Which Envelopes Work Best With a Franking Machine?

Most mailrooms spend a long time choosing the right cartridge and almost no time choosing the envelope. That is a mistake. A franking machine can only print as well as the surface underneath it, and the wrong envelope stock will turn a perfectly good cartridge into a patchy, smudged impression that Royal Mail may not accept.

The short answer

The best envelopes for a franking machine are plain, uncoated, wove-finish envelopes in white or a pale shade, with a smooth flat top right corner and no flap seam or window in the print area. Franking ink is a liquid inkjet ink, so it needs a slightly absorbent paper surface to soak into and bind. Coated, glossy, laminated or heavily textured stock leaves the ink sitting on the surface, where it smears the moment the envelope is handled. If your packaging is dark, shiny or not paper at all, frank onto a label instead.

Why the envelope matters as much as the ink

A franking head fires tiny droplets of ink at an envelope travelling past it at speed. The droplets have to land in the right shape, then fix to the paper before anything touches them. Envelope choice affects both.

Paper that is too smooth and sealed lets droplets spread sideways before they dry, which thickens the lines of the 2D barcode and softens the edges of the text. Paper that is too rough or fibrous wicks the ink along the grain and creates feathering. Somewhere in between sits ordinary uncoated wove envelope paper, which is what franking machines were designed around and what our range of franking envelopes is built from.

If your prints have already started looking thin or striped, envelope stock is worth checking before you replace anything. Our guide to faint and streaky franking impressions walks through the other usual causes.

Paper finish: uncoated beats coated every time

This is the single biggest factor. Avoid anything described as gloss, silk, satin, laminated, varnished or wipe-clean. These finishes are designed to stop liquid penetrating the surface, which is exactly the opposite of what you need.

Also be wary of recycled envelopes with a very high fibre content and a visibly rough face. They are fine for the address, but the franking area can feather noticeably. If sustainability matters, look for a recycled stock with a smooth wove finish rather than a rustic one.

Envelopes sold specifically for metered mail take the guesswork out of it. Our approved meter posting envelopes use a paper face chosen for ink absorption and a construction that feeds cleanly.

Envelope colour and barcode readability

Royal Mail's requirement is simple in principle: the franking impression must be clear, complete and legible, printed in the top right-hand corner. In practice, colour is what usually breaks that.

Mailmark impressions carry a 2D barcode printed in approved blue ink, and scanners read that barcode by contrast between the ink and the background. A white or near-white envelope gives maximum contrast. A mid-blue, navy, dark grey or strongly saturated envelope can reduce that contrast to the point where the barcode will not read reliably, which risks the item being handled as unpaid or surcharged. Pale cream, pale grey and manila usually scan without trouble, but the darker the stock, the more risk you take on.

If you want the full picture on how the barcode works and what data it carries, see our plain English guide to Mailmark, and Royal Mail's own official franking pages.

Watch the top right corner

The franking impression always goes in the top right corner, so that corner needs to be flat and uninterrupted. Three things commonly get in the way.

Flap seams and glue lines

On some envelope constructions the side seam or the underside of the flap runs into the top right area, creating a raised ridge. The print head skips over the ridge and you get a broken line straight through the impression. Wallet envelopes with the flap on the long edge are generally safer than pocket envelopes with a deep side seam.

Windows

A window film in or near the print area is a guaranteed smudge. Franking ink will not dry on the film at all. Standard DL and C5 window envelopes place the window well clear of the top right corner, but non-standard layouts, double-window designs and inserting machine envelopes should be checked before you commit to a pallet. If you run an inserter, our folding and inserting machine envelopes are cut for that workflow.

Contents shifting

A thick, loose insert that bunches up under the top right corner lifts the paper away from the print head. Well filled envelopes with an evenly distributed insert frank far more consistently than half empty ones.

Weight, thickness and staying in the right format

Envelope choice affects your postage bill as well as your print quality. A Royal Mail Letter must be no larger than 240mm x 165mm x 5mm and weigh no more than 100g. Go over any one of those and the item becomes a Large Letter, which costs more.

The envelope itself eats into that budget. A heavyweight 120gsm C5 envelope can be enough to tip a borderline mailing over the 100g line where a 90gsm one would not. For a large run, that is real money. For franking specifically, 90gsm to 110gsm is a good window: light enough to protect your format, stiff enough to feed straight without buckling.

Royal Mail's guidance on preparing and posting franked mail covers how items should be separated and presented once they are franked.

Self seal or gummed: does it matter?

For print quality, no. Both feed and frank identically. The difference is workflow and shelf life.

Self seal envelopes are faster for hand-packed runs and avoid the moisture that comes with wetting a gummed flap. Gummed envelopes store better over long periods and are usually cheaper per thousand. One practical point: if you seal by hand with a damp sponge, let the flap dry before franking. Damp paper absorbs ink unevenly and the impression will bleed.

When to frank onto a label instead

Some items should never be franked directly. Poly mailers, jiffy bags, dark or strongly coloured envelopes, glossy presentation packaging and anything with an uneven surface all belong in the label category. Franking labels give you a clean white surface with the correct absorbency, which means the impression reads first time regardless of what it is stuck to.

Our article on when to use franking labels covers placement and the common mistakes that lead to rejected items.

A quick pre-run checklist

• Is the envelope uncoated with a smooth wove finish?
• Is the top right corner white or pale, flat, and free of seams, windows and print?
• Is the stock between roughly 90gsm and 110gsm?
• Will the filled envelope stay under 240mm x 165mm x 5mm and 100g if you need Letter rates?
• Have you run five test envelopes from the actual batch, not a sample from last year's stock?
• Are you leaving the impressions to dry before stacking or bundling them?

That last point catches a lot of people. Franking ink is dry to the touch quickly but not instantly. Stacking envelopes face down straight off the machine, or sliding them against each other, is the most common cause of a smeared barcode on an otherwise perfect run.

Frequently asked questions

Can you frank onto a brown envelope?

Yes, standard manila brown envelopes usually frank and scan without any problem because the contrast with blue ink is still strong. Darker kraft or heavily recycled brown stock is riskier, so run a test before committing to a large mailing.

Why does my franked impression smudge when I touch it?

The most common cause is a coated, glossy or laminated envelope surface that the ink cannot soak into. Switch to an uncoated wove envelope and allow the impressions a short time to dry before stacking or bundling them.

Do window envelopes cause problems with franking?

Only if the window sits in or near the top right corner where the impression is printed, because franking ink will not dry on the plastic film. Standard DL and C5 window envelopes keep the window well clear of the print area and are perfectly safe.

What weight of envelope is best for a franking machine?

Around 90gsm to 110gsm is the sweet spot for most machines. It is stiff enough to feed straight without buckling, but light enough that it does not push a borderline mailing over the 100g Letter limit.

Does the envelope I use change which ink cartridge I need?

No, the cartridge is determined solely by your franking machine model, not by the envelope. You can identify the correct one using our guide to choosing a franking ink cartridge.

Getting the envelope right costs nothing and removes one of the most frustrating causes of wasted postage. If you are choosing stock for a new run, browse our envelope range or check the correct cartridge for your machine in our franking machine supplies. Not sure what fits your setup? Our cartridge identification guide will narrow it down in a minute, or get in touch with our team and we will check it for you.