Injection Molding Machine
Mold Opening & Closing Cylinder

Open Fast.
Close Carefully.
Protect the Mold.

A precision injection mold costs $50,000 to $500,000 or more. The mold opening and closing cylinder moves the moving platen that carries half of this investment — opening the mold to release the part, then closing it for the next shot. Closing sounds simple. It is not. A stuck part, a fallen fragment, or a misaligned core pin between the mold halves will be crushed when the mold closes — destroying cavity surfaces that took weeks to machine. The closing phase must detect any obstruction and stop before damage occurs.

384 KN

Max Thrust

50–140mm

Bore

≤700mm

Stroke

25MPa

Working Pressure

Two Opposite Motions, Two Different Engineering Problems

The cylinder performs two opposite motions every cycle — and each places different demands on the hydraulic circuit and the cylinder's response characteristics:

Opening — Break Apart and Clear

The mold halves are locked together under hundreds of tonnes of clamping force. The opening cylinder must first break the mold halves apart (overcoming the residual clamping force and any vacuum between the mold faces), then retract the moving platen to full-open position as fast as possible. The initial break-open force is the highest force the cylinder produces. Once the halves separate, the retraction force drops to the platen's mass and rail friction — and the speed should increase to minimise cycle time. The part either stays on the core (to be ejected by the ejection cylinder) or falls free by gravity.

Closing — Three Phases, One Chance

Closing is the dangerous motion. The cylinder advances the moving platen through three distinct phases: fast traverse (full speed, low pressure — covering 80% of the stroke in minimum time), slow approach (reduced speed, reduced pressure — the "mold protection" zone where the machine detects any resistance that shouldn't be there), and final close (full pressure — seating the mold halves together and engaging the clamping lock-up). If the slow approach detects unexpected resistance, the machine stops and reverses — before the mold is damaged.

Korea Ever-Power manufactures the mold opening and closing cylinder with proportional-valve-compatible port sizing and low-friction seals that respond smoothly across the full speed range — from the fast traverse to the slow mold-protection crawl. Browse the complete injection moulding machine cylinder family.

Injection Molding Machine Mold Opening and Closing Cylinder

Technical Specifications

Parameter	Value
Product	Injection Molding Machine Mold Opening and Closing Cylinder
Function	Traverse the moving platen — open and close the mold
Bore Diameter	50 mm – 140 mm
Rod Diameter	28 mm – 100 mm
Stroke	≤ 700 mm
Maximum Thrust	384 KN (bore 140 mm / pressure 25 MPa)
Working Pressure	Up to 25 MPa
Certification	ISO 9001 · 100% hydrostatic tested

Mold Protection — The $500,000 Reason for Slow Closing

Mold protection is the most important function of the mold opening/closing cylinder — more important than speed, more important than force. Here is what it prevents:

Stuck part between mold halves

If the ejection cylinder failed to push a part out completely, it hangs between the mold halves. Closing the mold at full speed and pressure crushes the part into the cavity surface — imprinting its shape into the polished cavity steel. Repairing the cavity requires re-machining, re-polishing, and possibly re-plating — weeks of mold shop time and $10,000–$100,000 in repair cost. The mold protection phase detects the 2–5 mm of resistance that the stuck part creates and stops the close before impact.

Debris on the mold face

A metal chip from a runner cut, a fragment of flash, or even a dropped tool can land on the mold face between cycles. Closing on this debris indents the cavity surface — a permanent defect that prints onto every part until the mold is repaired. The mold protection phase runs at low pressure — if the cylinder encounters any resistance beyond the expected platen friction, the machine stops.

Misaligned core pin or slide

Complex molds have moving cores and slides that must retract before the mold closes. If a core pin fails to retract, the mold half closes onto it — bending or breaking the pin and potentially cracking the cavity block. The mold protection phase gives the machine controller time to verify all core and slide positions before committing to full closing force. Contact the hydraulic cylinder engineering team for mold protection specifications.

Three-Phase Closing — Fast, Slow, Lock

Phase 1 — Fast traverse (0–80% of stroke). The moving platen advances at maximum speed, covering most of the open gap in minimum time. Hydraulic pressure is moderate — just enough to overcome the platen mass and rail friction. This phase determines the "dry cycle time" (the minimum time for the machine to open, eject, and close without any injection) — faster traverse = more parts per hour.

Phase 2 — Mold protection (80–98% of stroke). The cylinder slows to a crawl — typically 5–20 mm/second. The hydraulic pressure drops to a low, adjustable value (the "mold protection pressure"). At this low pressure, even a small obstruction — a stuck part, a chip, a core pin — generates enough resistance to stop the platen. The machine controller monitors the platen position; if it does not reach the "mold closed" position within the expected time window, it alarms and reverses.

Phase 3 — Final lock-up (last 2%). The mold halves touch. The cylinder builds to full system pressure to seat the halves together, compress the mold parting line, and — on toggle-clamp machines — drive the toggle to its over-centre locked position. The clamping force (which is much larger than the cylinder's traverse force) is then held by the toggle lock or by a separate high-pressure clamp circuit.

Two Machine Architectures — One Cylinder, Different Roles

Toggle and direct-clamp injection molding machine architectures

Toggle-clamp machines. The mold opening/closing cylinder drives a toggle linkage — a system of hinged arms that amplifies the cylinder's force at the end of travel. The cylinder provides the traverse motion (opening and closing the toggle arms); the toggle's over-centre geometry provides the clamping force (typically 5–10× the cylinder's thrust). The cylinder is relatively small (50–100 mm bore) because it only needs to drive the toggle — the toggle does the heavy clamping.

Direct-hydraulic-clamp machines. No toggle — the cylinder provides both the traverse motion and the clamping force directly. The cylinder bore is larger (100–140 mm range) because it must produce the full clamping force without mechanical advantage. Some designs use a separate high-pressure short-stroke "booster" cylinder for the final clamping force, with the opening/closing cylinder providing the long-stroke traverse only.

Korea Ever-Power manufactures the mold opening/closing cylinder for both architectures — with the bore diameter, rod diameter, and stroke matched to the specific machine's toggle geometry or direct-clamp force requirement. The 50–140 mm bore range covers small to mid-sized injection moulding machines across both architectures.

Mold opening/closing cylinder in injection molding plant

Manufacturing

Korea Ever-Power mold opening/closing cylinder production

The mold opening/closing cylinder must transition smoothly from fast traverse to the slow mold-protection crawl without stiction or jerky motion. If the cylinder sticks at the start of the slow phase, the platen jumps forward — potentially bypassing the mold protection zone entirely. Korea Ever-Power specifies low-friction seals (polyurethane piston seal, PTFE-bronze rod seal) to minimise the difference between static and dynamic friction, ensuring smooth speed transitions at all flow rates. The bore is honed to Ra 0.2–0.4 µm. Chrome plating is 30–50 µm (clean environment). Every cylinder is hydrostatic tested at 1.5× working pressure and functionally tested for smooth speed transition from full speed to the mold-protection crawl speed — verifying that the cylinder responds proportionally without stick-slip at the critical transition point.

OEM & ODM

What You Provide

Machine clamping architecture (toggle or direct), platen mass, required traverse speed (open and close), mold protection zone length and speed, maximum force (break-open and close), stroke, system pressure, toggle geometry (if applicable — link lengths, pivot positions), and the platen/tie-bar assembly drawing showing cylinder mounting positions.

What the Factory Delivers

Engineering drawing with bore, rod, stroke, low-friction seal specification, port sizing (for proportional valve compatibility at the required traverse and mold-protection speeds), and mounting dimensions. Hydrostatic test + speed-transition functional test. Seal kits. Browse the complete injection moulding machine cylinder family.

FAQ

Does this cylinder provide the clamping force that holds the mold closed during injection?

On toggle machines: indirectly — the cylinder drives the toggle to its over-centre position, and the toggle geometry provides the mechanical clamping force (which is much larger than the cylinder's thrust). On direct-clamp machines: yes — the cylinder (or a combination of this cylinder plus a short-stroke booster) provides the clamping force directly. Korea Ever-Power sizes the bore diameter based on which architecture the machine uses — toggle machines need less cylinder force because the toggle amplifies it.

How fast should the mold protection zone be?

Typically 5–20 mm/second, covering the last 20–50 mm of closing travel. Slower = safer (more time for the controller to detect resistance and stop) but slower = longer cycle time. The optimum balance depends on the mold's value, the part's complexity (how likely a stuck part is), and the production volume (higher volume justifies more aggressive cycle times). The cylinder must respond smoothly at these very low speeds — any stick-slip can be misinterpreted by the controller as an obstruction, causing false stops.

How does this cylinder coordinate with the ejection and shooting cylinders?

Through the machine controller's sequencing logic: the mold must be fully open (confirmed by the opening/closing cylinder's position sensor) before the ejection cylinder extends. The mold must be fully closed (confirmed by the same sensor or a toggle lock sensor) before the shooting cylinder advances the nozzle. These interlocks prevent the ejector pins from extending into an open mold area (safety risk) or the nozzle from docking to an unclamped mold (injection blowout risk). Browse the full hydraulic cylinder product range.