Polycarbonate sheet - the cost of custom production

Polycarbonate is extruded into sheet by large production lines. The more specialty lines typically extrude 3,000 lbs/hr, while commodity lines extrude 5,000-10,000 lbs/hr. For some of the large lines, if they take ten minutes to change dimensions, they could easily generate over 1,000 lb of off specification material, which will either need to be recycled or scraped.

Non-standard width

The maximum width of the sheet is governed by the width of the die installed on the line; most large lines have a die that can produce 96” wide sheet. The extrusion lines produce most efficiently when they are running at maximum throughput, which means running the maximum width of 96”. One option that sheet extruders have is to cut the sheet in half while the sheet is being produced, this process will give two sheets of 48” wide. The widths of 48” and 96” are some of the most common widths. Because they can be produced cheaply they have become industry standards and are nearly always in stock at the major producers and distributors.

If a customer needs a non standard width, this can be achieved by extruding 96” wide material through the die and then cutting down the edges by in line saws. Any off cut material can then either be recycled or sold as scrap. If a customer needs dimensions such as 95” wide or 47” wide, there will not be much scrap generated, even though the material would still need to be custom produced. If the customer needed a width such as 75”, proportionally more scrap would be generated and the cost to produce would go up. For widths of say 60”, the scrap ratio would be too high and the producer could stop the line and block off part of the die to limit the width of the sheet being produced. The stoppage would obviously lead to lost production and then the machine would be run at a lower, more inefficient rate because the die width would be lower. All of these factors add to the cost.

Non-standard length

The length of the sheet is more easily controlled. During production an in-line cross cut saw is used to cut across the sheet. The length can be set to almost any value (as long as it is not too small). It is therefore possible for a producer to make custom lengths without too much additional cost.

Non-standard thickness

For stocking purposes the major producers have standardized on a number of thicknesses – 0.060”, 0.118”, 0.177”, 0.236” are some examples. These sizes are normally carried in stock in both 48” x 96” and 72” x 96”. Adjusting to another thickness really only involves some minor changes to the die and chrome polishing rolls; these changes are quick and do not generate much off specification production. As a consequence, non-standard thicknesses are not difficult to produce, but manufacturers normally insist on a reasonable minimum order size. Also manufacturers will normally only produce non-standard thicknesses against an order, as they do not have a general need for the material. Because the material is custom produced, manufacturers often quote a long lead-time.

Non-standard color

For custom colors, introducing a new color to the line causes a lot of scrap changeover material between the production of the old color and the production of the new color. The lines are not shut down and cleaned between color changes, as that would be too inefficient. Manufacturers dislike frequently changing colors, and often plan large color runs to improve efficiency. Asking a manufacturer to stop the production of a clear material to produce a few hundred pounds of a red material is not likely to be received well, as the change-over produced from going from clear to red and then back to clear is likely to be many thousands of pounds. A manufacturer can not offer a competitive price if they produce ten or more pounds of scrap for every pound of good product.

Summary

Standard production sizes and colors have been established to improve efficiency and reduce cost. If a customer needs a non-standard product it is likely to be more expensive and require greater lead-time and larger minimum orders. Non-standard colors are the most costly, followed by non-standard width, followed by non-standard thickness, with non-standard lengths being reasonably cheap to produce. Considering these factors during product design can help in minimizing later costs and ensuring availability for the customer.

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.