Everything you need to know about PBT Injection Molding

The polymerization process of PBT is mature, the cost is low, and the injection molding process is easy.

The performance of unmodified PBT is not good. In practical application, PBT should be modified. Among them, glass fiber-reinforced modified brand accounts for more than 70% of PBT.

PBT material properties

PBT material has excellent toughness and fatigue resistance, good heat and weather resistance, good mechanical properties, and low water absorption.

Modified PBT plastic material to enhance, flame retardant and other modifications, to improve its heat resistance, flame retardant, dimensional stability, and flexural modulus, used in automotive, electronic, and electrical industries.

Preparation of PBT injection molding materials

Before injection, it should be dried and the moisture content should be controlled below 0.02%.

When using hot air circulation drying, when the temperature is 105℃, 120℃, or 140℃, the corresponding time is not more than 6h, 4h, or 2h. The thickness of the material layer is less than 30mm.

Injection molding process parameters of PBT materials

Injection temperature: the decomposition temperature of PBT is 280℃, so it is generally controlled between 235 ℃ and 245℃ in actual production.

Injection pressure is generally 50 ~ 100MPa.

Injection rate PBT cooling speed is fast, so a faster injection rate should be used.

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

The mold temperature is generally controlled at 70 ~ 80℃, and the temperature difference of each part is not more than 10℃. The forming cycle is generally 15 ~ 60s.

Precautions for PBT materials

1. The use of recycled materials and new materials in the proportion of 25% ~ 75%.

2. The use of a release agent generally does not use a release agent, when necessary, the silicone release agent can be used.

3. The downtime of PBT processing is within 30min, and the shutdown can be stopped when the temperature is reduced to 200℃.

In the case of production after a long-term shutdown, the material in the cylinder should be emptied and new material should be added to carry out normal production.

4. The post-treatment of products generally does not need to be processed, if necessary at 120℃ treatment 1 ~ 2h.

Molding performance of PBT

1. PBT hygroscopic property is small, but at high temperatures, water is more sensitive, molding processing will make PBT molecular degradation, color dark, and surface marks, so usually should be dried.

2. PBT melt has excellent fluidity, so it is easy to form thin-walled, complex shape products, but pay attention to mold overflow, and nozzle salivation phenomenon.

3. PBT has an obvious melting point, when the temperature rises above the melting point, the fluidity will increase suddenly, especially attention should be paid to.

4. PBT molding processing range is narrow, cooling crystallization is fast, coupled with good fluidity, especially suitable for rapid injection.

5. PBT has a larger shrinkage rate and shrinkage range, and the shrinkage rate difference in different directions is more obvious than in other plastics.

6. PBT response to the notch, sharp Angle is very sensitive, in these positions are prone to stress concentration, so that the bearing capacity is greatly reduced, prone to rupture when the force or impact, so in the design of injection molding plastic parts should pay attention to this point, all corners, especially the internal corner should try to use arc transition.

7. The elongation of pure PBT can reach 200%, so the products with small depressions can be forced to demold from the mold.

But AFTER FILLING it WITH glass fiber OR PACKING, ITS elongation DECREASES greatly, IF there is a depression in the product, it cannot implement compulsory demold.

8. The flow channel of the PBT mold should be short and thick when possible, and the round flow channel has the best effect.

General modified and unmodified PBT can be used in ordinary runners, but glass fiber-reinforced PBT application of hot runner molding has a good effect.

9. Point gate and the latent gate of the shear action is large, can reduce the apparent viscosity of the PBT melt, is conducive to injection molding, is often used gate, and gate diameter should be biased.

10. It is better to have the gate directly in front of the core cavity or core, to avoid spraying and minimize the filling when the molten material flows in the mold cavity. Otherwise, the product is easy to produce surface defects and performance of deterioration.

PBT main injection molding conditions

1. Barrel temperature. Cylinder temperature selection of PBT forming is very important, such as low temperature, and bad plasticization will cause products to lack material, depression, uneven shrinkage, and luster phenomenon.

If the temperature is too high, will cause nozzle salivation serious, overflow edge, color and luster, and even degradation.

Usually, the barrel temperature is controlled at 240~280℃, and glass fiber-reinforced PBT is controlled at 230~260℃. The nozzle temperature should be controlled at 5~10℃ below the temperature of the front section of the barrel.

2. Mold temperature. The die temperature is directly related to the dimensional stability, warping deformation, forming cycle, and crystallinity of the product.

PBT is easy to crystallize, even at room temperature crystallization is also very fast, so the mold temperature does not need to be too high, usually 40~60℃, and glass fiber reinforced PBT mold temperature is slightly higher, usually 60~80℃.

3. Injection pressure. PBT melt viscosity is low, fluidity is good, can use the injection pressure of medium degree, it is 60~90MPa commonly, glass fiber enhances PBT for 80~100MPa, usually injection pressure increases along with the increase of plastic part thickness, but does not exceed 100MPa, can make de-molding difficult otherwise.

Process characteristics of PBT

PBT has an obvious melting point, melting point is 225 ~ 235℃, is a crystalline material, and crystallinity can reach 40%.

The viscosity of PBT melt is not as affected by temperature as the shear stress. Therefore, the injection pressure has an obvious effect on the fluidity of PBT melt in injection molding.

PBT in the molten state of good fluidity, and low viscosity, second only to nylon, in the injection molding, is prone to the “flow” phenomenon. Anisotropy of PBT injection molded products. PBT is easy to degrade in water at high temperatures.

PBT material requirements for injection molding machine

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

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

2. Should be selected gradient type three screw, the length-diameter ratio of 15 ~ 20, the compression ratio of 2.5 ~ 3.0.

3. Self-locking nozzle should be selected and equipped with a heating and temperature control device.

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

PBT products and die design

1. The thickness of the product should not be too thick, PBT is sensitive to the notch, therefore, the product at the right Angle and other transitions should be connected by an arc.

2. The shrinkage rate of unmodified PBT is larger, 1.7% ~ 2.3%, and the injection mold should have a certain demould slope.

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

4. The diameter of the gate should be large.

5. The mold needs to be set with the temperature control device. The highest mold temperature shall not exceed 100℃.

6. Flame retardant PBT forming, die surface to chrome plating, to corrosion.