Injection Molding vs. CNC Machining Comprehensive Comparison

In the field of modern manufacturing, choosing the appropriate production process is crucial for balancing cost, quality and efficiency. The two mainstream technologies - injection molding and CNC machining service- have their own strengths but also overlap to some extent, and each performs well in specific scenarios. 

Injection molding is a combined additive and subtractive technology for mass-producing plastic parts. CNC Machining is a subtractive process that excels in manufacturing with high precision and medium to low production volumes, and is suitable for metal products. This article will provide a detailed introduction to their principles, performance, applications, and costs, helping manufacturers, engineers, and product designers make informed decisions.

1. Core Principles: How Injection Molding Works

Injection molding technology involves injecting materials into custom molds to form uniform and complex components. This process relies on repeatability and is suitable for large-scale production.

First, place the plastic particles into a heated barrel. Inside the barrel, there is a screw-type conveyor that will heat the plastic to a temperature range of 180°C to 300°C (the specific temperature depends on the material type, such as ABS, PP, etc.). Once the plastic melts, the screw will inject the material into the mold under high pressure to ensure complete filling of the mold cavity.

This mold is usually made of steel or aluminum and cools the molten plastic until it solidifies - the time required for this process varies from a few seconds to several minutes, depending on the thickness of the part. Finally, the mold opens and the finished part is removed. Some subsequent processing may be carried out, but for well-designed molds, no processing is necessary.

2.Core Principles: How CNC Machining Works

CNC machining is a subtractive process that uses computer-programmed tools to remove material from a complete workpiece to achieve the desired shape. This process is highly favored for its high precision and flexibility, and is suitable for small-batch production.

This process begins with the three-dimensional CAD model of the component. The software converts this model into a CAM program, which then transforms the design data into digital codes (G codes/M codes) that can be operated by the CNC machine.

The workpiece is fixed onto the machine. Then, the machine will use rotating cutting tools to remove the excess material. CNC machines can operate 3 to 5 axes: 3-axis machines handle basic 3D shapes, while 5-axis machines will rotate the workpiece and the tool to achieve complex shapes. After the processing is completed, the parts may undergo deburring or surface treatment to improve their appearance and functionality.

3.Key Performance Metrics: A Side-by-Side Table

The table below compares Injection Molding and CNC Machining across critical manufacturing metrics, helping to identify which process aligns with specific project goals.

Performance Metric	Injection Molding	CNC Machining
Initial Setup Cost	High ($10,000–$150,000+ for steel molds)	Low-to-Medium ($500–$5,000 for CAD/CAM setup)
Unit Cost	Very low ($0.10–$5) for high volumes (>10k units)	Moderate-to-High ($5–$50+) for low volumes (<1k units)
Production Speed	Fast (10–1,000 parts/hour, depending on size)	Slow (1–50 parts/hour, due to cutting time)
Tolerance Precision	Good (±0.02–0.10 mm for plastics)	Excellent (±0.001–0.02 mm for metals/plastics)
Material Compatibility	Best for thermoplastics; limited for metals (MIM)	Excellent for metals, plastics, composites, wood
Design Flexibility	High for moldable shapes; limited by mold design	Very high (handles complex 3D geometries)
Waste Generation	Low (excess plastic can be recycled)	High (30–70% material waste from cutting)

4.Material Suitability: Which Process Handles What?

The selection of materials is the primary factor. Both of these technologies can handle plastics and metals, but their respective advantages are quite different. The following table lists the common manufacturing materials.

Material Type	Injection Molding Suitability	CNC Machining Suitability
Thermoplastics (ABS, PP, PVC, nylon)	Excellent—high flowability and repeatability; ideal for complex parts	Good—works for rigid plastics but slower than molding; best for low-volume plastic parts
Metals (aluminum, steel, titanium, brass)	Limited—only via Metal Injection Molding (MIM); high cost and setup time	Excellent—handles all metals; excels at precision metal components (e.g., aerospace parts)
Composites (carbon fiber, fiberglass)	Moderate—requires specialized molds for fiber-reinforced plastics (FRP)	Good—cuts composites cleanly but needs sharp tools to avoid fiber fraying
Thermosets (epoxy, phenolic resins)	Good—cures in the mold; used for heat-resistant parts (e.g., electrical insulators)	Poor—brittle and prone to cracking during cutting

4.1 Ideal Applications for Injection Molding

Injection molding technology excels in the mass production of uniformly sized components. The initial mold cost can be offset by the relatively low cost per unit. Its main application areas include:

• Consumer Electronics: Phone cases, laptop bezels and headphone housings - all of them need to maintain consistent sizes and extremely high aesthetic standards. Apple produces millions of iPhone back covers each year through injection molding technology.

• Automotive Components: The interior components and those beneath the hood are made possible by the efficiency of the injection molding process and the durability of the materials.

• Medical Devices: Disposable items like syringes, IV connectors, and surgical tool handles—molding ensures sterility (via autoclavable materials) and batch-to-batch consistency.

• Packaging: Plastic bottles, caps, and food containers—molding produces lightweight, leak-proof parts at rates of 500+ units per hour.

  
  

4.2 Ideal Applications for CNC Machining

CNC Machining is the go-to choice for low-to-medium volumes, precision parts, or projects requiring rapid iteration. Key applications include:

• Aerospace & Defense: Turbo blades, aircraft structural components, and missile guidance components - all of these require extremely high precision (±0.005 millimeters) as well as compatibility with high-strength metals.

• Custom Tooling: Mold cores, fixtures and positioning devices - CNC machines can quickly produce these one-time or small-batch tools, which will then be used in other manufacturing processes (such as injection molding).

• Prototyping: Early-stage product prototypes (e.g., a new camera body or robotic arm component)—CNC machining allows designers to test 3D geometries in real materials without investing in molds.

• High-End Medical Equipment: Surgical instruments (e.g., scalpels, forceps) and implantable devices (e.g., hip stems)—precision and biocompatible materials (titanium, stainless steel) are non-negotiable here.

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5.Cost Analysis: Short-Term vs. Long-Term

Cost is often the deciding factor between the two processes, and the “break-even point” (where injection molding becomes cheaper) depends on production volume.

Short-Term Costs (Low Volume: <1,000 Units)

The cost of CNC machining is low. However, injection molding requires the payment of the cost for custom molds, which is not cost-effective for small batch production. For instance, the cost of producing 500 plastic brackets through CNC machining might be $2,500, while injection molding would cost $12,500.

Long-Term Costs (High Volume: >10,000 Units)

Injection molding technology. Once the cost of the mold is recovered, the unit cost will drop significantly. For 100,000 plastic brackets, the cost of injection molding could be $10,500, while CNC machining would require $500,000. The break-even point is usually between 1,000 and 5,000 pieces, depending on the complexity of the part and the type of material.

Hidden Costs

• Injection Molding: Mold maintenance (cleaning, repairs) and material waste from “flash” (excess plastic around parts) add 5–10% to total costs.

• CNC Machining: Tool replacement (cutting tools wear out after 100–1,000 parts) and material waste (up to 70% for complex shapes) can increase costs by 15–20%.

  
  

6.Design Constraints: What Each Process Can (and Can’t) Do

Designers must tailor part geometry to the chosen process—ignoring constraints leads to costly rework or failed parts.

Injection Molding Constraints

• Draft Angles: Parts must have a slight taper (1–5°) to eject from the mold without damaging the part or mold. Vertical walls without draft will stick, causing defects.

• Wall Thickness Uniformity: Walls should be 1–4 mm thick and consistent. Thick sections cool slower, creating “sink marks” (dents), while thin sections may not fill fully.

• Undercuts: Internal recesses (e.g., a hook inside a housing) require complex, expensive molds with sliders or lifters. Simple molds cannot produce undercuts.

CNC Machining Constraints

• Tool Accessibility: Cutting tools can only reach areas within their length and angle. Deep cavities or narrow channels may be impossible to machine if the tool can’t reach the bottom.

• Part Size: Limited by the CNC machine’s bed size (standard 3-axis machines handle parts up to 1m x 1m; larger parts need specialized equipment).

• Thin Walls: Thin sections (<1 mm) are prone to vibration and bending during cutting, leading to inaccuracies. Machinists often need to add temporary supports (removed post-processing).

7. FAQ: Common Questions About Injection Molding vs. CNC Machining

1. Which process is better for prototyping?

CNC Machining is superior for prototyping. It requires no mold, so you can produce a single prototype in 1–2 days (vs. 2–4 weeks for a mold). This allows fast design iterations—critical for testing form and function.

2. Can Injection Molding produce metal parts?

Yes, but only through the metal injection molding (MIM) process. MIM technology involves mixing metal powder with a binder, injecting it into a mold, removing the binder, and then subjecting it to sintering. MIM technology is costly and slow, so for most metal components, numerical control machining remains the preferred method.

3. What’s the maximum tolerance each process can achieve?

CNC Machining reaches tighter tolerances: ±0.001–0.02 mm (common for metals). Injection Molding typically hits ±0.02–0.10 mm (plastics), as material shrinkage during cooling affects precision.

4. Which process is more environmentally friendly?

The waste generated by injection molding is less: the excess plastic can be recycled, and the material utilization rate is very high. However, CNC machining wastes 30% to 70% of the workpiece material, but the metal chips can usually be recycled.

5. How long does it take to start production?

CNC Machining starts quickly: CAD/CAM setup takes 1–2 days, and production can begin the same week. Injection Molding takes longer: mold design and manufacturing take 2–4 weeks (steel molds) or 1–2 weeks (aluminum molds) before production starts.

6. Can CNC Machining handle complex geometries?

Yes - five-axis CNC machines can create extremely complex shapes by rotating the workpiece and the tool along multiple axes. Injection molding can also handle complex structures, but it is limited by the design of the mold.

7. Which process is cheaper for 5,000 units?

It depends on part complexity. For a simple plastic part (e.g., a cap), injection molding may be cheaper (mold cost $10k + $0.5/unit = $12.5k total). For a complex metal part (e.g., a gear), CNC machining may be cheaper (no mold + $10/unit = $50k total vs. $50k mold + $2/unit = $60k total).

8. Which process is better for high-temperature parts?

CNC Machining is better for high-temperature metals (e.g., Inconel, titanium), which retain strength at 1,000°C+. Injection Molding can use heat-resistant plastics (e.g., PEEK, which withstands 250°C), but these are expensive and have lower strength than metals.

H2.8 Conclusion: Choosing the Right Process

There is no “better” process—Injection Molding and CNC Machining serve different needs. To decide:

• Choose Injection Molding if: You need high-volume (10k+ units) parts, work primarily with plastics, and prioritize low unit costs. It’s ideal for consumer goods, automotive components, and disposable medical devices.

• Choose CNC Machining if: You need low-to-medium volumes (<5k units), require ultra-precision or metal parts, or need rapid prototyping. It’s perfect for aerospace parts, custom tooling, and high-end medical equipment.

Ultimately, the best choice depends on balancing four factors: output, material type, precision requirements, and budget. For mixed projects, many manufacturers will combine these two processes - using CNC machining to process metal inserts and using injection molding to produce plastic casings. By understanding the advantages and disadvantages of each, you can optimize your manufacturing process to achieve the optimal balance of quality, speed, and cost.