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.
What to Expect by Substrate
Flame treatment raises surface energy on most low-energy polymers, but the achievable level and the tolerance for heat differ enough to change how the station is set up:
| Substrate | Response | Practical caution |
|---|---|---|
| Polypropylene | Excellent — the classic flame substrate | The usual reason the equipment exists. Wide, forgiving process window. |
| Polyethylene, including HDPE mouldings | Excellent | Thick sections tolerate the heat well; thin blow-moulded walls distort readily. |
| Glass-filled polyolefins | Very good | Filler raises heat tolerance, widening the window further. |
| ABS and styrenics | Good | Lower softening point — gloss can be disturbed before treatment is adequate. |
| Polycarbonate and acrylic | Moderate; use caution | Optical surfaces haze easily. An electrical method is usually the better choice. |
| TPE and TPV | Effective but demanding | Soft surfaces gloss or distort. Traverse speed must be tightly controlled. |
| Thin film and thin-wall parts | Not recommended | Thermal mass is too low. Use corona for web, atmospheric plasma for mouldings. |
Troubleshooting Flame Treatment
| Symptom | Likely cause | Correction |
|---|---|---|
| Sooty deposit on the part | Rich flame — too much gas for the air supplied | Lean the mixture to slightly oxidising. A rich flame contaminates the very surface it is meant to activate. |
| Surface glossy, distorted or sink-marked | Flame too close, or dwell too long | Increase standoff or traverse speed. Thin sections have little thermal mass and reach softening quickly. |
| Dyne level good at the centre of the part, poor at the edges | Burner shorter than the part, or contoured geometry moving out of the oxidising zone | Measure across the whole treated area. Contoured parts may need a profiled burner or a robot path holding constant standoff. |
| Treatment level drifts over a shift | Gas or air supply pressure varying, or burner ports partially blocked | Regulate and monitor the air-to-gas ratio rather than setting it once. Clean burner ports on a schedule. |
| Good dyne reading, adhesion still fails | Contamination, most often mould release or plasticiser bloom | Flame has some cleaning capability but it is not a degreaser. Remove the contaminant upstream. |
| Results vary with ambient conditions | Combustion air humidity and temperature affecting the flame chemistry | Log ambient conditions with dyne readings before concluding the burner has drifted. |
| Bond good immediately, weaker the next day | Activation decay | Flame decays more slowly than corona, but it decays. Establish and enforce a treat-to-bond window. |
Safety and Installation Requirements
Flame treatment is the only common pretreatment that brings an open flame and a fuel gas supply into the production cell, which puts it under a different set of obligations from the electrical methods:
- Flame supervision and automatic gas shut-off are the central safety interlocks. Loss of flame must close the gas valve immediately, or unburnt fuel accumulates in the enclosure.
- Purge before ignition. The sequence must clear the enclosure of any accumulated gas before an ignition attempt, and must lock out after a failed attempt rather than retrying indefinitely.
- Combustion product extraction. The flame consumes oxygen and produces carbon dioxide, water vapour and combustion by-products in an occupied space.
- Air-to-gas ratio control, not just setting. Because the ratio governs both treatment quality and safety, it belongs on a regulated and monitored supply rather than on a manually set valve that drifts.
- Hot surfaces and radiant heat around the burner, requiring guarding and consideration of the operator environment.
- Facility acceptance. Some plants and some insurers will not accept an open flame process at all, which is frequently the deciding factor in favour of electrical air plasma despite flame being technically suitable.
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.
Should the flame be rich or lean?
Slightly lean, giving an oxidising flame. A rich flame deposits soot onto the surface, contaminating exactly the area being prepared, and the resulting bond failure is often misdiagnosed as under-treatment. Because the air-to-gas ratio governs both treatment quality and safety, it belongs on a regulated and monitored supply rather than a manually set valve.
Can flame treatment be used on thin-walled parts?
Generally not. Thin sections have too little thermal mass, so they reach softening temperature before the surface is adequately treated, showing up as gloss change, distortion or sink marks. Thin film belongs on corona treatment, and thin-walled mouldings on atmospheric plasma, both of which deliver activation without the thermal load.
What safety systems does a flame treatment station need?
Flame supervision with automatic gas shut-off on flame loss, a purge cycle before ignition that locks out after a failed attempt rather than retrying, extraction for combustion products, regulated air-to-gas supply, and guarding for hot surfaces. Some plants and insurers decline open-flame processes outright, which is often what decides an installation in favour of an electrical method.
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Frequently Asked Questions
How does flame treatment change a plastic surface?
Controlled combustion of a hydrocarbon fuel generates the flame plasma, which oxidises the polymer surface without affecting the bulk properties of the part. The oxidation happens through several processes at once: homogeneous combustion of the premixed reactants, transport of the combustion products to the surface, and heterogeneous reaction of those products with the polymer. The adiabatic flame temperature is around 3,300F (1,816C).
Where in the flame should the part sit?
In the luminous, oxidising zone -- the hottest region, where free-radical species including hydroxyl, carbonyl, carboxyl, ether and ester rise sharply. The optimal distance is typically 3/8 to 1/2 inch (9.5 to 12.7 mm) above the reducing zone. The treating portion of the flame extends about 1-1/2 inches (38.1 mm) beyond the flame tip, with roughly the first 1/2 inch giving the highest level of treatment. The surface should never contact the reducing zone.
What does the flame colour tell you about the mixture?
A deep bluish violet, going almost transparent as gas is reduced, means the mixture is gas-lean -- that colour comes from excited CH radicals. Green means gas-rich, from excited C2 molecules. Increase the gas further and the radiation turns yellowish as carbon particles form. Ideal flame chemistry leaves an oxygen concentration of 0.1 to 0.5 percent in the plasma after the combustion reaction.
What are the process control variables?
Three: flame chemistry, the distance of the substrate from the flame, and the dwell time of treatment. The molar ratio of fuel to oxidiser is probably the single most important parameter. Stoichiometric combustion -- no excess oxygen or fuel -- is roughly 10:1 for natural gas and 24:1 for propane.
Ribbon burner or drilled port?
Ribbon burners are the later generation and the more widely adopted at industrial scale: crimped stainless steel ribbons set into a body, giving a large flame surface, good flame stabilisation, and a pattern customisable by slot width and ribbon configuration. They are the usual choice for treating polymer films. Drilled port burners are normally made in stainless steel and aluminium. Burner selection drives the uniformity and completeness of treatment, so part geometry and substrate both feed into it.
What are flashback and flame lift?
Two instabilities caused by an imbalance between the flow velocity of the fuel/oxidiser mixture and the burn velocity of that mixture. Flashback develops in a premixed burner when the burning velocity of the flame exceeds the stream velocity leaving the port, which lets the flame propagate back into the mixing chamber. A well-designed burner is stable enough that neither occurs.
