Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are two types of aromatic polyester plastics.
PET has a highly symmetrical molecular structure, which gives it good crystal orientation capabilities. As a result, it has excellent film-forming and molding properties.
The molecular chain structure of PBT is similar to that of PET, and most of its properties are the same. The main difference lies in the molecular backbone, which changes from two methylene groups to four, resulting in a more flexible molecular structure and better processing performance.
1. The difference between PBT and PET
- PBT has lower strength and stiffness
- Better impact resistance
- Lower glass transition temperature
2. Applications and characteristics of PET
2.1 Application
Within the classification of plastics, PET is designated number one, with a variety of applications:
- Can be spun into polyester fiber, commonly known as polyester.
- It can be used to produce films for recording, photography, insulation, product packaging, etc.
- Suitable for blow molding various bottles, such as Coke bottles, mineral water bottles, etc.
- It can be applied to electrical components, bearings, gears, etc.
2.2 Injection molding process conditions
Drying: PET is highly hygroscopic, so it needs to be pre-dried. The recommended drying conditions are 120~165℃, 4 hours, and the humidity is less than 0.02%.
Melting temperature: 265280°C for the unfilled version, and 275-290°C for the glass-filled version.
Mold temperature: 80~120℃.
Injection pressure: 300~1300 bar.
Injection speed: Use a higher injection speed without causing brittleness.
Runners and gates: Conventional gate types can be used. The gate size is 50~100% of the part thickness.
2.3 Chemical and physical properties
The glass transition temperature of PET is approximately 165°C. The crystallization temperature range is 120~220℃. PET is highly hygroscopic at high temperatures. Adding a crystallization accelerator can increase the crystallinity of the material.
Products made from PET have a glossy finish and a high heat distortion temperature. Special additives such as mica can minimize warpage. Even with unfilled PET, transparent products can be achieved by using lower mold temperatures.
3. Applications and characteristics of PBT
3.1 Application
Typical applications include:
- Household appliances (food processing blades, vacuum cleaner components, electric fans, hair dryer housings, coffee containers)
- Electrical components (switches, motor housings, fuse boxes, computer keyboard keys)
- Automotive industry (radiator grilles, body panels, wheel covers, door and window components).
3.2 Injection molding process conditions
Drying: PBT is prone to hydrolysis at high temperatures, so pre-drying is crucial. Recommended drying conditions are 120°C for 68 hours or 150°C for 24 hours, with humidity below 0.03%.
Melting temperature: 225~275℃, recommended temperature: 250℃.
Mold temperature: 40~60°C for unfilled materials. Proper mold cooling design is critical to reducing part warpage.
Injection pressure: medium, up to 1500 bar.
Injection speed: Because PBT cures very quickly. So use a faster injection speed.
Runners and Gates: Round runners are recommended to increase pressure transfer. A variety of gate types are available. The gate diameter is 0.8~1.0 times the thickness of the part.
3.3 Chemical and physical properties
PBT is one of the toughest engineering thermoplastics. It has excellent chemical stability, mechanical strength, electrical insulation properties, and thermal stability. Its hygroscopicity is weak. The unfilled type of PBT has a tensile strength of 50 MPa. The tensile strength of the glass-filled type is 170 MPa. Too much glass content can make the material brittle.
