Introduction to PBT

Polybutylene terephthalate (PBT for short) is a series of polyesters, which is made of 1.4-pbt butylene glycol and terephthalic acid (PTA) or terephthalic acid ester (DMT) by polycondensation, and is made of milky white through the mixing process. Translucent to opaque, crystalline thermoplastic polyester resin. Together with PET, it is collectively known as thermoplastic polyester, or saturated polyester.

PBT was first developed by German scientist P. Schlack in 1942, then developed industrially by Celanese Corporation (now Ticona) and marketed under the trade name Celanex, which was launched in 1970 as a 30% glass fiber reinforced plastic under the trade name X-917, later changed to CELANEX. Eastman launched a product with and without glass fiber reinforcement, under the trade name Tenite (PTMT); in the same year, GE also developed a similar product with three varieties of unreinforced, reinforced and self-extinguishing. Subsequently, world-renowned manufacturers such as BASF, Bayer, GE, Ticona, Toray, Mitsubishi Chemical, Taiwan Shin Kong Hefei, Changchun Synthetic Resins, and Nanya Plastics have successively entered the production ranks, and there are more than 30 manufacturers worldwide.

As PBT has heat resistance, weather resistance, chemical resistance, good electrical characteristics, low water absorption, good gloss, widely used in electronic appliances, automotive parts, machinery, household products, etc., and PBT products and PPE, PC, POM, PA, etc. together known as the five major general engineering plastics. PBT crystallization speed, the most suitable processing method is injection molding, other methods are extrusion, blow molding, coating, etc.

Typical application scope

Household appliances (food processing blades, vacuum cleaner components, electric fans, hair dryer shells, coffee utensils, etc.), electrical components (switches, motor housings, fuse boxes, computer keyboard keys, etc.), automotive industry (lamp trim frames, radiator grille windows, body panels, wheel covers, door and window components, etc.).

Chemical and physical properties

PBT is one of the toughest engineering thermoplastics, it is a semi-crystalline material with very good chemical stability, mechanical strength, electrical insulation properties and thermal stability. pbt has good stability under environmental conditions. pbt has very weak moisture absorption properties. The tensile strength of non-reinforced PBT is 50 MPa, and the tensile strength of glass fiber additive type PBT is 170 MPa. too much glass fiber additive will cause the material to become brittle. the crystallization of PBT is very fast, and uneven cooling will cause bending deformation. For the material with glass fiber additive type, the shrinkage rate in the process direction can be reduced, and the shrinkage rate in the vertical direction is basically no different from the normal material. The shrinkage rate of general PBT materials is between 1.5% and 2.8%. The shrinkage of materials containing 30% glass fiber additives is between 0.3% and 1.6%.

The characteristics of PBT injection molding process

The polymerization process of PBT is mature, low cost and easy to mold and process. The performance of unmodified PBT is not good, and the actual application of PBT should be modified, of which, glass fiber reinforced modified grades account for more than 70% of PBT.

1, PBT has an obvious melting point, melting point of 225 ~ 235 ℃, is a crystalline material, crystallinity up to 40%. the viscosity of PBT melt is not affected by temperature as much as shear stress, therefore, in injection molding, the injection pressure on PBT melt fluidity is obvious. PBT in the molten state of good fluidity, low viscosity, second only to nylon, in molding easy to occur “PBT molded products are anisotropic, and PBT is easy to degrade under high temperature in contact with water.

2、Injection molding machine

When choosing a screw type injection molding machine. The following points should be considered.

① The amount of material used in the product should be controlled at 30% to 80% of the rated maximum injection volume of the injection molding machine. It is not appropriate to use a large injection molding machine to produce small products.

② should be selected with a gradual three-stage screw, length to diameter ratio of 15-20, compression ratio of 2.5 to 3.0.

③It is best to use self-locking nozzle with heating and temperature control device.

④In molding flame retardant PBT, the relevant parts of the injection molding machine should be treated with anti-corrosion.

3、Product and mold design

①The thickness of the products should not be too thick, and PBT is sensitive to the notch, so the transitional places such as the right angle of the products should be connected by arcs.

②The molding shrinkage of unmodified PBT is large, and the mold should have a certain slope of demoulding.

③The mold needs to be equipped with exhaust holes or exhaust slots.

④The diameter of the gate should be large. It is recommended to use circular runners to increase the pressure transfer. Various types of gates can be used and hot runners can also be used. The gate diameter should be between 0.8 and 1.0*t, where t is the thickness of the plastic part. In case of submerged gates, a minimum diameter of 0.75mm is recommended.

⑤ The mold needs to be equipped with temperature control device. The maximum temperature of the mold should not exceed 100℃.

⑥For flame retardant grade PBT molding, the surface of the mold should be chrome plated to prevent corrosion.

Setting of process parameters

Drying treatment: PBT material is easily hydrolyzed at high temperature, so it needs to be dried before processing. It is recommended to dry in hot air at 120℃ for 4 hours, and the humidity must be less than 0.03%.

Melting temperature: 225℃～275℃, recommended temperature: 250℃.

Mold temperature: 40℃～60℃ for unreinforced material. Mold cooling should be uniform to reduce the bending deformation of the plastic parts, and the recommended diameter of the mold cooling cavity channel is 12mm.

Injection pressure: medium (generally 50 to 100MPa, maximum to 150MPa).

Injection speed: Injection rate PBT cooling speed is fast, so a faster injection rate should be used. The fastest possible injection rate should be used (because PBT solidifies quickly).

Screw speed and back pressure: The screw speed for molding PBT should not exceed 80r/min, and is generally between 25 and 60r/min. The back pressure is generally 10%-15% of the injection pressure.

Attention

①The use of recycled material The ratio of recycled material to new material is generally 25% to 75%.

②The use of mold release agent Generally, no mold release agent is used, and silicone mold release agent can be used if necessary.

③Shutdown processing The shutdown time of PBT is within 30min, and the temperature can be lowered to 200℃ when shutdown. When producing again after long-term shutdown, the material in the barrel should be emptied and then new material should be added for normal production.

④ Post-processing of products Generally, no treatment is required, and if necessary, 1~2h treatment at 120℃.

PBT special screw

For PBT, which is easy to decompose, sensitive to pressure and needs to add glass fiber, the PBT special screw produces stable pressure and uses double alloy to improve wear resistance for the material with glass fiber (PBT+GF).