Specifying engineering plastic sheet and rod is a paperwork exercise before it is anything else. Two buyers can order the same material at the same price, and one receives stock they can machine the same afternoon while the other receives sheet that is out of tolerance, scratched, or impossible to trace back to a resin batch. The difference is almost never the supplier’s honesty; it is the order text.
What follows is the set of fields we ask for when a customer sends an enquiry, and the reasons each one matters on the shop floor. None of it is exotic, but leaving any single field open means the supplier has to guess, and a guess is what turns up at your incoming inspection.
The five fields every plastic order needs
Every usable engineering plastic sheet and rod order answers five questions: grade code, form, thickness with tolerance, finish, and certification. A purchase order that says “PEEK sheet, 20 mm” is missing at least three of them.
Grade code is not the same as polymer family. “PEEK” tells you the polymer; “PEEK 450G” or “PEEK GF30” tells you the actual compound, its filler content and its processing history. Unfilled PEEK and 30% glass-filled PEEK are different materials with different machining behaviour, different wear rates and different prices. Form covers sheet, rod, tube and profile: the same grade exists in all of them, and the extrusion route differs between them, which changes internal stress and therefore flatness.
Thickness is meaningless without a tolerance. A nominal 20 mm sheet may be supplied at +0.2/-0.0 mm or at plus or minus 0.5 mm, and those two impose completely different machining allowances. Finish describes surface condition and colour: natural, black, sanded or annealed. Certification is the paper trail: batch number, resin origin, and where relevant a food-contact or electrical declaration. To see our standard range before writing any of this down, the products page lists what we hold in stock.
| Field | What to write | Why it matters |
|---|---|---|
| Grade code | PEEK 450G, POM-C, PA66 natural | Filler and process change wear and creep |
| Form | Sheet, rod or tube, with dimensions | Decides extrusion route and internal stress |
| Thickness tolerance | 20 mm +0.2/-0.0 mm | Sets the machining allowance you keep |
| Finish | Natural, sanded or annealed | Affects cleanability and post-machining movement |
| Certification | Batch-linked certificate | Makes later traceability a single email |
Form and the manufacturing route behind it
Form drives the manufacturing route, and the route drives the properties you will actually machine. Extruded sheet is pushed through a die and cooled under tension; compression-moulded sheet is pressed and cooled without that directional pull. Extruded rod is usually the most economical and most dimensionally consistent option for small diameters, while tube and hollow bar remove the need to bore out a solid billet.
The same polymer behaves differently depending on how it was formed. In our experience, extruded POM-C sheet machines cleanly in both directions, but a heavily drawn thin sheet can hold more residual stress along the extrusion direction, and that shows up later as movement after machining. If the part is thin and flat, ask for the manufacturing route, not just the grade. We keep a material library organised by family and form, which is the fastest way to see whether the grade you have in mind exists as the shape you need.
Thickness, flatness and what tolerances are realistic
Thickness tolerance and flatness are different specifications and should never be merged into one line. Thickness is a calliper reading; flatness is a deviation across the whole plate. A sheet can hold thickness within 0.1 mm and still bow by several millimetres across a metre.
Typical commercial thickness tolerances for extruded engineering sheet run from about plus or minus 0.1 mm on thin gauge to plus or minus 0.5 mm on heavy plate, with tighter figures available on request. Flatness is normally quoted per unit length rather than as a single number, because a long plate accumulates deviation. If your part is a seal face or a slide surface, specify flatness separately and say which face is the datum.
Thicker is not automatically safer. Buying 30 mm to machine down to 18 mm adds cost, machining time and a fresh set of internal stresses. Order close to final thickness where the grade allows it.
Finish, colour and edge condition
Finish and colour are cosmetic only when the part is invisible. On a food-contact or a cleanroom part, surface roughness affects cleanability and particle generation, so “natural, sanded” is a real engineering specification.
Colour matters for a different reason: a black grade is usually pigmented and often filled, so black POM and natural POM are not interchangeable in wear behaviour. Colour also lets you distinguish grades on the shelf, which matters once you hold several similar-looking polymers. Edge condition is the field most often forgotten. Do you want saw-cut edges, milled and squared edges, or fully finished edges ready to install? Saw-cut sheet arrives quickly and cheaply but leaves a rough edge that must be trimmed, while cut-to-size orders arrive closer to net shape and reduce your internal handling.
Certificates, traceability and lot identity
A certificate is only useful if it can be tied to the material in your crate. Ask for the lot number to appear on the delivery note, the certificate and the label. If a problem appears six months later, that number is the whole investigation.
Two documents are commonly confused. A material certificate states that the grade supplied conforms to a specification; a test report gives measured values for a specific batch. For most engineering work a batch-linked certificate is enough, but for regulated work you want measured data and a stated retention period. Grade naming is its own source of errors, particularly with filled compounds, and our guide to grade naming explains how the codes map to fillers and properties.
Questions buyers ask about specifying sheet and rod
Can I specify only the polymer and leave the grade to the supplier?
You can, but you are then accepting whatever compound the supplier stocks. For structural or wear parts this is a false economy, because a different filler content changes stiffness, friction and creep. Name the grade where the part carries load or slides.
How tight a thickness tolerance should I ask for?
Ask for the loosest tolerance your process can accept, then hold the line there. Tighter tolerance narrows the number of suppliers who can serve you and usually adds cost. Reserve tight calls for sealing faces and bearing fits.
Do I need certification if I am not in a regulated industry?
Not always, but batch identity is cheap insurance. Even in general engineering, being able to trace a batch shortens any later quality discussion from weeks of argument to a single email.
Who decides the final tolerance, me or the supplier?
You do, because you own the drawing. The supplier can tell you what is economical and what is achievable, but the acceptance criteria are your call. Put them in writing on the order, and talk to us before the first crate ships rather than after.
Need a material recommendation?
Send us the application and we will reply with two or three candidate grades and a price.
