Pet Carrier Padding: Foam Refill Options

Pet carrier production desk · Updated 2026-10-06 · 16 min read

A custom pet carrier brief for pet carrier accessory is quoted against three variables before the sample is cut: fabric weight, hardware grade and print method.

Why Padding Fails: Compression Set, Not Wear

The intuition about a flattened pad is that it has worn out. It has not. Nothing has been removed from it — the foam has the same mass it always had. What has happened is that the cell structure has stopped pushing back, and the correct name for that is compression set.

Compression set is measured by compressing a sample to a defined fraction of its thickness, holding it at an elevated temperature for a defined period, releasing it and measuring the thickness it does not recover. The standard method for flexible cellular materials is published by ASTM, and the result is reported as a percentage of the original thickness permanently lost. A pad that has taken 18% set has lost nearly a fifth of its height without losing a gram.

The mechanism is cell wall fatigue. A flexible foam is a network of polymer cell walls, and each compression cycle flexes them. Most recover elastically; a small fraction of the walls rupture or take a permanent bend each time. The fraction is small per cycle and it accumulates — which is why the loss is invisible month to month and obvious at the end of a year, and why it is worse on a pad that is compressed deeply and repeatedly than on one that is loaded lightly for the same total time.

Two variables dominate the rate. The first is density: a low-density foam has thinner cell walls and a higher proportion of open cells, and it sets faster. The second is deflection depth: a pad compressed to 25% of its thickness sets slowly, one compressed to 70% sets several times faster. That second point is the design argument for a thicker pad — a 40 mm pad compressed by 15 mm is deflected 37%, while a 20 mm pad compressed by the same 15 mm is deflected 75% and will be flat in half the time.

The practical consequence for a refill programme is that the replacement pad should not be identical to the original. It should be one density step higher, because the customer is replacing it precisely because the original density proved insufficient for how they used the carrier.

Foam Families and What Each Number Means

Three families cover carrier padding, and the numbers on their data sheets mean different things, which is the source of most specification errors.

Flexible polyether polyurethane is the default. It is open-cell, breathable, cheap, available in a wide density range and easy to cut and shape. Its weakness is hydrolysis: polyether is better than polyester-based polyurethane but it still degrades slowly in warm humid conditions, and it oxidises, which is why an old pad yellows and then crumbles at the edges. Density range in service is 20-50 kg/m³.

EVA (ethylene vinyl acetate) is a closed-cell foam supplied as sheet or as moulded parts. It does not absorb water at all, it has excellent recovery, and it is the right choice for a base board or a structural insert rather than for comfort padding, because closed cells make it firm and slightly springy rather than soft. Density 60-200 kg/m³, but the published density of a closed-cell foam is not comparable with that of an open-cell one — it includes the gas in the cells.

Viscoelastic, or memory, foam is an open-cell polyurethane modified for slow recovery and high energy absorption. It distributes pressure extremely well, which is genuinely valuable for an old or a thin-coated animal, and it has two drawbacks: it is temperature-sensitive, becoming noticeably firmer below about 15 °C, and it has poor airflow, so it runs warm. Used as a 10-15 mm topper over a conventional base rather than as the whole pad, it gives the benefit without the heat.

The numbers. Density in kg/m³ tells you how much polymer is in the pad and predicts durability. IFD, or indentation force deflection, measured at 25% deflection on a defined indenter, tells you how firm it feels; it is reported in newtons. Compression set tells you how long it lasts. Rebound resilience, reported as a percentage, tells you how lively it is. All four are needed; quoting density alone is the commonest error, because two foams of identical density can differ by a factor of two in IFD.

The service column is the one that matters commercially: it is measured to a 20% thickness loss under a standard cyclic load, and it is the number that decides whether a programme sells refills annually or every three years.

Density, IFD and Thickness: Choosing the Combination

Choosing a pad is a three-variable problem with a constraint, and the constraint is the animal.

By mass. Up to 5 kg, a 15 mm pad at 30 kg/m³ and 25-30 IFD is correct — soft enough to be comfortable, dense enough to last. From 5 to 8 kg, 20 mm at 30-35 kg/m³ and 30-35 IFD. From 8 to 15 kg, 25-30 mm at 35-40 kg/m³ and 35-45 IFD, because a heavier animal needs more deflection depth before it bottoms out on the shell. Above 15 kg, 40 mm at 40-50 kg/m³ with a firmer IFD, or a two-layer construction.

Bottoming out is the failure to design against. If the pad compresses fully under the animal, the load path goes straight to the shell and the animal is effectively lying on a board. The check is simple: the pad should retain at least 40% of its thickness under the static load of the animal. A 20 mm pad under a 12 kg animal at 30 kg/m³ will retain close to that; the same pad under 18 kg will not.

Firmness perception is not linear with IFD and it is worth knowing why. A thin firm pad and a thick soft pad can feel similar at first touch and behave completely differently under load, because the thick pad has more travel before it stiffens. Where a pad is described as firm in a product brief, the specification should be written as an IFD plus a thickness rather than as an adjective.

Two-layer construction is the best answer for heavier platforms and for old animals: 10-15 mm of viscoelastic over 20-25 mm of standard polyether. The top layer distributes pressure, the base layer provides travel and durability, and the combination keeps airflow better than a solid viscoelastic pad because the base layer is open-cell and breathable.

Cost. Foam is priced by volume and by density, so a 20 mm pad at 30 kg/m³ costs roughly 1.6 times what a 15 mm pad at 25 kg/m³ does, and the step to 40 kg/m³ adds another 35%. The service-life column is what justifies it: the denser pad lasts nearly twice as long, so the annualised cost is lower even though the unit cost is higher.

Cover Construction and Access Design

A refillable pad is a system of foam plus cover, and the cover usually decides whether the system works. Three things have to be right: the fabric, the closure, and the fit.

Fabric. The cover takes the abrasion, the moisture and the washing, so it is specified independently of the foam. A 200-300 denier polyester with a light PU or TPU back coating is the mainstream choice: it wipes clean, it resists claw damage better than an uncoated weave, and it does not wick. A knitted face is more comfortable and less cleanable. Where the pad is used without a separate liner, the cover should additionally be specified for fluid resistance, because the alternative is a foam core that is ruined by one accident.

Closure. A zipper on three sides is the correct design for a refillable cover: it lets the pad be inserted flat without being folded, and folding a foam pad to get it into a cover is how corners are damaged. The zipper should be coil #3 or #5 with a pin-lock slider and a garage — a small fabric flap at the top that stops the slider touching the animal. A cover with a zipper on one side only is cheaper and is the usual reason a refill is returned as "does not fit".

Fit tolerance. The cover is cut 3-5 mm larger than the foam in each dimension. Too tight and the foam is compressed by the cover itself, which adds pre-load and accelerates set; too loose and the foam moves inside and bunches at one end. For a refill pad going into an existing shell, the tolerance stack has to account for the shell pocket as well — a pocket that has stretched with use needs a slightly larger pad to sit flat, and this is why refill pads are offered in a small size range rather than as one size.

Venting. A sealed cover with a coated face traps air and the pad feels dead when it is compressed. Two or three small vented panels, or a cover face in an air-permeable knit with a coated underside, lets the air out and the pad responds properly. This is a detail almost nobody specifies and it is the difference between a pad that feels expensive and one that feels like a board.

Refill Options: Full Pad, Topper and Inserts

Three refill formats exist and they solve different problems. Offering all three is usually unnecessary; choosing the right two is what makes a programme work.

Full replacement pad is the default and the right answer when the original has taken a set. It restores the carrier to its as-new height, and because it is a complete component it can also correct a specification mistake — a programme that launched with 25 kg/m³ and saw complaints can ship a 35 kg/m³ refill and end them. The cost is the highest of the three and the pack size is the largest, which matters for postal distribution.

Topper is a 10-15 mm layer that sits over the existing pad. It is the right answer when the base is sound but comfort is the complaint, and it is the cheapest item to ship because it rolls. A viscoelastic topper is the classic use: it converts a firm pad into a pressure-distributing one without changing the base. Toppers are also the easiest item to sell as an upgrade rather than as a repair, which changes the whole commercial framing.

Shaped insert is a cut piece that addresses a specific geometry — a corner bolster, a head rest, a base wedge that corrects a slope. Inserts are the highest-margin format and the most design-dependent, because each is cut to a platform template. They are also the format most likely to be copied, so they are usually tied to a specific platform rather than offered generically.

Loose fill is worth mentioning to rule out. Shredded foam or fibre fill is cheap and it migrates: within a few weeks it collects at the lowest point and the pad is thick in one place and empty in another. A refillable system should always use a cut pad, never loose fill, unless the cover is baffled like a duvet — which costs more than the pad it replaces.

Hygiene: Moisture, Washability and Odour

A pad sits in contact with an animal for hours at a time and it will eventually get wet. Hygiene is therefore a design requirement, not a nice-to-have, and it drives three decisions.

Moisture path. An open-cell polyurethane foam holds water; a 20 mm pad can absorb several times its own mass and will stay damp for a day or more in the middle. The correct construction is a coated cover over an open-cell core with a vented face, so liquid does not reach the core, plus a core that can be removed and dried if it does. Where a carrier is marketed for an animal that is not reliably clean, the core should be closed-cell EVA or a spacer knit instead, because neither absorbs.

Washing. Covers are specified for machine washing at 30 °C with 20 cycles to no visible change in coating adhesion or seam integrity. The foam core is not machine washable and it should not be — agitation tears open-cell foam. The correct instruction is cover washing plus core air-drying, and it should be printed on the label, because a cover that goes in the machine with the foam still inside comes out with a torn core.

Odour. New foam has a characteristic smell from residual blowing agent and catalyst, and it dissipates over roughly 7-14 days in open air. Two controls apply: foam is specified to a low-emission grade, and pads are aired before packing rather than bagged straight off the cutting machine. A pad bagged wet from cutting is the usual origin of a "chemical smell" complaint, and the fix is a 48-hour airing stage rather than a different foam.

Antimicrobial treatment is offered as an add-on and it is worth being precise about what it does. A durable antimicrobial finish on the cover reduces odour-causing bacterial growth on the fabric surface; it does nothing for the core and it does not make an uncleanable pad cleanable. It is worth specifying on premium platforms and it needs its own registration and documentation in most markets.

Emissions, Certification and Market Compliance

Foam is a chemically complex material and the compliance burden is real, but it is manageable if it is handled at specification rather than at shipment.

Emissions. Flexible polyurethane foam can emit volatile organic compounds, principally residual isocyanate-reactive species, catalysts and blowing agents. Low-emission foam grades are certified under voluntary schemes that set limits and require chamber testing, and specifying one of them removes the need for ad-hoc testing on every lot. The certificate is held at the foam supplier and is passed down the chain with the lot number.

Restricted substances. The general screen is REACH Annex XVII for the EU market, administered by ECHA, and the textile-specific OEKO-TEX Standard 100 certificate under OEKO-TEX is the most efficient single document because it covers the cover fabric, the thread and the foam in one assessment. Flame retardants are the ingredient most likely to appear in a foam and most likely to be restricted, and they should be explicitly declared as absent unless a market requires them.

California. A Proposition 65 assessment is made at the component level, with the reference being OEHHA. In practice a certified low-emission foam with a declaration file satisfies it without a warning label, and the assessment costs nothing if it is done before production rather than after a retailer asks.

Documentation per lot. Each foam lot should arrive with a certificate of analysis stating density, IFD and compression set measured on that lot, not a generic data sheet. That is what makes a specification enforceable: if a refill pad arrives at 24 kg/m³ against a 30 kg/m³ specification, the certificate is the evidence and the lot is rejected at incoming inspection instead of in the field a year later.

Refill Programme: Sizing, MOQ and Inspection

Refill pads are cut-and-sewn items with a fabric cover, so they sit on normal sewn-goods economics with one advantage: they are flat, so they pack densely and ship cheaply compared with their value.

Sizing. A refill range is offered in three or four footprints that map to the platform size bands, plus a thickness option. Cutting pads to four templates rather than one costs nothing at the cutting stage and it removes the fit complaints that come from a one-size approach. Each template carries the platform revision code, for the same reason every other spare part does.

MOQ. 500 pieces per colourway, met across the templates in a range because the cover fabric is the shared element. A programme offering a standard refill and a viscoelastic topper in one cover colour meets the commitment once for both. Sampling takes 6-10 working days and bulk production 35-50 days after approval.

Inspection. AQL 2.5 applies, with foam-specific defect definitions: a density deviation beyond 5% from specified is critical, a cover seam that has split is critical, a dimensional deviation beyond 3 mm is major, and a foam surface tear at a corner is major. Compression set is verified on three pads per lot by the accelerated method, with 10% as the acceptance figure, and IFD is measured on three pads with a tolerance of plus or minus 15% around the specified value.

Packing. Pads ship flat, ten to a carton, not rolled, because a foam pad rolled for a season takes a curvature that does not fully relax. Where a topper is shipped rolled for postal reasons, the instruction to unroll and rest for 24 hours is printed on the bag. Our production team runs refill pad lots through the same SGS-verified production base as the carrier programme, on T/T 30/70 and FOB Xiamen terms.