Home › Blog › Medical › Medical Imaging Insulation, Flex Circuit & EMI Shielding › Material selection
Polyimide Dielectric, Conductive Gasket, or PORON Cushion: Choosing Medical Imaging Insulation and Shielding
Three jobs get confused on a medical imaging board because they all involve a thin die-cut part: insulating a conductor from its neighbor, shielding a circuit from interference, and cushioning a fragile detector from shock. They need opposite material properties, so the first task is to name which job each part does. The families on the H-O imaging page sort by exactly that distinction, and this guide walks each by the decision it wins, with methods named by designation.
Insulate, shield, or cushion: name the job first
A dielectric insulator must not conduct: its job is to hold off a voltage between two conductors, and it is chosen for dielectric strength and thickness. An EMI shield must conduct: its job is to carry interference currents to ground and close a gap in a shielded boundary, and it is chosen for conductivity and compression. A cushion does neither: its job is to absorb shock and vibration so a detector or board survives handling.
Specifying a conductive gasket where a dielectric belongs shorts the very gap it was meant to insulate, and specifying an insulator where a shield belongs leaves the boundary open. So the families are sorted by job, with dielectric behavior cited per the ASTM D149 method and the relevant biocompatibility basis per the ISO 10993 series on each maker TDS.
The dielectric layer: polyimide film
Where a thin, tough, heat-stable insulator has to hold off a working voltage in a tight space, polyimide film is the default: it has high dielectric strength per unit thickness and tolerates the heat of nearby electronics. The general-purpose polyimide HN grade is the workhorse insulator and slot liner, and where the film must also be bonded into a stack the bondable FEP-coated polyimide carries a heat-seal layer so it laminates without a separate adhesive.
The thickness is chosen to the working voltage with margin, per ASTM D149, because a film sized to the nominal voltage with no margin is the classic late breakdown.
The shield: conductive elastomer, foil, and sponge
Where a gap in a shielded enclosure has to be closed electrically, three conductive forms cover the range. A conductive EMI shielding elastomer is a compressible gasket that seals a seam both environmentally and electrically, for a lid or a removable panel. A conductive foil tape bridges a seam or wraps a cable where a thin, conformable conductive path is wanted.
And a conductive silicone sponge closes a low-closure-force seam where a solid gasket would need more force than the lid can deliver. The form is chosen by the closure force and the seam geometry, and all three only work if the mating surfaces are conductive where they touch.
The cushion: PORON medical microcellular urethane
Where a detector, sensor, or populated board has to survive shock and vibration, the part is a cushion, and PORON Medical microcellular polyurethane is chosen for its controlled compression and slow recovery, which absorbs energy without taking a permanent set the way a cheaper foam would. A silicone foam covers the same cushioning job where higher temperature tolerance is needed.
The cushion is neither a dielectric nor a shield, and the mistake to avoid is asking one thin pad to cushion and insulate and shield at once, when those are three different materials chosen for three different properties.
Insulation, Shielding, and Cushioning Families by Job
| Family | Job | Property that matters | Conducts? | Where it lives |
|---|---|---|---|---|
| Polyimide film | Dielectric insulator | Dielectric strength / thickness | No (insulates) | Slot liners, barriers |
| Conductive elastomer | EMI seam gasket | Conductivity + compression | Yes | Lids, removable panels |
| Conductive foil tape | Seam bridge / wrap | Thin conductive path | Yes | Seams, cable wraps |
| Conductive silicone sponge | Low-force EMI seam | Soft conductive seal | Yes | Low-closure-force seams |
| PORON Medical urethane | Shock cushion | Energy absorption / set | No | Detectors, fragile boards |
Illustrative and relative only; confirm each grade on its current TDS. Dielectric strength per ASTM D149; biocompatibility basis per the ISO 10993 series where the part contacts a patient-facing surface; shielding effectiveness is a system property verified by the device builder.
Two rules that sort the page
First, never let one part do two electrical jobs: a dielectric insulates and must not conduct, a shield conducts and must close to ground, and confusing them either shorts the gap or opens the boundary. Second, a shield only works if its mating surfaces are conductive where they touch and it has a path to ground, which is a grounding-and-contact question the companion design article on this estate walks in detail. Pick the property to the job, then the form to the closure force and geometry.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer and not the device maker. We take polyimide film, conductive EMI elastomer, conductive foil tape, conductive silicone sponge, and PORON Medical microcellular urethane from the material manufacturers and die-cut, kiss-cut on liner, laminate, and kit them to your drawing, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We recommend a family from whether the part insulates, shields, or cushions, then size it to the working voltage, the closure force and seam geometry, or the shock duty, with grade-level values confirmed against the maker's current TDS. Emissions qualification, dielectric verification, and device clearance stay with the device owner.
Frequently asked questions
What is the difference between a dielectric insulator and an EMI shielding gasket?
Opposite jobs. A dielectric insulator must not conduct: it holds off a voltage between two conductors and is chosen for dielectric strength and thickness, such as a polyimide film. An EMI shielding gasket must conduct: it carries interference currents to ground and closes a gap in a shielded boundary, such as a conductive elastomer. Specifying a conductive part where a dielectric belongs shorts the gap it was meant to insulate, and specifying an insulator where a shield belongs leaves the boundary open.
How thick should a polyimide dielectric film be for my working voltage?
Thick enough to hold off the working voltage with margin, sized against the dielectric strength on the maker TDS per ASTM D149. A film sized to the nominal voltage with no margin is the classic late breakdown, because surface contamination, a pinhole, or an edge can lower the real breakdown below the bench value. Tell us the working voltage and any creepage and clearance constraint, and we recommend a thickness and grade with margin built in.
Which conductive form do I use for an EMI gasket: elastomer, foil, or sponge?
By the closure force and seam geometry. A conductive elastomer gasket seals a seam both environmentally and electrically at moderate to high closure force, for a lid or removable panel. A conductive foil tape bridges a seam or wraps a cable where a thin conformable path is wanted. A conductive silicone sponge closes a low-closure-force seam where a solid gasket would need more force than the lid can deliver. All three require conductive mating surfaces to work.
Can a flex-circuit coverlay be die-cut and laminated by H-O?
Yes. We die-cut polyimide film, including a bondable FEP-coated grade that carries its own heat-seal layer, to the coverlay or stiffener outline and laminate it into the stack. The bondable grade laminates without a separate adhesive, which keeps the stack thin. Send the coverlay or stiffener geometry and the stack-up, and we cut and laminate the polyimide to your drawing with material traceability.
Why does my EMI shield not stop interference even though the gasket is installed?
Almost always because the mating surface is not conductive where the gasket touches. A conductive gasket sitting on an anodized or painted flange never reaches bare metal, so the interference current has no path to ground and the shield does nothing. The gasket also has to compress enough to make continuous contact. The companion design article on this estate walks grounding paths, contact area, and seam continuity, which are the usual reasons a correctly chosen gasket still fails.
What cushion protects a PCB or imaging detector from shock and vibration?
A microcellular cushion chosen for controlled compression and slow recovery, which absorbs energy without taking a permanent set. PORON Medical microcellular polyurethane is the usual choice, and a silicone foam covers the same job where higher temperature tolerance is needed. The cushion is neither a dielectric nor a shield, so it is specified separately for its energy absorption and set behavior, sized to the shock and vibration the assembly sees.
Are these internal parts subject to a biocompatibility requirement?
Only where they contact a patient-facing surface or a fluid path; a purely internal insulator or shield generally is not, but the device owner makes that determination. Where a basis is needed, we convert materials whose makers publish one per the ISO 10993 series, such as cytotoxicity, sensitization, and irritation endpoints, and pass it through with traceability. Tell us whether the part is internal-only or patient-facing and we point you at a material whose maker publishes the basis you need.
Does a conductive part need a conductive adhesive?
Only if the adhesive is in the current path. If a conductive gasket is held by an adhesive on a non-contact face and grounds through its compressed body against bare metal, a standard adhesive is fine. If the adhesive itself has to carry current to ground, then a conductive adhesive or a contact area free of adhesive is needed. Tell us how the part grounds and we lay out the adhesive so it does not block the path it depends on.
Can H-O die-cut fine features like light-mask windows and slots?
Yes. Precision die-cutting handles fine apertures, slots, light-mask windows, and tight registration on thin polyimide, foil, and foam, including kiss-cutting on a liner so the line places the part cleanly. Send the feature dimensions and tolerances and we confirm what is cuttable on the chosen material and quote it. Very fine features on very thin films are exactly the work die-cutting is suited to.
My insulation film broke down in service but passed at the bench. Why?
Usually because it was sized to the nominal voltage with no margin, and a real-world factor lowered the breakdown below the bench value: surface contamination, humidity, a handling crease, a pinhole, or an edge effect at a cut. The defense is to size the dielectric thickness to the working voltage with margin per ASTM D149 and to keep edges and creepage paths clean. The companion design article on this estate covers dielectric breakdown in service.
Does H-O make finished medical devices or certify the electronics?
No. H-O converts the documented insulation, shielding, and cushioning materials to your drawing, with material traceability and lot-code TDS records that support your file. Designing the board, qualifying its emissions, verifying its dielectric performance, performing any biocompatibility testing per the ISO 10993 series, and holding device clearance belong to the device owner and its test houses. Our role is bounded: die-cut, kiss-cut, laminate, and kit the parts to your drawing.
What do I send H-O to get an insulation, shielding, or cushioning quote?
Tell us which job the part does, then send the specifics: for a dielectric, the working voltage and the barrier or slot geometry; for a shield, the seam geometry, the closure force, and whether the mating surfaces are conductive; for a cushion, the shock and vibration duty and the part it protects. Add any biocompatibility basis needed and the quantities. With that, engineering recommends a family and grade and quotes a die-cut part. The companion how-to-spec article on this estate is a full checklist.
Get an insulation, shielding, or cushioning recommendation
Tell us whether the part insulates, shields, or cushions, then the working voltage, the seam geometry and closure force, or the shock duty, and we will recommend a family and grade and quote a die-cut part to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
ApplicationMedical Imaging Insulation, Flex Circuit & EMI ShieldingRead →
GuideWhy the Shield Did Not Shield: Grounding Paths, Flex-Circuit Coverlay, and Dielectric BreakdownRead →
GuideHow to Spec Medical Imaging Insulation, Shielding, or Cushioning: What to Send H-O for a QuoteRead →
IndustryMedical hubRead →
Request a quote for your medical imaging insulation or shielding part
Send H-O the board or seam geometry and the electrical duty—engineering reviews every RFQ and converts samples on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D149 (dielectric breakdown voltage and dielectric strength of solid insulating materials), cited by designation for the polyimide dielectric basis.
- ISO 10993 series (biological evaluation endpoints) and USP Class VI, cited by designation where a part contacts a patient-facing surface or fluid path.
- Manufacturer technical data sheets for the polyimide film, conductive EMI elastomer, conductive foil tape, conductive silicone sponge, and PORON Medical microcellular urethane grades named on the parent application page.
This article summarizes material-selection and design principles for medical imaging insulation, flex circuits, and EMI shielding using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values are maker technical-data-sheet figures and must be confirmed on the manufacturer's current TDS at your installed condition. Standards (ISO 10993, USP Class VI, ISO 11607, IEC, ASTM, UL) are cited by designation for context, not as a claim of qualification or registration.
H-O Products is a precision die-cut converter, not a raw-material producer and not the device maker; biocompatibility testing, sterilization validation, device clearance, and final-assembly certification belong to the device owner and its test houses.