When considering an actuator for an injection molding application, review three positions to determine the impact on cylinder type and capacity: initial breakaway, movement through the stroke, and final positioning. Physical force requirements for each (both set and pull) will provide insight into the most limiting conditions and direct selection of the most appropriate product type. Determine limiting forces, then determine size.
Core Set Considerations
For most moveable core applications, initial movement and core weight forces are very low relative to the force needed to preload the slide during injection. For applications holding the core with a heel block, choosing cylinder size for only movement and lift reduces cylinder requirements significantly. Preloading the movable core in the set position hydraulically without a lock can be problematic, as any amount of exposed molding surface creates increased force requirements and requires larger cylinders and dedicated hydraulic pressure during injection. Available hydraulic flow in gallons per minute (gpm) from the press or auxiliary pump may be insufficient to provide adequate cylinder speed, especially for longer strokes.
Consider alternatives such as preloading hydraulic locking cylinders. With very high output force in the set position, these cylinders typically have much smaller bore sizes, lower hydraulic requirements, and higher speeds.
As some presses remove hydraulic pressure to core circuits during injection, the use of hydraulic cylinders without a heel block or preloading locking cylinder may not be possible. For presses that drop hydraulics, ensure locking cylinders maintain preload after locking without pressure or that an independent hydraulic source is available. Some customers use check valves with standard hydraulic cylinders. However, this adds complexity, and without a dedicated hydraulic supply, check valves may pass some fluid, and pressure may drop.
When considering an actuator for an injection molding application, review three positions to determine the impact on cylinder type and capacity: initial breakaway, movement through the stroke, and final positioning.
Core Pull Considerations
Initial force to pull is sometimes demanding due to material shrinkage around the core or if the core is set into a shallow taper with preload. Once initial retract movement occurs, the remaining stroke and hold in the final pull position usually requires low forces that are not limiting. To reduce overall breakaway forces, providing tapers over eight degrees or a slight touch or gap to the taper using a core stop to absorb preload and time the final position is helpful.
Long, penetrating cores with aggressive shrinkage will likely create a limiting condition and require larger bore hydraulic cylinders to pull them free, unless an internal diameter taper can be added to the part to improve release. When using preloading hydraulic locking cylinders, use caution, as many provide large output forces but low retract forces. Larger bore models may be available from some manufacturers to accommodate breakaway demands when retracting the core.
Mold Forces from Injection and Shrinkage
Force on the slide core due to injection (Fi) is a function of the nozzle injection pressure (P) and the core exposed surface area projected along the axis of movement (Ap).
Fi=Ap x P
For fully exposed cores, the projected area is best visualized by thinking of slicing the core perpendicular to the direction of movement and measuring the resulting region.
While it is possible that some pressure drop will occur in the cavity and pressure at the core may be less than nozzle pressure, it is often as likely that peak pressure will increase due to processing demands. Cavity pressure is sometimes difficult to predict, even with mold flow analysis, so using nozzle injection pressure is recommended.
After injection, the plastic part cools and may shrink around the core, developing friction that resists movement. Calculating the resisting force Fr is more challenging and seems to be highly dependent on plastic material type and amount of cooling.
Cylinder Extend and Retract
Projected Area (Ap) Calculations
For a square 2 by 2-inch core body with a complex surface at the tip yet fully exposed to plastic, the projected area Ap= 2 by 2-inch = 4 square inches. Similarly, for a core with a fully exposed 4-inch diameter spherical end, the projected area Ap is the same as the circular area of a cut through the core body perpendicular to the surface A = (π x 42)/4 = (3.14 x 4 x 4)/4 = 12.56 square inches. By using the full core area, flash of any shutoff areas are included.
Many CAD systems allow for calculation of projected area from the core perpendicular to the axis. Consider including any shutoff area in case of flash.
Pressure (P) and Injection Force (Fi) Calculations
Injection pressure is calculated by multiplying the screw ratio (typically about 10:1) by the hydraulic injection pressure to the injector (generally 1,000 to 2,500 psi) for a resulting typical nozzle injection pressure between 10,000 psi and 25,000 psi.
For the square body core and 10,000 psi nozzle pressure Fi = 10,000 psi x 4 sq. in. = 40,000 lbs. For the spherical core and 10,000 psi nozzle pressure Fi = 10,000 psi x 12.56 sq. in. = 125,600 lbs.
To provide a cylinder force greater than the injection force, it is necessary to find a cylinder with a bore diameter and available hydraulic pump pressure combination that exceeds this amount plus a safety factor of 10 to 25% or more.
Applying a Safety Factor of 25%
For the square body core F = 10,000 psi x 4 sq. in. x 1.25 = 50,000 lbs. For the spherical core F = 10,000 psi x 12.56 sq. in. x 1.25 = 157,000 lbs.
The formula for force (C) from a hydraulic cylinder is the Differential Area (D) multiplied by the Hydraulic (H) core pull circuit system machine pressure. C = D x H
The differential Area for extend (De) is the area of the cylinder bore De=(3.14 x B2)/4 where B is the bore diameter. The hydraulic pressure is typically 1,500 psi to perhaps 2,500 psi. A 2-inch bore cylinder would have the following area De=(3.14 x 2 x 2)/4=3.14 sq. in. Using H=2,500 psi, Ce=2,500 psi x 3.14 sq. in. = 7,850 lbs.
Note as the forces increase, the amount of hydraulic volume necessary to move the standard hydraulic cylinder would be substantial.