Steel, polypropylene, polyester and high-strength polyester are all excellent strapping materials. That poses a problem for those trying to select the best strapping material for their application. Each strapping product excels in certain characteristics, and each works well where those characteristics satisfy a packaging need.
So the question becomes, "Which strapping is appropriate for your customer's application?"
To being to answer that, we must do two things. We must take a look - by testing - at a few characteristics of strapping. And we must relate those characteristics to various types of packages and loads.
Material Cost: How much should I spend?
- Polypropylene strapping is the lowest cost strapping material, followed by polyester strapping, then high-strength polyester strapping, then steel.
- In choosing the strapping product for your application, the material cost should be a decisive factor only when two or more materials can protect the package or unit-load about equally well.
- For some packages, one strapping material protects substantially better than others. In these situations, its performance benefits most certainly outweigh its cost.
- Strapping is purchased for its reliability and product protection; the strength of the strap becomes a by-product of the strapping characteristic that contributes to the protection required.
- Ultimate strength of a strap is measured and compared by the strapping break strength and the tensile strength.
- Break strength is just that: the tensional force, in pound, required to break a strap. Tensile strength is harder to envision because it is a derived number, expressed in pounds-per-square inch. Tensile strength is equal to break strength divided by cross-section area.
- Why is tensile strength important? It provides the basis for comparing the strength of strapping materials themselves, without reference to the dimensions of the specific samples.
Here is an example of the comparison between the Break Strength and Tensile Strength on 1/2" x .020 heavy duty steel strapping:
||Break Strength (lbs.)
||Tensile Strength (lbs./sq. in.)
|Heavy Duty Steel Strapping
Thoughts for Consideration:
- Steel strapping has almost twice the tensile strength of the strongest plastic strapping.
- The break strength concept provides more useful numbers than tensile strength. Not only are they smaller numbers, but they quantify a familiar measure: pounds.
- When a strap fails, inadequate strength is rarely to blame. More often, failure results from poor application or handling practices.
- If the packages are heavy enough to require a strapping of great strength, and strong enough to withstand the application of strapping at high tensions (and not subject to shrinking or settling), steel is often the choice.
- If they are not that heavy and strong, one of the plastics may be called for. Given the relatively small strength differences among them, the plastic selected should be chosen on criteria other than strength.
Working Range of Strapping Materials:
In the example above, steel strapping has a break strength of about 1475 pounds. But it is applied at tensions whose typical upper level is only 700 pounds.
Polyester and polypropylene strapping of the same dimension may have break strengths of 600 pounds or more. But they are applied at tensions whose typical upper levels are only 300 to 200 pounds respectively.
Strapping equipment limitations help explain the vast difference between each material's break strength and its applied tension levels.
Working range is a range applied tension whose minimum may be virtually zero on a fragile package and whose maximum is the highest tension level at which strapping is applied in actual situations.
**It is within the working range that strapping does nearly all its work securing a package.**
- One reason the working ranges of plastic strapping are lower than that of steel strapping is because the breaking strengths of plastic are lower.
- Plastic strapping is so much softer than steel. At high tension, it is more likely to be milled by the feedwheel or cut by the grippers of a strapping tool or machine.
- All plastic strapping has a much lower working range than steel strapping of a similar size.
- The working range determines how tightly a strapping material can be applied.