Como são feitos os moldes de injeção?

An injection mold is made by reviewing the part design, selecting tool steel, designing the mold structure, machining cavity and core inserts, finishing the surface, assembling the tool, and validating it through mold trial. For process context, see our injection molding process guide, our injection mold complete guide, and our injection molding supplier sourcing guide before comparing quote, cost, lead time, and supplier capability.

What Happens Before a Single Cut Is Made in Mold Manufacturing?

Pre-machining work is the DFM review, mold structure design, steel planning, simulation, and machining preparation completed before cutting steel. Before any metal is cut, mold manufacturing begins with a critical design review phase—analyzing the customer’s part design, selecting mold type and steel grade, performing mold flow simulation, and producing detailed 3D mold design data. In our factory, we spend roughly 20–30% of total mold lead time in this pre-machining phase, and it’s the most important investment we make.

The starting point is a Design for Manufacturability (DFM) review of the customer’s 3D part model. We check for minimum wall thickness (our rule: 1.0 mm minimum for most resins), consistent wall thickness to prevent warpage and sink marks, adequate draft angles (1°–3° minimum), undercut features requiring sliders or lifters, and gating locations that will fill the part without weld lines in critical areas.

After DFM sign-off, our mold designers use CAD software (typically CATIA or Solidworks with specialized mold design modules) to create the full mold assembly: cavity and core inserts, mold base, runner system, cooling channel layout, ejection system, and any side-action components. This design package typically runs 200–600 pages of engineering drawings and 3D data files.

How Does Mold Flow Analysis Improve the Mold Design?

Análise do fluxo do molde³ is a simulation step that predicts fill, pressure, cooling, weld lines, air traps, sink, and warpage before machining. Mold flow analysis improves mold design by simulating plastic fill, pack, and cooling stages before the mold is cut—identifying potential defects like weld lines, air traps, sink marks, and warpage before they become expensive steel modifications. We run mold flow on every production mold we build; it’s non-negotiable in our operation.

The simulation solves the coupled fluid dynamics and heat transfer equations for the specific resin, part geometry, proposed gate location, and cooling channel layout. Outputs include fill time maps showing how the melt front progresses, pressure distribution at fill end, temperature maps identifying hot spots in the tool, weld line positions, and predicted warpage after ejection and cooling.

We’ve seen mold flow analysis save $20,000–$80,000 per project by identifying problems before they’re machined in. A common example: a customer proposes a single center gate for a rectangular 400×300 mm part. Simulation reveals that single-gate filling creates weld lines across two functional snap-fit features. We redesign to a two-gate hot runner system—adding $3,000 to the mold cost but eliminating what would have been $15,000 in gate modifications and weeks of delay after the mold was already built.

What Steel Is Used and How Is It Prepared for Machining?

Mold steel is chosen by production volume, resin behavior, surface finish, corrosion risk, tool life, and heat-treatment requirements. Injection mold steel is selected based on required production volume, plastic material, and surface finish, with P20, H13, S136, and NAK80 being the most commonly used grades—then cut to rough block dimensions by the steel supplier, stress-relieved, and inspected before machining begins. In our factory, we’ve learned that steel quality is never a place to cut costs; substandard blocks with inclusions or internal stress cause catastrophic cracking during EDM operações.

For a typical P20 mold base, we receive pre-hardened blocks at 28–32 HRC from our certified steel suppliers, already cut to within 5–10 mm of final block dimensions. Before any machining begins, we perform ultrasonic testing on blocks thicker than 80 mm to check for internal voids or segregation—defects that won’t appear until a tool cavity is cut and loaded in production.

For H13 and S136 molds that will be hardened after roughing, we rough-cut all features with 0.3–0.5 mm stock remaining on all surfaces, send the block to heat treatment (hardening to 50–54 HRC for H13, 48–52 HRC for S136), then finish-machine to final dimensions. This sequencing is critical: you cannot hardening after finishing—the thermal distortion from quenching would destroy the precision you spent dozens of hours achieving.

How Is CNC Machining Used to Create Mold Cavities and Cores?

CNC machining is the main steel-removal process used to cut cavity, core, insert, and mold-base geometry from 3D mold data. CNC machining creates mold cavities and cores by removing material from steel blocks using multi-axis milling centers with carbide tooling, following the 3D mold design data in multiple sequential operations from rough material removal to semi-finishing to fine finishing passes. In our factory, CNC machining accounts for 40–60% of total mold manufacturing time and requires the most skilled programming and setup.

The machining sequence for a typical cavity block proceeds in three phases. Roughing uses large end mills (20–32 mm diameter) at high feed rates to remove the bulk of material—leaving 0.5–1.0 mm stock on all surfaces. We use high-speed roughing toolpaths that maintain consistent chip load regardless of geometry changes, protecting the machine spindle and extending cutter life.

Semi-finishing Ferramentas de moldagem por injeção de precisão mostrando componentes da cavidade e do núcleo usinados a partir de aço ferramenta

Finishing passes use ball-nose end mills as small as 0.5–1.0 mm diameter for detailed features, running at 20,000–40,000 RPM spindle speeds with step-over distances of 0.05–0.1 mm to achieve Ra 0.4–0.8 µm surface roughness before polishing. For a complex A-side cavity with a 300×250 mm surface area, finishing alone can take 16–40 hours of CNC time.

What Role Does EDM Play in Making Injection Molds?

EDM is the spark-erosion process used for sharp corners, deep ribs, narrow slots, text, and mold features that milling cutters cannot reach. EDM (Electrical Discharge Machining) creates sharp internal corners, deep ribs, fine text engravings, and complex geometry that CNC milling cannot reach—by eroding metal with controlled electrical sparks between an electrode and the steel workpiece without mechanical contact. We use EDM for 60–80% of our production molds, particularly for features like sharp rib bottoms under 0.3 mm radius and deep slots with aspect ratios above 5:1.

There are two primary EDM processes in mold making. Sinker EDM (also called die-sinking or ram EDM) uses a shaped graphite or copper electrode that erodes the steel to create a negative mirror image of itself. We machine custom electrodes on dedicated graphite CNC machines, then program sequences of roughing and finishing spark conditions to achieve the final surface quality. Tolerances of ±0.005 mm are routinely achieved.

Wire EDM uses a continuously moving brass wire (0.1–0.3 mm diameter) to cut precise 2D profiles through the full thickness of a block. We use wire EDM for ejector pin holes, core pin holes, and cutting precision through-slots that would require multiple CNC setups to achieve the same straightness.

The main limitation of EDM is speed—it’s 5–10× slower than CNC milling for material removal. We always machine as much as possible with CNC and reserve EDM for features that truly require it. Strategic EDM placement is one of the key skills that separates efficient mold shops from slow ones.

How Is the Mold Surface Finished and Polished?

Mold surface finishing is the grinding, polishing, or texturing process that controls part appearance, release behavior, and surface quality. Mold surface finishing progresses from stone grinding to progressively finer diamond polishing compounds, ultimately achieving mirror-like surfaces for optical and cosmetic applications, or specific texture finishes (VDI/Mold-Tech) for functional parts. Polishing is the most labor-intensive single step in mold making—and the hardest to automate. In our factory, our polishing team is among the most specialized; mirror-finish polishing is a skill that takes years to master.

The polishing sequence starts with grinding stones (600 grit → 800 grit) to remove EDM recast layer and machining tool marks. We then progress through diamond pastes: 6 µm → 3 µm → 1 µm → 0.25 µm for SPI B-2 standard (commercial polish); continuing to 0.1 µm diamond and 50 nm oxide polishing compound for SPI A-1 mirror finish.

For textured surfaces (SPI C or D standards, or specific VDI texture patterns for rubber-touch finishes), we skip fine polishing and instead chemically etch the steel after all machining is complete. Texture depth determines required additional draft angle: every 0.025 mm of texture depth requires an additional 1° of draft angle per side. We ensure draft angles are approved before any texturing is applied—removing texture from an already-textured surface requires significant rework.

What Happens During Mold Trial and First Article Inspection?

Mold trial is the first production test that checks filling, ejection, dimensions, surface quality, and correction needs before mold shipment. Mold trial (also called “T1 trial”) is the first production test of the finished mold—running it on the target injection machine with the specified resin to evaluate fill, surface quality, dimensional accuracy, and ejection function before the mold is shipped to the customer. In our factory, T1 trials are the most intense days of the mold-making process; every defect found is an opportunity to improve before the mold reaches the customer’s facility.

Before the first shot, we dry the specified resin to manufacturer requirements, set initial process parameters based on material data sheets (melt temperature, mold temperature, injection speed, and cooling time), and install the mold with tooling clamps verified to the machine platen specification. We run the first shots with slow injection speed and monitor fill on a shot-by-shot basis, increasing speed incrementally until the part fills 95%, then adjusting holding pressure to complete pack.

First Article Inspection (FAI) documents that every dimension on the customer’s drawing is within tolerance. We use a CMM (Coordinate Measuring Machine) for all critical dimensions, reporting actual versus nominal for each measurement point. Typical FAI reports for automotive or medical parts contain 50–200 measured dimensions with tolerances as tight as ±0.01 mm for critical features.

After T1, most molds require minor corrections: adjusting gate size for fill balance, adding vent slots where air traps cause short shots, polishing witness marks, or making steel-safe dimensional corrections. We build steel-safe margins into critical features so controlled steel removal can correct measured deviations after first article inspection.

What Should Buyers Check Before Approving a Mold?

The buyer approval checklist is DFM evidence, steel grade, mold design, machining records, T1 samples, FAI data, and open corrections. Buyers should check the DFM review, mold steel grade, cavity and core design, CNC and EDM plan, polishing standard, T1 trial samples, FAI report, and correction list before approving mold shipment. These records prove whether the supplier controlled the mold-making process or only reacted after defects appeared.

Perguntas mais frequentes

Quanto tempo é necessário para fazer um molde de injeção?

A simple prototype or single-cavity mold can take about 2 to 5 weeks, while a complex production mold with sliders, lifters, or multi-cavity layouts typically requires 8 to 12 weeks. The biggest time drivers are CNC machining hours (especially for hardened steel), EDM electrode manufacturing, heat treatment turnaround, and surface polishing. At our Shanghai facility, we run an in-house mold manufacturing line that supports 100+ mold sets per month, which helps compress lead times compared to outsourced tooling shops. Buyers should always confirm lead time during the DFM review stage.

What is the first step in making an injection mold?

The first step is a Design for Manufacturability (DFM) review, not machining. Engineers check wall thickness, draft angle, ribs, bosses, undercuts, gate locations, and material flow before any steel is cut. This review catches issues like impossible-to-machine undercuts or uneven wall thickness that would cause sink marks during production. Skipping DFM is the single most expensive mistake in mold making — problems found after machining cost 5 to 10 times more to fix. A thorough DFM review typically takes 3 to 5 business days depending on part complexity. This check should be documented before mold shipment approval.

Why are CNC machining and EDM both used in mold making?

CNC machining removes most steel quickly and creates the main cavity, core, and mold-base geometry with high precision. EDM is reserved for features CNC cannot reach — sharp internal corners, deep narrow ribs, fine text engravings, and complex undercut geometries. Using both processes together produces the best result: CNC for bulk material removal and overall form, EDM for detail and precision features that would break a cutting tool. In practice, a typical mold uses roughly 70 percent CNC and 30 percent EDM by machining time. This check should be documented before mold shipment approval.

How do mold makers choose mold steel?

Mold steel is chosen based on production volume, resin abrasiveness, corrosion risk, surface finish requirements, expected tool life, and budget constraints. For low-volume runs under 100,000 shots, pre-hardened P20 steel offers good machinability and a lower overall cost. For high-volume production above 500,000 shots, hardened H13 or S136 provides superior wear resistance and can be polished to an optical mirror finish for transparent parts. Stainless grades like S136 are strongly preferred when molding corrosive resins such as PVC or flame-retardant compounds. This check should be documented before mold shipment approval.

What happens during the first mold trial?

The first mold trial, commonly called T1, is the moment of truth — it tests whether the finished mold can produce acceptable parts on a real injection molding machine. The team sets initial process parameters, runs 20 to 50 shots, and evaluates fill pattern, dimensions, cosmetic quality, and flash. Most molds require minor corrections after T1, such as adjusting gate size for fill balance, adding vent slots, or adjusting cooling. A detailed First Article Inspection (FAI) report documents every dimension against the customer drawing tolerances. This check should be documented before mold shipment approval.

Can ZetarMold help review an injection mold project before cutting steel?

Yes. ZetarMold provides a free moldability review for injection mold projects before any steel is cut. You can send 3D files (STEP or IGES), 2D drawings with tolerances, resin requirements, cosmetic surface specifications, annual volume, and any special requirements. Our engineering team will review the design, identify potential manufacturing issues, and provide recommendations on steel selection, gate location, and expected tool life. With 20+ years of injection molding and tooling experience in our Shanghai factory, we can help you avoid costly design mistakes early. This check should be documented before mold shipment approval.

Summary: How Is an Injection Mold Made?

An injection mold is made through a controlled sequence: DFM review, mold design, steel preparation, CNC machining, EDM, polishing, assembly, T1 trial, correction, and first article inspection. The strongest mold suppliers can explain each step, show evidence for each gate, steel, machining, and inspection decision, and connect those choices to production quality and total project cost.

Request a Free Quote → Send your 3D file, drawing, resin target, annual volume, and cosmetic requirements. ZetarMold will review manufacturability, mold-making risk, tooling lead time, and production assumptions before final quote.

1. EDM: EDM refers to electrical discharge machining removes steel with controlled sparks to create sharp internal corners, ribs, slots, and fine mold details. ↩

2. DFM review: DFM review refers to design for Manufacturability review checks whether the part can be molded before tooling steel is cut. ↩

3. Mold flow analysis: Mold flow analysis refers to computer simulation that predicts plastic fill, weld lines, pressure, cooling behavior, sink risk, and warpage before machining. ↩