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Mastering Anodizing Masking: The Key to Perfect Edge Definition
Why Standard Masking Fails in the Anodizing Tank
Most other finishing processes are similar in concept: A layer of a wet or dry material is applied on the surface of a part, then it dries or is cured, adding to the thickness of the part overall. When an area needs to be kept free of said coating, a plug, cap or tape is applied, then removed after drying/curing. The area adjacent to the tape, cap or plug has a sharp cut off line where coating is masked off.
Anodizing, as you likely already know if you’re an aluminum or titanium anodizer, is a bit different in that respect. For those who are unaware, the anodizing process essentially speeds up the oxidation of the surface of aluminum or titanium. Because of this, it makes the part slightly smaller – and importantly, it physically alters the surface of the part, rather than adding a coating to the top.
How does this affect masking? For tape, in a specific way with regards to applying the tape.
In many cases, in particular Type 2 anodizing, a dye is added during the process. This colors the part. But because the surface of the part is physically changing during the anodizing process, masking tape doesn’t function quite as adequately – well, immediately after tape application anyway. That’s because the adhesive is now touching a surface that will be altered at the microscopic level in the anodizing tanks.
What can go wrong? A lot - if the tape is applied and then immediately introduced to the anodizing process, say in an hour or two. This can cause jagged or uneven lines where the tape meets the anodized area. See image here on the right ---->
Why does this happen? And what can be done to prevent it? It helps to understand a bit about how tape works to solve the anodizing / tape masking problem.
Most masking tapes for the metal finishing industry have adhesive that are called pressure sensitive adhesive systems, or PSA's. That means pretty much exactly what it says: Pressure is required to fully activate the adhesive, so that it will start and continue to flow over time into the microscopic crevices of a part.
Did you see that one phrase? “Flow over time”? That’s the key here.
While not ideal for short lead times, it is necessary for anodizers to allow enough time for the adhesive to flow out. Because when that occurs, it creates better “stop off” along the edge of the tape where the areas to be anodized meets the surface to be masked.
How long is “long enough”?
Ideally, 24 hours. Minimum? Probably 4 to 6 hours, though that’s unlikely to get the best results. Many anodizers will mask parts one day, then anodize the following day. That tends to net at least 12+ hours of dwell time for the tape adhesive to flow out properly and achieve good anodizing masking stop off. Imagine that the lighter pink "adhesive" in the image at right is the "flowed out over time" adhesive - note it is deeper into the crevices of the metal, creating better stop off.
Pro-Tip: For the best results in Type II anodizing, apply tape 24 hours in advance. If you're in a rush, a minimum of 4 hours is required to allow the PSA to flow into the metal's microscopic crevices.
And how much pressure is needed? Well, you don't need to drive a truck over the tape on the part. But it does require some moderate "pushing down" to get that adhesive flowing. There are various ways to ensure good pressure is applied – some anodizers use a rubber roller, others may have masking personnel go over a part carefully with gloves applying moderate pressure, and still others may use a plastic tool to “swipe” over the surface, particularly the edges of the tape. Maybe you have an idea that works best for your needs. Experiment! See what works best for your process.
However it is accomplished, just remember that pressure and time are the keys to anodizing masking tape success.
Viadon slits tape to width and die cuts several tapes to size that are anodizing masking materials and sends free samples for testing, including:
We also stock anodizing caps and plugs.
3 Steps for Better Anodizing Masking Stop-Off
To achieve sharp, clean lines and prevent chemical leaks during the anodizing process, follow these three critical steps for tape application:
- Apply Moderate Pressure: Use a rubber roller, a plastic "swipe" tool, or gloved hands to firmly press down the tape. This activates the Pressure Sensitive Adhesive (PSA), forcing it to begin its "flow".
- Allow Sufficient Dwell Time: For the best results, let the masked parts sit for 24 hours before placing them in the tank. At a bare minimum, allow 4 to 6 hours of dwell time to ensure the adhesive has physically filled the microscopic crevices of the metal surface.
- Ensure Edge Definition: Pay special attention to the tape edges during application. Proper "flow out" creates the essential "stop-off" barrier that prevents dyes and acids from seeping under the mask and creating jagged lines.
Frequently Asked Questions: Anodizing Masking Success
1. Why does my masking tape leak during anodizing?
Masking leaks usually happen because the adhesive hasn't fully "flowed" into the metal's surface. This is caused by insufficient dwell time (not waiting long enough after application) or low pressure during application. If the adhesive doesn't seal the microscopic crevices of the aluminum, acids will seep under the edge.
2. How long should masking tape sit before anodizing?
Ideally, you should allow for 24 hours of dwell time. This gives the Pressure Sensitive Adhesive (PSA) enough time to physically flow and create a permanent "stop-off" barrier. At a minimum, wait at least 4 to 6 hours before placing masked parts into the anodizing tank.
3. What is the best masking tape for aluminum anodizing?
The best choice is a high-performance polyester tape with a chemical-resistant adhesive. Top industry standards include:
4. How do I get sharp masking lines on anodized parts?
To achieve sharp, crisp lines, you must burnish the edges of the tape using a rubber roller or a plastic "swipe" tool. This ensures the adhesive is fully activated along the border where the anodizing stops, preventing the "jagged" effect caused by seepage.
5. Can I use regular painters tape for anodizing?
No. Regular painters tape or standard crepe paper masking tape will fail in the acidic environment of an anodizing tank. The adhesive will break down, contaminating the bath and leaving a sticky residue on the part that is difficult to remove without damaging the finish.
If you have questions about masking for anodizing or need tapes, caps or plugs for the process, send us an email, give us a call or check out our full line of masking products here at Viadon.com.
Viadon. A cut above.