The choice between injection molding and CNC machining for plastic parts is determined by the production quantity, the part complexity, the material requirements, and the development timeline. Injection molding requires a custom steel mold that costs 5,000 to 50,000 dollars and takes 4 to 12 weeks to fabricate, but produces parts with per-part costs as low as 0.10 to 2.00 dollars at high volumes. CNC machining requires no tooling investment and can start production within days of drawing approval, but the per-part cost is relatively constant at 5 to 50 dollars regardless of quantity. The crossover point where injection molding becomes more economical is typically between 500 and 5,000 units for simple parts and between 100 and 1,000 units for complex parts where the CNC machining time per part is high. The crossover point shifts with the part material, the geometry complexity, and the machine time available at the CNC supplier, and is best determined by requesting quotes for both processes on the specific part design rather than relying on general rules of thumb. The cnc turning services option for plastic cylindrical parts is particularly cost-effective for quantities up to 10,000 units because the turning cycle time is fast enough that the per-part cost remains competitive with injection molding for simple geometries like bushings and spacers.
The material properties comparison between CNC machined and injection molded plastic parts reveals important differences that affect the application suitability. CNC machined plastics have isotropic mechanical properties because they are machined from extruded or compression molded stock that has consistent properties throughout the cross section. Injection molded plastics may have anisotropic properties caused by the flow orientation of the polymer molecules during injection, and the shrinkage and warpage that occurs during cooling can introduce residual stresses that affect the dimensional stability. The mechanical strength of a CNC machined plastic part is typically 10 to 20 percent higher than an injection molded part made from the same material grade because the molded material has internal stress concentrations and possible voids from the molding process. The surface finish of CNC machined plastics is typically superior to injection molded plastics because the cutting tool produces a smooth surface without the flow lines and knit lines that are characteristic of injection molded parts. The cnc machining parts option is preferred for plastic components that require tight tolerances below plus or minus 0.003 inches, smooth surface finishes below Ra 32 microinches, or zero internal voids that could affect the part performance in pressure or vacuum applications.
The lead time comparison is often the deciding factor for product development projects with tight launch schedules. CNC machined plastic prototypes can be delivered in 3 to 10 business days from drawing release because the CAM programming and machine setup can be completed in a single shift. Injection molded parts require 4 to 12 weeks for mold design and fabrication before any parts can be produced, and the mold typically requires one or more trial runs and modifications before it produces parts that meet the dimensional specifications. The hybrid approach that starts with CNC machining for initial production and transitions to injection molding for volume production is the optimal strategy for most new product launches, capturing the time-to-market advantage of CNC machining while achieving the long-term cost advantage of injection molding. The transition timing should be planned during the product development phase, with the CNC supplier and the molding supplier collaborating on the design features that must differ between the machined prototype and the molded production version. The buttress thread calculator is a reference tool for designing threaded fasteners in plastic parts that will be machined or molded, because the buttress thread form distributes the thread pull-out load over a larger stress area in the plastic material than standard thread forms, reducing the risk of thread stripping in the softer plastic material.
The tooling cost for injection molding is the primary economic barrier that limits the process to higher production volumes. A simple two-plate mold for a plastic part with no side actions costs 5,000 to 15,000 dollars, while a complex mold with multiple slides, lifters, and hot runner systems costs 30,000 to 80,000 dollars. The mold cost is amortized over the production quantity, so the per-part tooling cost at 1,000 units with a 10,000 dollar mold is 10 dollars per part, while at 100,000 units the per-part tooling cost is only 0.10 dollars. The mold life for steel production molds is 500,000 to 5,000,000 cycles, while aluminum prototype molds are limited to 5,000 to 50,000 cycles. The CNC machining alternative has no tooling cost, making it the economical choice for quantities below the crossover point even though the per-part machining cost is significantly higher than the per-part injection molding cost at production volumes.