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Figure 22. Hydraulic clamping system
Mold set-up is much easier with a hydraulic clamp than with a toggle clamp since hydraulic clamp tonnage can be reached anywhere along the clamp stroke. Mold setup is accomplished by setting the clamp position from the machine’s control center.
Hydromechanical clamps are commonly used on very large injection molding machines, i.e., over 1000 tons. In the hydromechanical clamp, a hydraulically actuated toggle mechanism pushes the moving platen at high speed to a point where the mold halves are nearly closed. A mechanical locking plate or links prevent rearward movement during final build-up to full clamp tonnage. Short-stroke hydraulic cylinders are used to move the platen the final short closing distance and develop full clamp tonnage. See Figure 23.
Clamp specifications
Key clamp specifications to consider in choosing an injection molding machine are:
• Clamp stroke
• Minimum mold thickness
• Clearance between tie bars
• Maximum daylight opening
• Platen size
• Clamp tonnage
Clamp stroke is the maximum distance(inches) the moving platen can travel. Clamp stroke is a major factor in determining the minimum mold thickness that can be used with the machine. Generally, clamp specifications also state the minimum mold thickness for which the clamp can develop its full tonnage.
Maximum daylight opening is the distance (inches) between the two platens when the clamp is completely open. This measurement is a major factor in determining the effective maximum mold thickness which takes into account the mold opening required for part ejection or removal. Complicated molds may require more opening space and rigid mounting surfaces, since high platen deflection under load could damage the mold. An allowable deflection of 0.001 in/ft of span with full clamp load on the center of the platen is, generally, considered acceptable.
Platen size is given in horizontal and vertical measurements (inches) for the full platen. Since there are tiebars running through the corners of the platens, mold-size limits are less than full platen size. A mold can extend between the tie-bars in either the vertical or horizontal direction but, generally, should not extend outside of the platens.
Clearance between tie-bars is given for the distance (inches) between the top tie-bars (horizontal) and sidebars (vertical). Since the tie-bars are fixed on most injection molding clamps, the distance between them dictates the maximum size of a mold that can be placed in the clamp.
Clamp tonnage is the maximum force which the clamp can develop. A clamping pressure of five-tonsper- square-inch of the projected area of the molding (including the runner system) is more than adequate for polyolefins. However, where packing is not a major factor, this pressure may be as low as 2 tons/in2. An industry rule-of-thumb is that a clamp force of 2 to 3 tons per in2 of the projected area of the part(s) and cold runner system is adequate for reciprocating-screwtype, injection-molding machines. Some thin-wall stack molds may require 5 tons/in2 for optimum performance.
Injection molds
There are many types of injection molds and tooling in use today, such as two-plate, three-plate and stack molds. Two and three- plate molds are more commonly used for heavy wall and non-packaging products. Both cold and hot-runner systems are used for two and threeplate molds. All stack molds use a hot manifold to convey the melt to the cavities. Each mold component must be machined and finished to exact dimensions with very tight tolerances and must be heat-treated to be able to withstand very high injection and clamp pressures. Injection molds are the most expensive molds used in plastics processing with very long lead times required for their design and fabrication. Every mold must be tested and debugged to prove-out the ejection system, cooling and/or heating system and operating components before it is placed in production.