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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

How is Clamp Tonnage Adjusted?

There are two types of systems used to generate tonnage, Hydraulic Cylinders and Toggle Linkage Systems. Many toggle systems used hydraulic cylinders to actuate them, but the actual tonnage is applied through the mechanical advantage of the linkage system.

Question:

How is clamp tonnage adjusted?

My Response:

On a toggle clamp, the tonnage is generated by the mechanical linkages and the
stretch of the tie bar. The movement of the rear platen (die height) adjusts
the clamp tonnage on this machine.

On a cylinder clamp, there is no toggle, just one or more hydraulic cylinders
holding the mold closed. The adjustment of the hydraulic pressure adjusts clamp
tonnage on this machine.

Additional Comments:

On a cylinder clamp, the tonnage is determined by a direct relationship between the hydraulic pressure and the clamp tonnage. ie. Max pressure = Max tonnage.

On a toggle clamp, the tonnage is determined by an indirect measurement of the tie bar stretch using a strain gauge on one or more tie bars. ie. Max strain = Max tonnage.

How is Toggle Clamp Tonnage Measured?

With toggle clamps, the only way to measure tonnage is to measure the strain (stretch) of the tie bar.

Question:

How do you measure the tonnage on a toggle molding machine:

My Answer:

With toggle clamps, the only way to measure tonnage is to measure
the strain (stretch) of the tie bar. Using the strain measurement of the tie
bar, the force on the tie bar can be calculated if the tie bar material is
known. If that is multiplied by the number of tie bars, the tonnage is
determined.

Most newer molding machines use a strain gauge mounted on or in
the tie bar. The best measurement can be obtained using an ultrasonic
measurement through the tie bar to determine the exact amount of stretch in
each tie bar. This equipment is expensive, but there are many machine
evaluation consultants you can hire to do this.

Short Shots to Correct Flash

The first step in troubleshooting flash is to determine whether it is a filling problem or a packing problem.

Question:

I have a part with flash. It ran many times in the past without flash, but recently we have been using a new machine and it is often creating flash. We have tried many adjustments including packing pressure, clamp tonnage, transfer pressure, etc.

Answer:

The first step in troubleshooting flash is to turn off packing and determine if you have flash during first stage injection. This will tell you immediately whether the problem is occurring during 1st stage injection or second stage packing. Ultimately, this will focus your attention very quickly.

Additional Thoughts:

In many cases, you see technicians adjust parameters such as packing pressure and clamp tonnage when the problem occurs during injection. The problem with this approach is that the technicians almost never return the clamp tonnage or packing pressure to their original settings when they ultimately fix the flash. As a result, the plant gets lots of other defects such as shorts, sinks, and burns afterwards.

Documentation is the First Step

Documentation is the first step to good troubleshooting, it is the first step to continuous improvement, it is the first step to process improvement, it is the first step to better maintenance, it is the first step to getting your technicians up to speed faster, etc.

Question:

During a recent on-site training session, I was asked by the plant manager: What is the first step we need to take to get better?

My Response:

Better documentation is the first step you need to take to get better. With respect to processing, if you do not properly and accurately document the process that is being used to make a good part, there is nothing to use as a basis of comparison when troubleshooting a bad part. Everything that happens to a process, machine, mold, material, or a piece of auxiliary equipment should be documented. This includes everyone from tooling and processing to maintenance and die setters. Essentially everyone who works on the equipment or process must document what has transpired.

For example, if you don’t document the time and reason for the swapping-out of a failed water temperature controller during 2nd shift, the 3rd shift will inaccurate information when troubleshooting defects that might be found on that machine at the start of their shift and may not be able to locate the root cause of the problem in a timely manner.

Additional Thoughts:

When trying top get new process technicians up to speed faster, good documentation significantly reduces the learning curve. For example, let’s suppose a new technician notices delamination on the parts as well as an increase in transfer pressure, but no other significant deviations in the current process from the documented standard. In this case, he/she can place attention on why the pressure has increased. This approach puts the focus on what likely caused the pressure to increase which is much more efficient than trying to remember all the things that might cause delamination. If they correct the cause for the pressure increase and the delamination goes away, the new technician has a very high level of confidence that they corrected the problem and can move on to the next problem.

Correct Short Shot Size

The purpose of a short shot during 1st Stage Injection is to ensure that all cavities remain short at transfer during normal process variation.

Question:

I was told to short the mold around 98% based on final part weight, is that enough?

My Response:

Basing any short shot on the final part weight is an inaccurate and misleading process. This is faulty because there are so many factors which contribute to the percentage of material that is added to the mold between when the first cavity fills and the part is completely packed out. Aspects such as wall thickness, material shrinkage, filling imbalance, degree of semi-crystallinity, pressure loss, material consistency, packing pressure, etc. all make a general rule based on final part weight impossible. because of these factors, a short shot of 95-98% based on a fully packed part will likely be full in most if not all cavities at the time of transfer as well as show screw bounce.

The best approach is to use a short shot percentage based on 100% being equal to the weight of the shot when 1) only one cavity fills and 2) no packing is present. We typically recommend 90-95% short under these circumstances. If the packing is established correctly, the process will have enough pressure to keep the screw moving forward to fill and pack out the parts without flash. This will compensate for 1-2 percent check ring variation as well as 2-3 percent material viscosity variation.

Additional Thoughts:

If you are processing very unstable material such as post-consumer regrind having as high as 10-20% viscosity variation from shot-to-shot, then a short shot as low as 75% may be necessary to maintain a consistent final part weight.