Polyester
Key Takeaways
- Polyester covers a family including PET and PBT with quite different processing behaviour.
- Grade matters as much as polymer family. Behaviour varies between polymer grades, so validate per grade rather than per material.
- Semi-crystalline structure makes it harder to bond than amorphous polymers such as polycarbonate.
- Secondary operations start at material selection. Bonding, printing and marking outcomes are largely decided before the part is molded.
What Is Polyester (PET and PBT)?
Polyester in engineering plastics normally means PET or PBT — semi-crystalline thermoplastics whose partly aromatic backbone gives good strength, chemical resistance and, usefully, a natural tendency to mark well under an infrared laser.
Polyester represents a broad category of polymers characterized by ester functional groups in their main chain. In the context of laser plastic processing, the most significant polyester material is polyethylene terephthalate, commonly known as PET. This versatile thermoplastic belongs to the polyester family and has become one of the world’s most important packaging materials while also serving critical roles in textiles, films, and engineering applications. Understanding how polyester materials respond to laser processing enables manufacturers to leverage these technologies across diverse applications.
PET is characterized by excellent clarity, strength, and barrier properties against gases and moisture. The material can be processed into films, fibers, bottles, and rigid containers, making it ubiquitous in modern manufacturing. A modified version, polyethylene terephthalate glycol (PETG), incorporates glycol modification to enhance processing characteristics and impact resistance while maintaining many of PET’s desirable properties.
Material Properties and Structure
Polyethylene terephthalate is a semicrystalline thermoplastic whose crystallinity can be controlled through processing conditions. Rapidly cooled PET remains largely amorphous and transparent, while slower cooling or subsequent heat treatment promotes crystallization and opacity. This variable crystallinity affects optical properties, mechanical strength, and behavior under laser processing.
PET offers exceptional dimensional stability, chemical resistance, and barrier properties. The material provides excellent resistance to moisture vapor transmission, making it ideal for food and beverage packaging applications. Electrically, PET serves as an excellent insulator with properties suitable for transformer insulation, flexible printed circuits, and electrical tape backings.
The material’s melting point around 260°C is higher than many commodity plastics, providing good thermal stability for demanding applications. However, this relatively high melting point also means PET requires more laser energy for cutting and welding compared to lower-melting thermoplastics.
PETG Properties
PETG modification reduces crystallization tendency, resulting in consistently amorphous, transparent material that is easier to process than standard PET. The material offers exceptional clarity, toughness, and high chemical resistance while maintaining excellent formability. PETG’s reduced melting point and broader processing window compared to PET make it particularly suitable for laser cutting applications.
Laser Cutting Polyester
CO2 laser cutting of polyester materials produces clean edges free of significant discoloration with limited melting when parameters are properly optimized. The material efficiently absorbs the 10.6 micrometer CO2 wavelength, enabling precise cutting of complex shapes.
PET and PETG have lower melting points compared to other thermoplastics such as acrylic or polycarbonate. This characteristic makes them more susceptible to heat damage during laser cutting, potentially causing excessive melting, poor cut quality, or deformation if settings are not carefully controlled. Successful processing requires lower power settings and higher cutting speeds compared to many other plastics.
When laser cutting polyester films, the sealed edges created by the laser beam eliminate the need for post-processing to prevent fraying or delamination. This sealed-edge characteristic is particularly valuable for multilayer film applications where maintaining layer integrity is critical.
Laser Engraving and Marking
Laser engraving of polyester produces depth with high contrast and limited melt-back at the ablation point. The material responds well to CO2 laser engraving, creating recessed marks with clean edges. However, Laser Marking of polyester (surface color change without material removal) using CO2 lasers is generally not recommended due to the tendency to create uncontrolled melting rather than clean color change.
Fiber lasers and UV lasers offer alternative approaches for polyester marking. These shorter wavelength sources can produce surface marks through mechanisms other than simple melting, potentially creating better results for applications requiring surface-level identification without material removal.
For high-contrast permanent marking on polyester products, laser-sensitive additives incorporated into the material formulation can dramatically improve marking quality and speed. These additives absorb laser energy and facilitate color-changing reactions, producing readable marks with standard laser marking equipment.
Laser Welding Polyester Materials
PET can be laser welded using transmission welding techniques similar to other thermoplastics. The relatively high melting point requires adequate laser power to achieve proper fusion, and the material’s sensitivity to thermal degradation demands careful control of energy input to avoid burning or charring at the weld zone.
PETG’s lower melting point and broader processing window make it somewhat easier to laser weld than standard PET. The material produces strong joints when properly welded, though its reduced crystallinity compared to PET may affect ultimate joint strength depending on application requirements.
Both PET and PETG require absorptive additives in one component for conventional transmission laser welding. Clear-to-clear welding techniques using specialized absorbers or longer wavelength lasers can produce joints in transparent assemblies without visible dark additives.
Film and Textile Applications
Polyester films are extensively processed using laser cutting technology. Applications include electrical insulation films, printed graphics, tape backings, and release films. The precision and speed of laser cutting combined with the sealed edges produced make it ideal for film converting operations.
Polyester textiles and woven fabrics represent another significant laser processing application area. Laser cutting fabric creates sealed edges that prevent fraying without additional finishing operations. Complex patterns and shapes can be cut rapidly without die tooling, enabling economical production of custom and short-run textile products.
Industry Applications
Polyester’s versatility and laser processability serve numerous industries:
- Food and beverage packaging with laser-marked product identification and lot codes
- Electrical insulation films with laser-cut custom shapes for transformers and motors
- Flexible printed circuit substrates with laser-drilled vias and cut patterns
- Protective films and tapes with laser-cut precision shapes
- Textile and apparel production with laser-cut patterns and engraved designs
- Medical packaging with laser-welded seals and identification marking
Processing Considerations
Several factors require attention when laser processing polyester materials. Adequate ventilation and fume extraction are essential as the material produces fumes during laser processing. While PET fumes are generally less hazardous than some plastics, proper safety measures protect workers and maintain air quality.
Material moisture content can affect laser processing results, particularly for cutting and welding operations. Conditioning materials to appropriate moisture levels before processing helps ensure consistent results.
For critical applications, pre-production testing with representative material samples is recommended to optimize laser parameters for the specific polyester grade, thickness, and processing requirements.
PET and PBT Are Not Interchangeable
Both are thermoplastic polyesters and they are routinely discussed together, but they
crystallise differently and that difference drives most processing decisions:
| PET | PBT | |
|---|---|---|
| Crystallisation rate | Slow — can be quenched to a clear amorphous state | Fast — crystallises readily, so mouldings are opaque |
| Typical form | Bottles, film, fibre, thermoformed packaging | Injection-moulded engineering components |
| Clarity | Achievable when quenched, which is the basis of the bottle industry | Not available; naturally opaque |
| Moisture sensitivity at melt | High — hydrolysis degrades molecular weight permanently | High — same mechanism |
| Laser marking | Moderate to good; aromatic backbone chars reasonably | Good, and better still in filled grades |
| Dimensional stability | Varies with crystallinity, which varies with cooling | Good and consistent |
| Main decorating use | Film and bottle printing, requiring corona treatment | Component marking for traceability |
Drying is not optional for either. Polyester hydrolyses at melt temperature in the
presence of moisture, and the molecular weight loss is permanent — the part may look
acceptable while having lost toughness that no downstream operation can restore.
Diagnosing Polyester Processing Problems
| Symptom | Likely cause | Correction |
|---|---|---|
| Parts brittle despite correct mould settings | Hydrolytic degradation from inadequate drying | Verify the dryer, not the mould. The loss is permanent and will not show as a visual defect. |
| Ink will not anchor to PET film | Untreated, or treatment decayed | PET starts around 41 dyne/cm and typically needs 46–52. Corona is the standard route; measure across the web width. |
| Mark contrast varies between mouldings | Crystallinity varying with cooling rate | Crystallinity changes surface appearance and therefore apparent contrast. Control mould temperature before adjusting the laser. |
| Clear PET part turns hazy after moulding | Crystallisation from slow cooling | Quench faster. Clarity in PET depends on preventing crystallisation, not on the resin alone. |
| Glass fibres visible in the mark on filled PBT | Polymer ablates below the threshold of the glass | Reduce energy per pass and add passes. |
| Weld strength inconsistent | Semi-crystalline structure scattering the beam | Control crystallinity through moulding conditions; expect a narrower window than an amorphous polymer. |
| Marks acceptable at inspection, poor code grade | Surface texture and gloss affecting readability | Fix the illumination geometry in the inspection method and grade to ISO/IEC 15415. |
Related Terms and Reading
- Carbonization
- Laser marking clear and transparent plastics
- Plastics laser marking solutions
- Polymer surface pretreatment
Applying this in production
The Sabreen Group provides independent engineering support for PET and PBT marking, printing and bonding. 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 the practical difference between PET and PBT?
Crystallisation rate. PET crystallises slowly and can be quenched to a clear amorphous state, which is what makes transparent bottles and film possible. PBT crystallises quickly, so mouldings are opaque but dimensionally consistent, which suits injection-moulded engineering components. They share a chemistry and a moisture sensitivity, but they are specified for different reasons.
Why is drying so critical for polyester?
Because the ester linkage hydrolyses at melt temperature in the presence of moisture, permanently reducing molecular weight and toughness. The damaging part is that a part can look entirely acceptable while having lost mechanical properties, so the defect is discovered in service rather than at inspection. Drying to the supplier specification is a requirement, not a refinement.
Does polyester laser mark well?
Reasonably. The aromatic backbone chars adequately, so PET and PBT give moderate to good contrast without an additive, and glass-filled PBT grades mark better still. The complication is crystallinity: it varies with cooling rate and changes surface gloss, which changes apparent contrast between mouldings that are otherwise identical.
What surface energy does PET film need before printing?
Commonly 46–52 dyne/cm, against roughly 41 untreated — a smaller increment than polyolefins require because PET starts higher. Corona treatment is the standard route in web converting. Measure across the full web width rather than at one point, since edge-to-centre variation is common and easily missed.
Why does a clear PET part turn hazy?
It has crystallised. Clarity in PET depends on cooling the part fast enough to freeze it in an amorphous state; slow cooling allows crystals to form and scatter light. This is a process control issue rather than a material fault, and it is why thick sections are much harder to keep clear than thin walls.
