|
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. The hydraulic is a simple system in understanding its mechanics, in that the clamp is a 3 platen system and uses a large hydraulic cylinder which applies pressure to our moveable platen.
To understand the setup of our clamp in the press for a hydraulic type of clamp the discussion shall be on a 3 platen system. Note that there are 2 platen systems but those well not be discussed here. As in a toggle press there is the front, A, hot half, or 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 front of a hydraulic ram 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 hydraulic cylinder mechanism and it is held in place by the tie bars and sets atop the machine frame.
The tie bars which hold and allow for alignment of platens and acts as restraints to our force applied via the cylinder ram to the platen, through our mold to our A platen and back unto the tie bar which are held in place through our platen and retaining bolts. In applying pressure to our mold the rear platen has the hydraulic cylinder bolted to it, and when we apply hydraulic fluid the ram extends and after the mold is closed the pressure builds with resistance.
In this configuration the tie bars are always locked and set in the 1 and 3rd platen as they do not move. The cylinder and ram applying pressure are located in the center of the platens on the back side of the B or moveable platen thus applying pressure to the mold area. Since we are using a hydraulic ram the adjustment for mold height is easy, for the movement of setup for opening and closing. In some systems mold height is done automatically, by the controller of the press.
What has to be set is the speed of action, since we are using a ram to move the mold open and close, there are issues with high gpm for fast movement and low gpm for high pressure, the speed of closing and opening is through a flow control valve and in some cases a choice of pumps. The slowing/speeding of press movement is done via position of clamp and is controlled on the screens in today's presses. Whereas the toggle press slows down naturally due to design a hydraulic press has to be programed in as to where it slows. (This is not to say a toggle does not need to be adjusted but due to defined stroke is typically left unchanged in most shops)
Some of the benefits of the hydraulic clamp are the unlimited mold size, (within the range of movement) and direct readout of clamp tonnage. Actual tonnage can be dialed in for the mold. Another aspect is that the ram is applying pressure to the center area of the mold thus platen deflection is not really an issue.
A benefit in optical molding and or cosmetics with a hydraulic press is that we can cause clamp movement, by lowering the available tonnage, or increasing injection pressure such that the force holding the mold closed is just minimal to injection force and as the plastic is injected into the mold the mold actually breathers or opens slightly and then as the plastic cools the mold closes up. What happens in this case is that the contact to the part surface is maintained and replication is positive to the outcome of cosmetics necessary to the product. Though this is somewhat uncontrolled, (as opposed to using a coining feature/ press) it can play an important role in being able to make good parts.
The author has experienced many times that client parts have run different in one type of clamping system over the other. In some cases new presses were purchased and the clamping system was not noted, resulting in having to place the mold back into the old press because that is where good parts could be produced.
There are many various configuration to hydraulic clamp presses, which have been developed over the years but the principals are all the same, in that once closed the press develops a clamp tonnage which can be dialed in for force, and in most cases this force is applied to the center of the mold though the platen. The setup of the positions, movements and speeds must be adjusted in a hydraulic press to avoid tool damage.
.
Silveysplastics@hotmail.com | 
