Flame Plasma
Key Takeaways
- An open gas-air or gas-oxygen flame oxidises and thermally activates the surface, removing contaminants as it does so.
- Exposure is measured in fractions of a second, which is what makes it viable inline.
- It handles chemically inert, very low surface energy plastics that other methods struggle with.
- Not for heat-sensitive or thin parts, which can warp or degrade under an open flame.
Flame Plasma
Flame plasma treatment activates a polymer surface by passing it rapidly through the oxidising region of a precisely controlled air–gas flame, raising surface energy so that adhesives, inks and coatings will wet and bond. Despite involving an open flame the process is not thermal in intent: exposure lasts milliseconds, the bulk part never approaches its heat distortion temperature, and it is the chemistry of the flame — not its heat — that does the work.
It is the workhorse pretreatment for high-volume three-dimensional polyolefin parts: automotive bumper fascias, fuel tanks, containers and closures.
The Chemistry of the Flame
A premixed air and hydrocarbon fuel — natural gas or propane — is burned at a ribbon burner. In the oxidising zone just beyond the visible blue cone, the flame is rich in atomic oxygen, hydroxyl radicals and other excited species. These abstract hydrogen from the polymer chain and graft polar oxygen-containing functional groups in its place. Polypropylene moves from roughly 29–31 mN/m to well above 40 dyne/cm in a single pass.
The air-to-gas ratio is the critical variable and the one most often set wrong. The mixture must run slightly lean — excess air — to produce the oxidising flame chemistry that activates the surface. A rich flame deposits carbon and can actively contaminate the part. This ratio should be metered and monitored, not adjusted by eye.
Process Variables
| Variable | Typical guidance | Failure mode if wrong |
|---|---|---|
| Air-to-gas ratio | Slightly lean / oxidising | Rich flame sooties and contaminates the surface |
| Flame-to-part distance | Part held in the oxidising zone beyond the inner cone | Too close melts or distorts; too far gives no activation |
| Dwell / line speed | Milliseconds of exposure | Excess dwell causes surface melting and gloss change |
| Number of passes | Usually one; occasionally two | Repeated passes accumulate heat in the part |
Why Flame Is Chosen
- Throughput and cost. On large, high-volume parts it is the lowest cost per unit of any effective pretreatment.
- Thick and contoured sections. It handles substantial three-dimensional geometry that a web-based corona treater cannot address at all.
- Slower decay than corona. Flame-treated polyolefin surfaces tend to hold their activation longer than corona-treated film, though decay still occurs and prompt bonding remains best practice.
- Robust and well understood. Burner hardware is mature, serviceable and straightforward to integrate.
The constraints are equally clear: an open flame requires appropriate facility safeguards and combustion controls; line-of-sight means recesses and internal surfaces go untreated; and heat-sensitive or thin-walled parts may not tolerate the process at all. Where those constraints bind, atmospheric plasma or cold gas plasma are the usual alternatives, and for the most intractable substrates Pyrosil flame plasma deposits a silicate layer rather than relying on activation.
Verification
Confirm treatment with wetting tension solutions to ASTM D2578 or contact angle measurement, sampling several locations on the part rather than one. Uniformity across a contoured part is the usual weak point, because flame-to-surface distance necessarily varies with geometry.
Related Terms and Reading
- Flame plasma treatment services
- Flame plasma surface modification of polymers for adhesion bonding
- Preparing plastics for painting
- Industrial liquid coating of polyolefin products
Applying this in production
The Sabreen Group provides independent engineering support for flame treatment process design, burner setup and air-to-gas ratio optimisation. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What is flame plasma treatment?
A high-temperature surface activation process using a controlled open flame — typically propane, butane or natural gas mixed with air or oxygen — to modify the chemical and physical properties of a surface. It oxidises and thermally activates the material, raising surface energy.
Which plastics is it used on?
Difficult-to-bond polymers, commonly polypropylene, polyethylene, PET and ABS. It prepares surfaces for adhesive bonding in automotive, appliance and construction assemblies, and ahead of printing operations.
How does flame compare with corona treatment?
Flame is hotter and more aggressive, which makes it well suited to thick or hard-to-treat plastics. Corona is better for thin films and temperature-sensitive materials. Flame also produces longer post-treatment shelf life on polyolefins because of its shallower treatment depth.
How does flame compare with atmospheric plasma?
Atmospheric plasma allows finer control of the chemistry and is gentler, while flame delivers higher activation power on inert, very low surface energy substrates and cleans contaminants at the same time.
What are the limitations of flame treatment?
Thermal stress rules it out for heat-sensitive or thin materials that could warp or degrade. It requires proper ventilation, flame shields and safety protocols. Surface uniformity also depends on precise nozzle placement and consistent part handling.
