Plastic injection molding is a cycle-critical process where every second of mold open, eject and core-pull time affects part cost. A pneumatic cylinder for plastic injection molding handles ejector plate return, core pulls, sprue pickers, part removal and gate cutting with speed and reliability. Carroland supplies high-force tie-rod cylinders, guided core-pull actuators and corrosion-resistant units for injection molding cells, with bore sizes from 32 mm to 160 mm and strokes up to 1,200 mm. For complementary clamp and locator stations on the assembly side, see our pneumatic fixture system for assembly lines guide.
Pneumatic Applications in Injection Molding
Injection molding machines (IMM) generate high clamping force but rely on pneumatics for several auxiliary motions:
- Ejector plate actuation — a 63–100 mm bore cylinder returns the ejector plate after the hydraulic ejector pushes parts off the core.
- Core pull and slide action — a 40–80 mm bore cylinder pulls side cores or slides to create undercuts such as threads, snaps and bosses.
- Sprue picker and runner removal — a compact 25–40 mm cylinder actuates a gripper or cutter that removes the cold runner from the mold.
- Part extraction robot — pneumatic cylinders drive the wrist or gripper axis of a pick-and-place robot that removes finished parts.
- Gate cutting and degating — a 32–50 mm bore cylinder drives a knife or shear to separate the part from the runner.
These motions are typically short, high-force and repetitive, with cycle times of 20–60 seconds depending on part thickness and material.
Force and Sizing for Core Pull and Ejection
Core pull cylinders must overcome the injection pressure that has locked the core in place. For a side core with 50 cm² projected area and 30 MPa plastic injection pressure, the holding force is roughly 15,000 N. A 100 mm bore cylinder at 0.7 MPa delivers about 5,500 N, so a mechanical locking mechanism or a larger 160 mm bore cylinder is needed. Many mold designers use a 90° toggle lock or a hydraulic intensifier for high-holding-force cores, and use the pneumatic cylinder only for unlocking motion.
For ejector plate return, a 63 mm bore cylinder at 0.5 MPa provides 1,560 N, which is sufficient for most small-to-medium molds. Stroke matches the ejector plate travel, typically 50–200 mm. For the full force and bore methodology behind these numbers, our ISO 15552 bore and force calculation guide covers the F = P × A derate workflow.
| Molding task | Typical bore | Typical stroke | Pressure | Key requirement |
|---|---|---|---|---|
| Ejector plate return | 63–100 mm | 50–200 mm | 0.5–0.7 MPa | Reliable end cushioning |
| Core pull / slide | 50–100 mm | 30–150 mm | 0.6–0.8 MPa | High force or mechanical lock |
| Sprue picker | 25–40 mm | 30–80 mm | 0.5–0.6 MPa | Fast response |
| Gate cutter | 32–50 mm | 20–50 mm | 0.6–0.8 MPa | Sharp, controlled cut |
| Part extraction robot | 40–63 mm | 100–300 mm | 0.5–0.7 MPa | Side-load resistance |
Heat, Oil and Contamination Resistance
Molds run at 40–120 °C, and the cylinder sits close to the hot platen. We specify high-temperature FKM seals for zones above 80 °C and heat shields or stand-offs to reduce conducted heat. The ejector area often contains oil mist, plastic dust and cooling water, so we use cylinders with rod scrapers, drain ports and corrosion-resistant rods to extend seal life.
Integration with the Molding Machine
Pneumatic cylinders in injection molding are usually controlled by the machine’s integrated valve manifold or by a separate PLC. We recommend 5/2 solenoid valves with fast response times and pressure sensors that confirm the cylinder reached the mold position before the next cycle step. M12 connectors and 24 VDC coils simplify wiring to the machine controller.
Carroland Injection Molding Solutions
Carroland supplies ISO 15552 tie-rod cylinders and guided actuators for injection molding machines, with options for high-temperature seals, rod scrapers and stainless rods. We support custom mounting brackets and stroke lengths to fit existing mold frames, with standard lead times of 10–15 working days and 5–7 day sample support for mold trials.
FAQ
What bore size is needed for an ejector plate return cylinder?
A 63–100 mm bore cylinder at 0.5–0.7 MPa is typical for ejector plate return, depending on the plate weight and mold size.
Can a pneumatic cylinder hold a core against injection pressure?
Only for small cores. For large projected areas, use a mechanical lock, toggle or hydraulic intensifier; the pneumatic cylinder then only handles the unlocking motion.
How do you protect pneumatic cylinders from mold heat?
Use FKM high-temperature seals, keep the cylinder away from direct contact with the hot platen, add a heat shield or stand-off, and ensure the FRL supplies clean, dry air to prevent seal degradation.