Pull-up is a secondary non-destructive test reference for closure application. It should be used as a reference guide. Removal torque and security are the main criteria for determining proper closure application.
A pull-up is the measurement of the mechanical relationship of the closure thread to the bottle thread. It is sometimes referred to as an “application angle.” It measures how far the closure has been applied to the bottle.
Silgan Closures technicians perform pull-up checks on plastic closures (as well as steel) to ensure that the closure is properly applied to the bottle within the specification range. Each closure will have a certain specification range that can vary depending on the bottle and the location of the pull-up indication mark. Always be sure to use the correct recommended range for the closure being applied.
There are several variables on how to properly measure based on the type of mark and where the mark is located on a closure. For instance, the mark can be a single knurl or a small rectangle. The mark on the bottle can be a variety of sizes, shapes and can be found at different parts of the closure. Sometimes, there is no indication mark and instead, the mold part line is used to measure.
How to perform a pull-up check on a plastic closure
Before you begin, you’ll need a closure size-specific pull-up gauge, like the one shown below:
The measurement is made from the pull-up indication mark on the closure to the indication mark on the bottle and is measured in 1/16” increments.
To measure, the zero point on the pull-up gauge should align with the indicator mark on the bottle. In the video example, the zero point on the bottle is the half moon shape found on the bottle transfer flange.
With the pull-up gauge aligned with the bottle, you read either counterclockwise (+) or clockwise (-) the number of 1/16ths to the mark on the closure, which is the closure pull-up indicator mark (in the video, it’s found at the vertical knurling).
The measurement shown in the video example is a negative 4 which means the mark on the closure is 4/16” in the clockwise direction from the pull-up mark on the finish and was properly applied within specification. In this example, the range is -2 to -9 for the type of bottle being used.
What the data means and why it matters
If you measure a pull-up that is outside the recommended range on the operating guidelines, it could mean a couple of things:
- If the readings are lower than the minimum measurement, it means that it is under applied. It could be an indication that the capper static torque setting is too low, there is an excessive amount of top load, or that the bottles are being allowed to rotate during the application of the closure to the bottle. This can potentially lead to loose closures, leaking product and tamper evidence issues.
- If the pull-up is in excess of the recommended range, then the closure has been overapplied. This could be an indication that too much application torque was used or that bottles are being allowed to rotate during the application of the closure to the bottle. This can potentially lead to leaking product or hard to open complaints from the customer.
- Measurements may vary depending on where the mark is located on both the closure and the bottle.
- Silgan Closures recommends that this and all quality checks be completed hourly and after extended periods of downtime.