Stretch film casting process

The molten resin extruded from the die is moulded into a substrate by a casting machine. This step is very important in the entire production process because the quality of the product is based on it. In the casting process, the molten resin is rapidly cooled and cured on quenching rollers, ideally with uniform cooling on both sides of the substrate sheet. In order to obtain a uniformly cooled substrate sheet, different methods are recommended depending on the resin type. For PP, a typical casting method with several small-diameter rolls and one large-diameter single roll (with water bath) is used as shown in Fig. 6.13. Recently, a large roll with a spray system has been developed with high cooling efficiency, which is compact and small enough for high-speed operation.

For PET, due to its low viscosity, the die head is located directly above the quenching roller, the key to rapid curing quenching is to quickly press the molten resin on the quenching roller, if the pressing effect is not good, it will not only reduce the cooling effect, but also between the molten resin and the quenching rollers will also entrain the air bubbles stay in the substrate will cause surface roughness or fine under the bubbles and other phenomena. There are many pressing methods to stick the molten resin to the quenching roller, such as ‘air knife’, ‘pressure roller’, ‘spray liquid’ and ‘electrostatic’. Air knife’, “pressure roller”, “spray liquid” and “electrostatic” and other methods, air knife and electrostatic method are PP and PET, respectively, commonly used methods.

In the case of PP, the molten resin is usually pressed onto a quenching roll using an air knife. The shape of the air knife and its contact points are critical to forming a stable substrate. For this reason, the design of the air knife is optimised by computer simulations analysing the air flow for practical applications. Out of the air velocity and pressure distribution, no optimisation was performed at this point. From the air velocity distribution graphs it was found that vortices were present in the air flow and subsequently negative pressures were generated in the upper part of the substrate in the pressure distribution graphs, in this case the shape and position of the air knife was determined by the simulation analysis in order to remove the negative pressures.

Chattering occurs in the presence of air flow, resulting in instability of the air knife in relation to the air thickness. The pressure pressed on the quenching roll is determined by several factors, such as the electrode voltage, the distance between the electrode and the quenching roll, the electrode diameter, and the electrical conductivity of the molten resin. In the case of electrostatic pressing, the action of the charge, as the moulding speed increases, the charge density decreases and the pressing pressure decreases, resulting in the entrapment of gas between the molten resin and the quenching roll. However, when the electrode voltage is increased and the charge density is increased, an arc discharge occurs.

Nanjing Suminoe Precision Machinery Co., Ltd. is a collection of scientific research, production, sales in one of the high-tech enterprises, and has always adhered to the integrity, equality, customer interests first business philosophy, in order to better improve the quality of equipment, step into a more professional standardised production track. We set up a plastic sheet, sheet, casting, bi-oriented, non-woven nonwoven meltblown cloth, film composite, renewable granulation and environmental protection recycling system and coating machine manufacturing system and other seven categories of equipment, independent research and development technology centre and production departments, to provide a professional full set of technical solutions to meet the special needs of each customer. Among them, the bi-directional stretch film manufacturing system has: BOPP bi-directional stretch film manufacturing system, PVC bi-directional stretch shrink film production line, PET bi-directional synchronous stretch film production line, BOPA bi-directional stretch film production line, BOPLA bi-directional stretch film production line and so on.