All custom parts begin with an engineering decision about what process can deliver the desired geometry, tolerances, and mechanical performance within budget. Choosing the wrong one not only doubles lead time but also consumes the tolerance stack or ties up working capital in premature hard tooling.
But choosing the right process takes a CAD file through to a functional production-quality part in days while setting up an economical course from prototyping to high-volume manufacturing.
There is no “best” way to manufacture. Effective manufacturing depends on many trade-offs between annual production volumes, microstructure of the base material, part complexity, and non-recurring engineering cost.
This guide will cover the primary manufacturing processes for custom parts that mechanical engineers have at their disposal, including mechanical limitations, tolerance ranges, economics, and DFM rules.
Process Selection: Upstream of Geometry and Material
A substantial portion of part failures encountered during pilot production are not design flaws; they are process mismatches.
Common failure modes include specifying a thin wall that leads to sink marks or short shots in injection molding, requiring tight linear tolerances across multiple press-brake sheet-metal bends, or engineering internal sharp corners that demand costly sinker EDM operations instead of standard multi-axis CNC milling.
Process selection dictates nominal wall thicknesses, required draft angles, metallurgical grain orientations, and machining datum strategies. When the fabrication method is established correctly upstream, DFM review cycles compress, inspection routines simplify, and unit manufacturing costs hit target margins.
The Custom-Part Manufacturing Landscape
The modern industrial processes of manufacturing are classified into five major families of processes:
- Subtractive Processes: Material is selectively removed from isotropic solid billets, rolled bars, and extruded sections by rotary cutting tools (such as 3-axis, 4-axis, and 5-axis CNC milling, CNC turning).
- Formative/Solidification Processes: Material is shaped by using dedicated molds, cavities, or dies (such as plastic injection molding, high-pressure die casting, investment casting, metal stamping).
- Sheet Processes: Material is formed, sheared, and joined as uniform-gauge rolled sheet metal (such as fiber laser cutting, CNC turret punching, press-brake air bending, and TIG/MIG welding).
- Additive Processes: Layers of polymer filaments, liquid photopolymers, or atomized metal powders are bonded together.
- Bridge/Hybrid Processes: The economic boundary between prototyping and production tooling is bridged (such as polyurethane vacuum casting, rapid soft tooling, secondary CNC machining of near-net-shape).
Subtractive Manufacturing: Accurate CNC Machining
Subtractive manufacturing is used for making precise and robust prototype parts, kinematic assemblies and mechanisms, and manufacturing parts in limited series.
Since toolpaths are calculated by computers through CAM software, even complicated shapes can be produced directly from solid materials without creating molds.
Strength and Accuracy of Mechanical Parts
In contrast with casting and additive manufacturing, subtractive manufacturing starts with pre-manufactured bars that are machined into desired forms. These materials have dense and homogeneous grains, low porosity, and isotropy due to mechanical work performed during production of the material.
Critical applications, such as the aerospace industry, medicine, or robotics, require using alloys, including 6061-T6, 7075-T6 aluminum, and 316L or 17-4 PH stainless steel. Their published characteristics remain true after cutting with rigid machine tools on several axes. To find out more about machine types, visit our page on machining processes and machine types.
Tolerance and Surface Finish Capabilities
Today’s 5-axis CNC machining centers regularly deliver linear tolerances of ±0.005 mm to ±0.025 mm (±0.0002 in to ±0.001 in) on important parts like bearing journals, locating dowels, and dynamic seals.
The standard surface finish for machined parts is in the range of Ra 0.8 μm to 1.6 μm (32–63 μin), which may be refined down to Ra 0.4 μm (16 μin) or mirror surface finishes using precision grinding, reaming, or abrasive finishing.
When CNC Machining Makes Sense
- Validation that needs real production material or engineering thermoplastics (PEEK, Delrin/POM-C, PTFE, nylon 6/6).
- Difficult parts with stringent GD&T (true position, coaxiality, tight flatness over large areas).
- From 1 piece up to 1,000 pieces where molds or dies aren’t amortized efficiently.
- Fast turnaround (2 to 5 business days after CAD freeze).
Process Limitations
- Flat Unit Cost Graph: The time spent on using CNC spindles, tool wear, and removal rate are linear; the cost per unit does not fall exponentially with increased numbers.
- Internal Shape Limitations: Completely enclosed cavities, deep slots, and sharp corners within the workpiece (which require radii equal to standard end mill diameter) cannot be machined without using multi-axis operations or wire EDM technology.
Formative Processes: Plastic Injection Molding and Metal Casting
Once volumes exceed hundreds or thousands, formative processes employ physical molds and dies to attain fast cycles and reduced cost per unit.
Plastic Injection Molding
Injection molding utilizes thermoplastic resins injected at very high clamp tonnage (100-1,500 tons) into hardened dies made out of either steel (H13, P20 or NAK80) or aluminum. Upon cooling through conformal channels within the die, the molded component is ejected by means of ejector pins.
- Non-Recurring Costs/Lead Times: There are upfront non-recurring costs (between $5,000 and $50,000+) that are required to pay for the mold manufacturing time (4 to 8 weeks). This involves tool steel machining, EDM spark erosion and T1 trial samples.
- Design for Manufacturing Requirements: Ensure consistent wall thicknesses (1.5mm to 3.5mm), 1° to 2° draft angle per side for ease of mold extraction, and avoid gates and knit lines in regions of high stress.
- Molding Techniques: Insert molding (with threaded brass ferrules), and Two-Shot Over-molding (with TPE/TPU coating on top of PC/ABS body).
Metal Casting – Die Casting versus Investment Casting
High-volume metal components should avoid wasteful machining of billets and lengthy cycle times by using casting techniques:
- High Pressure Die Casting (HPDC): It is the best process to be used for Aluminum (A380, ADC12), Zinc (Zamak 3), and Magnesium metal alloys having production ranges from 3,000 to above 100,000 units. This process offers thin-wall castings (down to 1.5 mm) with a cycle time of a few seconds. Nevertheless, mold filling through turbulence creates problems of entrapped gases inside the casting.
- Investment Casting (Lost Wax): Most appropriate process for complex shapes, high-temperature superalloys, and Stainless-Steel components. Offers near-net shape with fine topography (Ra 1.6 to 3.2 microns).
Sheet Metal Fabrication: Enclosures, Panels, and Brackets
Sheet metal fabrication uses flat stock (aluminum, cold-rolled steel, galvanized sheet, stainless steel) converted into enclosures, panels, and brackets by means of laser cutting, CNC punching, press-brake forming, and hardware installation.
- Ideal Application Range: Thin-walled enclosures, 19-inch rack-mounted chassis, brackets, and protective covers for surfaces that have a low thickness-to-surface-area ratio. Little or no tooling is needed; hence, the process is efficient.
- Tolerances: High-powered CNC fiber lasers maintain tight cutting tolerances of ±0.10 mm (±0.004 in). On the other hand, press-brake forming has limitations due to material grain orientation, spring-back, and material thickness; hence, linear formed tolerances are ±0.30 mm to ±0.50 mm.
- Prototyping for Production Run: For sheet metal enclosure prototypes that need a quick turnaround, sheet metal fabrication houses provide rapid batch fabrication without the need for dies.
Additive Manufacturing and Polyurethane Vacuum Casting
Industrial 3D Printing
Industrial additive manufacturing (AM) involves polymer powder bed fusion (SLS, MJF), resin stereolithography (SLA), and direct metal laser sintering (DMLS).
AM is very effective for lightweight topology-optimized structures, internal conformal fluid passageways, and quick concept testing. For understanding differences between additive manufacturing techniques and soft tooling, see our engineering review of rapid prototyping methods and applications.
Polyurethane Vacuum Casting
Vacuum casting (urethane casting) acts as a critical intermediary stage between 3D printing and injection moulding.
A master pattern from SLA generates a room temperature vulcanized (RTV) silicone mold. Degassed liquid polyurethane resin undergoes curing in the RTV silicone mold, producing 10 to 50 parts from each Mold.
The resultant parts mimic production thermoplastics (ABS-like, polypropylene-like, elastomer Shore A, clear polycarbonate-like) in their molded texture for functional prototyping, clinical trials, and usability testing.
Engineering Process Selection Matrix
The following table provides an overview of the engineering capabilities and economic characteristics of custom manufacturing processes:
| Manufacturing Process | Standard Linear Tolerance | Surface Roughness (Ra) | Tooling / NRE Cost | Initial Lead Time | Optimal Volume Range |
| CNC Machining (3-Axis / 5-Axis) | ±0.005 to ±0.05 mm | 0.4 – 1.6 μm | Negligible (Modular Fixturing) | 2 – 5 Days | 1 – 1,000 units |
| Plastic Injection Molding | ±0.05 to ±0.15 mm | 0.2 – 1.6 μm (SPI-A2 to SPI-C1) | High ($5,000 – $50,000+) | 4 – 8 Weeks | 2,000 – 100,000+ units |
| High-Pressure Die Casting | ±0.10 to ±0.25 mm | 1.6 – 3.2 μm | High ($8,000 – $40,000+) | 6 – 10 Weeks | 3,000 – 50,000+ units |
| Sheet Metal Fabrication | ±0.10 to ±0.50 mm | N/A (Mill finish / Powder coat) | Low (Setup / Soft tooling) | 3 – 7 Days | 10 – 2,000 units |
| Industrial 3D Printing (SLS/MJF) | ±0.15 to ±0.30 mm | 6.3 – 12.5 μm | None | 1 – 3 Days | 1 – 50 units |
| Polyurethane Vacuum Casting | ±0.15 to ±0.30 mm | 0.8 – 1.6 μm | Low ($300 – $1,500) | 7 – 12 Days | 10 – 50 units |
Hybrid Paradigm: Near-Net-Shape Casting and Secondary Machining
Engineering parts designed for performance almost always use more than one process to manufacture them. High-tech manufacturing usually requires a hybrid near-net manufacturing process.
According to this process, casting or extruding is used to make the macro-envelope, fluid pockets, thin cooling fins, and mounting bosses. No raw material is wasted, and no spindle hours are spent doing roughing.
CNC machining centers are responsible for the secondary process – cutting O-ring grooves, making bearing seats within the H7 tolerance limits, drilling and tapping for fasteners, and machining flat surfaces for sealing.
Dealing with suppliers who have the capability of providing precise CNC machining along with rapid tooling services reduces potential quality problems. As long as the machining engineers know about the parting line of the casting, ejector pin locations, and clamp deflections, it is possible to design fixtures that won’t allow any warping during machining.
Engineering Decision Model: Four Key Criteria
For a custom part design, consider the following four engineering factors while choosing a process:
- Materials Selection: Does the custom part design need high-strength wrought alloys (6061 / 7075 aluminium, titanium), high-temperature engineering polymers (PEEK, Ultem), or just commercial-grade resins? High-alloy demands lean towards CNC Machining; complex thin-walled plastic shell design favors injection molding.
- Production Quantity Per Year: Under 500 parts per year, the capital cost of tooling makes the use of injection molding or die casting impractical relative to CNC machining or additive processes. Over 2,500 parts per year, tooling amortizes quickly, resulting in low part cost.
- Tolerances & GD&T: If a design includes features requiring tolerances < ±0.05 mm, bearing concentricity <0.02mm, and sealing flatness <0.025mm, then a secondary operation of CNC milling/turning/precision grinding is required.
- Turnaround Time: For functional components needing to be made in 72-120 hours for active testing or fit checks, the quickest path lies with subtractive machining and/or additive manufacturing.
Conclusion
Efficient mechanical engineering requires the balance of design intention and production capability. The least expensive components will be designed from scratch while keeping manufacturing considerations in mind.
Engineers can mitigate development risks and move into volume manufacturing efficiently by doing prototypes using CNC and 3D printing technology, planning the wall thickness and draft angle in consideration of eventual molding or casting, and collaborating with manufacturing experts for DFM analysis at an early stage of development.
About the Author:
LS Manufacturing Engineering Team provides rapid prototyping, low-volume production, and mass production with CNC Machining, Injection Molding, Sheet Metal Fabrication, 3D Printing, Die Casting, Vacuum Casting, and Extrusion for engineers and procurement teams around the world.
Located in Dongguan, Guangdong, the team has delivered more than 250,000 different parts to clients in over 150 countries since 2012. Learn about their manufacturing services and find out how DFM support, multiple manufacturing process capabilities, and certified quality management minimize development risk from concept to volume production.