Custom Filtration & Venting: Reticulated Foam Filters & ePTFE Pressure-Equalization Vents

Reticulated foam is an open-cell polyurethane foam whose cell walls (the thin windows between struts) have been removed, leaving a skeletal, fully through-porous network. Because there are no walls to block the flow, air and fluid pass through it with low restriction, which is exactly what you want in a coarse filter or a porous spacer. It is specified by pores per inch (PPI): a low PPI is coarse and open, a high PPI is finer and captures smaller particles at a higher pressure drop.

It is the opposite of a closed-cell sealing foam, which is built to keep fluid out rather than let it through. H-O converts uncoated, acrylic-coated and PVC-coated reticulated grades to your drawing.

Work from the particle you must catch and the airflow you must pass. A coarse, low-PPI grade passes high airflow at low restriction and traps larger debris; a finer, high-PPI grade catches smaller particles but adds pressure drop and clogs sooner, shortening service life. The general rule is to pick the coarsest grade that still meets the capture requirement, so airflow and service life stay high.

Send the particle size you need to capture, the airflow the filter must pass, and the face area you have, and the grade can be matched against the manufacturer's data; the final pore count and gauge are confirmed on the technical data sheet.

They solve different problems. A foam filter is a through-porous screen that lets air or fluid pass while trapping particles; it is not a barrier to liquid or dust. A pressure-equalization vent is a thin ePTFE membrane that lets an enclosure breathe, equalizing internal and external pressure as temperature or altitude changes, while resisting liquid water and dust because PTFE is strongly hydrophobic.

So a vent's job is to relieve pressure without letting water and dust in, supporting the enclosure's IP rating per IEC 60529, whereas a filter's job is to capture particles in a flow. If you need both, they are usually two different parts: a filter in the air path and a vent on the sealed enclosure.

Start from the open area, because the restriction of both a filter and a vent scales with the area presented to the flow: a larger face passes more air at the same pressure drop, and a part that is too small chokes the flow and drives the pressure drop up. For a vent, size the membrane area to the breathing need (the volume and how fast it heats or cools); for a filter, size the face to the airflow at an acceptable pressure drop and to a sensible service interval before clogging.

The illustrative helper on this page shows the area-to-airflow relationship in relative terms. The actual numbers come from the manufacturer's airflow-versus-pressure-drop curve and a bench test, not from a rule of thumb, so send the airflow, the available area and the allowable pressure drop.

The enclosure carries the rating; the membrane supports it. IEC 60529 defines the IP code as a property of an enclosure tested as a whole, against ingress of solids and water. A pressure-equalization vent contributes to that result by sealing an opening with a membrane that breathes but keeps water and dust out, so the enclosure can equalize pressure without breaking its seal.

But the rating that you can claim, for example IP67, belongs to the assembled, tested enclosure, including its seals, gaskets, fasteners and the vent together. So the honest framing is that the vent and its adhesive part support the IP rating; the device maker validates the enclosure's IP rating by test. H-O converts the membrane and the adhesive carrier; it does not certify the enclosure.

When the enclosure the filter sits in carries a flammability requirement. Many electronics, telecom and industrial cabinets must meet a flammability class, and a foam filter inside one has to satisfy that class rather than become the weak point. UL 94 is the flammability test standard for plastic materials, and HF-1 is its horizontal-burn rating for low-density foamed materials, so a reticulated grade carrying a UL 94 HF-1 rating lets the filter meet the cabinet's flammability class while still passing airflow at a coarse pore count.

The rating is grade- and thickness-specific and is a property of the material as tested; the assembly is qualified by the device maker. Call out the flammability requirement on the drawing so the flame-rated grade is quoted rather than a standard one.

Biocompatibility is evaluated against ISO 10993, and it is a property of the material as evaluated and ultimately of the finished device, not something a converter certifies. ISO 10993 is the framework for the biological evaluation of medical devices; the relevant parts depend on the contact type (skin, fluid path, and so on) and the contact duration. A material can be evaluated by its manufacturer against the relevant parts, and that evaluation is documented on the data sheet.

H-O converts open-cell reticulated polyurethane in grades whose manufacturers report this evaluation, to your fluid-pad outline, but the device-level biocompatibility file, the testing of the finished device for its specific use, belongs to the device maker. So the honest framing is: the material is evaluated to the relevant ISO 10993 parts per the manufacturer; H-O does not certify a finished device.

Tell us the contact type, the duration, and any cleaning or sterilization the part must survive.

All three are the same open-cell reticulated polyurethane filter foam; the difference is a surface treatment. The uncoated grade is the base coarse filter, with the lowest restriction at a given pore count, and is the default for fan and vent screens. The acrylic-coated grade adds an acrylic surface treatment for extra durability and a cleaner handling surface, useful where the filter is handled often or runs a little dustier.

The PVC-coated grade adds a PVC coating for additional surface robustness and wipe-down service in dustier or wetter intakes. Both coatings add a small amount of restriction over the uncoated grade at the same PPI, and neither closes the pore structure that does the filtering. Choose the coating for the surface duty and the PPI for the capture; confirm both on the data sheet.

A few things move a recommendation from a guess to a real direction: the job (filter, vent, or fluid pad), the airflow you must pass and the pressure drop you can accept, the particle size to capture (for a filter) or the target IP rating and enclosure volume (for a vent), the available open area or footprint, and any flammability (UL 94 HF-1) or medical fluid-contact (ISO 10993) requirement.

Add the part geometry or a drawing, the adhesive and liner needs, and the prototype and annual volume, and engineering can match a family, a grade direction and a converting approach, then confirm the grade-level values against the manufacturer's data sheet. The "What to send H-O" box below lists these. If you only know the symptom, "the enclosure pulls in moisture," "the fan screen clogs," that is a fine starting point too; describe it and we will work back to the spec.

H-O is a precision converter, not a raw-material producer. We do not extrude, foam or cast the reticulated polyurethane or the ePTFE membrane; we buy sheet, roll and slab stock from the material manufacturers and convert it to your drawing, by die-cutting, kiss-cutting, laser and waterjet cutting, adhesive lamination, slitting and kitting, with material traceability and lot-level data-sheet records.

Every filtration and venting part is made-to-order; we do not carry finished parts in stock and we do not advertise a no-minimum policy, though prototype quantities through full production runs are equally welcome and the minimum varies by material and part.

Prototype and production timing is summarized in the process strip near the top of the page and on the quote form. Send your drawing or describe the enclosure through the form below for a specific quote.