[Weekly Topic] Special lecture on polymer materials: “Plastics” – Learn a little every day (10) PBT “Improve yourself””

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Starting this month, learning activities will be carried out in the polymer materials sector; we hope that fellow enthusiasts will come to participate, offer valuable suggestions, engage actively in learning, and those who conduct in-depth discussions will receive substantial rewards in terms of wealth and attractive bonuses~~! To encourage everyone to come and participate in discussions more actively, as well as to study more diligently, the top ten participants each time will receive a “Motivation Reward for Active Learning”. Some people might think that it’s terrifying to spend their working days doing nothing useful, performing menial tasks, not knowing what to do or what to learn; day after day, year after year, with no vision for their future We must start working hard from now on; even if we learn a little knowledge every day, it will add up over time and help us enrich ourselves. We need to set clear goals – only by filling our minds first can we enrich other aspects of our lives as well. . . . . . Just spend a short ten-odd minutes every day, and you will see the future! ! ! (Anyway, I have learned it; whether you learn it or not is up to you. Besides those experts like Hai Chuan, there are still others who know it.) . . ) Enough talk – let’s start with the basics today!! Physical Properties of PBT Material
Chemical name: Polybutylene terephthalate
Glass transition temperature (Tg): 20°C; Melting point (Tm): 222°C
Density: 1.31 g/cm³
Water absorption: 0.34% (at 23°C under equilibrium conditions)
Shrinkage upon molding: 1.7–2.3%

Mechanical Properties
I. Effect of glass fibers on the mechanical properties of PBT
1. The heat deformation temperature increases significantly.
2. All types of mechanical strength increase substantially; these values are better than those of MPPO, POM, and PC under similar conditions.

II. Effect of temperature on the mechanical properties of PBT
1. As temperature rises, the tensile strength, flexural strength, and unnotched impact strength of PBT decrease, while its notched impact strength increases.

III. PBT resin has a relatively low notched impact strength and is sensitive to notches.

IV. PBT material exhibits excellent dynamic mechanical properties as well as good creep resistance.

Thermal Properties of PBT Material
I. As the content of glass fibers increases in PBT material, the heat deformation temperature rises significantly. The heat deformation temperature of pure PBT is 60°C, while that of PBT reinforced with 30% glass fibers is 210°C.
II. The molecular structure of PBT is symmetrical, exhibiting high geometric regularity, which enables it to crystallize easily.
III. The Tg value of PBT is quite low, at only 30°C; it begins to crystallize at 40°C. Therefore, at a molding temperature of around 60°C, sufficient crystallization occurs to obtain products with the desired properties.

Electrical and Chemical Properties
I. PBT molecules do not contain strongly polar groups like those found in polyamides. Their symmetrical molecular structure and geometric regularity result in excellent electrical properties.
II. Another advantage of PBT’s electrical properties is that they change little under the influence of factors such as temperature, time, and frequency.
III. Being a crystalline material, PBT can resist most chemical reagents except strong acids and bases at room temperature, thus having good chemical resistance.

Heat Resistance and Aging Resistance
I. PBT polymers contain ester bonds, so they cannot withstand hot water or steam.
II. PBT polymers have low internal stress, resulting in poor resistance to stress cracking.
III. The thermal properties of PBT depend on the content of glass fibers and the degree of crystallization. Therefore, when the degree of crystallization is high, PBT can withstand temperatures slightly below its melting point for a short period without external forces.
IV. PBT exhibits excellent heat aging resistance; even after prolonged exposure to high temperatures, its physical properties hardly decline, and its performance remains stable.
V. When exposed to hot water for a long time, its macromolecules undergo hydrolysis, leading to a decrease in molecular weight, which in turn reduces both the degree of polymerization and strength.

Molding Processing of PBT
Mold Temperature
1. For non-reinforced grades, the typical temperature range is 38–60°C. Higher temperatures usually result in a smoother and more glossy surface of the molded parts.
2. For reinforced grades, it is recommended to use mold temperatures above 66°C. Operating within the range of 66–107°C can also improve the flowability of the reinforced resin, as well as the strength of the bonds between particles and the overall appearance of the product.

Screw Speed
1. Adjusting the screw speed so that it rotates throughout the cooling cycle without delaying the overall cycle time helps reduce damage to the glass fibers during the plasticizing process, especially for grades with lower reinforcement levels.
2. The recommended screw speed depends on the diameter of the screw, according to the formula: RPM = …

Injection Pressure
The principle for selecting injection pressure is to use the lowest pressure that still meets the requirements regarding performance, appearance, and molding cycle time. Since PBT is a material that flows easily, moderate pressure is usually sufficient. However, for products with complex geometries or thin walls, moderate to higher injection pressure is required. For non-reinforced grades, the typical pressure is 55–70 Mpa, while for reinforced grades it is 65–120 Mpa. Under normal conditions, an appropriate holding pressure is 60–80% of the injection pressure. Thin-walled products with small gates require only moderate holding pressure, whereas thick-walled products with large gates need higher holding pressure and a longer holding time.

Cycle Time
1. The cycle time mainly depends on the thickness of the part.
2. For thin-walled products with wall thicknesses of 0.76–1.52 mm, the total cycle time is usually 10–18 seconds.
3. For thick-walled products with a wall thickness of 3.8 mm, the total cycle time is typically around 40 seconds.

Everyone is welcome to listen to my broadcasts. I hope those who, like me, want to gain knowledge every day will listen. Let’s learn together every day and discuss various topics! 【Special Lecture】Special lecture on polymer materials: “Plastics”; continuously being updated. http://bbs.hcbbs.com/thread-1256917-1-1.html