1. Requirements for raw materials: Polyethylene blown film mainly includes three kinds of heavy packaging film, general packaging film and agricultural film according to the requirements of use. The molecular weight of the polyethylene resin is expressed by the size of the melt index (MI). If the melt index is small, the molecular weight is higher and the tensile strength is higher. Conversely, if the melt index is large, the molecular weight is lower and the tensile strength is lower.

     2. Uniformity of the head ring gap: the uniformity of the head ring gap directly affects the uniformity of the product thickness. Has an important impact on the quality of the product. The die gap is generally 0.6 to 1 mmm, and the ring gap of the die is adjusted with a steel feeler gauge to ensure that the parts are evenly distributed. Temperature control: PE has good thermal stability. <300 ℃ is not easy to decompose without contact with oxygen. > 300 ℃ contact with oxygen can easily cause cross-linking.

     Polyolefin blown film generally ranks higher than the temperature of the machine head, and the PE has good thermal stability. This is because the olefin is not easy to decompose, the body temperature is high, the plasticization is sufficient, and the fluidity is good.

     The temperature of the LDPE hopper should not be too high to prevent the adhesion of the pellets. The barrel temperature can reach 180 ℃. The temperature of the head can be 10-20 ℃ lower than the body temperature. To produce irregular flow, which cannot be smoothly expanded and stretched. This is because the tensile strength of the film is also low, and the surface gloss is poor, the transparency is reduced, and even patterns like wood annual rings and unmelted crystal nuclei (fish eyes) appear. At this time, the impact strength of the film also decreases, and in severe cases, the weld seam of the film is obvious. Adjust the traction ratio after the belt is stable to make the thickness even. Uneven film thickness, adjust bolts, uneven layout can be adjusted with air volume. The latter part reduces the air volume of the wind ring. The air volume decreases, the local temperature increases, and the compressed air further swells to solve the uneven local thickness. However, the temperature of the machine head must be uniform.

     3. Temperature: Temperature control is the key in the blown film process and an important parameter affecting quality. The advantage of this temperature control method is that the high temperature time that the material is subjected to is short and it is not easy to decompose, which can reduce the number of disassembly of the head and improve the production capacity of the machine (see Table 1).

Table 1 Blowing film forming temperature

material	SPVC (High Speed Film)	HPVC (Heat Shrinkable Film)	LDPE	PP	PE-PP LDPE	PP
body	165～175	170～185	130～160	190 ~ 250	120～170	181～210
Machine neck	170～180	180～190	160～180	240～250	210～220	210～220
Nose	185～190	190～195	150～160	230～240	200	200

     4. Cooling and setting: when the tube bubble is just extruded from the machine head, the temperature is high and it is in a semi-flow state. Now the cooling capacity is insufficient, the films adhere to each other under the pressure of the traction roller, and the capacity of the host cannot be volatilized. The amount of air supply around the cooling air ring should be uniform, otherwise the film thickness will be uneven, otherwise the film thickness will be uneven. The distance between the wind ring and the die is 30-100mm.

     5. Drafting ratio: the ratio of the traction speed to the linear speed of the extrusion is the traction ratio, usually controlled at 4-6, the traction ratio is the longitudinal draft multiple, generally controlled at 2.5-3, which is easier to operate, and the film The vertical and horizontal strengths are similar. If the inflation ratio is too large, the inflation film will swing like a snake, making it difficult to control the uniform thickness.