PET and PI are both flexible polymer films that can be used as substrate platforms in printed heating elements. Choosing between them, however, is not simply a matter of comparing two material names.
In a flexible heating-film project, the substrate supports the printed electrical circuit and becomes part of the heater’s mechanical, electrical and thermal construction. Final heater performance also depends on the resistive circuit, conductive electrodes, insulation structure, adhesive or lamination system, electrical connections and the way the heater is integrated into the finished product.
For this reason, PET versus PI should be treated as an engineering-selection question rather than a “which material is better?” comparison.
This article explains the role of the substrate, the general engineering differences between PET and PI, and the application information an OEM team should consider before selecting a preliminary substrate direction.
What Role Does the Substrate Play in a Flexible Heating Film?
In a screen-printed flexible heater, the substrate provides the film platform that supports the functional printed circuit.
- supporting printed conductive and resistive materials
- forming part of the electrical insulation structure
- providing mechanical support during manufacturing and assembly
- helping define the dimensions and flexibility of the heating element
- interacting with adhesive, lamination and protective structures
- becoming part of the heater’s overall thermal and mechanical system
The substrate is therefore important, but it does not determine complete heater performance by itself.
Voltage, resistance and power are primarily related to the electrical design of the heating circuit. Final temperature and thermal response also depend on heat transfer, mounting conditions, surrounding materials, insulation, airflow, control strategy and the complete product structure.
For a broader explanation of the electrical principle and printed circuit structure, see how a printed flexible heater works.
| The more useful engineering question is not: “Which material is better?”It is: “Which substrate platform is appropriate for the electrical, thermal, mechanical and assembly requirements of this application?” |
What Is PET in a Printed Heating Film?
PET stands for polyethylene terephthalate, a polyester film material used in electrical insulation, flexible electronics and printed-circuit applications.
Specific PET grades can be designed with characteristics such as heat stabilization, surface treatment or primers to support printed electronics and conductive-ink processing.
In a printed heater, PET can act as the flexible substrate supporting the functional electrical structure.
The important point is that “PET” describes a material family rather than one universal heater specification.
- surface condition
- dimensional stability
- processing behavior
- thickness
- adhesion characteristics
- compatibility with printing and lamination processes
The selected PET film therefore needs to be considered together with the printed materials, process conditions and complete heater construction.
Toreada supports PET heating film as one substrate direction within its custom screen-printed heating-film platform.
Generic PET material properties should not be treated as finished Toreada heater specifications.
What Is PI in a Printed Heating Film?
PI stands for polyimide, another family of flexible polymer films widely used in electrical and electronic applications.
Polyimide films are commonly used as dielectric substrates in flexible printed circuits and related electrical structures. Specific PI grades are also designed for manufacturing processes that require characteristics such as dimensional stability, adhesion and compatibility with printed circuitry.
In a printed heating element, PI can serve as the flexible substrate supporting the functional circuit.
PI is often considered when the project involves a more demanding thermal environment or manufacturing process. However, this does not establish a universal operating-temperature rating for a finished PI heating film.
A raw-film datasheet describes a particular film grade under defined test conditions. It does not automatically define:
- the allowable temperature of the resistive material
- the electrode-system limit
- the adhesive limitation
- the connection limitation
- the insulation-system rating
- the finished heater operating range
Toreada supports PI heating film as another substrate option for custom projects.
The substrate still needs to be evaluated as part of the complete heater architecture.
PET vs. PI Heating Film: Key Engineering Differences
PET and PI are both flexible substrate platforms, but they are not interchangeable in every application.
The comparison below is intended as an engineering-selection framework rather than a performance ranking.
| Engineering Factor | PET Substrate Direction | PI Substrate Direction | What the OEM Should Verify |
|---|---|---|---|
| Substrate role | Flexible polyester film supporting the printed structure | Flexible polyimide film supporting the printed structure | Complete heater construction |
| General material character | Established platform for electrical insulation and printed electronics | Established dielectric platform for flexible circuits and electrical structures | Specific film grade |
| Thermal environment | Selected PET grade and complete construction should be checked against actual conditions | Often considered where thermal demands are more severe | Installed and processing conditions |
| Mechanical / flexibility | Flexible; behavior depends on grade and complete construction | Flexible; behavior depends on grade and complete construction | Static bend, repeated movement and dimensions |
| Electrical insulation | PET films are used in electrical-insulation applications | PI films are used as dielectric and electrical-insulation substrates | Film grade and complete insulation structure |
| Mounting / lamination | Must be compatible with adhesive and mounting structure | Must be compatible with adhesive and mounting structure | Full assembly stack |
| Printed-circuit compatibility | Surface treatment and stabilization may be relevant | Grade and process compatibility may be relevant | Paste system, adhesion and processing |
| OEM decision factors | Application-specific | Application-specific | Thermal, electrical, mechanical and assembly requirements together |
Neither substrate should be selected from this table alone.
The appropriate direction depends on the complete product requirements.
How Should the Thermal Environment Affect Substrate Selection?
Thermal environment is one of the first factors engineers consider when comparing PET and PI, but it should be evaluated more broadly than a single maximum-temperature value.
- What are the normal operating conditions?
- Is the heater powered continuously or intermittently?
- What surface is the heater attached to?
- Is the heater enclosed?
- Are nearby components generating additional heat?
- Can heat accumulate within the product?
- What ambient conditions are expected?
- Does the manufacturing process expose the substrate to elevated temperatures?
- Are adhesives, protective films or connectors present with their own limitations?
PET film grades can be engineered for greater dimensional stability during printed-electronics processing, while PI films are commonly considered in flexible-circuit applications involving more demanding thermal conditions.
This does not mean that PET is simply a “low-temperature material” or that PI automatically defines a “high-temperature heater.”
The correct comparison is between the actual application conditions and the specific heater construction under consideration.
How Do Mechanical Requirements and Dimensions Affect the Choice?
Both PET and PI are flexible films, but the mechanical behavior of a finished heater depends on more than the substrate family.
- overall heater dimensions
- required outline and cutouts
- installation curvature
- available installation space
- whether the heater remains flat after installation
- whether it is bent during assembly
- whether movement is static or repeated
- wire and connection locations
- adhesive or laminated structures
A raw flexible film may behave differently after conductive materials, resistive layers, insulation, adhesive, reinforcement and electrical connections are added.
For this reason, claims such as “PI is more flexible than PET” are not sufficiently precise without defining the actual film grade and complete heater structure.
The more relevant question is whether the selected construction can support the mechanical requirements of the product.
If repeated movement or significant bending is expected, this should be identified at the beginning of the project so that it can be considered during heater design and prototype evaluation.
How Do Electrical Insulation Requirements Affect PET vs. PI Selection?
Both PET and PI film families are used in electrical-insulation applications.
However, raw-film dielectric properties should not be treated as the dielectric specification of a finished heating element.
- film grade
- film thickness
- printed electrode geometry
- resistive circuit design
- insulation and protective layers
- edge distances
- connection areas
- adhesives and laminates
- manufacturing conditions
- operating environment
- test method
A raw-film datasheet may list dielectric properties measured on a specific material thickness under a defined laboratory method.
That value does not automatically represent the electrical withstand capability of a finished heater containing printed conductors, edges, connection points and additional layers.
Electrical insulation requirements should therefore be defined and verified at the heater or product level.
How Do Printing, Adhesion and Manufacturing Processes Affect the Substrate Decision?
The substrate must also be compatible with the heater manufacturing process.
Screen-printed heating elements rely on controlled deposition and processing of functional materials. The substrate therefore needs to remain suitable through printing, drying, curing and subsequent lamination or assembly steps used for the particular heater design.
For PET, surface treatment, primers or heat stabilization may be relevant to characteristics such as conductive-ink adhesion and dimensional control.
For PI, specific film grades may also be selected according to printing, adhesion and processing requirements.
This leads to an important distinction: Choosing PET or PI is only the first level of material selection.
Substrate family → Specific film grade → Surface condition → Printed materials → Printing and curing process → Lamination or protective structure → Finished heater
A material that appears suitable from a generic polymer datasheet may still require process evaluation before it is used in a specific heater design.
Toreada’s confirmed manufacturing capabilities include screen printing, silver electrode printing and carbon resistive paste printing, together with PET and PI substrate options.
The exact substrate and printed structure should be evaluated for the individual project rather than assumed to be identical across all heater designs.
How Should Adhesive Backing, Lamination and Surrounding Materials Be Considered?
A flexible heater rarely operates as an isolated film.
It normally becomes part of another assembly.
- bonded to a metal surface
- attached to glass
- integrated with a plastic component
- laminated into another material structure
- covered by insulation
- installed inside an enclosed product
- combined with an adhesive backing system
These surrounding materials can affect both thermal behavior and mechanical integration.
An adhesive layer, for example, introduces its own:
- temperature capability
- bonding characteristics
- thickness
- thermal-transfer behavior
- surface-compatibility requirements
The mounting surface can also change how heat is transferred away from the heater. A metal component, polymer housing and insulated assembly may produce different thermal results even when the electrical heater design is unchanged.
The allowable conditions of the complete assembly may therefore be constrained by another material or connection within the structure rather than by the PET or PI substrate itself.
Substrate selection should therefore include the mounting and lamination concept from an early stage.
Toreada offers adhesive-backing options for custom heating-film projects, with the mounting structure to be reviewed according to the application requirements.
Why Is a Material Datasheet Not a Finished-Heater Specification?
This distinction is particularly important when comparing PET and PI.
A raw-film technical datasheet describes the properties of a specific material grade under defined test conditions.
- mechanical properties
- dimensional behavior
- electrical properties
- thermal characteristics
- film thickness
- test methods
These values are useful engineering inputs. They are not automatically finished-heater specifications.
Raw PET or PI film → Printed conductive electrodes → Printed resistive circuit → Insulation or protective structure → Electrical connections → Adhesive or lamination structure → Finished heating element
The heater is then integrated into another product structure:
Finished heating element → Mounting surface → Product enclosure and surrounding materials → Power supply and control system → Actual operating environment
Each stage can affect the limitations and behavior of the complete system.
A temperature, dielectric or mechanical value from a generic PET or PI datasheet should therefore not be presented as the corresponding specification of a finished heating film.
Material data supports material selection. Finished-heater performance needs to be evaluated at the heater and application level.
When Is PET a Reasonable Design Direction?
PET can be a reasonable substrate direction when the selected PET-based construction is compatible with the electrical, thermal, mechanical and assembly requirements of the application.
Relevant considerations include whether:
- the expected thermal environment is compatible with the selected construction
- the required circuit can be manufactured on the selected PET substrate
- electrical-insulation requirements can be addressed by the intended structure
- dimensions and geometry are compatible with the film construction
- the mounting or lamination method can be integrated appropriately
- surrounding product materials are compatible with the heater design
PET should not be selected simply because it is assumed to be the lower-cost option, and it should not be rejected automatically because the application involves heating.
The decision should be based on the actual product requirements.
Toreada’s PET heating film platform can be considered when PET is an appropriate substrate direction for the heater architecture under development.
When Is PI a Reasonable Design Direction?
PI can be a reasonable substrate direction when the project requirements favor a polyimide-based construction.
Relevant considerations may include:
- a more demanding thermal environment
- manufacturing conditions that favor a PI-compatible structure
- electrical-insulation requirements
- dimensional or mechanical requirements
- integration with surrounding materials
- the intended assembly architecture
PI should still be selected because of defined engineering requirements rather than because it is perceived as the automatically “higher-performance” material.
A PI substrate alone does not guarantee:
- a particular heater temperature
- longer service life
- greater heating uniformity
- higher electrical power
- greater energy efficiency
- improved reliability
These characteristics depend on the complete heating element and product design.
Toreada’s PI heating film platform can be evaluated when the project requirements indicate that PI is an appropriate substrate direction.
What Information Should an OEM Provide Before Selecting PET or PI?
A substrate recommendation becomes more meaningful when the heater manufacturer understands the actual application.
Useful starting information includes:
1. Application and Heating Function
Explain what component or area needs to be heated and what the heating function is intended to achieve.
2. Heater Dimensions or Drawing
Provide the available installation area, required heater outline and any cutouts or restricted zones.
3. Required Heating Area
Identify which area requires active heating and whether any regions need to remain unheated.
4. Supply Voltage
The available voltage is a key input for electrical circuit design.
5. Target Power or Resistance
If electrical power or resistance has already been defined, provide the target. If not, the discussion can begin with the intended heating function and operating conditions.
6. Thermal Requirements
Describe the expected operating environment and thermal objective. If a temperature target exists, define where the temperature is measured, what surface or component the measurement refers to, the surrounding conditions, and whether the heater is mounted or tested free-standing.
7. Mounting Surface
Identify whether the heater will be attached to metal, glass, plastic, textile or composite structure, or another material. The mounting surface can substantially affect heat transfer.
8. Adhesive or Lamination Requirements
Indicate whether the project requires adhesive backing, lamination, protective layers, or integration into another multilayer structure.
9. Mechanical Requirements
Describe whether the heater remains flat, follows a curved surface, is bent during assembly, or experiences repeated movement.
10. Electrical Connection
Provide requirements for wire outlet position, lead-wire length, terminal, connector, and connection orientation.
11. Surrounding Materials and Available Space
Identify nearby materials and any relevant installation thickness or clearance limitations.
12. Drawings, Photos or Reference Samples
These can help clarify integration requirements more effectively than dimensions alone.
13. Project Stage
Indicate whether the project is currently at concept, engineering development, prototype, validation, or production-planning stage.
Complete specifications are not always necessary during the first discussion.
The initial review should determine which information is already defined, which items remain open and whether PET or PI is the more reasonable direction for further development.
When Should the Project Move to Prototype Evaluation?
A project should normally move into prototype evaluation once the preliminary substrate direction and basic electrical and mechanical architecture are sufficiently defined.
A practical sequence is:
Application requirements → Preliminary PET or PI direction → Electrical design → Heater geometry → Connection structure → Mounting / lamination concept → Prototype evaluation
At this stage, the prototype can help determine whether the selected substrate direction works within the actual heater and product structure.
Depending on the project, evaluation may include:
- physical dimensions and fit
- electrical resistance
- electrical power
- wire and connector arrangement
- mounting or bonding method
- thermal response in the intended assembly
- interaction with surrounding materials
- mechanical integration
- areas requiring circuit or structural revision
Prototype evaluation is particularly important because heater behavior can change after bonding, lamination or integration into the intended product.
The purpose is therefore not simply to ask whether PET or PI can “withstand heat.”
| The more useful question is: “Does the complete PET- or PI-based heater construction meet the requirements of this application?” |
If changes are needed, the substrate, circuit, mounting method or other parts of the heater architecture can be adjusted before the project moves further toward production.
Discuss a PET or PI Heating Film Project
If you are developing an OEM product that requires a flexible printed heating element, substrate selection can begin with the application rather than with a predefined preference for PET or PI.
Toreada supports custom screen-printed heating-film projects with PET and PI substrate options, silver electrode printing, carbon resistive paste printing, custom voltage, resistance, power and circuit design, as well as lead-wire, terminal, connector and mounting options.
You can discuss your application requirements with Toreada and provide available drawings, specifications, photos or reference samples for preliminary technical review.