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: Clearing Up Surfaces the Mold Could Not
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.
Vapor polishing uses solvent vapor to level a thin skin after molding. High gloss polish is what the mold can do instead. Annealing is the stress relief step people pair with vapor. Lenses and light pipes are the clear parts that ask for it.
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.
Roles
Not sure which gloss route
Vapor polishing treats the part. High gloss polish is in the steel. Annealing is the heat step that may come with it.
| Page | Use it for |
|---|---|
| Vapor polishing (this page) | Solvent vapor on the molded part |
| High gloss polish | Gloss cut into the mold |
| Annealing | Stress relief before or after a solvent step |
| Lenses | A clear part that may need the surface |
| Light pipes | Another clear optical part |
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)All processes
Every molding process and secondary operation.
All processesFAQ
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.
Is vapor polishing the same as polishing the mold?
Mold polish is in the steel and repeats on every shot. Vapor polishing treats the part after it is molded. Start with high gloss polish when the mold can hold the surface.
Which parts usually ask for vapor polishing?
Clear lenses and light pipes. Name the resin on the RFQ. The solvent process is set per resin before polishing.
Next step
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