Plastic hollow container production process, equipment and development (1)

Injection blow molding is a molding method for producing hollow articles by combining injection molding and blow molding. This is a molding process that goes from one injection phase to one blow molding phase. It first uses the injection molding machine to form the parison, then pulls the core together with the parison from the die, transfers it into the blow molding mold body, and finally blows the compressed air to cool it.

The use of injection-pull blowing technology can improve the uniformity and dimensional accuracy of the product's wall thickness, improve mechanical strength, impact resistance and transparency. Injection blown hollow products are widely used in the beverage and cosmetics packaging industry.

Injection blow molding develops rapidly because injection blow molding has the following advantages:
(1) Without flashing, the resin yield is high and there is no need to make repairs.
(2) After the parison is injection molded, the weight deviation is very small.
(3) There is no seam on the product, and the parison has a closed bottom, so the strength is good.
(4) The dimensional accuracy of the mouth is high, and the wall thickness of the mouth can be arbitrarily adjusted, especially for products with threaded bottle mouth.
(5) It is possible to inject multiple products at one time, resulting in high production efficiency.
(6) A wide range of injection molding materials can be injection molded PVC, PET, PAN, PP, PE, and so on.

Injection blow molding process in three stages

(1) injection molding parison: Note-blowing method is first to inject the molten material into a parison placed in a mandrel, the melt is cooled in the parison, and then shrinks on the mandrel, forming a viscoelastic premolded type The blank is opened in the parison mold, and the mandrel with the parison is transferred to a new blow-molding position according to the procedure for blow molding.

(2) Blow molding: After the mandrel is transferred to the blow molding station, the blow mold locks the mandrel with the preform and immediately passes compressed air to the mandrel core so that the parison is in air pressure. It separates from the mandrel wall and begins to expand until it expands to the contour of the blow mold. After the parison is molded in the blow mold, the mandrel with the blown product is turned to the mold release position.

(3) Product demoulding: The molded products transferred to the demoulding station are ejected from the mandrel or directly transported to the packaging station.

From this it can be seen that the basic structure required for injection molding should be:

I. Must have the same injection molding machine as the ordinary injection molding machine to plasticize, melt and injection molding parisons.

II. There are two sets of molds that are parison molds (formed parisons) and the other are blow molds (formed products). The injection mold and the blow mold include two mold halves. The mold is a cavity of the parison. The blow mold is a cavity of the product and has a positioning stop. When closed, it can wrap the mandrel.

III. There are sequential devices and positioning devices that control the displacement or displacement of the mandrel, and can accurately and timely control the mandrel's conversion station.

IV. A mandrel ejection mechanism completes the shedding procedure of the product.

Injection blow-molding process In general, the relationship between the quality of the blow-molded product and the injection molding process is closer than the blow-molding process. The formation of the parison is a critical part of the entire molding cycle. In this program, plastics are plasticized. There is no cooling device on the parison molds. If temperature control is performed on the parison molds, the temperature of the parisons must be balanced because most of the parisons are light and heavy, and the thickness is not distributed. The phenomenon of uniformity, so the injection of too much material or different types of plastic, will cause the weight of the container to change.

The structure of the mandrel and parison mold should be designed according to the nature of the material. In addition to the density of the plastic, the quality of the injected parison should be adjusted by injection efficiency, injection pressure, molding pressure, molding cycle, and the amount of release agent.

At the beginning of molding, the initial parison cannot be blown, and the parison shrunk on the mandrel must be torn off. It must be injected several times to stabilize the heat in the parison mold and the process is stable before it can start. And for each core The stick is all the same. If the parison mold is too cold, the parison shrinks so severe that it cannot be blown into the desired hollow product; similarly, the mandrel overheat parison will also shrink on the mandrel. If the parison sticks anywhere in the mandrel, the wall thickness of the hollow product will change. Therefore, it is required that the temperature of the mandrel, the parison mold and the melt should be suitable. For example, if the mandrel is too hot and the temperature of the mandrel is high, the melt sticks to the mandrel. Since many parisons have different thicknesses, the temperature of the initial material of the mandrel is not uniform. If a certain portion is too hot, the thin portion of the parison will become thinner after being blown. The thicker part of the parison is hotter and first blows. If this is the case, the melt will flow too fast, causing the cold and thin parts to keep up with the material, which leads to local voids.

In the demoulding stage, the hollow product's weight, size and presence of flash, missing material and holes are examined. If a thin part is present, it can be used to solve this problem by dry blending a small amount of hard zinc oleate or some type of release agent before pre-molding. It can be seen that the most important issue in the injection molding process is to solve the most suitable process conditions. First, the bottomed parison is injected and then the blow molding takes place under the most suitable process conditions. This is different from ordinary injection molding.

After the first injection molded billet is moved from the bottomed parison to the blow mold, it is often in a high temperature state of about 100 to 150° C., and it is difficult to release the core and the parison at a low temperature.

The second part of the mouth of the container is not stretched, the deformation of the injection molding is maintained as it is, and the fusion of the material flow at the top is also problematic, so here is the place where the strength of the container is the weakest. In order to improve the shortcomings in this area, formulas have to be selected for some plastics. For example, when a stearic acid metal salt or amide compound is added to the composite modified high-impact polystyrene, the parison and the core are excellent in releasability during injection molding, and the bottle mouth is improved. The quality of blow molding at the site.

Thirdly, when the bottomed parison is separated from the nozzle part of the mandrel and turned to the blow mold, there is a phenomenon of drawing between the parison and the molten resin sticking to the nozzle, and sometimes the wire sticks to the parison, so that not only The appearance of the product is impaired, and once it is mixed with the food in the container, it is not allowed from the sanitary point of view. This situation requires changing the molding conditions and resin modification. For example, it has been reported that adding a proper amount of rubber (2-7%) to general purpose PS can change the sticking phenomenon, and that the diameter of the rubber particles also affects the drawing behavior. When the rubber particles are as large as 10-30 μm, they are effective. To prevent the wire-drawing phenomenon, this is much larger than ordinary injection-molded rubber particles.

The production of polyethylene terephthalate (PET) blow-molded bottles can be divided into injection stretch blow molding (abbreviated injection blowing) and extrusion stretching blowing (pinch extrusion blowing) according to preform preforming. . Among these two molding methods, the injection-pull blowing process is easy to control, and the production efficiency is high, and the waste products are less and more general. PET blow-molding can be divided into two categories, one being pressurized bottles, such as bottles filled with carbonated beverages. The other type is a non-pressure bottle, such as a bottle filled with water, tea, oil and the like. The tea beverage bottle is a modified PET bottle blended with polyphenylene naphthalate L-ester (PEN) or a composite bottle of PET and thermoplastic polyarylate. It is a thermos bottle on the classification, and it can withstand more than 80° C., and the water bottle is cold. Bottle, no requirement for heat resistance. In the remolding process, the hot bottle is similar to the cold bottle. The pressure bottle forming process in cold bottles is now mainly discussed.