The rheological property of PET is better in the melting state, and the effect of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of melt is mainly changed by pressure.
1. Treatment of plastics.
Because PET macromolecules contain lipid groups and have certain hydrophilicity, the granules are sensitive to water at high temperature. when the water content exceeds the limit, the molecular weight of PET decreases, and the products become colored and brittle.
Therefore, the material must be dried before processing, the drying temperature is 150 ℃, more than 4 hours, generally 170 ℃, 3-4 hours.
The air shot method can be used to test whether the material is completely dry.
2. Selection of injection molding machine.
Because the PET is stable after the melting point for a short time and the melting point is high, it is necessary to choose the injection system with more temperature control sections and less self-friction during plasticizing, and the actual weight of the product can not be less than 2 × 3 of the machine injection volume.
Based on these requirements, Ramada has developed a series of small and medium-sized PET special plasticizing systems in recent years.
The clamping force is greater than 6300t/m2.
3. Mold and gate design.
PET bottle embryo is generally molded with hot runner, it is best to have a heat insulation plate between the mold and the injection molding machine template, its thickness is about 12mm, and the heat insulation plate must be able to withstand high pressure.
Exhaust must be adequate to avoid local overheating or fragmentation, but the depth of the exhaust port is generally not more than 0.03mm, otherwise it is easy to produce flying edge.
4. Melting temperature.
It can be measured by aerial shooting method.
The GF-PET of reinforcement can be set at 290C-315C, etc.
5. Injection speed.
Generally, the injection speed should be fast, which can prevent premature solidification during injection.
But too fast, high shear rate makes the material fragile.
The injection is usually completed in 4 seconds.
6. Back pressure.
The lower the better, to avoid wear and tear.
Generally, it does not exceed 100bar.
It is usually not necessary to use it.
7. Detention time.
Do not use too long residence time to prevent the decrease of molecular weight.
Try to avoid temperatures above 300 ℃.
If the downtime is less than 15 minutes.
Only air-shot treatment is required; if it takes more than 15 minutes, clean with viscosity PE and lower the barrel temperature to PE temperature until the machine is turned on again.
8. Matters needing attention.
If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opaque.
The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent if there is little crystallization.
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