Clamping Concepts

The injection molding machine clamp is used to close the mold, hold it closed during the injection and curing of the plastic material, and open the mold for the removal of the formed part. There are 3 different types of clamp design:

  1. Straight hydraulic clamp
  2. Linkage or toggle clamp
  3. Hydromechanical clamp

1- Straight hydraulic clamp

This design uses hydraulic fluid and pressure to open and close the clamp and to develop the force required to hold the mold closed during the injection of plastic. The basic concept is to direct hydraulic fluid to the booster tube to move the clamp ram forward. Oil fills the main area by flowing from the tank through the prefill to the main area. as the ram moves forward, a slight vacuum is developed in the main area, pulling fluid from the tank into this chamber. Once the clamp is closed, the prefill valve is closed, trapping the oil in the main cylinder area. High-pressure fluid is put into this area, compressing in this area. The maximum pressure is controlled by a pressure control valves, which closely controls the clamp tonnage ( the max. hydraulic pressure times the area it pushes against.)

To open the clamp, hydraulic fluid is directed to the pullback side of the cylinder while the prefill valve is open, with fluid from the main cylinder being returned to the tank. One of the major advantages of the straight hydraulic clamp is its very precise control of the clamp tonnage.

2- Linkage or Toggle Clamp

This concept uses the mechanical advantage of a linkage to develop the force required to hold the mold closed during the plastic injection portion of the cycle. Normally the linkage design is done in such a way that slowdowns are built in. The advantage of a toggle clamp is that less hydraulic fluid is required to open and close the clamp. A disadvantage is that the clamp tonnage is not precisely known.

A small hydraulic cylinder travels at a constant speed with the slowdown for mold close built into the linkage. The mechanical advantage of the linkage is extremely high so the relatively small closing cylinder can develop high tonnage.

3- Hydromechanical Clamp

This design uses a mechanical means for high speed close and open. A short stroke cylinder is used to develop tonnage identical to the straight hydraulic design. This concept is said to offer the advantage of toggle clamps for high-speed close and open, and the advantage of a straight hydraulic for precise control of the clamp tonnage. The hydromechanical design normally has a high-speed clamp close and open device which is usually a hydraulic cylinder or actuator. The closing and opening mode occurs with relatively low force. Once the clamp is closed, a blocking action takes place allowing a large -diameter hydraulic cylinder to build tonnage similar to the straight hydraulic design.

When the clamp is to be opened, the blocking member is removed, and the clamp opens rapidly. The blocking action is normally a mechanical device,  and the tonnage action is done by hydraulic; hence the name hydromechanical.

Comparison of Clamp Designs

Over the years many arguments have been presented showing each clamp design concept to be superior to the others. In reality, each concept has merit.

The straight hydraulic design has proved over the years to have long-term reliability, excellent control of low-pressure mold protection, and exact control of tonnage, and it will not allow the clamp t be overstressed due to high injection forces.

The toggle clamp has extremely fast closing and opening actions and is typically lower in cost than the straight hydraulic, but this energy is small compared to the total energy usage of the machine. With good lubrication, the toggle bushings and pins last well, but they still must be reworked after several years of service.

The toggle design will also develop higher than lockup tonnage if the clamp is overpowered by the injection end, or due to temperature buildup in the mold. The hydromechanical tends to have the advantages of the straight hydraulic, whereas the toggle is more complex because of the block action required. The debate over the three clamp concepts will continue for many years.



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