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Definitions:
Clamp: an appliance with opposite parts that may be brought closer together to hold or compress something.
Clamping Capacity: the largest rated projected area of cavities and runners that an injection press can safely hold closed at full molding pressure.
Clamping force: the force applied to the mold to keep it closed, and opposing the pressure exerted by the injected plastic acting upon the projected area of cavities and runners.
The clamp mechanism for molding machines are of two basic types, toggle and hydraulic. There are list of pluses and minus with both systems and should one be using an electric press as the conversion to such presses seems to be high one than uses a toggle.
The toggle mechanism is a mechanical advantage type system, which can be up to 50:1 advantage. It is made up of linkages and uses either a hydraulic ram and or in electric servo motors to clamp over the toggle so as to provide the force onto the mold. There are two types or subgroups of toggles, locking and non-locking. The locking type is of "clamp over" meaning that as the toggle clamps over we have a high pressure and then a reduced pressure on the clamp/mold as the toggle goes from 180 degrees (straight) to a slight bend the toggle now in a locked position, no force is necessary now. Non-locking remain in a straight position and require continual force on the cylinder or servo to hold toggle the mechanism straight, as injection force is applied.
To understand this it is important to understand that a toggle works on a 3-platten press in that the front or A, or hot platen is stationary, (bolted to the machine frame) and that it is aligned to the other 2 platen via tie bars. The B, moveable, cold platen is attached to the toggle on the back side, the opening between the A and B platen is where we mount the molds. The 3rd platen is at the back of toggle mechanism and it is held in place by the tie bars and resting on the weighs of the machine frame.
Since the toggle mechanism has a fixed distance between the 3 and B platen, any adjustment for mold height moves this whole mechanism, which is the 3rd platen, toggle mechanism and B platen. This is done via the nuts on the back of the tie bars which are rotated to move the 3rd platen and all attachments forward or back as necessary. (Note: each machine Mfg. has their own method to rotate, lock and set but all move in unison so as to maintain uniformity) the force applied to the mold is via the movement of the toggle to platen, but then the retention of this force is held via the tie bars.
On a toggle one can adjust the relative clamp pressure by adjusting the mold height, this is by trial and error as parts are made, and in some presses today there is an actual readout, but it is calculated as the mechanical action does not allowed for a direct digital read out unless one was to install a load cell in line with the molds.
Another aspect of the toggle clamp being mechanical is that the toggle may be a 2 point location to the platen and or a 4 point to the platen. Some refer to this as single toggle or double toggle. What it means is the toggle is either attached center line to the platen (single) or at top and bottom of platen (double). This is where the force is applied, directly to the platen and then transmitted to the mold mounted in the area between A and B platens. Dependent on platen design, stiffness, and or thickness, when combined with less than ideal mold to platen size, bending of platen may occur in a double toggle set up.
In troubleshooting issues with a toggle clamp press the following should be considered when reviewing:
1. Wear in pivot points
2. Was previous press only a 2 point versus now a 4 point (or hydraulic)
3. In running the production order did the tie bars and platens heat up thus causing thermal expansion thus we lost the clamp tonnage
4. Did we move from a locking to non-locking type or vice versa?
5. Is tie bar strain uniform on all tie bars?
6. Is the front platen secured to the frame?
a. This is the authors note: I have experienced the front platen breaking its retaining bolts to the frame, causing repeated replacement of tie bars and nozzles. Upon injection the entire injection and clamp ends tended to lift up at their contact points.
To be continued.
silveysplastics@hotmail.com
360-882-3183 .
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