WHERE DO LEAKS COME FROM?
The blow molding of bottles involves the stretching of plastic under air pressure to force the bottle wall material against the internal walls of a mold to give a bottle its final shape. In the case of extrusion blow molding, the plastic is melted in the extruder barrel by mechanical working and electrical heating to achieve a viscosity that can be shaped into a cylinder (parison) that is then welded into a body with only one opening through which the compressed air is introduced to perform the final shaping. Why do holes appear in the final product? First, the amount of stretching can exceed the ability of the plastic to stay together and the bottle wall can thin down to nothing, causing a tear. Second, impurities (dirt) can be introduced into the plastic through the recycling of scrap that is blended with virgin resin and it’s also possible that dirt particles can find their way into the resin storage boxes (gaylords) or silos. In this case, the molten plastic does not adhere to the dirt particle and it causes a weak point in the bottle wall. Third, blow-molded bottles can contain several welded areas, such as at the base of the bottle, on the sides below the neck to make a taper, and around the inside of handles formed by welding and cutting out the center of the handle so the user’s hand can pass through (milk gallons).
WHY ARE LEAKY BOTTLES A PROBLEM?
One might think that losing a few bottles because of leaks would not be a major cost to a blow molder. That is right, when the leaker is discovered, rejected, and recycled into a new bottle by re-melting the plastic. However, after a bottle has been labeled, filled with a liquid and capped, then packed into a box that is stored and transported stacked up eight feet high, one leaker can cause a much larger loss in a damaged product that has to be scrapped. Another example is in the home and office water delivery business. When a bottle is upside-down on a dispenser, it is the vacuum in the bottle that keeps the water from running out onto the floor and requiring the repair or replacement of the floor. A small leak in the base of the 5-gallon bottle will allow this damage to occur, which the water supplier normally has to pay to repair.
WHAT MAKES BOTTLE LEAK DETECTION DIFFICULT?
A common type of leak detector operates on the pressure-drop principle, meaning that a known pressure of air is created in the bottle and the system is sealed off. If the drop in the pressure occurring over a set time exceeds a certain limit, it indicates that the bottle has a leak. Leak testing takes place immediately after blow molding or, in the case of purchased bottles, immediately before filling. In either case, the line speeds can be in the thousands of bottles per hour. This limits the time to test each bottle to a few seconds. Because leaks are small and the pressure that is used to test the bottles is low so that the bottles are not distorted, the detection of a pressure drop inside the bottle requires very sensitive instruments called transducers. The evolution of automation equipment and digital computers over the years has made leak detectors more accurate and also made them a source of quality control data that documents the performance of the blow molding operation.
WHAT TYPES OF LEAK DETECTORS ARE AVAILABLE?
Several different designs have been in the market. In one type, air at a known pressure is introduced into a tank of fixed volume and then this air is released into the bottle and the final pressure in the bottle is measured to see if it achieves the expected value. This method relies on all bottles being of the same size.
The most common type of leak detector is the pressure drop test as described above. The simplest have a single test head and a conveyor that indexes to bring each bottle under it. With the bottle stationary, an air cylinder moves the test head down onto the neck of the bottle. A soft rubber seal prevents leakage from the contact area between the test head and the neck of the bottle. A valve opens and allows air to fill the container to set pressure and then closes. The pressure in the bottle attains a level and settles down to a steady reading and then the test period begins. If the pressure stays above a set level, the bottle is good. Otherwise, the bottle is rejected from the conveyor.
In addition to detecting leaks, pressure-decay bottle inspection units can often be set up to test for the following:
- Wall thickness variation
- Choked / obstructed necks
- Defective neck finish
- Fallen bottles
Click Link for Leak Testing Videos: https://amslerequipment.com/en/leak-testers/
Screenshot of Leak Tester User Interface
Another test method is the use of a vacuum pump to create a negative pressure inside the bottle when compared to the atmospheric pressure in the room outside the bottle. This test can work better in some cases involving cracks, rather than holes in the wall of the bottle. In this case, if the low pressure inside the bottle rises by a given amount during a set time, due to the entrance of air from the outside into the bottle, the test shows the presence of a leak.
Leak testers can have a single head or multiple heads. These can be stationary or “flying”, meaning that the heads follow the bottles on the conveyor as they pass by without stopping the conveyor. The flying-head leak tester can test faster than a stationary-head tester and multiple-head testers are able to test faster than a single-head leak detector. Beyond the design of the leak detector, the speed that a leak detector can achieve is dependent on the bottle size (the smaller, the faster) and the size of the smallest hole that the operator wants to detect. The smaller the hole (.006 inches is a reasonable lower limit), the longer it takes for the test to occur and the fewer bottles can be tested per hour.
Pictured Above Manual Table Top Leak Tester