A complete breakdown of the 7 subsystems inside an injection mold - from cavity and core to cooling and venting. Covers each component's function, material, and how it affects part quality and cost.
An injection mold isn't one part. It's a system of 7 distinct subsystems, each one directly affecting part quality, cycle time, and per-unit cost. Undersized cooling channels or the wrong steel grade for your core will show up as warped parts, slow cycles, or a mold that wears out thousands of shots ahead of schedule.
Most guides on this topic either confuse mold components with machine parts (the hopper, barrel, and clamping unit are parts of the machine, not the mold) or list component names without explaining why they matter.
If you're evaluating injection molding manufacturers, understanding what's inside the mold is just as important as understanding the molding process itself. This guide covers the actual components inside the mold. What each one does, what it's made from, and how your choice of components affects the parts that come out the other end.
Injection mold components are the individual parts and subsystems inside an injection mold that shape, fill, cool, and eject a plastic part during the molding cycle. They don't include the machine itself (the barrel, hopper, or hydraulic system) - those are injection molding machine parts.
A standard injection mold contains 7 functional systems:
Each system has specific components made from specific materials for specific reasons. Here's the full breakdown.
Here's every major component at a glance. Use this as a reference. Each system is explained in detail below.
|
Component |
System |
Function |
Typical Material |
|
Top/rear clamp plates |
Mold base |
Mount the mold to the machine |
S50C / S55C carbon steel |
|
A plate (cavity plate) |
Mold base |
Houses the cavity |
P20 pre-hardened steel |
|
B plate (core plate) |
Mold base |
Houses the core |
P20 pre-hardened steel |
|
Spacer block (C plate) |
Mold base |
Creates space for ejection stroke |
S50C carbon steel |
|
Support pillars |
Mold base |
Prevent plate deflection under pressure |
S45C hardened steel |
|
Cavity |
Molding |
Forms the external shape of the part |
H13, S136, NAK80 |
|
Core |
Molding |
Forms the internal shape of the part |
H13, S136, NAK80 |
|
Slides/sliders |
Molding |
Create side features and undercuts |
H13 tool steel |
|
Lifters |
Molding |
Release internal undercuts during ejection |
H13, SKD61 |
|
Inserts |
Molding |
Replaceable sections for wear-prone areas |
Beryllium copper, H13 |
|
Sprue bushing |
Feed |
Entry point for molten plastic |
Hardened tool steel |
|
Runner |
Feed |
Channels plastic from sprue to gates |
P20, H13 (hot runner) |
|
Gate |
Feed |
Final entry point into the cavity |
Varies by gate type |
|
Cold slug well |
Feed |
Traps cold material before it reaches the cavity |
Same as runner plate |
|
Ejector pins |
Ejection |
Push the part out of the mold |
SKD61, SKH51 HSS |
|
Ejector sleeves |
Ejection |
Eject cylindrical or ring-shaped features |
SKD61 tool steel |
|
Return pins |
Ejection |
Reset the ejector plate to starting position |
SUJ2 bearing steel |
|
Ejector plates |
Ejection |
Hold and drive the ejector pins |
S50C carbon steel |
|
Water lines |
Cooling |
Circulate coolant to control mold temperature |
Stainless steel, copper |
|
Baffles/bubblers |
Cooling |
Direct coolant flow in tight spaces |
Stainless steel, brass |
|
Vent slots |
Venting |
Allow trapped air to escape (0.02–0.05 mm gaps) |
N/A (machined into parting line) |
|
Guide pins |
Guiding |
Align the two mold halves |
SUJ2 bearing steel |
|
Guide bushings |
Guiding |
Receive guide pins with precise clearance |
Bronze, oil-impregnated |
The mold base is the steel skeleton that holds every other component in position. It's not glamorous, but if the base deflects under clamping pressure, nothing else works right. You'll get flash, uneven wall thickness, or premature wear on cavity and core surfaces.
A standard mold base includes:
Most mold makers don't build their own bases from scratch. They order standardised bases from suppliers like DME, Hasco, or PCS, then machine the cavity and core pockets in-house. It's faster, more consistent, and standard practice across the industry.
Sidenote: Standardised mold bases follow either DME or Hasco sizing conventions. If you're sourcing from China, confirm which standard your mold maker works to before production starts. Mixing DME-spec components with a Hasco-spec base creates alignment problems that are slow and expensive to fix mid-project.
The molding system is the core of the mold - literally. It's the cavity, core, and any moving components (slides, lifters, inserts) that shape your part. Every surface the plastic touches during molding is part of this system.
The cavity forms the outside surface of your part. The core forms the inside. Together, they create the negative space that the molten plastic fills.
Here's the practical difference: the cavity side is usually the "show surface" (the side the customer sees), so it gets a better surface finish. The core side is where ejector pin marks end up. When you're reviewing a mold design, check which side is A (cavity) and B (core) - it determines where parting lines and witness marks appear on your finished part.
Cavity and core steel selection depends on your production volume and resin. H13 tool steel handles high-volume runs of abrasive glass-filled resins. S136 (a mirror-polish stainless) is the go-to for optical or medical parts that need a flawless surface. NAK80 is pre-hardened and easy to machine - good for medium volumes where you need fast mold delivery.
If your part has features that can't be formed in the direction of mold opening - holes on the side wall, clips, undercuts, snap fits - the mold needs slides or lifters.
Slides move perpendicular to the mold's opening direction. An angled pin (horn pin) or hydraulic cylinder drives them in and out. They handle external undercuts - side holes, windows, external clips.
Lifters move at an angle during ejection, driven by the ejector plate. They handle internal undercuts - ribs, hooks, or internal clips that would trap the part on the core.
Both add cost. A mold with 4 slides and 2 lifters might cost 30–50% more than one without, and takes longer to build. If you're still in the design phase, consider whether you can redesign the part to eliminate undercuts - it's the single fastest way to reduce mold cost. The advantages of injection moulding are well-documented, but they depend on designing parts that are actually mouldable without excessive tooling complexity.
Inserts are replaceable sections of the cavity or core that you can swap without rebuilding the entire mold. They're used for:
The feed system is the plumbing that gets molten plastic from the machine nozzle into the cavity. It has 4 components: sprue, runner, gate, and cold slug well.
The sprue is the main entry channel where plastic flows from the machine nozzle into the mold. It's tapered, wider at the base, so the solidified sprue pulls free cleanly when the mold opens. A hardened steel sprue bushing sits at the top and takes the wear from repeated nozzle contact.
Runners are channels that distribute plastic from the sprue to individual cavities (in multi-cavity molds) or to the gate (in single-cavity molds).
Two types of runners:
|
Feature |
Cold Runner |
Hot Runner |
|
How it works |
Plastic solidifies in the runner and is ejected with the part |
Heated channels keep plastic molten - no runner waste |
|
Material waste |
15–40% of shot weight becomes waste (regrindable) |
Near-zero runner waste |
|
Upfront cost |
Lower (simpler mold design) |
20–50% higher total mold cost (manifold, heaters, controllers) |
|
Cycle time |
Longer (runner must cool before ejection) |
10–30% shorter (no runner to cool) |
|
Best for |
Low-to-medium volumes, frequent colour changes |
High volumes, expensive resins, tight cycle time targets |
|
Maintenance |
Simple |
Requires specialised maintenance of heaters and nozzles |
For low-volume injection molding projects, cold runners are almost always the right call. Hot runners make economic sense when material savings and cycle time reduction offset the higher mold investment - typically above 100,000 units per year.
The gate is the narrowest point where plastic enters the cavity. Gate design affects fill pattern, packing pressure, cycle time, and the visible mark left on your part. Common types include edge gates, sub gates (tunnel gates), hot tip gates, and direct/sprue gates.
Gate placement matters more than most buyers realise. A gate in the wrong spot causes weld lines where two flow fronts meet, uneven packing, or visible marks on a cosmetic surface.
A small pocket at the end of the sprue or runner that catches the first slug of cold plastic. Without it, that cold material enters your cavity and causes surface defects or weak spots in the finished part.
The ejection system pushes the finished part off the core after it cools. Poor ejection design causes warping, white marks, or cracked parts, and the risk is highest on thin-walled components where the part has little structural rigidity to resist uneven force.
Cooling accounts for 60–80% of the total injection molding cycle time. That makes the cooling system arguably the most important factor in production efficiency.
The system works by circulating water (or occasionally oil for high-temperature resins) through channels drilled into the mold plates. The goal is uniform cooling across the entire part - uneven cooling causes warping, sink marks, and dimensional variation.
Three things you should know:
When plastic enters the cavity, it displaces air. That air needs somewhere to go. If it can't escape, you get burn marks (trapped air compresses and heats up), short shots (air blocks the flow), or visible weld lines.
Vents are shallow channels machined along the parting line, typically 0.02–0.05 mm deep - thin enough to let air pass but not plastic. Additional venting paths come from ejector pin clearances, insert gaps, and sintered metal vent plugs for blind areas that can't reach the parting line.
Venting is one of those things that barely gets mentioned in mold design discussions but causes a disproportionate number of quality problems. If your molder reports burn marks or incomplete fills, inadequate venting is one of the first things to check.
Guide pins and guide bushings are the alignment system that ensures the cavity and core meet precisely every time the mold closes. They're usually in all 4 corners of the mold.
Guide pins are hardened steel cylinders (typically SUJ2 bearing steel) pressed into one mold half. Guide bushings - bronze or oil-impregnated sleeves - sit in the other half and receive the pins. The fit is tight enough to prevent misalignment but loose enough for smooth operation over millions of cycles.
Worn guide components cause parting line mismatch, flash, and uneven wall thickness. They're wear items - plan to inspect and replace them on schedule.
Material selection is where mold cost, durability, and part quality intersect. Here's what's used where and why.
|
Material |
Hardness (HRC) |
Key Properties |
Best For |
Industry Standard |
|
P20 pre-hardened steel |
28–34 |
Good machinability, moderate wear resistance |
Mold bases, low-to-medium volume cavities |
AISI P20, DIN 1.2311 |
|
H13 tool steel |
48–52 |
High toughness, thermal fatigue resistance |
High-volume cavities and cores, hot runner components |
AISI H13, DIN 1.2344 |
|
S136 stainless steel |
48–52 |
Corrosion resistance, mirror-polish finish |
Medical, optical, and food-grade parts |
AISI 420 mod, DIN 1.2083 |
|
NAK80 pre-hardened |
37–43 |
Mirror-finish capable, no heat treatment needed |
Medium volumes, fast mold delivery |
Daido Steel NAK80 |
|
SKD61 tool steel |
48–52 |
Thermal crack resistance, good toughness |
Ejector pins, lifters, slide components |
JIS SKD61 |
|
Beryllium copper |
36–42 |
3–5x thermal conductivity vs steel |
Inserts in hot spots, conformal cooling sections |
ASTM C17200 |
|
Aluminium 7075-T6 |
N/A (Brinell ~150) |
Lightweight, fast heat dissipation, easy to machine |
Prototype molds, short-run production (<10,000 shots) |
ASTM B209 |
A few practical notes:
P20 is the default for anything that doesn't need extreme hardness. It machines fast, polishes well enough for most industrial parts, and doesn't require heat treatment. According to First Mold's mold lifespan data, P20 handles approximately 300,000 cycles, while 2738 steel reaches around 500,000.
H13 pays for itself on long runs. If you're planning 500,000+ shots, the extra upfront cost of H13 over P20 is offset by reduced maintenance and longer mold life. H13 typically lasts 800,000 to over 1,000,000 cycles.
Beryllium copper is expensive but solves problems nothing else can. If you have a thick section that keeps warping because it cools slower than the rest of the part, a beryllium copper insert at that spot conducts heat away 3–5x faster than the surrounding steel.
Every part quality issue traces back to a mold component. Here's the direct relationship:
|
Part Defect |
Mold Component Cause |
|
Warping / dimensional variation |
Uneven cooling (inadequate water channels), wrong steel grade causing thermal distortion |
|
Flash (excess material at parting line) |
Worn guide pins, mold base deflection, damaged cavity/core shut-off surfaces |
|
Burn marks |
Poor venting (blocked or missing vent channels) |
|
Sink marks |
Insufficient gate size, uneven cooling near thick sections |
|
Short shots (incomplete fill) |
Undersized runner or gate, poor venting blocking flow |
|
Ejector pin marks on cosmetic surface |
Poor ejection layout, insufficient pins causing uneven force |
|
Surface defects (flow lines, cold slugs) |
Missing cold slug well, poor gate location |
This is why asking your molder "what steel are you using for the cavity?" and "show me the cooling channel layout" aren't annoying questions - they're the two decisions that most affect whether your parts come out right.
When sourcing injection molding from China, factory capability in mold design and component selection is as important as the molding process itself. A mold built with the right steels, adequate cooling, and proper venting will produce consistent parts for hundreds of thousands of cycles. A mold built to minimise cost will produce problems.
Chinese factory gate prices run 30 to 60% below equivalent European and North American suppliers, with single-cavity prototype molds ranging from $1,000 to $3,000 and multi-cavity production molds from $5,000 to $15,000+. Of the sample factories in the 2026 China injection molding supplier analysis, 36.7% hold IATF 16949 certification, the standard that requires documented process controls for tooling consistency and corrective action across production runs.
The main components of an injection mold fall into 7 systems: mold base structure (clamp plates, A/B plates, spacer blocks), molding system (cavity, core, slides, lifters, inserts), feed system (sprue, runner, gate), ejection system (pins, sleeves, return pins), cooling system (water lines, baffles), venting system (vent slots along the parting line), and guiding system (guide pins and bushings).
Mold components are the parts inside the mold tool itself - cavity, core, ejector pins, runners, cooling channels. Machine parts are the equipment that operates the mold - the barrel, hopper, screw, clamping unit, and hydraulic system. They're separate things, though many guides conflate them.
Most structural components use P20 pre-hardened steel (AISI P20, DIN 1.2311). High-wear surfaces like cavities and cores use H13 (DIN 1.2344) or S136 stainless (DIN 1.2083) tool steel. Ejector pins use SKD61 or high-speed steel. Thermal management inserts use beryllium copper (ASTM C17200). Prototype molds often use aluminium 7075-T6 for faster machining and lower cost.
Slides are mold components that move perpendicular to the opening direction to form side features and external undercuts. Lifters move at an angle during ejection to release internal undercuts. Both are needed when a part has features that can't be formed in the mold's line of draw - like side holes, clips, or internal hooks.
Cooling accounts for 60-80% of the molding cycle time. Uneven cooling causes warping, sink marks, and dimensional inconsistency. Conformal cooling channels (3D-printed to follow part contours) can reduce cycle time by 30-40% compared to conventional straight-drilled channels, but cost more to manufacture.
A cold runner lets plastic solidify in the runner channels and ejects it as waste (regrindable). A hot runner keeps plastic molten in heated channels, eliminating runner waste. Hot runners add 20-50% to total mold cost but save material and cycle time - they're cost-effective for high-volume production or expensive resins.
Mold life depends on the steel grade and maintenance. P20 cavities handle approximately 300,000 cycles. H13 cavities can reach 800,000 to over 1,000,000 cycles with proper maintenance. The SPI mold classification system ranges from Class 105 (prototype, <500 shots) to Class 101 (high production, 1,000,000+ shots). Guide pins and ejector pins are wear items that need periodic inspection and replacement regardless of cavity life.
The two dominant standards systems are DME (North American) and Hasco (European). Both define mold base sizes, pin diameters, and component interfaces differently. Chinese mold makers typically work with DME, Hasco, or their own LKM standard. Always confirm which standard your mold will follow before ordering components from a different supplier.
If you're evaluating molds or sourcing injection molded parts, the factory's mold-building capability tells you as much as their molding capability. Ask about steel grades, cooling layouts, and component standards before you commit.
Knowing which questions to ask when reviewing supplier profiles online makes a real difference in narrowing the field. The short video below covers what to look for.
Haizol connects buyers with verified injection molding factories across China. Each injection molded factoryprofiled by equipment, certifications, and mold-building capability. Submit an RFQ with your part drawing, and you'll receive comparable quotes from capability-matched factories within 24 hours.
