Coating after molding
- The housing can use any resin and look any color outside
- Strong shielding from a continuous metal layer
- Adds a coating step, masking, and adhesion testing
Process guide
EMI Shielding Coating
Plastic lets radio frequency noise straight through. When a product moves from a metal box to a molded housing, the shielding the metal gave for free has to be added back, usually as a thin conductive layer on the inside.
The job
A conductive skin, continuous at every seam
The coating only works as well as the joints between coated parts.
An EMI shield is a conductive enclosure around the electronics. On a molded housing, that enclosure is built from a thin metal or metal filled layer on the inside faces of each part, connected at the seams and grounded to the circuit board or chassis.
Coating the flat walls is the easy part. Shields usually leak at the seams between the housing halves, around openings for connectors and displays, and where screws or snap fits should make electrical contact but do not. The coating choice and the housing design have to be settled together.
Methods
Ways to put a shield on the inside of a housing
Coating thickness, cost, and adhesion differ a lot between methods.
| Method | How it works | Strengths | Limits |
|---|---|---|---|
| Conductive paint | Spray paint loaded with silver, copper, or nickel particles | Flexible, works on most resins, easy to mask | Thickness varies in deep corners; overspray needs control |
| Vacuum metallizing | Aluminum evaporated in a vacuum chamber deposits on the part | Thin, even layer on open shapes | Line of sight only; deep recesses coat poorly |
| Electroless plating | Copper then nickel built up chemically on a prepared surface | Uniform coverage into ribs and recesses; strong shielding | Needs a platable resin or etch step; usually coats both sides unless masked |
| Conductive gaskets | Formed in place or cut gaskets that bridge seams between coated parts | Closes the seam, which is where most shields leak | An extra part or dispensing step; needs a groove or land molded in |
Housing design
Molded features that make coating work
These are cut into the mold, so they need to be on the drawing before steel is ordered.
- Overlapping or tongue and groove seams, so coated faces meet across the joint
- Flat lands at screw bosses where coating contacts a grounding path
- Radii on inside corners, since sprayed coatings thin out at sharp corners
- Clear boundaries for masking, such as a step between coated and uncoated areas
- Grooves or lands sized for a conductive gasket where one is planned
Alternative
Coat the housing or mold it from conductive resin
Both are real options. They fail in different ways.
Conductive or shielding resin
- No step after molding
- Fillers change strength, surface, and color choices
- Shielding depends on how evenly fillers disperse, including at knit lines
Keep reading
Related guides
Shielding built into the resin, and the housings that need it.
EMI shielding compounds
Shielding filled into the resin.
EMI shielding compoundsConductive compounds
Resins that carry charge.
Conductive compoundsElectronic enclosures
Housings for boards and devices.
Electronic enclosuresPainting molded parts
Spray finishes on the outside.
Painting molded partsChrome plating plastics
Decorative plating, a relative of shielding plating.
Chrome plating plasticsIn mold electronics
Circuits printed into the part itself.
In mold electronicsProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about EMI shielding coatings
Does the coating go inside or outside the housing?
Almost always inside, where it is protected from wear and does not affect the look of the product. The outside can then be molded in color, painted, or textured as usual.
How is shielding performance checked?
Surface resistance of the coating is checked on parts. The finished product is then tested for emissions and immunity to the standards its market requires, since the seams and openings decide the result.
Will the coating flake off?
Adhesion depends on the resin, surface cleanliness, and any release agent on the part. Adhesion tests on production parts are part of qualifying the coating.
Can ventilation slots stay open?
Yes, if they are small compared with the wavelengths that need blocking. Long slots act like antennas, so many small holes shield better than a few long slots.
Next step
Send the housing with the seams marked
Upload CAD for each housing part and mark faces that need shielding. The review covers the molded housing, painting of outside faces, and assembly. NetProto manufacturing is ISO 9001 certified.