Every plastic part you touch—from a smartphone case to a dashboard trim piece—was formed by a mold. Every metal bracket, washer, or automotive body panel was stamped by a die. Behind these billions of parts are precision mold components that must be accurate to microns, polished to mirror finishes, and hard enough to survive millions of cycles. China has become a global powerhouse for CNC machined die parts and injection mold machining, supplying mold bases, cores, cavities, and sliders to toolmakers worldwide. This guide covers the critical machined components for plastic injection molds, stamping dies, and die-casting molds—materials, machining strategies (hard milling vs. EDM), surface finishing, and practical sourcing tips from Chinese manufacturers.
Mold components are unlike any other machined parts. They must be defect-free, heat-treated, and polished to a near-optical finish.
Post-mold shrinkage compensation. Plastic parts shrink after molding (ε = 0.3-2.0%). Mold cavities must be machined oversized to compensate. The geometry must be perfectly matched to the part CAD with draft angles and parting lines.
Extreme surface finish. A fine-polished cavity (SPI A-1) has Ra 0.012μm. Any tool mark, scratch, or pinhole will transfer to every molded part. Polishing is as critical as machining.
Hard material machining. Most mold components are heat-treated to 48-55 HRC (pre-hardened) or even 60-62 HRC for wear surfaces. Cutting hardened steel requires high-speed hard milling (HSM) with carbide or CBN tools, or EDM (electrical discharge machining).
Tight fit and alignment. Core-to-cavity clearances for dynamic components (sliders, lifters) are often 0.01-0.02mm. Guide pins and bushings require h6/H7 fits. Mismatches cause flash or stuck parts.
Complex 3D geometries. Freeform surfaces, deep ribs, undercuts, and cooling channels require 5-axis machining or EDM.
Full traceability. Mold component suppliers provide material certifications (hardness, cleanliness), inspection reports, and often heat treatment charts.
Chinese CNC shops specializing in mold components typically hold ISO 9001, have 3- to 5-axis machining centers, CNC EDM (sinker and wire), high-speed milling spindles (>20,000 RPM), and CMMs with scanning capabilities. Major clusters are in Shenzhen, Dongguan, Suzhou, Ningbo, and Huangyan (Zhejiang).
An injection mold consists of two halves: the A-side (cavity) and B-side (core). Within them are many moving parts. Injection mold machining focuses on these components.
The mold core and cavity form the plastic part's shape. Their machining sequence:
Rough cut from pre-hardened steel block (e.g., P20, 718H, or 1.2738) on a 3-axis VMC.
Heat treatment if using through-hardening steel (e.g., H13, S136, or 420 stainless). Pre-hardened steels (30-36 HRC) are often machined without final heat treat.
High-speed hard milling (HSM) of the cavity surfaces with ball end mills. Stepover 0.05-0.2mm for fine finish.
EDM (sinker) for sharp internal corners, deep ribs, or areas inaccessible to cutters.
Polishing (manual or automated) to required SPI finish.
Coating (e.g., PVD TiN, CrN, or DLC) for wear resistance and release properties.
Critical tolerances for a core/cavity:
Overall dimensions: ±0.01mm
Parting line flatness: 0.02mm over whole surface
Wall thickness (core to cavity gap): ±0.02mm
Surface roughness after polishing: SPI A-2 (Ra 0.025μm) for high-gloss parts, SPI B-1 (Ra 0.05μm) for general purpose.
Rib width and depth: ±0.01mm
Chinese shops use high-speed machining centers (e.g., Makino, Mikron, or domestic brands) with spindle speeds 20,000-40,000 RPM. For EDM, they employ CNC sinker EDM with C-axis and tool changers to machine multiple electrodes.
Sliders (slides) and lifters are moving components that form undercuts. They are machined from wear-resistant tool steels like H13, D2, or A2, heat-treated to 50-54 HRC. Their inclined guide surfaces require precise angular milling or grinding. Wear plates (brass or graphite) are often added to slides.
Typical tolerances:
Slide guide angle: ±0.01°
Slide travel fit: H7/g6 sliding fit (0.005-0.015mm clearance)
Lifter hole position: ±0.01mm
Many Chinese mold shops offer complete slide assemblies machined and fitted in-house.
Hot runner components (manifolds, nozzles, and tips) require precision machining of flow channels and sealing surfaces. Manifolds are often machined from stainless steel 420 or H13. The flow channels are drilled and then polished to Ra 0.4μm to prevent material hang-up. Nozzle tips have small orifices (0.5-3mm) requiring micro-drilling and lapping.
Chinese specialists in hot runner parts use deep hole drilling machines and flow polishing (abrasive flow machining) to achieve smooth internal surfaces.
Progressive stamping dies cut and form metal sheet. Their components endure extreme impact and abrasion. Stamping die parts are typically machined from tool steels (D2, M2, A2, DC53) heat-treated to 58-62 HRC.
A punch is a cylindrical or shaped rod that shears the hole. It requires precision O.D. grinding, and often a stripper shoulder. Die buttons have a precisely ground bore (H6 or H7) with clearance (+0.01-0.03mm larger than punch).
Machining process:
CNC turning of blank.
Heat treat and cryogenic treatment.
OD grinding (centerless or cylindrical).
Wire EDM of shaped profiles (square, hex, or special forms).
Surface polishing of cutting edge.
Tolerances for punches:
OD: h6 (e.g., 10mm -0.009/-0.013)
Roundness: 0.002mm
Perpendicularity of end face to axis: 0.005mm
Surface finish (cutting edge): Ra 0.2μm
Chinese shops often have wire EDM machines (AgieCharmilles, Sodick, or domestic) for complex punch profiles. They may also offer TiCN or TiAlN coating to extend punch life.
Die plates (pierce die plate, stripper plate) are large plates with many holes for punches and bushings. They are machined from D2 or 4140 steel, heat-treated and ground flat. Hole positions are critical: true position ±0.005mm. This requires high-precision coordinate grinding or wire EDM.
Chinese mold component suppliers often use jig grinders or high-precision machining centers with measuring feedback to achieve these tolerances.
Die-casting molds for zinc, aluminum, or magnesium alloys experience high thermal stress and erosion. They require hot work tool steels like H13 (1.2344, SKD61) heat-treated to 46-50 HRC. Components include cores, cavities, slides, and cooling inserts.
The main difference from injection molds is the need for cooling channels near the surface to control thermal cycling. These channels are often drilled with angled intersecting holes. Conformal cooling channels (3D curved) can be made by additive manufacturing (DMLS) then finish-machined – a growing niche among Chinese mold makers.
Surface finish for die-casting molds is generally SPI B-1 (Ra 0.05μm) or rougher, as parts are often post-machined.
Choosing the right steel is critical for mold life and part quality.
P20 / 1.2738 (30-36 HRC pre-hardened): General-purpose injection mold base plates, support pillars. Easy to machine, cheap. Not for high-wear or high-gloss parts.
718H / 1.2738HH (33-38 HRC): Improved purity, used for larger molds requiring better polishability.
S136 / 420 stainless (48-52 HRC after heat treat): Corrosion-resistant and high polishability. For medical, food, and optical plastic parts. Requires EDM or hard milling after heat treat.
H13 / 1.2344 / SKD61 (46-52 HRC): Hot work steel for die-casting and high-temperature injection molding. Good toughness and thermal fatigue resistance.
D2 / DC53 (58-62 HRC): High carbon, high chromium for stamping dies, punches, and wear plates. Very high wear resistance but brittle.
A2 / A8 / O1: Oil-hardening tool steels for smaller die components requiring toughness and good machinability.
Copper alloys (e.g., C17200 beryllium copper): For fast cooling inserts (high thermal conductivity) in injection molds. Machinable, but beryllium requires safety precautions.
Aluminum 7075 or 6061: Used for prototype molds and low-volume production (under 10,000 shots). Soft, machined directly.
When specifying material, request ultrasonic testing (UT) for internal defects, especially for large blocks. Many Chinese suppliers provide UT reports from the steel mill.
Plastic injection molds are polished to specific SPI (Society of Plastics Industry) grades. Chinese shops follow these standards:
SPI A-1 (Ra 0.012μm, diamond buff): Optical clear parts, lenses, CDs. Requires hardened steel (S136, H13) and meticulous polishing.
SPI A-2 (Ra 0.025μm, fine buff): High-gloss cosmetic parts (e.g., phone cases).
SPI A-3 (Ra 0.05μm): General gloss parts.
SPI B-1 (Ra 0.05μm, paper stone): Semi-gloss, textures.
SPI C-1 (Ra 0.10μm, 600 stone): Matte finish.
Polishing is done manually by skilled mold polishers. Chinese mold shops often have dedicated polishing teams trained to SPI grades. They also offer texturing (EDM texture, chemical etching) for leather, grain, or matte finishes.
For stamping dies, cutting edges are polished to Ra 0.2μm, but the rest of the surface may be left with ground finish (Ra 0.8μm).
Mold component quality is verified through:
CMM scanning: Contact or non-contact (laser) measurement of freeform surfaces. Many Chinese shops have Zeiss, Hexagon, or Mitutoyo CMMs with scanning probes.
Optical comparators: For small inserts, sliders, and punch profiles.
Surface finish measurement: Contact profilometer or glossmeter.
Hardness testing: Rockwell HRC on the surface and sometimes inside (after heat treat).
Non-destructive testing: MPI (magnetic particle) or dye penetrant for cracks after heat treat.
Fit checking: Assembly of core and cavity with shims or blue dye.
First article inspection reports (FAIR) for mold components should include the above data, plus a check sheet of critical dimensions.
Practical selection steps:
Step 1: Look for mold-making experience. Ask for examples of molds they have produced (photos, cycle life, SPI grade). Shops that make complete molds generally have better component machining skills.
Step 2: Check equipment. High-speed machining centers (HSM) with spindle speed >20,000 rpm, CNC sinker EDM, wire EDM, and CMM. For large molds (1m+), they need large-travel VMCs.
Step 3: Evaluate heat treatment and coating. Do they have in-house vacuum heat treatment or a trusted partner? Can they apply PVD coatings (TiN, TiAlN, DLC)?
Step 4: Assess polishing capabilities. Ask about SPI grade they can achieve. Request a polished sample or a surface finish report.
Step 5: Order a trial component. Start with a simple core or cavity insert. Inspect dimensions, hardness, and polish. Then proceed to more complex parts.
Major Chinese mold clusters: Huangyan (Zhejiang) – known as the “Mold Capital”; Shenzhen and Dongguan – high-precision molds for electronics; Suzhou and Ningbo – automotive and home appliance molds.
Mold components are custom and often low volume (1-50 pieces). Pricing benchmarks:
Core or cavity insert (100x100x30mm, P20 pre-hard, 3-axis machined): $150-300
Same in H13 hardened (hard milling + EDM): $300-600
Slider (complex, with wear plates): $200-500
Punch (10mm dia, 100mm length, D2, ground): $15-30
Hot runner nozzle (complete assembly): $50-150
Lead times: For a simple insert, 2-3 weeks; for a complex cavity with EDM and polishing, 4-6 weeks. Heat treatment and coating add 1-2 weeks. Shipping: air 3-7 days, sea 30-45 days.
MOQ: Usually 1-10 pieces for mold components (since each mold is unique). Many shops accept single-piece prototype orders.
Poor steel quality (inclusions). Causes pinholes after polishing. Prevention: specify ESR (electro-slag remelted) steel and request UT report.
Heat treatment distortion. Core/cavity warps after hardening. Prevention: specify double tempering and stress relieving. Hard mill after heat treat rather than before.
EDM recast layer (white layer). Causes micro-cracks and poor polish. Prevention: specify no recast layer or require EDM finish with fine settings and post-EDM polishing.
Inaccurate parting line matching. Flash on molded parts. Prevention: require 3D scanning of both core and cavity, with a mismatch report<0.02mm.
Sharp corners in cooling channels. Stress risers that crack in die-casting molds. Prevention: specify drilled intersecting holes with radiused intersections (use ball end mill or pre-drill).
The mold industry is evolving rapidly in China:
Conformal cooling. Additive manufacturing (DMLS) builds cooling channels that follow the cavity contour, reducing cycle time. Then EDM or hard milling finishes the cavity.
Automated mold polishing. Robotic arms with abrasive tools are being adopted to reduce manual labor and improve consistency.
Digital twin for mold trials. CAE simulation used before machining to predict warpage and shrinkage, then compensates the cutter path.
High-speed hard milling replacing EDM. For many applications, 5-axis hard milling (40,000 rpm) with small ball end mills can finish ribs and corners that once required EDM, saving time and eliminating electrode costs.
Precision mold components from China are integral to global manufacturing. Chinese CNC shops have developed expertise in injection mold machining, stamping die parts, and mold core and cavity production, using high-speed milling, EDM, and advanced polishing. By selecting a shop with proper equipment, heat treatment, and quality control, and by clearly specifying material grades, hardness, and SPI finish, you can obtain mold components that perform reliably for millions of cycles. Start with a trial insert, verify their workmanship, and build a partnership.
Need precision mold or die components from China? Contact us with your 3D CAD files and material/SPI requirements. We will match you with qualified Chinese mold shops that specialize in cores, cavities, sliders, punches, and complete mold bases. Free DFM feedback and quoting available.
A: Pre-hardened steel (e.g., P20, 40-36 HRC) is delivered already hardened and can be machined directly, suitable for molds up to a few hundred thousand shots. Through-hardened steel (e.g., H13, 48-52 HRC after heat treat) is machined in soft state (15-20 HRC), then heat-treated, then EDM/hard milled. It offers higher wear resistance and longer mold life (millions of shots).
A: Yes, many specialized mold shops can achieve SPI A-1 (Ra 0.012μm) on materials like S136 (420 stainless) or H13. It requires fine EDM or hard milling followed by diamond polishing. They should provide a surface finish measurement report. For large molds, A-2 is more common.
A: For dynamic shut-offs (sliders, lifters), clearance is typically 0.01-0.02mm. For static shut-offs (parting line areas), 0.02-0.05mm. These clearances must be uniform to prevent melt flow. Specify them on the drawing and request an assembly check.
A: Many have in-house vacuum heat treat furnaces, or long-term partners. They should provide hardness test reports (e.g., HRC at surface and core). For large molds, request a process control chart (time-temperature).
A: Specify that intersecting drilled channels must be deburred and cleaned. For plugs, specify pipe thread (NPT or BSP) with thread sealant. After machining, require a pressure test (e.g., 150% of working pressure) with no pressure drop. Some Chinese shops offer this.
A: Some can, but beryllium is toxic in dust form. Shops must have proper dust extraction and personal protective equipment. Many avoid it. Ask if they have beryllium machining experience and safety measures.
A: For a small to medium mold (200x200mm), Chinese shops typically deliver in 4-6 weeks. Larger molds (1m) take 8-12 weeks. This includes design, CAM programming, roughing, heat treat, finish machining, EDM, polishing, and assembly.
Ready to source precision mold and die components from China? Send your part CAD and mold specifications. We'll connect you with experienced Chinese mold component manufacturers capable of high-speed hard milling, EDM, and SPI-grade polishing. Fast quoting and DFM feedback available.
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