Although both diamond netting and square netting are extruded, the processes used to produce the products result in quite different product characteristics, which may be important in some applications. Diamond netting is produced with the strands in each direction overlaying each other. The joint structure of the product can be described as a strand over another strand. The overall thickness of the product will essentially be twice the thickness of each strand, although there may be some melding of the strands at the joint. Normally, the strands in each direction are produced uniformly, and the product looks the same from one side of the material as the other. This type of netting is marketed as “bi-planar”, as both sides are structurally the same, the strands forming a plane or channel along the net.
The joint structure on square netting is quite different. The two strands that form the joint, the “machine direction” strand running the length of the roll and the “cross direction” strand running the width of the roll, are not stacked one over the other as in diamond netting. Generally, the machine direction strand is thicker than the cross direction strand. The overall thickness of the netting is mostly comprised of the machine direction strand thickness.
In diamond netting, the overall thickness of the netting is the summation of the thickness of the two overlaying strands. For example, two strands each .015″ thick overlaying each other results in a diamond net that is .030″ thick overall. Whereas, a .030″ thick square netting will likely have a .030″ machine direction strand and a smaller cross direction strand, perhaps around .024″ thick, for example.
In looking at a cross section of a diamond net joint, the strands are joined in the middle of the joint, whereas with square net the strands are joined on the bottom side of the joint. As a result of this joint configuration, square netting usually has one side of the material that is smooth, that is, both the machine and cross strands are flush on the same plane, and one side that is “bumpy”, where the machine direction strand has a higher profile than the cross strands.
Hole Shape and Size
Beyond the obvious fact that the diamond net process produces netting with diamond-shaped holes and the square net process produces net with square holes, there are wide varieties that are available with both processes.
As mentioned earlier, most diamond netting is produced with the strands at 90-degree angles. However, the process allows for more acute angles as well. Products with machine direction angles at 60 degrees are also common. Further, a single strand can be held constant, running in the machine direction, while the second strand is produced angular over the constant strand. This produces netting with a more rhombus-like shape. These are process variations that can be done irrespective of the tooling used to produce the product.
Likewise, the square net process can be manipulated to produce various hole shapes. The cross direction strand can be altered to make a square or rectangular shaped hole. Square net can also be stretched in one or two directions to make even more exaggerated rectangular hole shapes.
Strand Shape and Number
The joint structure, as discussed previously, is determined by the process itself; that is, whether it is a diamond net or square net process. The hole size and shape are variables that can be controlled within the process. And, although the strand count can be manipulated by the strand angle and the line speed, it is primarily determined by the tool or die that is used in the process. Obviously, there are many different tools used to produce the wide variety of products that are available.
The tool contains a fixed number of slots through which the molten resin flows, producing the strands. A net that has 8 strands per inch in each direction, for example, would be produced from a tool with the proper number of fixed slots in it. If one wanted a net that has 9 strands per inch, it would require a different tool, if all other process parameters were held constant.
The strand shape is also determined by the tooling. Most slots are square, rectangular or trapezoidal in shape, although there are tools with round holes rather than slots. Even though a slot may be square, the strands themselves are not square, because the molten plastic strands become more rounded as it exits the tool and ultimately cooled in a quench tank. Even though there is not a direct relationship between the shape of the slot and the shape of the strand, a tall and narrow slot will produce a strand that is taller and narrower than a strand produced by a square or round slot.
When the netting is oriented or stretched, the strand shape is altered greatly. In this process, there is less control over the ultimate strand shape, as the material is being heated and stretched. Generally, the shape of an oriented strand is somewhat hour-glassed, with the center of the strand necking down and the strands becoming wider as it approaches the joint. Oriented netting also has a higher profile joint, where the joint thickness is greater than the strand thickness.
All of these product characteristics are important in understanding the role of plastic netting in the filtration industry.