Specification and print guidance that applies directly to the materials and components that move through an automotive supply chain.
A working guide to the six numbers that decide whether a woven sack survives the trip from your filling line to your customer's store. Written for buyers who have to sign off the specification.
How to get printed woven sacks that still look right after a year in a warehouse. Colour choice, artwork preparation, and the storage habits that quietly ruin a good print job.
A finished car contains very little woven polypropylene. The materials that go into making it are a different story, and a surprising number of them travel as 20 or 25 kg bags rather than in bulk tankers.
Follow the chain backwards from the assembly line. Tyres need carbon black, silica, sulphur and a list of accelerators. Interior trim and bumpers are moulded from polypropylene compounds filled with talc, calcium carbonate or mica. Under the bonnet, reinforced plastics use chopped glass fibre. Every one of those arrives at the compounder or the moulder as a bagged powder, granule or pellet, because they are consumed in batches that are too small and too varied to justify a silo.
That is the market we serve with this range of sacks. It is not glamorous work, but the requirements are unusually strict, because a contaminated batch in an automotive compound is expensive in a way that a contaminated batch in a fertilizer blend is not.
Carbon black is the reason automotive customers often end up with a more expensive sack than they expected.
It is an extremely fine powder with a particle size measured in tens of nanometres. It is also black, and it stains permanently. Two consequences follow directly.
First, it will escape through any opening. A stitched closure, which is perfectly adequate for rice or fertilizer, leaves a line of needle holes through the fabric. Fill a sack with carbon black and those holes become a slow leak. The material does not pour out; it migrates, and by the end of a lorry journey the inside of the trailer is grey and the outer bag is filthy.
Second, any dust that escapes goes everywhere. Customers have told us about consignments arriving with the outer layer stained so badly that the receiving warehouse rejected the pallet on sight, even though the product inside was perfectly good.
What we specify for carbon black: a coated fabric to close the weave, a heat sealed or valved closure rather than a stitched one, and fabric in a dark colour where the customer will accept it, because a dark sack hides the staining that a white one advertises.
Sulphur, accelerators, activators, antioxidants and zinc oxide all move in sacks, usually at 25 kg. Several of them are supplied as pastilles or granules rather than powders, which makes the sack easier to specify.
Two things matter here. The first is that many of these materials are hygroscopic to some degree, so a coated fabric is usually the right call. The second is that several of them carry regulatory marking requirements, and the panel those markings sit on has to be legible and durable for the shelf life of the product.
Precipitated silica is the modern addition to that list. It is a very fine white powder, increasingly used in low-rolling-resistance tyres, and it behaves much like carbon black in a bag: it finds every gap and it does not come out of fabric once it is in. Coated fabric and a sealed closure, every time.
Resin is the easiest of the automotive materials to bag, and it is worth saying so because customers often over-specify it out of caution. Pellets are typically three millimetres across. They will not sift through a 80 gsm woven fabric, they are not moisture sensitive in the short term, and they do not need a coating.
A standard uncoated sack with a stitched closure is the right answer for pellets, and adding a coating to it is money spent on nothing.
Mineral fillers are the opposite. Talc, calcium carbonate and mica are all fine powders, and all three are consumed in large volumes by compounders making filled polypropylene for interior parts. They dust, they do not like damp, and they need a coated bag with a proper closure.
Chopped glass fibre deserves its own line. It is a skin irritant, so the closure has to contain it properly and the bag has to be tough enough not to abrade through on the corners. We specify coated fabric and a heat sealed top as standard, and we would push back on anything lighter.
The automotive sector does not stop at the factory gate. Independent workshops and parts distributors consume a steady stream of bagged material: blast media, sand, absorbent granules, refractory products for foundry work, and any number of consumables that arrive in 25 kg sacks and are decanted into smaller containers on site.
Requirements here are looser than at the compounder, which usually means the classic construction is the right answer and the buying decision comes down to price and print. That is fine, and it is a large part of what we make.
A starting point for the common automotive-chain materials. Treat it as a conversation opener rather than a fixed price list, because the handling between the filling head and the customer matters as much as the product does.
| Material | Starting specification |
|---|---|
| Carbon black | 25 kg, 100 gsm, hot-melt coated, valved or heat sealed |
| Rubber chemicals and accelerators | 25 kg, 90 gsm, coated, heat sealed |
| Precipitated silica | 20 kg, 100 gsm, coated, heat sealed |
| Plastic resin pellets | 25 kg, 80 gsm, uncoated, stitched |
| Talc, calcium carbonate, mica | 25 kg, 100 gsm, coated, stitched |
| Chopped glass fibre | 20 kg, 100 gsm, coated, heat sealed |
| Workshop blast media | 25 kg, 90 gsm, uncoated, stitched |
Automotive buyers tend to specify a sack and then hold a retained sample against every future order. It means the question "is this the same as last time?" has an answer that does not depend on anybody's memory. It is a small administrative habit that saves an enormous amount of arguing, and we are happy to retain a sample from every run we do for you, whether your industry requires it or not.
If you are buying sacks for an automotive material and you are not certain that your current specification is right, send us one of your existing bags and tell us what goes into it. We will measure it and tell you plainly whether it is over-specified, under-specified, or about right.
Two reasons. It is extraordinarily fine, so it escapes through anything with a gap in it, and it stains permanently. A sack that weeps carbon black ruins the inside of a lorry and the clothes of everybody who unloads it. That is why we default to a coated fabric and a closed closure rather than a stitched one for that material, even though a stitched sack is cheaper.
Technical, usually. Automotive supply chains run on material specifications and batch traceability, so tell us early which standard your buyer requires. We would rather tell you at the enquiry stage whether we can satisfy it than have you discover a gap when the goods reach a port.
Yes, and for automotive materials we would encourage it. A printed batch or a print position reserved for a stencil or label is far more reliable than marking a sack with a pen after it has been filled. Tell us what needs to appear and where it will be read.
Woven polypropylene handles tropical ambient temperatures without difficulty. What it does not enjoy is a closed steel container parked in direct sun for weeks, where internal temperatures climb much higher than the outside air. If your consignment is going to sit like that, say so, because it changes our recommendation on coating and on pallet configuration.
Tell us the material, the fill weight and how it is handled. We will suggest a fabric, a coating and a closure, and explain why each one is there.
