose based on timeline, tolerance, and whether design is locked. Above 1,000 pcs: injection molding pulls ahead and the gap widens rapidly. Above 10,000 pcs: injection molding is 3-10x cheaper per part.
Detailed example — 75g PA66 bracket, 50x50x30 mm: CNC machining: $22/part (1 hr setup + 15 min/part at $60/hr + $8 material). Injection molding: $12,000 mold + $1.20/part (material $0.35 + machine time $0.45 + labor $0.40). Total cost: 100 pcs: CNC $2,200 vs IM $12,120. 500 pcs: CNC $11,000 vs IM $12,600. 1,000 pcs: CNC $22,000 vs IM $13,200. 10,000 pcs: CNC $220,000 vs IM $24,000. The mold pays for itself between 500-600 parts.
When to Choose CNC Machining
Prototyping and design iteration (1-50 pcs): No mold means design changes cost zero in tooling. CNC parts in 3-5 days let you test, modify, and re-make overnight. Bridge production (50-500 pcs): While the injection mold is being built (3-6 weeks), CNC parts keep your assembly line, testing program, or customer demos running. Large-format parts (over 500×400 mm): CNC machines handle large plastic sheets and blocks that would require enormous and expensive injection presses. Ultra-tight tolerances (plus or minus 0.05 mm or better): CNC holds tighter tolerances than injection molding for most geometries. Low annual volume ongoing: If annual demand stays below 500 pcs, the mold may never amortize — CNC is the permanent production solution.
When to Choose Injection Molding
Production volumes above 1,000 pcs/year: The mold cost amortizes to pennies per part at scale. Per-part cost drops 80-95% versus CNC at volume. Cosmetic surface quality: Molded surfaces replicate polished mold steel — CNC leaves tool marks that require secondary finishing for cosmetic parts. Thin walls and fine detail: Injection molding achieves wall thicknesses down to 0.3-0.5 mm and replicates sub-millimeter detail that CNC tools cannot physically reach. Material properties through orientation: Glass-filled materials gain directional strength from fiber orientation in molding — machined parts have random fiber orientation from the stock material. Consistent batch-to-batch quality: Once the mold is qualified, every shot produces the same part. CNC parts have operator-to-operator and setup-to-setup variation.
Design Rules for Process Selection
- Start with CNC, transition to molding: The most cost-effective product development path: CNC machine 10-50 prototypes for design validation, then invest in an injection mold once the design is locked. The prototype phase informs gate location, wall thickness sensitivity, and tolerance requirements — all valuable inputs for mold design that reduce the risk of mold modifications.
- Design for your production process from day one: Even if you are starting with CNC, design the part as if it will eventually be molded: uniform wall thickness (avoid thick sections that are easy to machine but impossible to mold without sink), draft angles on vertical surfaces, and generous radii instead of sharp internal corners. A part that machines beautifully but cannot be molded requires redesign before tooling — doubling your engineering cost.
- CNC for complex 3D surfaces: Freeform surfaces, undercuts (accessible by 5-axis), and deep pockets with flat bottoms are CNC strengths. Injection molding the same features may require side actions, lifters, or collapsible cores that add thousands to mold cost. If the part has complex 3D geometry that requires 3+ side actions to mold, CNC may be cheaper even at moderate volumes (1,000-2,000 pcs).
- Mold for multi-cavity cost reduction: A single-cavity mold produces one part per cycle. A 4-cavity mold produces four parts per cycle with roughly 50-70% more mold cost — not 4x. For high-volume parts (50,000+/yr), multi-cavity molds are the standard. CNC has no equivalent — 4 parts always cost 4x as much as 1 part.
- Material stock availability limits CNC: CNC machining requires the material to be available in sheet, rod, or block form. Some engineering plastics (PPS, PPA, specialty grades) are not stocked in machinable forms and must be injection molded. Check material availability before committing to a CNC-only strategy for exotic thermoplastics.
- Combine both for hybrid manufacturing: The hybrid model: injection mold a near-net-shape blank with all cosmetic surfaces and fine details, then CNC machine only the critical tolerance features (bearing seats, seal faces, mating surfaces). This delivers injection molding per-part economy with CNC precision where it matters. The approach is standard in automotive and medical — the blank costs $1-3 from molding, and the machining adds $2-8 for the tight features. Total: $3-11/part versus $15-50 for full CNC.
Process Selection by Application
Industry Application Matrix
| Ngành công nghiệp | Typical Parts | Material/Grade | Key Requirement |
|---|---|---|---|
| Prototyping / R&D | Functional prototypes, form/fit testing, design iteration | CNC (1-50 pcs) in 3-10 days | Speed and design flexibility; cost secondary |
| Bridge / Pre-Production | Customer samples, testing, regulatory submission | CNC (50-500 pcs) while mold is built | Match production material and finish |
| Low-Volume Production | Specialty equipment, replacement parts, custom tooling | CNC (100-1,000/yr ongoing) | Mold never amortizes; CNC is permanent solution |
| High-Volume Production | Consumer goods, automotive, medical disposables | Injection (10,000+/yr) with multi-cavity | Per-part cost below $2; consistent quality |
Cost Decision Framework
Cost comparison formula: CNC total cost = (Setup time x Shop rate) + (Cycle time/part x Shop rate x Quantity) + (Material cost/part x Quantity). Injection total cost = Mold cost + (Material cost/part + Machine cost/part + Labor cost/part) x Quantity.
Typical shop rates: CNC plastic machining: $50-80/hr (3-axis), $80-150/hr (5-axis). Injection molding: machine rate $25-50/hr (shared across cavities).
Decision rule: If (CNC unit cost x Quantity) is greater than (Mold cost + IM unit cost x Quantity), injection molding is cheaper. Solve for the break-even quantity: Q = Mold cost / (CNC unit cost – IM unit cost). For our 75g bracket example: Q = $12,000 / ($22 – $1.20) = 577 parts. Below 577, CNC wins; above, injection molding wins. Every part has its own number — this formula gives you the answer in 30 seconds.
Common Mistakes and Solutions
| Defect | Appearance | Root Cause | Solution |
|---|---|---|---|
| Designing a CNC-only part blind to molding | Part has non-uniform walls and zero draft | Designing only for the immediate process | Design with molding rules from day one — uniform walls, draft, radii |
| Underestimating mold lead time | Project delayed because the mold is taking forever | Assuming mold = 2 weeks; reality is 3-8 weeks | Plan 6 weeks for mold build; use CNC bridge production in parallel |
| Choosing injection too early | Mold modification cost exceeds original mold cost | Design not yet validated; changes require steel-safe mods | Use CNC prototypes to validate design before committing to mold steel |
| Choosing CNC for annual volume over 2,000 | Per-part cost never decreases; margin erodes | No tooling to amortize; labor and material cost linear | Run the break-even calculation; if volume supports it, invest in mold |
Why Choose Nylon Plastic for Your Project
Precision Manufacturing
30+ CNC & injection molding cells under one roof
ISO 9001:2015
Certified quality system, full inspection reports
15-25 Day Lead Time
Fast turnaround with expedited options available
Global Shipping
Air & sea freight to North America, Europe, Asia
Download Our CNC Machining vs Injection Molding Guide
Free PDF reference guide with technical data, design rules, and supplier checklists.
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Câu hỏi thường gặp
When should I choose CNC machining over injection molding?
Choose CNC when: (1) Quantity is under 250-500 pcs — the mold cost dominates and CNC is cheaper in total. (2) You need parts in under 2 weeks — CNC delivers in 3-10 days versus 3-8 weeks for molding. (3) The design is not yet finalized — CNC lets you iterate without tooling modification cost. (4) Tolerances must be tighter than plus or minus 0.10 mm. (5) The part is very large (over 500 mm) or requires complex 3D surfaces that would need expensive mold side actions. (6) Annual volume stays below 500 pcs ongoing — the mold never amortizes.
What is the cost break-even volume between CNC and injection molding?
For a typical palm-sized part (50-100g): break-even is between 250 and 1,000 pcs. A simple part with a $5,000 mold breaks even at roughly 150 pcs. A complex part with a $30,000 mold breaks even at roughly 2,000 pcs. Use the formula: Break-even Q = Mold cost / (CNC unit cost – IM unit cost). For quick estimates: if the mold costs $10,000, CNC unit cost is $25, and IM unit cost is $2, the break-even is 10,000/(25-2) = 435 parts. Below this number, CNC is cheaper; above it, injection molding is cheaper. The formula accounts for all variables and takes 30 seconds to calculate.
Which process produces more precise parts — CNC or injection molding?
CNC machining produces more dimensionally precise parts in nearly all cases: plus or minus 0.05-0.10 mm typical versus plus or minus 0.10-0.30 mm for injection molding. However, injection molding produces more consistent parts batch-to-batch — once the mold is qualified, every shot is nearly identical. CNC parts vary with setup, tool wear, and operator. For absolute dimensional accuracy on a single part: CNC wins. For part-to-part consistency at volume: injection molding wins. The ideal combination: injection mold to near-net shape, then CNC machine only the critical tolerance features.
Can I combine CNC machining and injection molding on the same part?
Yes — this is called hybrid manufacturing and it is widely used in automotive, medical, and industrial applications. The most common approach: injection mold the part blank with all cosmetic surfaces, thin walls, and fine details, then CNC machine only the critical tolerance features — bearing seats, seal faces, flatness-critical mounting surfaces. The molded blank costs $1-3, and the machining adds $2-8 for the precision features. Total per part: $3-11 versus $15-50 for full CNC or plus or minus 0.15 mm tolerance from molding alone. This approach is standard for high-volume precision components and worth considering any time you need molding economics with machining precision.
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