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Clamp Tonnage in Preform Molding

All too often I go into a plant and work on a machine that is always set to full clamp tonnage. This is especially true when machines other than Husky or Netstal are used. New tools should always be tested in how little tonnage they can take without flashing. Processors are afraid of the mess that flashing a tool can create and are therefore reluctant to experiment. However, dialing in the minimum tonnage is an important component in maximizing tool life. The lower the tonnage, the longer the taper locks hold up, the vents stay open, and the longer the tool will be in operation.
What determines necessary tonnage? The clamp has to be held closed when material pressure pushes against it. When the material pressure overcomes the clamp tonnage the tool flashes. Let’s consider when it is possible to flash the tool. That is either
– at the end of injection or
– during hold time
During injection the screw or shooting pot fills the cavity. At the start of injection the cavity is empty and very little material pressure is exerted. This makes it possible to clamp up and inject simultaneously, a cycle time-saving measure optional on many machines that should always be used in preform molding. When the cavity is full the machine should switch to hold where less pressure at slower speed allows material delivery to account for shrinkage. In PET molding we always switch on distance (not on time or pressure) and the point where we do is called transition or switch-over point. When the transition point is set too small a number the machine tries to inject into the already filled cavity and flashing is possible depending on how the maximum pressure is set. Many processors are unsure how to set the transition point and there is good reason for it as it depends on the wall thickness of the preform.
Let’s first think about why we separate between injection and hold and what happens in either process. PET has a solid density of 1.335 g/cm3 and a melt density of 1.15 to 1.2 g/cm3. In the heated state PET molecules push each other away and so require more space, hence the lower density. As the material temperature drops during hold the PET molecules pack more tightly together and the density increases. We call this shrinkage and it is a common feature of all thermoplastics. When the cavity is full at the end of injection the temperature of the material has cooled to a degree unknown to us. PET directly pushed against the cold mold walls is already at solid density while the material in the center is still at melt density.
We can attempt to calculate the difference in densities as a percentage and convert that to distance. Here is how this is done:
Difference between solid and melt density is 1.335 – 1.175 = 0.16
Percentage of this difference to solid density: 0.16 / 1.335 *100% = 12%
If we assumed that the material is still at melt density after injection 12% of the total shot from the shotsize to the cushion position should be during hold. This is a reasonable calculation for preforms with a wall thickness of 2.5 to 4 mm. It doesn’t work for very thin and very thick preforms for different reasons. With very thin preforms there is a large amount of the total material in the cavity exposed to the cold mold wall and therefore the overall temperature is lower. With very thick preforms it is the long injection time that cools the material down more. In either case, the material is colder and there is therefore less distance needed to make up for the shrinkage. The percentage then drops to 8% or even 5%.
We can now calculate with some degree of accuracy where the transition point should be. Here is an example:
Shotsize: 120 mm
Cushion position: 5 mm
Total stroke: 120 -5 = 115 mm
At 5%: 115 * 5 / 100% = 5.8 mm
At 12%: 115 * 12 /100% = 13.8 mm
We have to add these numbers to the cushion position of 5 mm and arrive at a transition point that is between 10.8 and 18.8 mm. In order not to flash the tool we might start with the higher number and work our way to the lower if that is justified. Using this method there is very little chance to flash the tool and it is therefore highly recommended to follow it when adjusting the tonnage.
Now that we do not have to worry about flashing the tool during injection how do we calculate the necessary hold pressure and what tonnage is required for a given pressure? When it comes to hold pressure less is always better up to the point when sink marks appear. Over-pressuring the preform can lead to gate problems and blowing difficulties. You can start with about 60% of the maximum injection pressure and reduce if necessary. I do not have a better calculation for that. Necessary clamp tonnage however can be calculated. The material pressure works on the circular surface where it is largest, that is the circular surface of what we refer to as the T-dimension, that is the outside of the threads. The neck support ring area is also pushing against the clamp but the underside of it is pushing in the opposite direction and therefore does not have any effect. Pressure times area results in force and it is this force that is trying to open the mold. Let’s use an example: (For those hydraulic machines that display hydraulic and not material pressure you will have to look up what the ratio between these two numbers is. When in doubt use 7, i.e. material pressure is hydraulic pressure times 7)
Neck finish: 28 mm (38 mm), cavitation: 48
T-dimension: 27.43 mm (37.19 mm)
Area: 27.43^2*3.14/4*48/100 = 283.5 cm2 (521.2 cm2)
At 300 bar we get: 300 * 283.4 = 88,620 kg or 85 tons but 300 * 521.2 = 156 tons with the 38 mm neck. We would want to leave a little room between the required and applied tonnage and might decide to run the 28 mm neck at 100 tons and the 38 mm one at 200 tons with the hold pressure at 300 bar.
I actually started up a new 16-cavity tool recently. T-dimension was 28.3 mm and I lowered the tonnage to 30 tons with a hold pressure of 250 bar. I did not do the calculation at the time but it comes out to 25.1 tons.
A reasonable approach may be to start at full clamp tonnage, optimize the hold pressure, and then lower the tonnage to the calculated value. Use small steps of 30 tons or so and watch for little protuberances at the parting line of the neck finish. Flash will appear there first and how much is acceptable is up to your customer. In a recent exercise I attempted to lower tonnage on a number of tools, some of which had already 4 million cycles under their belt. I was able to lower tonnage on all but one tool, in some cases significantly. Lower clamp tonnage helped with a defect often called a “vent burn”, that is a whitish area on the threads that is the result of insufficient venting. So there are many reasons to run the lowest clamp tonnage!

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