What are the methods for extruding and blowing hollow plastic products?

(1) Continuous extrusion inflation method. The sequence of continuous extrusion and blow molding of hollow plastic products: the blow molding process extrudes the molten tube blank that has been mixed and plasticized uniformly from the extruder to a certain length - the molding die for the product is moved from the inflation and cooling station to Below the molten tube blank → the blank tube material enters the forming mold → the mold is closed, and the tubular feed is cut off → the forming mold is moved to the inflation cooling position (another forming mold moves simultaneously to the bottom of the tube blank) → mold opening, forming plastic products The two forming molds are moved back and forth to exchange positions, so that the extrusion and blow molding of hollow plastic products can be continuously carried out. This production method has the advantages of simple equipment structure, low investment, and easy to master the production process and operation. At present, this method of producing hollow plastic products is widely used in small and medium-sized plastic products factories.

(2) Transposition feeding extrusion blowing method. The production method of extruding and blowing hollow plastic products by transposition feeding is basically the same as that of continuous extrusion and blowing, except that the feeding method is controlled by the reversing valve (the forming mold is fixed), and the melt is alternately supplied to the two forming molds, and then Inflation molded articles.

(3) Rotary disc continuous extrusion blowing method. The continuously extruded plasticized molten blank tubes are respectively supplied to multiple forming dies that rotate around the axis of the turntable. With the rotation of the turntable, the actions of clamping the mold, cutting the blank tube, inflating and cooling, and opening the mold to release the plastic product are completed.

In the extrusion and inflation molding of large-scale hollow plastic products, due to the large amount of materials used for one-time molding, the extruded molten material is first stored in a storage cavity during production, and when the amount of material is reached, the molten material is quickly pushed and melted. The material enters the product forming mold through the blank tube, and then blows and cools to form.

Heating temperature control during blister molding

Transparent thermoplastics are amorphous, but as far as our Shenzhen packaging factory understands: not all amorphous thermoplastics are transparent in blow molding, such as colored and modified amorphous plastics. Partially crystalline thermoplastics are no longer transparent due to the partial orderly arrangement of molecular chains, and the degree of transparency varies depending on the degree of crystallinity. Amorphous and partially crystalline thermoplastics have an operating temperature range that will be described later. Below the glass transition temperature Tg (formerly known as the softening temperature), thermoplastics are generally very brittle (such as ordinary polystyrene PS), and the rigidity (modulus E) and strength (σ) of thermoplastics vary with It decreases with increasing temperature, but the deformability increases. When the material is at the working temperature, the effect of the raw material on the appearance of the finished product must also be sufficiently rigid. The coefficient of thermal expansion can be considered to increase linearly with temperature within a certain temperature range.

From the above analysis, it can be seen that if the vacuum forming product is used at high temperature, or it needs to be sterilized for a short time, then a higher temperature should be used when the material is vacuum forming. For partially crystalline thermoplastics, the vacuum forming temperature should be at least above the middle temperature of the melting range of the crystalline body. The shape change of the amorphous thermoplastic is also reduced if molded at higher temperatures. In actual production, if these factors are taken into account, the following problems will arise: the sheet will not be produced because the vacuum forming machine cannot reach the required vacuum forming temperature (when passing through the machine cannot get a smooth and flat surface), or the melt strength of the material is too low, causing too much collapse, or the material is too tightly bonded to the die. Thermoplastics such as CPET, which tend to crystallize when heated and shaped, are an exception. CPET is an amorphous polyester containing a crystalline nucleating agent. In the heating process of the sheet with air support, the PP sheet should be slightly recessed to accommodate the expansion and wrinkling as much as possible; if the heated sheet cannot be supported by air during or after heating, in addition to linear expansion, Depression becomes an operating factor for plastic materials and is related to the strength of the material at the molding temperature and to the internal stresses introduced during the production of the sheet.

The thermoplastic sheet expands up to the softening temperature. ②When the softening temperature (glass transition temperature) is exceeded, the frozen stress will be released; highly oriented sheets, such as OPS or LDPE, the main chain stretches in the mold frame and when the softening point is reached; other materials, such as PP Or PVC, continue to expand until they reach molding temperature. ③ When the contact plate is heated, the expansion will change the contact pressure, and adhesive waves (adhesion strechs) will be generated on the surface of the formed sheet. ④When radiant heating is performed, two problems will arise about the influence of material properties on the blister processing. a. If the sheet is not supported and dented, the expansion due to its own weight must be added to the thermal expansion. Radiant heating may produce insufficient heating, or thermal damage to the sheet. When producing thin PP materials, the method of freezing stress must be used when heating on a vacuum forming machine, otherwise, it is very difficult for the sheet not to collapse. b. Where the molding material is supported by air to prevent sagging and sag, in order to keep it level, wrinkles will be generated, and if there is a significant temperature difference between the convex and concave parts, the quality of the molded product will be negatively affected. It is not possible to calculate the actual amount of dents for molding materials with severe dents.