In the preform molding process, the best conditions are as follows: the lowest possible temperature, the shortest possible time, rapid, uniform and complete melting, minimizing the decrease in IV, minimizing AA, and being as transparent as possible. The related process conditions are:
Molding temperature refers to the temperature of the barrel and the hot runner. Only 30% of the heat in the molding process comes from external heating, and 70% comes from internal frictional heat. Therefore, in addition to suitable heating, shear heat is also used.
Injection and holding pressure
The injection is to fill the mold with molten material to overcome the resistance in the flow path. For preforms, it is best to have three stages of speed and pressure, decreasing in order.
The injection speed is too slow, cutting is not enough, before it is full, it is cooled, causing the product to be full or underfilled; too fast, the exhaust in the cavity is not enough, resulting in insufficient filling, shrinking, AA high.
Holding pressure has two important functions: to prevent backward flow of the melt and to ensure cooling under pressure (to increase the cooling effect). Too high will cause overfilling and expansion molds, etc. Internal stresses will be high and may also crystallize. Too low will cause shrinkage, preform deformation (cooling is not enough), gate problems such as pinholes, bubbles, etc., because the cooling rate at the gate decreases. The pressure holding time should also be appropriate. Too short can also cause pinholes, drawing, etc.
The pressure relief is to reduce the pressure in the hot runner, to prevent the clogging of the gate, the needle valve action is not flexible and so on. But too much will cause shrinking, drawing, pinholes and so on.
Back pressure is the kneading force exerted by the hydraulic system on the screw during the rotation of the screw driven by the oil motor. Role: To strengthen the plasticization of PET and eliminate bubbles. When turned on, it can be adjusted to 0. When the preforms are pulled up, they are slowly added. The back pressure added to the preform without bubbles or defects is the appropriate back pressure. Excessive shearing is too strong, and problems such as poor molding, gate blocking, and pyrolysis can occur.
The buffer is the margin in front of the screw head after each injection. Too little will cause poor molding and too much will cause PET decomposition. Generally, it is adjusted slowly from as little as possible, and the amount of the preform is not fogged or crystallized.
PET is opaque, and the preform is transparent, relying on cooling. Poor cooling will reduce the cooling rate of the preform, which will lead to shrinkage, preform deformation and affect the cycle time. To avoid this situation, we must do: water treatment, regular cleaning of the water channel, check the water flow and water pressure, core and type Cavity washable and so on.
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