Monday, December 28, 2009

Nominal thickness and thickness specifications

With ever increasing demands to lower production costs, extruded Polycarbonate sheet manufacturers become ever more inventive in their ways of achieving these cost savings. These costs savings sometimes have no affect on the quality of the product, however, sometimes they do.

One of the well-known ways that manufacturers reduce cost is using regrind in the product instead of 100% prime virgin resin. Sometimes the level of regrind can be as high as 60 or 70%. Because the regrind level significantly affects the optical and mechanical properties of the sheet, at HighLine Polycarbonate we refuse to use any regrind in our product.

One of the less well known ways that manufacturers save costs and decrease product quality is by using thickness control. Many manufacturers do this in two ways:

The first way is by selling what has been termed a nominal thickness. If you ask for 1/8” thick Polycarbonate sheet, the thickness that you will be supplied is not 0.125” but rather 0.118”. The manufacturers refer to 0.118” sheet as nominal 1/8” sheet. In the same way, if you ask for ¼” sheet, you will be supplied 0.236” sheet. This nominal thickness means that the manufacturers use about 5.6% less material to make the sheet and as a result save a significant amount of money. Often this nominal thickness is not an issue for the customer, especially if they understand that they will be receiving 0.118” sheet instead of 0.125” sheet. However, if your application requires 0.125” sheet, make sure that you specify that the sheet must be 0.125” when you purchase it.

The second way that manufacturers use thickness control to save cost is by using a thickness specification. The argument is that it is impossible to produce exactly 1/8” thick sheet and that they need a thickness specification range. Some Polycarbonate sheet manufacturers claim a specification range of -5% to + 5% while some claim -10% to + 10%. With the 5% figure, this means that a nominal 1/8” sheet could be between 0.112” and 0.124”. Although this specification sounds reasonable on the surface, it should be understood that manufacturers have sheet thickness control developed to an art form. If the manufacturers claim to have a thickness specification of -5% to +5%, they actually target production in the range of -5% to -4%. Modern sheet extrusion lines can very accurately control sheet thickness and this can lead to huge savings for the sheet manufacturers. As a result of this thickness control, it is actually very rare to receive sheet thicker than 0.113” when ordering 1/8” sheet; these two methods of saving costs actually save sheet manufacturers around 10% on Polycarbonate resin costs. When they purchase tens of millions of pounds of resin a year, the savings are huge.

While all of these savings sound harmless, it should be remembered that Polycarbonate sheet is often used for protective screens, machine guards, and layers in bullet resistant laminates. Shaving material off the thickness of the sheet could, in some circumstances, compromise the application if the customer does not properly understand both nominal thickness and the thickness specification.

At HighLine Polycarbonate, we have taken two measures to prevent problems. Firstly, we make clear that we are supplying 0.118” sheet and that if the customer wants 1/8” sheet, we supply 0.125” sheet instead.

Secondly we have a thickness specification of -0% to +10% [Although in the interest of complete disclosure we typically run in the range of -0% to +1%]. Both of these measures mean that the minimum thickness that you will get if you order 1/8” sheet is 0.125” and not the 0.113” that some other manufacturers supply.

Our advice is to make sure that you understand what thickness sheet that you actually need for your application and then discuss both the nominal thickness and thickness specification with your supplier to ensure that you receive the sheet that you need.

1 comment:

  1. Great and very useful information. The trend in the industry to lower the gauges of the sheets has been going on for some time now. While in cast products this represents a very high risk of failure, extrusion is definitely a more controlled way to approach this issue.

    An important suggestion is to evaluate the final application before deciding to use a particular product and thickness and not to base the decision solely on the cost.

    Good job, keep it up!