What Is the 720 Rule for Anodization? Explained Plainly

The 720 rule is a simple math rule that helps people make aluminum look pretty and strong. When we put aluminum in a special bath with electricity, it grows a coat that protects it. This is called anodization. The 720 rule tells us how long to keep the aluminum in this bath.

The 720 Rule Formula & How It Works

The 720 rule is easy to use. Just remember:
Amps × Time = 720

This works for 1 square foot of aluminum to get a coat that is 1 mil thick (that’s very, very thin – about as thick as a piece of paper).

Formula Basics

Let’s break it down:

1. We need to know how big our part is
2. We need to know how thick we want the coat to be
3. We need to know how much electricity we use

For example, if we use 12 amps of electricity, we need to keep our part in the bath for 60 minutes.
12 amps × 60 minutes = 720

The rule uses amp-minutes per square foot. This is just how much electricity we use over time for each square foot of aluminum.

Key Variables Explained

The 720 rule has two main parts we can change:

1. Surface area: How big is your aluminum part?
   • Big parts need more electricity or more time
   • We measure in square feet (ft²)
   • If your part is 2 square feet, you need twice as much (1440)
2. Target thickness: How thick do you want the coat?
   • Standard thickness is 1 mil (0.001 inch)
   • For half as thick (0.5 mil), you need half the number (360)
   • For twice as thick (2 mil), you need twice the number (1440)

Practical Applications of the 720 Rule

Common Use Cases

The 720 rule works best for these cases:

• Type II anodizing: This makes a medium coat (0.5 to 1.0 mil thick)
• Type III anodizing: This makes a thick, hard coat (1.0 to 4.0 mils thick)
• Aerospace parts: When need to be strong
• Car parts: For parts that need to look nice and last long
• 6061-T6 aluminum: This type of aluminum works very well with the rule

The rule is used every day in shops that make pretty, colored aluminum parts. Many shops use it to coat thousands of parts each day.

When the Rule Fails

The 720 rule does not work well for:

• Type I anodizing: This uses chromic acid instead of sulfuric acid
• Aluminum with lots of copper or silicon: These metals make the anodize coat grow differently
• Very complex-shaped parts: Hard-to-reach spots may get less coating

If you work with special aluminum alloys, you might need to adjust the rule.

Step-by-Step Calculation Guide

Let’s see how to use the 720 rule step by step:

1. Measure your part’s surface area
   • Length × Width for each side
   • Add up all sides
   • Convert to square feet (1 ft² = 144 in²)
2. Decide how thick you want the coat
   • Type II: Usually 0.5 to 1.0 mil
   • Type III: Usually 1.0 to 4.0 mils
3. Calculate the amp-minutes needed
   • Amp-minutes = 720 × Surface area (ft²) × Thickness (mils)
4. Choose your current (amps)
   • Most tanks use 12–24 amps per square foot
5. Calculate your time
   • Time (minutes) = Amp-minutes ÷ Current (amps)

Surface Area Estimation Hacks

It can be hard to measure every part of a complex shape. Here are some tricks:

• For long, thin parts (like tubes): Length × Perimeter
• For flat parts with holes: Outer area – hole area + (hole perimeter × thickness)

Don’t worry about being perfect! A 10% error is usually OK in this process.

Critical Best Practices

Avoid These Mistakes

Many people make these common mistakes with the 720 rule:

1. Putting too many parts in the tank
   • This can make the electricity spread unevenly
   • Parts might get different coat thicknesses
2. Ignoring temperature
   • Cold tanks make thick, soft coats
   • Hot tanks make thin, hard coats
   • The best temperature is usually 68–72°F
3. Poor sealing after anodizing
   • The anodize coat has tiny holes that need to be closed
   • Bad sealing leads to stains and weak coats
4. Wrong calculations
   • Forgetting parts of the surface area
   • Using the wrong units (inches vs. feet)

Pro Tips for Repeatable Results

To get the same good results every time:

1. Check your power supply
   • Make sure it gives steady current
   • Calibrate it regularly
2. Test coat thickness
   • Use tools like eddy current testers
   • Check several spots on each part
3. Keep good records
   • Write down all settings for each batch
   • Note any problems or special cases
4. Clean parts very well before anodizing
   • Dirt and oil make bad spots in the coat
   • Use the right cleaners for aluminum

If you do precision machining on your aluminum parts before anodizing, you’ll get better results.

Real-World Data and Case Studies

Let’s look at some real examples of the 720 rule in action:

Industry Adoption

According to finishing industry data:

• Over 60% of Type II/III anodizers use the current density method (720 rule)
• Shops using the 720 rule finish jobs 15–20% faster than those using voltage control
• Surface area estimates are usually within 10% accuracy

Limitations with Special Alloys

Parts with high copper or silicon (more than 5%) need special care:

• The 720 rule may need to be adjusted up by 10–30%
• These alloys grow oxide differently than pure aluminum
• Testing is needed for the best results

FAQs About the 720 Rule

Summary: The 720 Rule Made Simple

The 720 rule is a helpful tool for anyone who works with anodizing aluminum. It tells you how long to run your process to get the coat thickness you want. Just remember the key formula:

Amps × Minutes = 720 (for 1 square foot, 1 mil thickness)

By understanding this rule, you can:

• Plan your work better
• Get consistent results
• Make beautiful, durable aluminum parts Whether you’re making aerospace parts, bike parts, or just trying to anodize at home, the 720 rule is your friend for getting good results every time.