All too often I go into a plant and work on a machine that is set to full clamp tonnage without consideration whether this is necessary or not. 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 pressure 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. When the transition point is set too high, the preform may not form entirely (short shot). Many processors are unsure how to set the transition point. Here is how to go about it.

 Let’s first think about why we separate 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 average 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 during injection. The idea here is that we calculate how much of the stroke needs to take place during hold to account for the increase in density. Here is how this is done (I use the middle value between 1.15 and 1.2 g/cm3):

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, we have to make up 12% of the total shot from the shotsize to the cushion position during hold. This is a reasonable calculation for preforms with a wall thickness of 2.5 to 4 mm. It doesn’t work as well 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 the preforms are not fully formed for example. Using this method there is very little chance to flash the tool during injection 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. All machines offer at least 3 hold pressures and you may reduce it to 50% and 40% for pressures 2 and 3. There is no ironclad calculation for that as it depends on wall thickness, material temperature, and material IV. It is best to start with a low pressure, increasing it only to overcome sink marks.

Necessary clamp tonnage however can be calculated. The material pressure works on the circular surface where it is largest, that is the ‘E’ dimension of the neck. While both the threads and the neck support ring are larger than the ‘E’ dimension, hold pressure for these geometries works both in the forward and backward direction and has therefore no net force pushing against the clamp.

Pressure times area results in force and it this force that is trying to open the mold. Let’s use an example. We have to use material and not hydraulic pressure for these calculations. The diameter of the cylinder that injects the material is always larger than the screw or shooting pot and the hydraulic pressure is amplified in the ratio between the area of the cylinder and the screw/shooting pot. For 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. (On electric machines the displayed pressure is the material pressure)

Neck finish: PCO 1881, 28 mm (SPI 38 mm), cavitation: 48

‘E’ diameter:  25.07 mm (34.8 mm)

Area: 25.07^2*3.14/4*48/100 = 236.8 cm2 (456.3 cm2)

At 300 bar we get: 300 * 236.8 = 71,040 kg or 71 tons but 300 * 456.3 =  137 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.

One caveat to this procedure is the concern that the mold “breaths”, that is slightly opens and closes as the material pressure increases without leading to flash. This is highly undesirable as it will actually increase wear. Therefore, watch the mold closely, maybe with the help of a dial gauge and increase the tonnage to a value that prevents this from happening.

I actually started up a new 16-cavity tool recently. ‘E’ dimension was 29.39 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 27.1 tons.

A reasonable approach may be to start at full clamp tonnage, optimize the hold pressure, and then lower the tonnage to a value somewhat above 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 also 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!

Ottmar Brandau has been working in the Plastics industry since 1978, and is the president of Apex Container Tech Inc. His latest book “The Rapid Guide to Perfect PET Bottles” describes 31 common defects and their solutions. It can be found at www.blowmolding.org/shop.
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