Extrusion Blow Molding of Plastic Hollow Products
DATE：2022/12/1 8:49:12 / READ： / SOURCE：This station
Extrusion Blow Molding of Plastic Hollow Products
Plastic hollow products (also called plastic containers) are a major type of plastic products, mainly including bottles, barrels, cans and boxes. These products can be produced by extrusion blow molding with an extruder, by injection blow molding with an injection molding machine, by welding and assembling with thin plates, or by rotary or hot extrusion and cold pressing.Extrusion blow molding hollow products commonly used plastics are: polyethylene, polypropylene, PVC and other resins. The molded hollow products are non-toxic, tasteless, light, corrosion resistant, sealable, beautiful and sanitary. It is widely used for packaging food, spices, chemical products, fuels and other items. The extrusion blow molding process of plastic hollow products is relatively simple:
The main and auxiliary materials are metered and mixed according to the requirements of the material formula for the products → they can be put into the extruder after being mixed and stirred evenly → they are plasticized into a molten state and extruded from the parison die mouth → they enter the molding die and cut off the tubular molten materials → they are closed → the parison is inflated with compressed air and the products are molded → the mold is opened after being cooled and shaped → the parts and components that take out the products to complete these actions constitute the hollow plastic products extrusion blow molding machine, Its structure is shown in Figure 11-1.
In addition, there is another production mode, that is, the forming part of the extrusion molding parison and the bulging part of the parison are two independent equipment. The structure of blow molding product device is shown in Figure 11-2.
The production process of polyethylene barrel extrusion blow molding is relatively simple: the production process (Figure 11-3) is to melt the PE resin through the extruder, extrude it into a tubular parison from the mold, place it in the blow molding mold, blow it with compressed air, and obtain products with the same shape as the inner cavity of the mold after cooling and shaping.
(1) Raw materials shall be selected according to the specification and volume of the product for extrusion blow molding of polyethylene barrel: the general rule is that for plastic barrel with smaller volume, low-density polyethylene with a melt flow rate (MFR) of 0.3~4g/10min shall be selected; High density polyethylene with a melt flow rate (MFR) of 0.05~1.2g/10min shall be selected for plastic drums with large volume (less than 25L); High molecular weight high-density polyethylene (HMWHDPE) shall be selected for large volume plastic drums (referring to hollow products larger than 25L).
The special PE resins for extrusion blow molding hollow products include DFDA-7143, DNDA-7145, DNDA-7146, DNDC-7148, DNDC-7150, DFDA-7340, DEX-8302, etc. produced by PetroChina Daqing Petrochemical Company. According to the requirements of product service conditions, a suitable resin brand is selected.
(2) Equipment conditions
① There are no special requirements for the structure of the extruder used for extrusion blow molding of polyethylene barrel. The general single screw extruder for extruding PE material can be used. The length diameter ratio of the screw is within the range of (20~25): 1, the compression ratio is 2~4, and the screw diameter should be determined according to the amount of material used for one-time molding of the extrusion blow molding barrel.
② Die for extruding tubular barrel blank The structure of the die for extruding tubular barrel blank is shown in Figure 11-4 and Figure 11-5.
It can be seen from Figure 11-4 that the mold structure for tubular barrel blank forming is very similar to that for plastic tube forming; The components, manufacturing materials and technical requirements of the die are also the same as those of the pipe forming die. The design can refer to the technical requirements of the die for pipe forming. The following provides several technical data required for mold design for reference during design.
a. The compression ratio is (2.5~4): 1.
b. Length of straight part of calibrating section L=about 88（ δ Is the die clearance, mm).
c. Rough estimation of the inner diameter of the die: D=D product (a is the blowing ratio).
③ Blowing hollow product forming mould The mould structure used for the forming of blowing hollow products is generally composed of two half moulds, and the opening and closing movement of the mould has horizontal movement or vertical movement; The hinge axis can also be used as the fulcrum, and the two half molds can be opened and closed along the arc line with the fulcrum axis.
In addition, for the products with concave bottom, the molding die should be designed into a three or four open structure to facilitate the smooth demoulding of the products.
The following points should also be paid attention to in the mold structure and application of hollow products formed by blowing.
① The junction of the die cavity surface should be an arc transition, and there should be no right angle transition line.
② The end of the forming die shall be provided with a clamping blank pipe orifice, from which the excess material of the product and the blank pipe before clamping and inflating shall be cut. The shape and size of the clamping opening are shown in Figure 11-5. The determination of several dimensions should take into account its influence on the forming and strength of products. The empirical data generally take h=1~2.5mm, H=3~5mm, β= 15°~45°。
③ In order to make the expanded parison close to the inner cavity wall of the mold and get a better product surface, an exhaust hole shall be reserved between the parison and the mold, and an exhaust hole of 0.2~0.5mm shall be drilled at an appropriate position or an exhaust groove of 0.1~0.2mm shall be reserved on the junction surface of the mold cavity.
④ The mold shall be made of materials with good thermal conductivity and capable of withstanding mold closing strength and blowing pressure. Common materials include aluminum, aluminum alloy, steel and stainless steel.
⑤ In order to speed up the cooling of products after blowing, if necessary, circulating cooling water shall be used to cool the molding mold of products. The mold temperature for polyethylene blowing products shall not be greater than 40 ℃ during operation.
⑥ The surface roughness of the inner cavity of the mold is not required to be high. If necessary, the inner surface needs to be sandblasted (40 meshes) to facilitate the escape of air between the mold and the surface of the product when inflating the product.
(3) The barrel temperature for plasticizing raw materials of the molding process extruder: 140~180 ℃ for LDPE and 150~210 ℃ for HDPE (the temperature increases gradually from the feeding section of the barrel to the homogenizing section).
The mold temperature of hollow products formed by blowing is 20~40 ℃. The inflation air pressure is 0.3~0.5MPa.
The blowing ratio is 1.5~3. Blow up ratio refers to the ratio of the transverse maximum diameter of the product after blow up to the diameter of the barrel blank tube (generally, a larger blow up ratio is used for small hollow products, while a smaller blow up ratio is used for large hollow products).
(4) Quality requirements The quality requirements of polyethylene plastic barrel shall comply with the provisions of GB 13508-92. The main contents are summarized as follows.
① The classification standard of barrels by use is: S represents barrels for food and medicine packaging, H represents barrels for chemical product packaging not listed as dangerous goods, and T represents special barrels for dangerous goods packaging.
② When threaded structure is used for opening and cover, it must be rotated for more than 1 turn from tightening to tripping.
③ The actual capacity of the barrel shall be greater than 5% of the nominal capacity.
④ The allowable deviation between the actual quality of the barrel and the approved quality is ± 8% (referring to the specification of the barrel 1~30L).
⑤ The allowable deviation between the actual overall dimension of the barrel and the design overall dimension is ± 5mm (referring to the outer diameter and height of the barrel when the barrel size is 1~30L).
⑥ Appearance quality of barrel: 1~5L barrel shall be free of bubbles, and when it is larger than 5L, no more than 2 bubbles with a diameter less than 2mm are allowed; Black spot impurities, with a length less than 1.5mm, shall not exceed 3 in 1~2.5L and 5 in 3~5L. Black spot impurities with a length less than 0.5mm shall not be counted, but penetrating impurities are not allowed; There shall be no cracks and holes, no boils with poor plasticization, no oil stains, no surface scratches, and the wall thickness and color shall be uniform.
Author：admin