Leak testers are viewed by some blow moulding companies as something that doesn’t directly add to production as it will not, for example, make more bottles per hour. However, leak testing does ensure that only quality bottles are delivered.
In many Western countries, if a pallet of filled product leaks in a supermarket, the pallet is returned to the supplier who will generally look for recompense for the cost from the blow moulding company. Faulty bottles that get out of your plant therefore do damage to your reputation and may also incur expensive re-testing of bottles offline.
Leak testers can prevent this, but there are several problems with their use. I will discuss how to overcome these later, but to start we need to discuss how leak testers work.
Firstly, they are not testing in absolute terms but are a “relative” tester. When setting up for a new bottle, the machine is first adjusted mechanically for the size of the bottle. The machine test parameters are then adjusted to pass a good bottle and then to fail a faulty bottle.
How do most people make a faulty bottle? They put a hole in it. If they are lucky they may use a drawing pin (0.6-0.7mm diameter) or a syringe (0.7mm). They make this “fail” but this is an extremely large hole that any tester can find and, as such, rarely do they set the threshold to levels well below this.
So how can you ensure that you have a good setting?
Answer: By making a small hole YOURSELF and the concept of “self test”.
Making “Good” Reference Holes
Many years ago we used small drills or heated wires to make holes in containers when testing sample bottles for clients. However, drilling is slow and the small drill bits break easily and are expensive. Plus the holes can sometimes be bigger or smaller than intended â€“ melted holes usually end up bigger than intended and are not easy to do.
Today we use off-the-shelf resilient wires that can be bought in most countries. One division of Nordson Corporation provides these wires for cleaning the nozzles of their hot melt glue systems. It has three sizes of wires it uses to prime hot melt glue guns. These wires are very good for making “reference holes” in bottles. The wires are available in sizes 0.007″, 0.011″ and 0.014″ or 0.17mm, 0.28mm and 0.35mm respectively.
Nordson also supplies a small pin chuck ideal for using with these wires, especially as many pin chucks made for small drills do not tighten up on the wire or are hollow allowing a full wire to slide down.
A pin chuck and three vials of wires costs approximately US$72-00 but will last a long time and also save time in making the holes. This is a small investment that will greatly improve quality.
The wire is used like a drill, first a small length is allowed to protrude out of the pin chuck and then, whilst in your fingers, twisted back and forth with a light pressure drilling into the container. It will take 1-2 minutes to make the hole. It is then necessary to extend a little more of the wire and ream the hole to make it clear.
Also place the test hole above or below the leak tester guide rails as dirt on these easily clog small holes (especially the 0.17mm holes). It’s good practice to ream the hole frequently when you use the test bottle. The 0.17mm holes are best made and used by the quality assurance people. On the factory floor, the 0.26mm hole is easier to use in a day-to-day production environment. You know the hole is correctly formed when you repeatedly put the pin in and there is minimal resistance, and there is a smooth fit between the pin and hole.
So why go to the trouble of using these wires?
Predominantly because they allow you to establish a reliable benchmark and also to see what pressure drop a hole causes. Plus they are real holes â€“ holes with a known size. When you start using real holes you will find a lot of machines with their current settings cannot reliably find the 0.17mm and maybe even the 0.26mm holes.
A lot of leak testers will find holes down to 0.3mm but have a lot of trouble finding anything smaller no matter how long the test time. Many machines are not consistent enough and as a consequence take a long time to find holes this small. For example, on our fast machines we take 0.05sec, 0.4-0.55sec and 4.0-5.5 sec test time to find a 0.17mm hole in a 100ml, 1000ml and five litre bottles respectively.
Instead of making real holes some tester manufacturers supply thin foils 0.16mm thick with a 0.16mm diameter hole. They suggest that this is taped over a hole in the container. The material is so thin that there are virtually no frictional losses. The 0.16mm hole in the thin plate has a pressure drop approx the same as a 0.4mm hole in a 1mm plate. However, any test orifice has to have a wall thickness similar to your range of bottles to give real feedback. So be aware that the above foils are giving you a false sense of quality. Even some calibrated orifices, if not similar in wall thickness to your bottle will give you erroneous feedback. Nothing is better than a real hole in your bottle as it truly emulates the faults that occur in the production process.
Many testers have a self test function that, on demand or at a preset count, opens a valve that has a test orifice. The self test works if the test orifice is detected and the bottle is rejected. If a machine automatically self tests every 500 bottles and tests 10 million bottles a year, 20,000 bottles would be wasted.
We have taken the self test further by making it check that the bottle to be self tested was “good” before doing the test. We then vent and recharge it but with the hole open. As such, the bottle is not wasted.
More importantly, a self test system gives operators feedback on what pressure drop happens when there is a hole in a container. They will see from using a self test system that bigger containers for the same size hole have smaller pressure drops than a small bottle. If your present leak tester does not show the final pressure in the bottle last tested on the display, it is less helpful to operators. The better machines give you this feedback and it is important for operators. Good feedback equals better testing.
We use a 0.4mm self test hole as smaller holes are easily blocked. If the margin between the final pressure of the “good bottle” test and the “self test” is too small, you are only just finding the big hole and need more time to test the container. If the margin is large, you know you can find a hole much smaller than 0.4mm.
A leak tester must be consistent day and night â€“ many are not. To check this, watch a row of bottles go through a machine. The final pressure in the bottle should remain almost constant.
On some machines it does appear this way but that is because the last digit does not have enough resolution. Our machines resolve to 0.01kPa, but many machines resolve up to 5-10 times this level. Some bottles vary only +/-0.01kPa, however, bottles from large multi-cavity moulds may vary more than +/-0.02kPa if there is a lot of variation in the moulds. If the variation is much larger this indicates that some of your cavities are very different and also possibly your extruder has started to excessively wear.
As such, a good leak tester is an accurate and consistent machine. If your final pressures are jumping around (there is little consistency), then it takes a lot of time to find that small hole. Unless the blow moulder is going very slowly, you may not have enough time. Furthermore, it will have a “high” limit to the minimum size it can find even with a long test time.
I also tell people just because we can find a small hole fast, if your production rate allows a higher test then use it to find even smaller holes. If containers are filled with liquids with low viscosity and low surface tension you need as much time as possible to find the smallest hole you can. But leave a margin if you need to catch up occasionally.
There are many things to consider with leak testing and this is a brief introduction.
I strongly encourage both bottle users and moulders to get a set of the hole making wires from Nordson. You will never look back as you set new benchmarks for the quality of the bottles you use or make. Profile Solutions clients have gained new clients as they show quality in “real terms” right in front of their client”s eye. Seeing makes believers and customers.
|Part Number||Part Description|
|901922||7 THOU INCH WIRE VIAL (0.17mm)|
|901923||11 THOU INCH WIRE VIAL (0.27mm)|
|901924||14 THOU INCH WIRE VIAL (0.35mm)|