Technical Disscussion

PET (Polyethylene terephthalate)

Drying and Crystallizing before Injection molding processing

It must be dried before molding. PET is very sensitive to hydrolysis. The conventional air heating-dryer are 120-165 C (248-329 F) for 4 hours. The moisture content should be less than 0.02%.

Adopt ODEMADE IRD system, the drying time only needs 15mins. Save energy cost about 45-50%. The moisture content can be 50-70ppm. (The drying temperature, drying time can be adjustable by customers' requirement on drying material, all the system is controlled by Siemens PLC ). And it is the processing with Drying & Crystallizing at a time.

Melt temperature

265-280 C (509-536 F) for unfilled grades

275-290 C (527-554 F) for glass reinforcement grade

Mold temperature

80-120 C (176-248 F); Preferred range: 100-110 C (212-230 F)

Material injection pressure

30-130 MPa

Injection speed

High speed without causing embrittlement

Injection molding machine:

Injection molding is mainly used to enhance the molding of PET. Usually, PET can only be formed by a screw injection molding machine.

It is best to choose a mutant screw with a reverse ring on the top, which has a large surface hardness and wear resistance, and the aspect ratio is not L / D = (15 ~ 20): 1 compression ratio of 3:1.

Materials with too large L / D stay in the barrel for too long, and excessive heat can cause degradation and affect product performance. The compression ratio is too small to generate less heat, is easy to plasticize, and has poor performance. On the other hand, the breakage of the glass fibers will be more and the mechanical properties of the fibers will be reduced. When the glass fiber reinforced PET is reinforced, the inner wall of the barrel is severely worn, and the barrel is made of a wear resistant material or lined with a wear resistant material.

Since the nozzle is short, the inner wall needs to be ground and the aperture should be as large as possible. The nozzle of the hydraulic brake valve type is good. The nozzles should have insulation and temperature control measures to ensure that the nozzles do not freeze and block. However, the nozzle temperature should not be too high, otherwise it will cause runny. Low pressure PP material must be used and the barrel cleaned before starting to form.

The main injection molding conditions for PET:

1. The temperature of the barrel.

The molding temperature range of PET is narrow, and the temperature will directly affect the performance of the product. If the temperature is too low, it is not good to plasticize plastic parts, dents, and lack of material defects; on the contrary, if the temperature is too high, it will cause splashing, the nozzles will flow, the color will become darker, the mechanical strength will decrease, and even degradation will occur. Generally, the barrel temperature is controlled at 240 to 280 ° C, and the glass fiber reinforced PET barrel temperature is 250 to 290 ° C. The temperature of the nozzle should not exceed 300 ° C, and the temperature of the nozzle is usually lower than the barrel temperature.

2. Mold temperature.

The mold temperature directly affects the cooling rate and crystallinity of the melt, the crystallinity is different, and the properties of the plastic parts are also different. Usually, the mold temperature is controlled at 100 to 140 °C. Smaller values are recommended when forming thin-walled plastic parts. When forming thick-walled plastic parts, it is recommended to have greater value.

3. Injection pressure.

The PET melt is fluid and easy to form. Usually, medium pressure is used, the pressure is 80 to 140 MPa, and the glass fiber-reinforced PET has an injection pressure of 90 to 150 MPa. The injection pressure should be determined by considering the viscosity of the PET, the type and amount of the filler, the location and size of the gate, the shape and size of the plastic part, the mold temperature, and the type of injection molding machine.

How much do you know about the processing of PET plastics?

1. Plastic processing

Since PET macromolecules contain a lipid base and have a certain hydrophilicity, the particles are sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of the PET decreases, and the product is colored and becomes brittle. In this case, the material must be dried before processing. The drying temperature is 150 4 hours, usually 170 3 to 4 hours. The air jet method is used to test whether the material is completely dry.

2. Selection of injection molding machine

PET has a short melting point and a high melting point, so it is necessary to select an injection system with a larger temperature control range and less self-heating during plasticization, and the actual weight of the product cannot be less than 2/3 of its weight. The amount of machine injection. Based on these requirements, in recent years, Ramada has developed a series of small and medium-sized PET special plasticizing systems. The selected clamping force is greater than 6300t / m2.

3. Mold and gate design

PET preforms are typically formed by hot runner molds. The heat shield between the mold and the injection molding machine is preferably insulated with a thickness of 12 mm, and the heat shield can withstand high pressure. The exhaust port must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port usually does not exceed 0.03 mm, otherwise flashing is easy.

4. Melting temperature

Measurement can be performed by air jet method. At 270-295 ° C, the level of enhancement of GF-PET can be set to 290-315 ° C.

5. Injection speed

The general injection speed is very fast, which prevents early curing of the injection. But too fast, the high shear rate makes the material brittle. The popup will usually be completed in 4 seconds.

6. Back pressure

The lower the better, so as not to wear. Generally no more than 100bar.

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