A brief analysis of PBT plastic properties and injection molding process conditions

Many people may not know what PBT is. PBT is a semi-crystalline material and it is one of the toughest engineering thermoplastics. Its internal molecular structure is very stable, has very good mechanical strength, electrical insulation properties and thermal stability, and is often used to make household appliances, electrical components, automotive parts, etc., and is widely used.

Here is a brief introduction to what PBT materials, PBT plastic prices and PBT characteristics and applications.

What is PBT material?

PBT is called polybutylene terephthalate. It is composed of 1.4-PBT butanediol (1.4-Butylene glycol) and terephthalic acid (PTa) or terephthalate (DMT). Condensed. PBT at room temperature is milky white translucent to opaque, crystalline thermoplastic polyester. It has high heat resistance, toughness, fatigue resistance, self-lubrication and low friction coefficient. Together with PET are collectively referred to as thermoplastic polyesters, or saturated polyesters.

PBT structure

The molecular chain structure of PBT is very similar to that of PET, and it is also composed of a flexible aliphatic hydrocarbon group, a rigid phenylene group, and a polar ester group.

The ester group is bonded to the phenylene group to form a large conjugated system. Unlike PET, the length of the aliphatic hydrocarbon group is large, so the influence of the flexibility factor in PBT is greater than the flexibility factor in PET, which brings two results. One is that PBT is less rigid than the PET molecular chain overall, which will lower the glass transition temperature and melting temperature of the material. The second is that the crystallization rate is larger when the melt is cooled to the solidified state, so the polymer The degree of crystallinity that can be achieved will be higher.

PBT plastic characteristics

PBT (polyethylene terephthalate #polyester) physical property parameters

PBT physical parameters

PBT is one of the toughest engineering thermoplastics. It is a semi-crystalline material with excellent chemical stability, mechanical strength, electrical insulation properties and thermal stability. These materials are very stable under a wide range of environmental conditions.

PBT has a weak hygroscopic property. The tensile strength of the non-reinforced PBT is 50 MPa, and the tensile strength of the PBT of the glass additive type is 170 MPa. Too much glass additive will cause the material to become brittle. PBT; crystallization is very rapid, which will cause bending deformation due to uneven cooling. For materials with a glass additive type, the shrinkage in the process direction can be reduced, but the shrinkage in the vertical direction to the process is essentially indistinguishable from that of the conventional material. Generally, the material shrinkage rate is between 1.5% and 2.8%. Materials containing 30% glass additives shrink between 0.3% and 1.6%. The melting point (225% C) and the high temperature deformation temperature are both lower than the PET material. The Vicat softening temperature is approximately 170 C. The glass trasitio temperature is between 22 C and 43 C. Due to the high crystallization rate of PBT, its viscosity is very low, and the cycle time of plastic parts processing is generally low.

PBT resin injection molding process conditions

Drying treatment: This material is easily hydrolyzed at high temperatures, so drying treatment before processing is very important. It is recommended to dry in air at 120C, 6~8 hours, or 150C for 2~4 hours. Humidity must be less than 0.03%. If drying with a hygroscopic dryer, the recommended condition is 150C for 2.5 hours.

Melting temperature: 225~275C, recommended temperature: 250C. Mold temperature: 40 to 60 C for unreinforced materials. The cooling channels of the mold are well designed to reduce the bending of the plastic parts. The loss of heat must be fast and even. It is recommended that the mold cooling channel has a diameter of 12 mm.

Injection pressure: medium (up to 1500 bar).

Injection speed: The injection speed should be as fast as possible (because PBT solidifies quickly). Runners and gates: Round runners are recommended to increase pressure transfer (empirical formula: runner diameter = plastic thickness + 1.5mm). Various types of gates can be used. Hot runners can also be used, but care should be taken to prevent leakage and degradation of the material. The gate diameter should be between 0.8 and 1.0*t, where t is the thickness of the plastic part. If it is a submerged gate, it is recommended that the minimum diameter is 0.75mm. Typical use household appliances (food processing blades, vacuum cleaner components, electric fans, hair dryer housings, coffee utensils, etc.), electrical components (switches, motor housings, Fuse box, computer keyboard keys, etc.), automotive industry (heatsink windows, body panels, wheel covers, door and window components, etc.).

a, mechanical properties: high strength, fatigue resistance, dimensional stability, creep is also small (very little change under high temperature conditions);

b, heat aging resistance: enhanced UL temperature index of 120 ~ 140 ° C (outdoor long-term aging is also very good);

c, solvent resistance: no stress cracking;

d, water stability: PBT is easy to decompose with water (use caution in high temperature, high humidity environment);

e, electrical performance:

1. Insulation performance: Excellent (wet, high temperature can maintain stable electrical performance, is an ideal material for manufacturing electronic and electrical parts); 2. Dielectric coefficient: 3.0-3.2; 3. Arc resistance: 120s f, molding processability : Ordinary equipment for injection molding or extrusion. Due to the fast crystallization speed and good fluidity, the mold temperature is also lower than other engineering plastics. When processing thin-walled parts, it only takes a few seconds, and for large parts, it only takes 40-60s;

Advantages of PBT

1. Mechanical properties The tensile strength is similar to the tensile modulus and nylon;

2. The friction coefficient is small and self-lubricating;

3. Low water absorption rate;

4. Excellent electrical properties;

5, good dimensional stability;

6. Excellent chemical resistance and oil resistance;

Processing of PBT plastics

Process characteristics

a, PET and PBT have a small water absorption, but both polymers are prone to hydrolysis at the temperature of the molten state, causing a decrease in performance, and drying must be performed before the molding process.

B, PET and PBT are both semi-crystalline polymers, both have a relatively obvious melting range and a low melt viscosity.

The C, PET and PBT melts have obvious pseudoplasticity characteristics, and the viscosity has a significant dependence on the shear rate. The temperature change has little effect on the melt viscosity of the two polymers.

D, both polymers have a large shrinkage rate and its fluctuation range.

processing method

Both PET and PBT can be injection molded and extruded. In addition, PET is also used for hollow blow molding.

Application of PBT (usually referred to as modified variety)

Unreinforced PBT is mainly used in the form of a film, which is used as a wrap-around insulating film for motors, transformers, printed circuits, wires and cables, and films can also be used to prepare composite films. The reinforced PBT can be used to fabricate structural components for electronic devices such as switch parts, bobbins, connectors, motor covers, covers, air conditioner blades, and the like. Enhanced PBT can also be used to make automotive parts such as switchboards, ignition coils, and valve components. The enhanced PBT is also used to prepare computer covers, mercury lamp housings, electric iron covers, baking machine covers, and can also be used to prepare gears, cams, buttons, and the like.

The specific uses are as follows:

a, electronic appliances: connectors, switch parts, household appliances, accessories parts, small electric cover (heat resistance, flame retardant, electrical absolutely