After the read, you will know about what are the process conditions of PBT injection molding.
With the rapid development of the plastics industry, new varieties of modified plastics are emerging, and polyethylene terephthalate (abbreviated as PBT) is an engineering material with excellent performance.
What is PBT material?
PBT is a milky white translucent to opaque, crystalline thermoplastic polyester. It has high heat resistance, toughness, fatigue resistance, self-lubrication, low coefficient of friction, weather resistance, low water absorption rate of 0.1%, and maintains various physical properties (including electrical properties) and electrical insulation in a humid environment, but has a high dielectric loss.
PBT is resistant to heat water, alkalis, acids, and oils, but is susceptible to erosion by halogenated hydrocarbons, has poor hydrolysis resistances, can be rapidly crystallized at low temperatures, and has good molding properties.
PBT crystallization speed is fast, the most suitable processing method is injection molding, other methods are extrusion, blow molding, coating, and various secondary processing molding, pre-drying is required before molding, moisture content should be reduced to 0.02%.
Modified PBT material is modified with reinforcement and flame retardant to improve its heat resistance, flame retardant, dimensional stability, and modulus, PBT (reinforced and modified PBT) is mainly used in automobiles, electronic and electrical appliances, industrial machinery and polymer alloy and blending industry.
Such as distributor, body parts, ignition coil skeleton, insulation cover, exhaust system parts, motorcycle ignition in automobiles, electronic and electrical industries such as television deflection coil, picture tube, and potentiometer bracket, companion output transformer skeleton, adapter skeleton, switch connectors, electric fans, refrigerators, washing machines motor end caps, bushings.
What are the PBT injection molding and processing properties?
1) The moisture absorption of PBT is small, but the moisture at high temperature is sensitive, and the molding processing will degrade the PBT molecules, darken the color and produce surface spots, so it should usually be dried.
2) PBT melt has excellent fluidity, so it is easy to mold thin-walled and complex-shaped products, but we should 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 sharply, which should be specially noted.
4) PBT has a narrow range of molding and processing, and it crystallizes quickly when it cools down, so it is especially suitable for rapid injection because of its good fluidity.
5) PBT has a large shrinkage rate and shrinkage range, and the difference in shrinkage rate in different directions is more obvious than in other plastics.
6) PBT is sensitive to the reaction of notches and sharp corners, and the stress concentration is easy to appear in these positions so that the load-bearing capacity is greatly reduced, and the rupture is easy to occur when the force or impact is applied, so attention should be paid to this point when designing plastic parts, and all corners, especially the internal corners, should be rounded as far as possible.
7) The elongation of pure PBT can reach 200%, so the products with small depressions can be forced off from the mold. However, after filling with glass fiber or filler, its elongation decreases greatly, and if there is a depression in the product, it is not possible to implement forced demoulding.
8) The runner of the PBT mold should be short and thick if possible, and the best effect is with the round runner. Generally modified and unmodified PBT can be used with normal runners, but glass fiber reinforced PBT can be molded with hot runners only with good results.
9) Point gate and latent gate have a large shearing effect and can reduce the apparent viscosity of PBT melt, which is favorable to molding, so they are often used.
10) The gate should preferably be facing the core cavity or core, so as to avoid injection and minimize backfill when the melt flows in the mold cavity. Otherwise, the product will easily produce surface defects and deteriorate its performance.
What are the main PBT injection molding conditions?
PBT injection molding condition 1) Barrel temperature
The choice of barrel temperature is very important to the PBT injection molding. If the PBT injection molding temperature is too low, the plasticization will be poor, which will cause a lack of material, depression, uneven shrinkage, and lack of luster in the products.
If the PBT injection molding temperature is too high, it will cause serious nozzle salivation, edge overflow, dark color, and even degradation.
Usually, the barrel temperature of the PBT injection molding is controlled at 240~280℃, and the glass fiber reinforced PBT injection molding barrel temperature is controlled at 230~260℃.
And the nozzle temperature of the PBT injection molding should be controlled at 5~10℃ lower than the temperature of the front section of the barrel.
PBT injection molding condition 2) Injection molding temperature
The mold temperature is directly related to the dimensional stability, warpage deformation, molding cycle, and crystallinity of the product. PBT is easy to crystallize, even at room temperature, so the BPT injection molding temperature is not too high, usually, 40-60℃, while the mold temperature of glass fiber reinforced PB is slightly higher, usually 60-80℃.
PBT injection molding condition 3) Injection molding pressure
The melt viscosity of PBT is low and the fluidity is good, so the PBT injection molding pressure can be moderate, usually 60~90MPa, and the injection pressure of glass fiber reinforced PBT is 80~100MPa, usually the PBT injection molding pressure increases with the increase of the thickness of the plastic parts, but do not exceed 100MPa, otherwise it will make the mold release difficult.
What are the problems to be paid attention to when processing PBT injection molding?
(1) The influence of resin viscosity on product performance
The greater the viscosity of PBT material, the greater the impact strength and elongation at break. Different viscosities have different product applications.
PBT materials with characteristic viscosity less than 0.85 are low-viscosity resins, mainly used for heat-resistant coatings such as cardboard and food packaging.
PBT materials with characteristic viscosity of 0.85 to 1.0 are medium viscosity resins that can be used for high precision engineering parts, cams, housings, office equipment, automotive parts, etc.
PBT material characteristics of viscosity greater than 1.0 high viscosity resin, commonly used in corrosion-resistant pumps, electric irons, coffee filter housing, etc.
(2) The effect of water content
If the water content increases, the product performance will drop sharply, so it is generally required to dry the material before PBT injection molding so that the water content can be reduced to less than 0.03%.
(3) Glass fiber content
Generally take the glass fiber content of 30%, the comprehensive performance index is better; glass fiber length in 0.1 ~ 1 mm, the best use of alkali-free twist 40 / 20 roving, glass fiber to be treated by KH-550 series of silane coupling agent is good.
(4) Forming equipment
Generally use the long glass fiber directly into the twin-screw extruder method, but also available short glass fiber reinforced PBT material. PBT injection molding temperature is generally 245 ~ 270 ℃, mold temperature of 80 ~ 90C, non-reinforced PBT injection mold temperature to be lower, 40 ~ 60 ℃.
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