No matter how beautiful mold design drawings are, if they can't be produced in factory—precision is wrong, surface finish is unsatisfactory—design is just a bunch of drawings.
Processing is "last mile" in turning a mold from drawings into a physical object, and it's also most prone to problems. This article breaks down six core processes of plastic mold processing. Each process includes reference parameters and common pitfalls, making it a useful guide for beginners and a training manual for experienced workers.

 

Key: Steel must be rough-machined before heat treatment, then finish-machined—finishing before heat treatment cannot guarantee dimensional accuracy.
Machining Allowance Allocation (Taking a cavity depth of 50mm as an example):
Roughing: 0.5~1.0mm; Finishing: 0.1~0.2mm; EDM/Polishing to Dimensions
Deformation amount from heat treatment (0.05~0.15mm) must be reserved within roughing allowance.
△ Mold Machining Process: Roughing → Heat Treatment → Finishing → EDM → Grinding → Polishing → Assembly → Inspection; allowance for each step is crucial.

CNC accounts for 60%~70% of the total mold machining. Strategies for roughing and finishing are completely different:

Order of heat treatment is crucial: Steel must be roughed first → stress relief/heat treatment (vacuum quenching HRC48~54) → then finished. Order is reversed—finish machining before heat treatment. Heat deformation (0.05~0.15mm) will directly render finished dimensions unusable.
Tool extension length specifications (anti-vibration tool): Tool diameter ≤ 6mm → Extension ≤ Tool diameter × 4; Tool diameter 6~12mm → Extension ≤ Tool diameter × 5; Tool diameter ≥ 12mm → Extension ≤ Tool diameter × 6
Exceeding recommended values requires reducing speed and feed rate; otherwise, vibration marks will directly ruin one cavity.
CNC machining of graphite electrodes: Graphite electrodes ("tools" used in EDM) must be machined separately on a dedicated graphite CNC machine. Steel cutting machines cannot be used—graphite dust is conductive and will short-circuit and burn circuit board if it falls into control cabinet of a steel machine. Graphite machining must be dry cutting (without cutting fluid), relying on a high-powered dust removal system to remove graphite dust.

 

Finishing Path Comparison (Influence of Stepper Pitch on Surface Roughness)
Stepper Pitch: 0.5mm, Ra: 3.2~6.3 Rough; Stepper Pitch: 0.2mm, Ra: 1.6~3.2 Medium; Stepper Pitch: 0.1mm; Ra: 0.8~1.6 Fine; Stepper Pitch: 0.05, Ra: 0.4 Mirror Finish.
△ Roughing vs Comparison of Finishing Strategies and Impact of Different Spacing on Surface Roughness

CNC machining relies on rotating cutting tools, which are limited by tool diameter and shape—deep and narrow grooves, clear corners, sharp corners, and fine ribs—areas where tool cannot reach must be addressed using EDM (Electrical Discharge Machining).
Core principle of EDM: Tool electrode (usually made of copper or graphite) does not contact workpiece. High temperatures (8000~12000℃) are generated through pulsed discharge to erode workpiece material. Discharge gap is only 0.01~0.5mm.
When is EDM mandatory?
Cavities with sharp internal angles (CNC tools are rounded and cannot eliminate right angles); Deep and narrow grooves (depth/width > 3, CNC tool rigidity insufficient); Fine ribs (rib width < 2mm, tool too thin and breaks); Mirror finish requirements (CNC finishing Ra≥0.4, EDM mirror finish Ra can reach 0.05)

Three key dimensions for electrode design:
Spark gap: Rough machining 0.2~0.5mm / Finish machining 0.05~0.1mm — This gap value should be subtracted from electrode size; Electrode wall reduction: Electrode shape = Cavity size − Discharge gap × 2; Number of electrodes: Copper typically uses 2~3 electrodes (for roughing → finishing), graphite uses 1~2 electrodes (lower wear).
EDM Discharge Principle (Side View) Layered Machining Strategy

 

Surface Roughness Grades of EDM Machining: Roughing, Ra 3.2~6.3; Semi-finishing, Ra 1.6~3.2; Finishing, Ra 0.4~0.8; Mirror Finish, Ra 0.05~0.2.
Copper electrodes are suitable for mirror finish (Ra<0.2), graphite electrodes are suitable for roughing/semi-finishing.
△ EDM Discharge Principle (top left), Layered Machining Strategy (top right), Surface Roughness Grade (bottom)

Slow wire EDM is the most precise method in mold processing, achieving a positioning accuracy of ±0.002mm. It is mainly used for: ejector pin holes, insert mating surfaces, slide grooves, irregularly shaped through holes, and other areas requiring high-precision contours.
Slow Wire EDM vs. Fast Wire EDM:
Slow Wire EDM (Recommended): Single-use electrode wire (copper wire φ0.1~0.3mm), machining accuracy ±0.002~0.005mm, surface Ra 0.8~1.6. Deionized water is used as medium.
Fast Wire EDM: Reusable molybdenum wire, accuracy ±0.01~0.02mm (5~10 times difference), surface Ra 3.2~6.3. Emulsion is used as medium.
Consensus among mold manufacturers: Slow wire EDM is used for all precision molds; fast wire EDM is only suitable for rough cutting or non-precision parts. The most easily overlooked aspects in slow wire EDM design:
① Starting hole (wire threading hole) – Wire EDM requires a starting hole for molybdenum wire to pass through, with a diameter ≥ φ1.5mm. Forgetting to mark starting hole position on drawing makes it impossible for machining operator to begin.
② Workpiece thickness limitations – Thickness of workpieces for wire EDM is typically ≤ 200mm (thicker workpieces experience extremely slow processing speeds and decreased accuracy).
③ One cut, multiple trims – Precision mating surfaces require "one cut, two trims" or even "one cut, three trims" (1 cut + 2-3 trims) for optimal surface finish and accuracy.

If there are gaps on parting surface, even the best cavity is useless – flash is inevitable. Grinding is final guarantee of parting surface accuracy.

Ironclad Rule for Parting Surface Matching: When matching parting surface, apply colorant to fixed mold side and wipe moving mold side clean. Close mold → tap lightly → separate → observe color distribution on moving mold. Contact area should be ≥ 80%, and color should be evenly distributed (not concentrated in a small area).

Mold polishing is purely manual work; machines can only assist. Polishing a set of mirror-finish molds can account for 30%~40% of the total processing time.
Polishing Grade Progression (Surface Roughness Ra increases from smallest to largest)
A0 Coarse Polishing: Ra 0.8~1.6 Oilstone #180~400 → A1 Semi-Fine Polishing: Ra 0.4~0.8 Sandpaper #400~800 → A2 Fine Polishing: Ra 0.1~0.4 Sandpaper #1000~2000 → A3 Mirror Finish, Ra <0.05 Diamond Paste W1~3.5.
Mold Polishing Grades for Different Product Surface Requirements
Toys/Daily Necessities: A0~A1 · Ra 0.4~1.6; Appliance Shells/Automotive Interiors: A1~A2 · Ra 0.2~0.8; Optical/Transparent Parts: A3 · Ra <0.05
⚠ Polishing Ironclad Rule: Polish step by step, do not skip grades!
Skipping directly from A0 to A2 results in deep scratches on the surface, requiring rework.
△ Polishing grade progression: Proceed step-by-step from coarse to fine. Skipping grades leads to deep scratches and rework.
Three common pitfalls in polishing:
① Skipping grades: Jumping directly from #400 sandpaper to diamond paste—scratches from #400 cannot be removed by diamond paste, a waste of effort. A step-by-step approach is necessary: oilstone → #400 → #800 → #1200 → #2000 → diamond paste
② Over-polishing: Spending too much time in one area causes localized depressions (invisible to naked eye, but leaving marks on finished product).
③ Single polishing direction: Polishing in one direction continuously leaves directional scratches on the surface. Change direction at each grade (90° intersection) to ensure scratches from previous grade are completely eliminated.

After all parts are machined, assembly is "final pass." Assembly is not just about tightening screws, but a strict sequence and inspection process.
Assembly Process:
Cleaning – Clean all parts with an air gun and alcohol, leaving no metal filings or oil stains.; Install guide pillars and bushings – Test mold closing to ensure smooth operation without jamming; Install cavity core – Confirm parting surface contact ≥80%; Install ejector pins and return pins – Manually push to confirm no jamming; Install sliders/lifters – Manually slide to confirm full stroke; Install cooling water channels – Perform air/water pressure test (≥8 bar) to check for leaks; Install locating rings and main runner bushings – Confirm nozzle fit; Final Inspection – Verify parting surface with Landon mold closing; use a feeler gauge to check parting surface clearance <0.02mm.

Select Machining Process from One Drawing

 

Precision Mating Surfaces: Wire EDM (cut one, repair two) + Grinding
Parting Surfaces: CNC Finishing → Grinding → Lamination
△ Quick Reference for Process Selection: Flat Surfaces → CNC, Deep Grooves → EDM, Through Holes → Wire EDM

Heat treatment alters hardness, wear resistance, and toughness of steel; surface treatment improves release properties and corrosion resistance. Both are performed during or after machining.

Important Notes on Heat Treatment:
Heat treatment after roughing (not after finishing), thermal deformation amount of 0.05~0.15mm must be reserved in roughing allowance; Vacuum quenching (not ordinary quenching) – Ordinary quenching causes surface oxidation and decarburization, resulting in direct scrapping of mold cavity; Tempering is mandatory after heat treatment—failure to temper will result in residual stress causing mold cracking during use.
Summary: Mold processing is a chain of interconnected processes—allowance of each process is left for next, and precision of next depends on reference of previous process.
Three key lines for beginners to remember:
① Processing sequence: Roughing → Heat treatment → Finishing → EDM → Wire cutting → Grinding → Polishing → Assembly
② Allowance allocation: Roughing: Leave 0.5~1mm → Finishing: Leave 0.1~0.2mm → EDM/Polishing to size
③ Precision transfer: Reference surface for each process must be established in first process, and all subsequent processes use this as reference.
One crucial point for experienced workers to remember: Polishing is not simply "making it shiny"—it's a step-by-step process; time saved by skipping steps will be entirely lost in rework.