High polish in the mold
- Every part comes out glossy with no extra step
- The polish is made once, in the steel
- Cannot fix gate marks, trimmed edges, or faces changed after molding
Process guide
Vapor Polishing
A clear part can come out of the mold with a hazy edge, a gate mark, or a trimmed face that is no longer glossy. Vapor polishing melts a thin skin of the surface just long enough for it to level out and clear up.
How it works
A breath of solvent, then a smooth skin
The surface is not ground or buffed. It is briefly dissolved and allowed to flow.
A heated solvent, often a chlorinated or specialty polishing solvent, is turned to vapor and directed at the part with a nozzle or in a closed chamber. The vapor condenses on the cooler plastic and dissolves the outermost layer. Surface tension pulls that thin liquid layer flat, fine scratches and tool marks fill in, and the solvent evaporates to leave a glossy surface.
It works on amorphous resins that the solvent can dissolve, such as polycarbonate, acrylic, polysulfone, and PEI, the same group that solvent bonds well. It is fast and reaches edges, holes, and inside corners where a buffing wheel cannot go.
Where to polish
Polish the mold or polish the part
On molded parts, the mold is usually the first answer. Vapor polishing covers what the mold cannot reach.
Vapor polish on the part
- Clears gate vestige, flash trim, and edges after molding
- Adds a handling step and solvent exposure on every part
- Rounds very sharp edges slightly as the surface flows
Uses
Where vapor polishing shows up
It is most common on low volume clear parts and on machined clear stock, and less so on molded parts.
| Situation | Why vapor polish |
|---|---|
| Light pipes and lenses with a trimmed gate | Clears the gate area so it does not scatter light |
| Clear medical and lab parts | Lets operators see fluid clearly through edges and internal channels |
| Display and demonstration parts | Glass like faces on low volume clear parts where the mold was not fully polished |
| Clear parts from machined stock | Removes tool marks that would otherwise need hand polishing |
The risk
Why vapor polished parts sometimes craze
The same solvent that smooths the surface will attack stressed plastic, and the damage can take hours or days to appear.
- Molded in stress near gates and thick to thin changes turns into fine cracks after polishing
- Annealing parts before polishing relieves much of that stress
- Too much vapor in one spot leaves sags, drips, or a soft, dimpled surface
- Polished parts need time to off gas before bonding, coating, or sealed packing
Keep reading
Related guides
Polishing in the steel, clear resins, and the parts that need clear faces.
High gloss polish
Getting gloss from the mold steel.
High gloss polishPolycarbonate vs acrylic
The two main clear resins compared.
Polycarbonate vs acrylicLenses
Clear optics and what spoils them.
LensesLight pipes
Carrying light from a board to the surface.
Light pipesSolvent bonding
The same chemistry used to join parts.
Solvent bondingPolysulfone (PSU)
Clear, tough, and heat resistant.
Polysulfone (PSU)Processes hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about vapor polishing
Can vapor polishing make a cloudy part clear?
Only if the cloudiness is on the surface, such as fine scratches or a matte finish. Haze inside the part, from moisture, degraded resin, or crystallinity, stays.
Does it change dimensions?
Very slightly. A thin skin flows, and sharp edges and fine texture soften. Critical sealing faces and snap features are usually masked or left unpolished.
Can polypropylene or nylon be vapor polished?
Not with the usual solvents. They resist them, which is one reason those resins are chosen for chemical service. Flame polishing is sometimes used instead on low volume parts.
Is vapor polishing safe for medical parts?
It is used on them, but residual solvent has to be controlled and verified for the application. That is a validation question for each device, not a general yes.
Next step
Mark the faces that must be clear
Upload CAD with the clear faces marked and the gate area you can accept. The review covers polish levels in the mold steel so clarity comes off the tool. NetProto manufacturing is ISO 9001 certified.