Introduction to PBT material characteristics and analysis of injection molding process

Introduction to PBT Material Characteristics and Analysis of Injection Molding Process

Polybutylene terephthalate, commonly referred to as PBT, belongs to the polyester family. This crystalline thermoplastic polyester resin is produced through polycondensation using 1.4-butanediol and terephthalic acid or dimethyl terephthalate. PBT, along with PET, forms what are known as thermoplastic polyesters or saturated polyesters.

Characteristics of PBT Materials

PBT materials are widely utilized in automotive applications, particularly in components such as decorative frames and reflectors that demand high aesthetic quality. These materials exhibit distinct properties that make them suitable for such functions.

Process Characteristics of PBT

The melting point of PBT is well-defined, ranging from 225℃ to 245℃. With a crystallinity level reaching up to 40%, PBT’s melt viscosity is mostly influenced by shear stress rather than temperature. In injection molding, this characteristic signifies that the injection pressure plays a critical role in determining the fluidity of the PBT melt. It is crucial to monitor the temperature fluctuations of the melt during the screw’s shearing action while plasticizing it.

PBT exhibits excellent fluidity along with low viscosity in its molten state, falling just shy of nylon in terms of processability, and may experience "casting" issues during molding. It's also essential to note that PBT molded products can show anisotropic properties and may degrade when exposed to high-temperature water.

Selecting a Screw Injection Molding Machine

When choosing an injection molding machine for PBT, consider the following:

• The material amount used should range from 30% to 80% of the maximum rated injection volume. Avoid using oversized machines for small products.
• A gradient three-stage screw with a 15 to 20 length-to-diameter ratio and a compression ratio between 2.5 to 3.0 is recommended.
• Utilizing a nozzle equipped with a heating and temperature control system is essential.
• For flame-retardant PBT, incorporate anti-corrosion treatment for the injection machine’s components.

Product and Mold Design Considerations

While designing molds for PBT, several key factors must be observed:

• The product thickness should be kept minimal, as PBT is sensitive to notching. Utilize rounded transitions at angles.
• Be aware of the significant molding shrinkage, which ranges from 1.7% to 2.3%, necessitating a slight demolding slope.
• Vent holes or grooves, ideally 0.013 to 0.038 mm deep, should be integrated into the mold.
• Use larger gate diameters for optimal material flow.
• Include a temperature control device, ensuring the mold temperature does not exceed 100℃.
• For flame-retardant PBT, chrome-plating the mold surface is advisable for corrosion resistance.

Raw Material Preparation and Injection Molding Parameters

Prior to injection molding, drying the PBT is crucial, ensuring the moisture content is below 0.02%. Optimal drying can be achieved at 120℃ for no longer than 4 hours using various drying techniques. In terms of injection parameters, the following guidelines should be followed:

• The injection temperature should be maintained between 235℃ and 245℃, while PBT's decomposition temperature is 280℃.
• Injection pressures typically fall within the 50 to 100 MPa range.
• Due to the rapid cooling of PBT, faster injection rates are recommended.
• Screw speed should range from 25 to 60 r/min, not exceeding 80 r/min, and for complex structures, it should be approximately 30 r/min.
• The mold temperature need be set between 70℃ and 80℃, with minimal temperature variation across areas.
• General molding cycles can vary from 15 to 60 seconds.

Finally, it is important to consult additional resources regarding ABS injection molding, PSU injection molding, and POM injection molding for comprehensive knowledge.