Squeeze testing, also sometimes known as simulated impact testing is a destructive test that can be performed on glass containers during the manufacturing process.
This is a test usually implemented in line and therefore to which all produced containers can be submitted.
It is common to see this test equipment installed in many plants of specific glass manufacturers groups but for many others glass manufacturers as well this is an equipment almost unknown or unheard off.
So it deserves that we look at its pros and cons.
Squeeze testing involves placing a glass container under stress to cause weak containers to break. Those “weak containers” have surface flaws or stress concentrators that lead to container breakage when squeezed.
The stress is caused by placing a load on one side of the container as the container rolls along a shoe or pad.
This method places a uniform stress all the way around the container during the test. Glass is a brittle material and like most brittle materials, it is strong in compression and weak in (tensile) tension. By loading the container in the described manner, the container walls receive an alternating stress. This stress causes the container to fail or break if the container has any structural flaws in its sidewall. "Good" containers withstand the stress and are undamaged.
When installed, it is the first equipment on the Cold End just after the annealing lehr. This helps prevent jams and damage to downstream inspection and handling devices.
The squeeze tester “rejects” - breaks! - all ware that is semi-broken or susceptible to brake in the inspection machines and therefore to damage or unsettle the inspection equipment.
So this not like the typical inspection machine that we have in the Cold End but rather more like an “in-line-destructive-test”.
In fact, perhaps – in my view – the biggest shortcoming of this equipment is the fact that it is not possible to collect the relevant data concerning the containers that are being “rejected” – broken! -by the squeeze tester.
Like I have referred in one of my previous posts: the task for the Quality Control / Cold End areas in a glass plant is not only guaranteeing that no defective containers reach the pallet and are not sent to the customer.
Also very important is to provide continuous feedback with reliable data for correction. That is, to provide information for correction of the manufacturing process, like: type of defect detected, affected mould numbers, occurrence percentages… and that you cannot get from a squeeze tester! - but you can from an inspection machine.
In the other hand, like any other inspection machine, we should have a challenge sample to assess the effectiveness of the squeeze tester at regular intervals. That is to verify if the structurally weak ware is being broken by the equipment.
Usually this procedure involves the simulation of a defective sample by merely damaging its internal surface with a scribe. The sample is then tested in the squeeze tester and checked if it is broken by the equipment.
Additional key checks shall be made to verify that the unit is set-up properly and that in fact is not a source of product contamination or damage.
Covers must be installed above the top of the finish of the containers to prevent that tramp (flying) glass – from breakages - does not gets inside them. The wider the finish of the container the greater the risk involved.
Any embedded glass – resulting from container breakages – should be removed from the pressure wheel so that subsequent containers will not be damage (frictive damage mechanism). Squeeze testers are equipped to prevent this but regular checks and maintenance should be in place to guarantee it.
Finally, a minimum air pressure set-point for the squeeze tester should be established and monitored regularly to assure that the defective ware is rejected – squeezed and broken! - appropriately.
Squeeze testers are not a consensual equipment in the container glass industry and there are arguments for and against its use.
Personally I have never had the opportunity to work on a daily basis with the equipment. So I do not have direct feedback to give.
But from what I know of the equipment, lead me to be a little bit jealous from my fellow colleagues that had the luck to count with the aid of such equipment in their lines.
I think that it is especially useful in situations that we are producing non-round containers or for containers where the standard automatic inspection is hampered due to the container geometry or special features.
In all situations I see it like a life insurance.