Compare best plastics for machining versus injection molding for optimal production choices

Compare Best Plastics for Machining vs Injection Molding for Optimal Production Choices

Prototype fast or scale smart? Choosing between CNC-machined plastics and injection-molded plastics starts with material behavior, part geometry, volume, and compliance. Use this guide—and the TaiwanMoldMaker.com network—to pick the right path from pilot to production.

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Quick answer: when to machine, when to mold

Choose CNC machining when…

Volumes are 1–500 pcs/variant (tooling ROI is weak).
You need tight tolerances quickly, or lots of design changes.
Stock shapes (rod/plate/tube) are available in your exact grade (medical, FR, UV, etc.).
Features are thick, localized, or deep that would make molding hard (long cooling, sink, slides).

Choose injection molding when…

Annual volumes are ≥1,000 pcs/SKU or multiple SKUs share a family tool.
You need repeatable cosmetics, low piece price, or overmold/IMD/two-shot integration.
Resin must be processed in production conditions (e.g., optics, controlled crystallinity).
You want traceability/MES and validated DOE → golden recipe for audits.

Typical break-even (guide): simple single-cavity aluminium tool justifies around 800–2,000 pcs; complex steel tools 5,000+ pcs. Your geometry and finish shift these ranges.

Plastics by process: what works best

Polymer (common names)	Machining suitability	Injection molding suitability	Notes / where it shines
POM / Acetal (Delrin®)	Excellent	Excellent	Low friction, crisp machining; great for gears, latches, precision clips.
PC (Polycarbonate)	Good (stress-relieve)	Excellent	Tough, clear; optics and housings. Avoid crazing with coolants/chemicals.
PMMA (Acrylic)	Excellent (polish to optical)	Good	Best clarity; more brittle than PC. Stress relief improves life.
PA6/PA66 (Nylon)	Good (watch fuzz/moisture)	Excellent (pre-dry)	Strong; absorbs moisture → dimension control in design.
PA12	Good	Excellent	Low moisture uptake; good for tubing, clips, outdoor parts.
PP / HDPE	Fair (burrs, soft)	Excellent	Cost-effective; great chemical resistance; cosmetics via texture.
ABS / PC-ABS	Good	Excellent	Stiff, paintless cosmetics with masterbatch + texture.
PEEK	Excellent (from rod)	Excellent (high-temp tools)	High-performance; machine for low volume, mold for scale.
PPSU / PSU	Good	Excellent	Repeated steam sterilization; medical trays/handles.
PVDF	Good	Excellent	Chemical resistance; lab/semicon fluid parts.
ETFE / FEP / PFA	Fair (can gum)	Excellent (high-temp)	Melt-process fluoropolymers; clean/chemical service.
UHMW-PE	Challenging (stringy)	Fair	Use sharp tooling; molding/secondary ops often better.
PTFE (Teflon®)	Excellent (from billet/sintered)	Not melt-moldable	Consider insert parts, compression molding, or coatings.
COC / COP	Fair (optics risky)	Excellent	Microfluidics/optics; prefer molding for stability.
TPE / TPU	Poor (gummy)	Excellent (esp. overmold)	Best for seals, grips; design mechanical locks.

Always verify the grade datasheet for FR (UL 94), medical/food contact, UV, and sterilization.

Tolerances, surface, and structure

Machining: ±0.05 mm (or better) is routine with good fixturing; surfaces can be polished, but anisotropy is minimal vs molding. Heat from cutting can induce stress—anneal/stress-relieve PC/PMMA/PEEK when critical.
Molding: Tight CTQs are achieved via scientific molding (V/P by pressure, cavity sensors, gate-freeze). Expect mold-induced anisotropy/warp—handled in DFM (flow/cool/warp) and DOE. Textures require draft 1.5–3.0°.

Cost & lead-time reality check

Factor	CNC Machining	Injection Molding
NRE (tooling)	$0–Low	Medium–High (Al → Steel)
Piece price	High	Low at volume
Lead time to FA	1–7 days	7–28 days to T0/T1 (tool build)
Design changes	Easy (modify toolpaths)	Costly (steel rework/new inserts)
Cosmetics	Tool-marks unless finished; paint/coating optional	Repeatable textures, IML/IMD, two-shot, overmold
Automation & data	Limited unless fixtured	MES/vision/traceability native

Decision guide (use this with your team)

Annual demand by SKU

< 800 pcs → Machine (or bridge tool if molding data is needed).
800–5,000 pcs → Al/MUD single cavity; compare to machining.
5,000 pcs → Steel tool; consider multi-cavity/family.

Geometry & finish

Thick/heavy mass, deep pockets → Machine first.
Paintless texture, tight knit-line control, overmold → Mold.

Material constraints

PTFE/UHMW-PE optics? → Machine.
COC/COP optics, TPE overmold, high-temp fluoropolymers → Mold.

Compliance & audits

Need DOE, CpK, genealogy → Mold with MES.
One-off fixtures, jigs, service parts → Machine.

Hybrid strategy (fast pilot → scalable production)

Phase 1: CNC from the production resin (or nearest equivalent) to validate fit/function/tests.
Phase 2: Bridge tool (aluminium/MUD) for T0/T1 data, cosmetic maps, ΔE/gloss, leak/functional tests.
Phase 3: Copy-cavity steel for ramp once demand lands; retain machining for fixtures and ECO buffers.

DFM tips to keep either path on schedule

Uniform walls (1.5–3.0 mm) for molding; add ribs instead of mass.
Gates & parting lines located away from cosmetics; plan ejector patterns.
Boss standards & inserts (heat-set/ultrasonic) to avoid thread failures.
Post-machining of molded parts for µ-level bores/threads when needed.
Stress relief of machined PC/PMMA/PEEK before critical inspection.

RFQ checklists (copy/paste)

For CNC machining

CAD (STEP/IGES) + 2D with GD&T and tolerances.
Material + grade (compliance needs), stock form/size, color.
Finish (polish/paint/anodize/coating), quantity by lot, inspection plan.

For injection molding

Target T1/SOP dates and milestones (DFM, T0, T1).
Material options & compliance (UL/food/medical/UV/FR).
Cavitation plan (single → family/multi), slides/hand-loads, overmold/IMD/two-shot if any.
Quality package (FAIR, CMM/scan, CpK targets), cosmetic map & texture codes.
Data/traceability (MES access: OEE, CpK, scrap, kWh/kg, genealogy).
Logistics (FA courier, export packing, Incoterms).

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Why TaiwanMoldMaker.com

One accountable owner from DFM to SOP across CNC + molding.
Bridge-to-scale strategy: machined pilots → aluminium/MUD → copy-cavity steel.
Audit-ready data (vision + MES) to accelerate approvals.

Start here:
Custom Mold & Design Maker → Mold Service → Injection Mold → Molding → Customer Examples → Contact