Plastic bars
DATE：2022/11/30 8:44:55 / READ： / SOURCE：This station
Plastic bars
Plastic bar is a kind of plastic product with the simplest structure. The reason why it is called a bar is that this product is a solid with a circular, square, rectangular, rhombic or triangular cross section. The purpose of producing this bar is to use mechanical processing (turning, milling, planing, drilling, filing, etc.) to manufacture some mechanical parts, such as gears, shaft sleeves, bolts, gaskets, shells and covers. The plastic parts molded by this production method are more economical and simple than those molded by injection molding.Plastic bars are extruded by an extruder. Common materials include engineering plastics such as polyamide, polyoxymethylene, polycarbonate, ABS, and general plastics such as PVC, polypropylene, polyethylene, and polystyrene.
The structure and layout of the equipment for bar extrusion and the main equipment in the extruder production line are basically similar to the equipment for extruding pipe by the extruder (see Figure 5-1). The only difference is the mold structure.
(1) Equipment conditions
① The universal single screw extruder is selected. The screw structure shall be configured according to different extrusion materials. For example, when extruding PA1010, PA66 and POM, the equidistant mutant screw structure shall be selected, and the compression ratio shall be between 3 and 4; When extruding ABS, PC, PPO and PSU materials, the equidistant gradual change screw structure shall be selected, and the compression ratio shall be between 3 and 3.5. The screw length diameter ratio is in the range of (20~30): 1. Note: The screw diameter should be slightly smaller than the product diameter (sectional area).
② The common structure of bar forming die is shown in Fig. 10-15. The size determination requirements of each part are as follows.
a. The surface of the cavity of the molten material runner formed after the assembly of the parts in the mold shall be smooth and streamlined; As shown in Figure 10-15, the flow passage is divided into compression angle, straight section and expansion angle of discharge port.
b. The melt compression angle taper at the feeding end is within the range of 30 °~60 °. The length of cone compression angle is within the range of 50~100mm.
c. The length of the straight section of the die at the setting position is taken as (6~10). D is the diameter of the die (d hour is the maximum). Generally, this size is equal to the outer diameter of the product (when the speed of pulling the product is equal to the speed of extruding the melted material from the die). There is a conical angle at the outlet, which is about 45 °, so that the molten material in the center of the formed bar can be fed quickly.
d. The structure of the sizing sleeve is shown in Figure 10-16. The structure is similar to the shaping sleeve for pipe forming. The jacket is filled with circulating cooling water. In order to improve the cooling effect, the cooling water channel in the jacket shall be spiral. The inner diameter shall be slightly larger than the outer diameter of the product, and the specific size shall be determined according to the shrinkage of the material used for the product. For example, when extruding PA1010 bar, the inner diameter of the sizing sleeve should be about 3.5% larger than the outer diameter of the product.
③ Traction device The structure of the traction device for bar extrusion molding is identical to that of the traction device for pipe extrusion molding. Crawler type is generally adopted, which can continuously change speed; The driving system can also be omitted. Two upper and lower tracks clamp the bar, and the driving force of the bar extruded from the die mouth drives the track to run. The actual pulling speed is the speed at which the bar is extruded from the die mouth, generally 0.01~0.5m/min.
(2) Forming process
① Production process sequence
Drying treatment of extrusion production materials → plasticizing and melting of raw materials in the extruder → molding of bar blank by die → cooling and shaping → traction → cutting → finished products
② Process conditions See Table 10-17 for process conditions of common engineering plastic extruded bars
(3) Key points of process operation
① The allowable moisture content of raw materials after drying treatment: 0.03% for polyamide, 0.03%~0.05% for ABS, 0.02% for PC, 0.03%~0.10% for POM, and 0.02%~0.04% for PSU.
② Pay attention to the heat insulation between the mold and the sizing sleeve, and it is recommended to use PTFE gasket. At present, the best heat insulation pad is made of metal and PTFE.
③ The bar blank formed by the mold as shown in Fig. 10-15 can be cooled and shaped without the setting sleeve. Several 5mm thick brass shaped plates can be added in the cooling water tank (see Fig. 10-17 for structure). The hole diameter of the shaped plate is arranged from large to small, but the hole diameter of the last shaped plate is larger than the outer diameter of the product, and the specific size should be determined according to the shrinkage of the material of the shaped product; The plate hole shall also have some taper (large inlet end and small outlet end), and the hole diameter at the feeding end shall be chamfered with an arc angle of 1~2mm.
④ Process temperature control. The melting temperature of molded products is generally controlled at 20~30 ℃ higher than the melting temperature of resin. When the temperature is too high, the products are easy to generate bubbles; The product strength is poor when the temperature is too low.
⑤ The mold temperature should be about 15 ℃ lower than the raw material plasticizing temperature. The temperature is too low, and the product surface is rough.
⑥ The internal stress shall be eliminated after the products are cooled and molded. Polyamide (nylon) products should be treated in 100 ℃ boiling water for about 3h after gradually heating up; Polycarbonate products shall be treated in hot air (130~140 ℃) oven for about 2h; Polysulfone products are treated in hot air (150~160 ℃) oven for about 2h.
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