Polyethylene terephthalate (PET) has: (1) excellent gas barrier properties; (2) pressure resistance and creep resistance (3) impact resistance; (4) transparency and surface gloss ; (5) It is odorless and fragrance-preserving; (6) It is safe in food hygiene, beautiful and easy to shape, and has been widely used in food containers. Today, the editor will give you a brief introduction to the injection molding process of PET preforms.
Characteristics of PET
PET is a high molecular polymer. The most important characteristic is its molecular weight. The molecular weight is the uniform number of molecules in the PET molecular chain. The longer the number of molecular chain connections, the better the physical properties of PET. PET is a crystalline resin with a specific gravity of about 1.4, a melting point of 245°C, and a molding temperature of about 290-315°C.
The PET plastic pellets generally used in the manufacture of containers must pay attention to the following characteristics:
Preform mold manufacturers
(1) Molecular weight;
(2) Acetaldehyde content;
(3) Diethylene glycol content;
(4) Carboxy;
(5) Percentage of copolymer;
(6) Melting point;
(7) Glass transfer point;
(8) Crystallinity.
PET raw material drying
PET is a hygroscopic material. If the moisture content is too high, it will be hydrolyzed under high temperature processing, which will break the PET molecular chain and produce acetaldehyde due to thermal decomposition. The physical and mechanical properties will be reduced, and the crystallization rate will increase. Part of the preform is crystallized and whitened, which affects the quality of the product. Before the preform injection molding, the PET pellets must be dried to a moisture content below 0.05%, so that a good non-crystalline transparent preform can be produced.
Preform mold The dryer uses dehumidified high-temperature air to circulate the PET pellets in the hopper. For dry air, its dew point temperature must be between -29°C and -40°C. The temperature of the dryer is set between 177°C and 182°C, and it is dried for about four hours.
Injection extension hollow molding
1. Basic principles
Firstly, the preform is produced by injection molding. This is to inject the melt into the mold cavity, and then rapidly cool it to make the preform temperature lower than the crystallization temperature range to make a transparent preform; the second step is to heat the preform, Make it higher than the glass transition point temperature, and then stretch, blow and cool to shape.
2. Processing method
There are two types: one-stage and two-stage.
One-stage injection and blowing are completed in the same machine. The injection molded preform is cooled to below the crystallization temperature range, but remains in a thermoelastic state, and finally blown into a bottle.
The two-stage type is completed by two machines, an injection molding machine and a hollow molding machine. First, after the preform is made by the injection molding machine, the preform is cooled to the solidification temperature and taken out, cooled to room temperature, and then sent to the hollow molding machine to heat the preform to the blowing temperature, and then blow, Cooling and forming.
Injection molding process conditions of PET preform
Except for the extrusion molding process, most of the PET preforms are made by injection molding.
In the entire preform process equipment, in addition to the injection molding machine, it also includes: hot runner mold, ester pellet dehumidification dryer, mold cooling ice water machine, preform removal robot, preform conveyor belt and injection environment control air conditioning dehumidifier, etc. , And each machine must cooperate with the action of the injection machine, so the injection molding machine plays the most important role in the preform molding process.
Generally, the main process control parameters of the injection machine are: (1) screw speed; (2) injection pressure; (3) injection speed; (4) barrel temperature; (5) nozzle temperature; (6) pressure holding; ( 7) Pull loose and retreat; (8) Switching speed of die; (9) Cooling time; (10) Cycle time. Each parameter is related to the quality of the injection product. Therefore, parameter control is very important. In order to achieve good process parameters, the performance of the injection machine must be designed in line with the process requirements.
Screw design
The screw design of the PET injection machine is very important to the preform injection molding process. The general screw used for PET injection has poor quality and is prone to problems. Therefore, the PET injection machine must have its own special screw. Because PET is a heat-sensitive material, it is easy to break the molecular chain at high temperature, reduce the molecular weight, produce acetaldehyde, and reduce the quality of the preform; and in the barrel, the screw is mainly rotated and compressed to cause the ester particles to generate shear heat and melt. If the shear heat generated by the screw compression is too large, the thermal cracking of the PET may occur. Therefore, the PET injection screw must be designed for low shear, and the target can be melted and crystallized PET ester pellets. Too high compression ratio or too short compression length will cause large shear; while the shear heat generated by the screw with too low compression ratio may not be able to completely melt the ester particles, causing the preform crystals to be white and atomized, and cannot Completely remove the air in the material tube, which is easy to cause bubbles in the preform
1. Treatment of plastics. Because PET macromolecules contain fat bases and have a certain degree of hydrophilicity, pellets are more sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET will decrease during processing and the products will become colored. Become brittle. Because of this, the materials must be dried before processing. The drying temperature is 150°C for 4 hours or more, generally 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry. The proportion of recycled materials should generally not exceed 25%, and the recycled materials must be thoroughly dried.
2. The injection molding machine uses PET because the stable time after the melting point is short and the melting point is high. Therefore, it is necessary to select an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual product (water-containing material) The weight cannot be less than 2/3 of the injection volume of the machine. Based on these requirements, Ramada has developed a series of PET special plasticizing systems in recent years. The clamping force is selected as greater than 6300t/m2.
3. Mold and gate design PET preforms are generally formed by hot runner molds. It is better to have a heat shield between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat shield must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise it is easy to produce flash.
PET preform mould
4. Melting temperature can be measured by air shot method. The temperature ranges from 270-295℃, and the enhanced GF-PET can be set at 290-315℃.
5. Injection speed Generally, the injection speed should be fast to prevent premature solidification during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds.
6. Back pressure is as low as possible to avoid wear. Generally not more than 100bar. Usually not needed.
7. Retention time Do not use too long retention time to prevent the molecular weight from falling. Try to avoid temperatures above 300°C. If the shutdown is less than 15 minutes. Only need to do air injection treatment; if it exceeds 15 minutes, clean with viscosity PE, and reduce the barrel temperature to PE temperature until it is turned on again. 8. Matters needing attention ⑴The recycled material should not be too large, otherwise it is easy to cause "bridging" at the feeding place and affect plasticization. ⑵If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity. The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent with less crystallization.
