Grammage is the mass of polypropylene in one square meter of sheet, expressed in grams. Together with gauge, it defines stiffness, strength and weight, and it is verified by weighing a sample of known area.
Why gauge alone lets a thinned-out sheet slip through
Gauge is the distance between the two faces of the sheet, and it can be measured with calipers in seconds. Grammage answers a different question: how much material is in that sheet. It is expressed in grams per square meter (g/m²) and equals the weight of a piece of sheet one meter on each side. The two measurements describe different things, and a sheet can meet one and fail the other, as the divider plant found out.
Density and geometry: how polypropylene is distributed in the twin wall
Polypropylene is one of the lightest plastics, with a density close to 0.9 grams per cubic centimeter. A solid plate one millimeter thick would weigh about 900 grams per square meter. Corrugated sheet distributes a similar amount of material, or less, across two thin faces separated by ribs: a four millimeter sheet at 650 grams per square meter carries less polypropylene than that solid plate and is much stiffer, because the distance between the faces works in its favor. Grammage tells you how much material went into that structure; gauge, how far apart the faces ended up.
Faces and ribs: where the difference in grammage shows
At the same gauge, a sheet with higher grammage has thicker faces and ribs. Thicker faces resist bending, impact and puncture better; thicker ribs carry more face-to-face load before they buckle. When grammage drops and gauge stays the same, the sheet keeps its measurement and loses exactly those properties. That explains why the cells in the opening opened up when the container was full: their walls had the same height and less material to hold their shape.
The same grammage in two different gauges: the risk of specifying a single number
If gauge alone leaves out the material, grammage alone leaves out the geometry. In our specification table, 600 grams per square meter is made in 2, 3, 3.5 and 4 millimeters, and 1,000 grams in 3, 3.5, 4 and 6 millimeters. With the same amount of material, the thicker gauge separates the faces further and gains bending stiffness, but it spreads the polypropylene into taller, slimmer ribs that give way sooner under a face-to-face load. That is why a useful specification always carries both numbers together.
Grammage ranges by gauge in our manufacturing table
Each gauge is made within a range of grammages. At Cassium, 2 mm sheet is produced between 350 and 600 grams per square meter; 3 mm, between 500 and 1,000; 3.5 mm, between 600 and 1,300; 4 mm, between 600 and 1,000; and 6 mm, between 950 and 1,300. Our standard combinations in 1.22 by 2.44 meter sheets are 2 mm at 400 g/m², 3 mm at 550, 4 mm at 600, 650 or 700, 5 mm at 1,000 and 6 mm at 1,300. Any other combination in the table is made to order, in the length, width and color you need.
Grammage, stiffness and weight: a balance you design
Within the range of a gauge, more grammage means more stiffness, more impact resistance and more tolerance to use cycles, in exchange for more weight per part and more cost per sheet, because resin is a significant share of the cost of the sheet. An internal divider that carries no load of its own can sit at the low end of the range; a tray that is stacked loaded or a returnable box that travels by truck calls for the high end. Raising grammage out of habit makes every part and every freight shipment more expensive, and lowering it to save money is paid for in damaged parts.
How a sheet gets thinned out without the calipers noticing
In extrusion, grammage depends on how much material the extruder delivers and on the speed at which the puller draws the web. With the same die and the same gauge, a faster line leaves thinner faces and ribs. That is how the same gauge is legitimately made in several grammages, and it is also how the price of a sheet is lowered without its measurement changing. That is why grammage must be written into the quote, the technical data sheet and the purchase order.
Variation within the same sheet: center versus edges
Grammage can also vary across the width of the sheet if the die distributes the flow unevenly. A sheet with more material in the center than at the edges yields parts that weigh and perform differently depending on the area of the sheet they came from. In an order of dividers cut from the same sheet, that difference shows up as cells that hold up well next to others that give way sooner. When you verify, take samples from several points across the width instead of settling for a single corner.
How to measure grammage at receiving without a lab
Measuring grammage takes a ruler, a utility knife and a scale. Accuracy depends on the size of the sample and the resolution of the scale, so it is best to choose the method based on the equipment you have on hand.
Sample method: a square of known area
Cut a 20 by 20 centimeter square, which equals 0.04 square meters, with straight, square cuts. Weigh it in grams and multiply the result by 25 to convert it to grams per square meter: a sample that weighs 24 grams corresponds to 600 g/m². Repeat with samples from the center and from both edges, and average them. With smaller squares, a cutting error of a couple of millimeters weighs more heavily on the result, and the scale needs a resolution of tenths of a gram.
Full sheet method: the shipping scale
If you do not have a precision scale, weigh a full sheet. A standard 1.22 by 2.44 meter sheet measures 2.98 square meters, so divide its weight in grams by 2.98. As a reference, a 4 mm sheet at 600 g/m² weighs about 1.8 kilos, and a 6 mm sheet at 1,300 g/m² comes to around 3.9 kilos. This method averages the entire sheet, center and edges included, and lets you compare two suppliers or two batches in minutes, just as the quality team in the opening did.
What to do with the result
Compare the result with the nominal grammage in your specification and with the tolerance you agreed on with your supplier. A value consistently below it, across several sheets and several pallets, points to a thinned-out sheet; a large difference between center and edges points to a distribution problem on the line. In either case, ask for the run record before you convert the batch. Polypropylene absorbs practically no water, so just make sure the flutes of the sample are dry before you weigh it.
How to specify grammage so your purchase order protects the part
A complete sheet specification includes gauge and grammage together, with their units, for example "4 mm, 650 g/m² corrugated polypropylene sheet", plus the size, the variant and the color. Add the acceptable grammage tolerance and the method you will use to verify it, so your supplier knows from the quote stage what its batch will be measured against. When you change suppliers or gauges, validate the new grammage with a sample in your process and under your real load before you commit volume.
When you do not yet know what grammage to order
If you are defining a new part, start from the application and let the grammage follow from it. Our Recommender proposes gauge, grammage and additives based on four questions about the application, the strength you need, the conditions of use and the features you are looking for. With that proposal, grammage stops being a guess and becomes the starting point for your test.
At Cassium we extrude sheet from 2 to 6 mm at our plant in Santa Catarina, Nuevo León, control grammage by batch and verify it by weighing samples of known size before releasing each run, with records under our ISO 9001:2015 quality management system. We send you a free sample with its technical data sheet so you can weigh it and test it on your line before your first order.
If a sheet with the right gauge started to flex, dent or open up cells on your line, check its grammage before anything else. At Cassium we make sheet from 2 to 6 mm with grammage controlled by batch, and we send you a free sample so you can weigh it and test it. Find your starting gauge and grammage with our Recommender at https://cassium.com.mx/en
