Introduction to PBT material characteristics and analysis of injection molding process

PBT stands for Polybutylene Terephthalate, a member of the polyester family known for its crystalline thermoplastic characteristics. As defined in recent findings, PBT is a milky white, translucent to opaque resin produced via polycondensation of 1.4-butanediol and terephthalic acid or dimethyl terephthalate. It is often grouped with PET as saturated polyesters or thermoplastic polyesters.

Characteristics of PBT Materials in Automotive Applications

PBT is widely utilized in automotive lighting products, especially for decorative frames and reflectors where aluminum plating is essential, making it important to meet stringent aesthetic standards. Let's delve into the specific traits of PBT.

Process Characteristics of PBT

Melting and Crystallinity

PBT exhibits a definitive melting point between 225°C and 245°C, boasting crystallinity as high as 40%. The viscosity of the molten PBT material is predominantly influenced by shear stress rather than temperature. Thus, during the injection molding process, the injection pressure considerably affects the flow characteristics of the PBT melt.

Fluidity and Degradation

In its molten state, PBT displays commendable fluidity, just after nylon. However, care must be taken to prevent "casting" during molding, as it can degrade when exposed to water at elevated temperatures.

Injection Molding Machinery Considerations

Selection Criteria

• Ensure that the quantity of PBT used aligns with 30% to 80% of the injection molding machine's maximum rated injection volume.
• Opt for a gradient three-stage screw, maintaining a length-to-diameter ratio of 15 to 20 and a compression ratio of 2.5 to 3.0.
• Utilize a nozzle equipped with a heating and temperature control device; self-locking nozzles are recommended too.
• For flame-retardant PBT, ensure the injection machine parts are treated for anti-corrosion.

Design Considerations for Products and Molds

Product Thickness and Mold Design

• The product's thickness must be moderated, as PBT is sensitive to notches; employ arcs to connect right-angle transitions.
• Recognize that unmodified PBT experiences a significant molding shrinkage, typically between 1.7% and 2.3%, necessitating adequate demolding slopes in mold design.
• Incorporate vent holes or grooves ranging from 0.013mm to 0.038mm in depth for optimal air release during molding.
• It is advisable to maintain a larger gate diameter to facilitate better flow.
• Temperature control mechanisms should be installed in molds, limiting the maximum temperature to 100°C.
• For flame-retardant PBT, chrome-plating the mold surface is essential to counter corrosion.

Preparation of Raw Materials

Prior to the injection molding process, PBT should be thoroughly dried, aiming for moisture content below 0.02%. Using dehumidifying dryers or hot air circulation, do not exceed a drying duration of four hours at 120°C.

Injection Molding Process Parameters

Temperature and Pressure Settings

• Keep injection temperatures between 235°C to 245°C, given that PBT decomposition occurs at 280°C.
• Generally, maintain injection pressures within the range of 50 to 100 MPa.
• Since PBT cools rapidly, higher injection rates are advisable.

Additional Parameter Recommendations

• The screw speed during molding should remain below 80 RPM, ideally between 25 and 60 RPM. For intricate designs, it may be controlled at 30 RPM.
• Back pressure should fall within 10% to 15% of the injection pressure.
• Regulate mold temperatures from 70°C to 80°C with a maximum allowable temperature difference of 10°C across different parts.
• The typical molding cycle time ranges from 15 to 60 seconds.

Other Important Notes

Recycling and Shutdown Procedures

• When using recycled materials, the ratio of recycled to new materials typically falls between 15% and 75%.
• Generally, no release agent is required, although silicone release agents may be employed as necessary.
• For shutdown procedures, keep the shutdown duration within 30 minutes, lowering temperatures to approximately 200°C. Upon resuming operations after a prolonged halt, empty the barrel of any remaining material before restarting production.
• Post-processing is usually unnecessary; if needed, process at 120°C for 1-2 hours.

For additional information on other materials, refer to the following:

ABS injection molding

PSU injection molding

POM injection molding