|
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 is the area of the machine where the mold is placed and is very often overlooked as an issue, or concern. In real terms the clamp is one classification of the machine size along with barrel capacity, basically stated Tons (of force, and ounces / grams / cu in cc of shot volume, respectively).
The force of the clamp is what is applied onto the mold. The amount of pressure applied has to be greater than the amount of injection force used to fill the cavities inside the mold. If the force is not adequate to hold close the mold than flash occurs and the tool can be damaged. In terms, one has to understand projected area of the tool, which is the area of the cavities and runner that are parallel to the parting line face of the mold, or if a 3 plate tool those additional areas that if viewed from the face of the A side of the tool that would be outside the cavity area. These areas would be calculated into square inches of area (or square cc of area) and that is the projected area.
In looking up the processing data for a material from the supplier's it is stated that a clamp tonnage per square inch of area is given, this typically being at anywhere between 2 and 5 tons per square area. An example for calculating clamp tonnage would be if the mold had 10 square units of area and tonnage was needed at 2 tons per square area the result is 20 tons of clamp force is necessary per manufactures recommendation. The upper limit per their recommendations is 5 tons per unit which than transfers to 50 tons of clamp force. All fairly straight forward.
In the processing of plastic on the machine we generate injection force which is the real force applied to projected area, not the recommended per manufacture data. If it were assumed that there was no pressure lose in the delivery system the calculation could be made that injection force in plastic psi divided by 2000 pounds per ton would equal tonnage used to inject, and that number times the projected area is what we need to compensate for with the clamp pressure. Thus if for example the press used 20,000 ppsi injection force we than would have 10 tons of force which when applied to our mold means 100 tons of clamp force is necessary.
It should be stated that the range of pressure loss through an injection molding systems can vary greatly dependent on the overall design of both the runner and cavity layout and thickness. Some references state that an average loss at approximately 50%, but the author cannot confirm this. Where this data and calculation comes into play is the area of troubleshooting and or design and placement of the mold in what machine. In many cases it has been found that flash has occurred with a tool, and by running the numbers it was found that the press was not capable, but at the same time it came back that if we could lower injection pressure to a set value that flash was eliminated and parts could be worried out.
It has been found that as wall thickness goes down a higher tonnage of clamp is necessary, while the inverse is true for thicker walled parts, that less tonnage is necessary. The problem in some of the cases is that flash occurs in the runner and gate area of the tool, due to pressure build up, via the restriction in the gating area.
Side Note:
If the tool is flashed while under clamp a deformation may happen to the mold steel. If this situation continues the damage get worst and the parting line of the tool must be repaired. The author has observed mold bases that appeared to be beaten with a ball peen hammer on the mold faces, but in reality it was only flash and stringing of the plastic which caused the issue.
The other issue in clamp tonnage is using the machine maximum tonnage when it is not needed. These cases can be small molds in relationship to the clamp platens and or molds that are large but projected area is small. How one sets the tonnage of the press, and or if it is allowed at all seems to vary per shop and processors, but to know what is going on with tonnage is required of all processors to avoid issues. Over time it has been observed that the maximum tonnage a press is capable of delivering diminishes a bit, this may be evident in tools that use to run in a press but now seem to flash a bit, at that point one might wish to check out the clamp mechanism for wear and or age.
SL SILVEY
360-882-3183
| 
Multi-cavity or even single cavity process issues?? Let us discuss and talk about it