Inside most film capacitors is a long strip of plastic film, often thinner than kitchen cling film, coated with a layer of metal only nanometres thick. That is metallized film, and its quality decides how the capacitor performs and how long it lasts.
This guide explains what metallized polyester film is, how it works in a capacitor, how it compares with polypropylene, and what to check before you buy it.
What Is Metallized Polyester Film?
It starts as thin BOPET polyester film. In a vacuum chamber, a metal such as aluminium, zinc or a zinc-aluminium alloy is evaporated and condensed onto the film. The metal layer is extremely thin, measured in nanometres rather than microns.
Metal thickness is usually specified indirectly, as surface resistivity in ohms per square. The lower the figure, the thicker the metal. For the base film itself, see our guide to BOPET polyester film grades.
How It Works in a Capacitor
Two metallized films are wound together so that the metal layer on each film acts as an electrode and the plastic between them is the dielectric. The ends of the winding are sprayed with metal, a step often called schooping, which connects all the turns to the leads.
The big advantage is self-healing. If a weak point in the film breaks down, the current at that spot vaporises the thin metal around it. The fault is isolated and the capacitor keeps working, with only a tiny loss of capacitance. Film-and-foil capacitors, which use separate metal foil electrodes, cannot do this.
Metallization Features to Know
- Margin. An unmetallized strip along one edge, so the electrodes do not short at the ends of the winding.
- Heavy edge. A thicker band of metal along the contact edge. It improves the connection to the sprayed end and helps carry current.
- Metal type. Aluminium resists corrosion well. Zinc and zinc-aluminium give good contact and self-healing but are more sensitive to moisture.
- Segmented or patterned film. The metal is divided into small segments linked by narrow fuses. If one segment fails, its fuse opens and only that small area is lost. This safety film is used for demanding AC duties.
Polyester vs Polypropylene Metallized Film
| Feature | Metallized polyester (PET) | Metallized polypropylene (PP) |
| Dielectric constant | About 3.2 to 3.3 | About 2.2 |
| Capacitor size for the same capacitance | Smaller | Larger |
| Dielectric losses | Higher | Very low |
| Typical upper temperature | Around 125°C | Around 105°C |
| Typical uses | DC circuits, decoupling, coupling, general electronics | AC motor capacitors, power factor correction, high-frequency and snubber circuits |
In short, polyester gives compact capacitors for DC and general electronics. Polypropylene wins where losses matter, as in AC power and high-frequency work, which is why most power capacitors use it.
What Buyers Should Check
- Base film thickness and tolerance. Thin films save space but leave little safety margin. Consistent thickness gives consistent capacitance and voltage rating.
- Surface resistivity. Ask for the specified range in ohms per square, and how tightly it is held across the width and along the roll.
- Margin and heavy edge dimensions. These must match your winding design.
- Slit width and edge quality. Clean, straight edges avoid shorts at the ends and help the end spray bond.
- Defects. Pinholes, scratches, wrinkles and uncoated spots all reduce voltage capability.
- Packing and storage. Metallized film, especially zinc, corrodes in moist air. It should arrive vacuum-sealed or with desiccant and be used soon after opening.
- Test data. Ask for breakdown voltage and resistivity results for each batch.
Beyond Capacitors
Metallized polyester is also used for flexible packaging, insulation facings, EMI shielding and decorative laminates. For these jobs, appearance and barrier properties matter more than electrical precision, so capacitor-grade film is not needed. Our post on polyester film uses covers this wider range.
Final Thoughts
Metallized polyester film gives compact, self-healing capacitors for DC and general electronics. Buy on consistency: base film thickness, metal resistivity, margin, edge quality and careful packing decide how well the finished capacitor performs.
Harnawa supplies electrical-grade polyester films and other materials used across the industries we serve, including capacitor construction. For high-temperature dielectrics, see our polyimide film. Contact us to discuss your film requirements.
Frequently Asked Questions
What is metallized polyester film?
It is polyester (PET) film coated in a vacuum with an extremely thin layer of metal, usually aluminium, zinc or a zinc-aluminium alloy. It is mainly used to make film capacitors.
Why is metallized film used in capacitors?
The metal layer acts as the electrode, so capacitors can be made very compact. Metallized film is also self-healing, which makes capacitors more reliable.
What is the difference between metallized polyester and polypropylene film?
Polyester has a higher dielectric constant, so capacitors are smaller, and it suits DC and general electronics. Polypropylene has much lower losses, so it is preferred for AC power and high-frequency use.
What does self-healing mean in a film capacitor?
When a weak spot breaks down, the current vaporises the thin metal around it. The fault is isolated and the capacitor keeps working with a tiny loss of capacitance.
How should metallized film be stored?
Keep it sealed in its original packing, in a dry, cool place, and use it soon after opening. Moisture can corrode the metal layer, especially zinc.
What is heavy edge metallization?
It is a thicker band of metal along the contact edge of the film. It gives a better connection to the sprayed end of the capacitor and helps it carry higher current.
