Engineering design resource
If you are choosing injection molding vs CNC for plastic parts, decide from volume, material history and feature definition—not a default tool. Molding forms resin in a cavity for repeat shapes; CNC machines stock when geometry, grade or low volume make tooling unnecessary. State resin, quantity and critical faces, then upload CAD for a reviewed quote.
Use this guide
CNC machining can support early functional parts, bridge quantities, thick sections, accessible pockets and precision interfaces without a dedicated production mold. The route depends on available stock, tool access, workholding, burr control, internal stress, moisture or conditioning and the anisotropy of some extruded stock forms. Geometry that looks simple in a model may require multiple setups or separate components, while very thin ribs and enclosed internal features may be unsuitable for machining.
Injection molding can support repeat production-intent geometry with draft, ribs, bosses, clips, textures and integrated features when the tool, material and process are developed together. It introduces tooling design, gate and ejector decisions, shrinkage, warpage, knit lines, sink, venting, cavity strategy and sampling. If a program moves from machined to molded parts, validate material grade, wall structure, interfaces, surface, dimensions and functional behavior again. A staged route can use machining for learning while the molded design and tool mature.
Design priorities
The decision should include both the part design and the commercial stage in which that design will be used.
Use stock-machined parts for early fit, fixture, assembly or functional learning when the geometry is accessible and the stock material is relevant. State which conclusions cannot transfer directly to molded behavior.
Use molding when production-intent wall, rib, boss, clip, texture and cavity decisions can be stabilized around a defined resin and tool. Include sampling, maintenance and change expectations in the program route.
Some molded parts need inserts, secondary machining, hardware, welding, marking or assembly. Define which features are tool-created and which are completed later, including datum transfer and inspection state.
Machining and molding can overlap during validation, tool build or supply transition. Keep revisions, material states and evidence separated so a machined sample is not mistaken for a released molded result.
Common review gaps
Even when dimensions match, feedstock, orientation, stress, surface and feature creation can produce different part behavior.
Stock and molded resin with similar names can differ in exact grade, additives, moisture history, orientation and residual stress. Use machined parts for defined learning and validate the released molded material separately.
Uniform walls, draft, ribs, bosses, gates, ejector access and undercuts may require a different architecture. Review the design for molding rather than asking the tool to reproduce billet-oriented geometry unchanged.
Tool construction, inserts, cavity count, sampling, maintenance, repair, storage and design changes are part of the program. Define who controls changes and what evidence supports repeat release.
Machined tool marks, molded texture, shrinkage, warpage, gate vestige and post-process operations affect dimensions and appearance differently. Compare final-state interfaces under the relevant process and conditioning.
Practical choices
The chosen route should preserve learning while making final material and geometry assumptions explicit.
Use sheet, plate, rod or block when tool access, stock properties, quantity and change frequency support machining. Define support, burrs, conditioning, threads, surface and dimensional state.
Use a tool-defined part when repeat geometry, integrated features and production-intent resin justify tool development. Plan DFM, tool construction, sampling, process window, secondary work and release evidence together.
Use machined parts for targeted fit or functional learning while developing a separate molded design and tool. Maintain revision and evidence boundaries, then validate molded parts against final requirements before transition.
Design decision table
Typical published capability for injection moulding against CNC machining of plastics. Achievable results depend on resin, geometry and supplier, and the project requirement is confirmed on the reviewed quote rather than from this table.
| Decision factor | Injection molding direction | CNC machining direction |
|---|---|---|
| Achievable tolerance | Commonly ±0.1 mm on small features in a stable resin, widening with wall thickness and shrinkage | Acetal at ±0.05 mm; PEEK and PEI at ±0.05 mm with annealing; unfilled nylon limited to about ±0.1 mm by moisture |
| Tooling cost and lead time | Aluminium prototype tooling commonly 2 to 4 weeks; production P20 tooling 8 to 16 weeks | None — parts come from stock |
| Quantity crossover | Economic from roughly 100 to 1,000 parts upward depending on part size | Economic below that, with setup dominating at 1 to 10 parts |
| Material properties | Isotropic, fully consolidated, with knit lines behind holes and bosses | Isotropic stock properties, with no knit lines and no moulded-in stress |
| Design constraints | Uniform wall of 1.0 mm to 3.0 mm, draft of 1° to 2°, ribs at 50 to 60% of wall, no undercuts without actions | Any geometry a cutter can reach; internal radii set by the tool |
| Shrinkage | 0.5% for ABS, 0.6% for polycarbonate, 1.5% for nylon, 2.5% for acetal — the tool is cut for one resin | None; the part is cut from stock at final size |
| Features only moulding gives | Living hinges, snap fits with fatigue life, textured surfaces, moulded-in inserts | Not reproducible by machining |
| Features only machining gives | Not applicable | Varying wall without sink, deep undercuts, one-off geometry changes |
| Surface finish | Follows the tool — a polished cavity gives near Ra 0.4 µm | Ra 0.8 to 1.6 µm from a finishing pass on acetal and PEEK |
| Stock availability | Resin grades are widely available | Machinable stock forms are narrower — some resins exist only as moulding grades |
| Design change cost | Steel-safe changes are straightforward; adding tool material means welding or an insert | A new programme and a new part |
| Where each wins outright | Unit cost at volume, consistency, and features only moulding creates | Low quantity, fast iteration, and tolerances tighter than moulding holds |
Turn the guide into an RFQ
Provide program stage and final-state requirements so machining and molding can be compared without claiming equivalence.
Complete packages move faster: revision-matched CAD, critical dimensions, quantity and material notes are enough to open engineering review across the network.
Get a QuoteSubmit the controlled model, drawing, revision, assembly context and functional requirements
State prototype, bridge and repeat quantities plus expected design-change timing
Define exact material grade, additives, color, conditioning and permitted alternatives for each route
Mark walls, ribs, bosses, clips, threads, inserts, undercuts, datums and critical interfaces
Specify finish, texture, gate or ejector restrictions, post-processing, inspection and packaging
Define tooling ownership, cavity and sampling needs, transition evidence and redesign authority
Questions before routing
These answers prepare the request; the reviewed quote controls project-specific commitments.
Close on geometry, different on behaviour. A machined part is cut from consolidated stock with isotropic properties and no moulded-in stress; a moulded part has knit lines behind every hole and boss that are weaker than surrounding material, fibre orientation in filled grades, and residual stress that later drives stress cracking in polycarbonate. Living hinges and fatigue-loaded snap fits only work moulded. Machining is also tighter — acetal at ±0.05 mm against moulding's ±0.1 mm.
Usually somewhere around 100 to 1,000 parts, depending on part size and geometry. Below that, machining or urethane casting is cheaper end to end. The other trigger is capability rather than cost: if the design needs a living hinge, a fatigue-rated snap fit, a textured surface or a moulded-in insert, machining cannot produce it at any quantity. Aluminium tooling at 2 to 4 weeks lead is the normal first step.
Yes, and it is common where a feature is tighter than moulding holds or would need an expensive tool action. Moulding gives about ±0.1 mm on small features; a machined bore or face on the same part reaches ±0.05 mm in acetal. Secondary machining also avoids side actions in the tool. The design point is to leave material where the machining will happen, rather than machining into a nominal wall and creating a thin section.
Machining, generally. Moulding commonly holds ±0.1 mm on small features and widens with wall thickness and shrinkage — which ranges from about 0.5% for ABS to 2.5% for acetal. Machined acetal holds ±0.05 mm, and ±0.025 mm with a stress-relieved blank and temperature control. The exception is nylon, where the material's 0.5 to 0.8% moisture-driven growth defeats tight tolerances by either route.
Because moulding forms the polymer under pressure and flow, and machining only removes material. Knit lines appear wherever two flow fronts rejoin, typically behind holes and bosses. Sink marks appear opposite thick sections. Living hinges depend on the molecular orientation that flow creates. Gate vestige and ejector witness are unavoidable. All of these are absent from a machined prototype, which is why a machined sample cannot fully validate a moulded design.
Next step
Apply the guide to a real drawing and RFQ package. MakeNexa routes capable suppliers from a global network covering competitor-class process categories, then returns a prepared quote or focused clarification for your revision.
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