Extruded plastic sheet quality is decided by three things long before anyone reads a datasheet: porosity, thickness consistency and flatness. They are related, they are frequently confused with one another, and each has a different process cause and a different way of specifying against it.
Top Plastic Co., Ltd. extrudes, relieves and machines stock shapes in Dongguan, and most of the sheet disputes we are asked to arbitrate come down to a buyer and a supplier using the same word for two different measurements. This article separates them.
How extrusion produces the sheet you receive
Polymer granules are dried, melted in a screw, pushed through a flat die and then drawn through polishing rolls before cooling. Those rolls set the thickness and the surface finish, while the cooling that follows sets the internal stress that the sheet carries to your machine.
The thermal gradient through the thickness is the part that matters later. The skin freezes before the core, so the core shrinks against a skin that has already set, and that mismatch leaves residual stress locked into the material. It is normal, it is measurable, and it is the reason a sheet that arrives flat can bow once it is cut. We list the grades we run and their extrusion behaviour on our materials pages.
Porosity: where bubbles come from and what they mean for machining
Bubbles usually come from moisture, from volatiles released by overheated polymer, or from air that was not vented before the die. Nylon and polycarbonate are the most moisture-sensitive of the common grades, and a drying fault typically shows up as a cloud of fine voids rather than as one large cavity.
For most machined parts, scattered fine porosity is harmless and will not change strength in any measurable way. It becomes a problem when a void lands on a sealing face, when a vacuum or pressure boundary is involved, or when a part is polished and the voids open into visible pits. If your application cannot tolerate any of those three, raise it at the enquiry stage, because the remedy is process control at the extruder rather than sorting after the fact.
Thickness tolerance: measuring and specifying it
Thickness is measured with a micrometer, not a caliper, and it is measured at agreed points. Left edge, centre and right edge, at both ends, is a reasonable default for a rectangular sheet. A single reading taken in the middle is not a measurement, it is a sample.
As a rule of thumb, extruded engineering sheet holds a nominal thickness within plus or minus two to five per cent, with the wider figure applying to heavier gauges. Below roughly 10 mm the achievable band is often tighter, while above 40 mm an asymmetric range is common, because the process does not cool symmetrically at that thickness.
State three things on the purchase order or the drawing: the instrument, the measuring points and whether the tolerance applies to the sheet as delivered or to the machined part. Leaving any one of them out turns a clean acceptance criterion into a negotiation after delivery, which is the most expensive place to hold that conversation.
Flatness and the difference from thickness variation
Flatness is deviation from a reference plane, and it is a separate measurement from thickness. A sheet can hold thickness perfectly and still bow, twist or dish. Flatness is normally checked on a surface plate, or with a straight edge and feeler gauge over a fixed span, and it should always be quoted across a stated length rather than as a bare figure.
PEEK sheet is a good illustration of why the distinction matters. A machined PEEK part can hold every dimension perfectly and still change shape in service if the sheet beneath it was never relieved, which is why we relieve stock before machining, and machined parts again afterwards, wherever the duty is dimensionally tight.
Extruded versus compression moulded sheet
Both routes produce usable stock, and the choice is usually made by thickness and by how much stress the part can tolerate. Extrusion gives long lengths, consistent thickness and a lower unit cost. Compression moulding gives lower internal stress and behaves better at very heavy gauges, where extrusion becomes difficult to control.
| Process | Typical thickness range | Porosity | Internal stress | Best suited to |
|---|---|---|---|---|
| Extruded | 1 mm to about 60 mm | Low, occasional fine voids | Higher, benefits from relieving | Long lengths, thin to medium sheet |
| Compression moulded | 10 mm to 100 mm and above | Very low | Lower | Thick blocks, stress-critical parts |
| Ram extruded | 10 mm to 150 mm | Low to moderate | متوسطة | UHMW-PE and high viscosity grades |
| Cast | 10 mm to 200 mm | منخفضة | Low to moderate | Cast nylon, large bushings and gears |
UHMW-PE sits at the edge of that logic. Its melt viscosity is so high that conventional extrusion is impractical, so the material is produced by ram extrusion or compression moulding instead. Whichever route is used, the delivered sheet is the starting point and not the finished part, which is worth remembering whenever extruded plastic sheet quality is judged on the pallet rather than on the machine. We supply both routes as cut-to-size stock shapes.
Questions buyers ask about extruded sheet quality
How much porosity is acceptable?
It depends entirely on the duty, so the honest answer is a question back. For a machined wear pad working in compression, fine dispersed porosity is of no consequence. For a sealing face or a vacuum boundary, the usual expectation is none visible on the finished surface, which has to be written as no voids on a machined face rather than as a percentage of the sheet volume. That single sentence about extruded plastic sheet quality is worth more than a page of general notes, because it is the one criterion an inspector can check without argument.
Can I measure flatness in my own warehouse?
Yes, with a granite plate and a dial gauge, or a straight edge and feeler gauges over the span your drawing names. Record the temperature as well, because a sheet measured next to a loading door in winter will read differently from one measured in a controlled room.
Why does a flat sheet bow after cutting?
Cutting removes material from one face and releases the stress balance that held the sheet straight. The remedy is a stress relief cycle before machining, and often a second one after roughing, so that the movement happens before the final finishing cut rather than after it.
Does a tighter thickness tolerance cost more?
Usually yes, because it is achieved through slower line speeds and more rejects, not through better intentions. Send the tolerance you genuinely need with the drawing and we will tell you what it adds to the price and the lead time, rather than quoting a figure the process cannot hold. Our contact page is the fastest route to that answer.
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