For almost all of the extrusion molding, injection molding, blow molding and vacuum molding methods, productivity is largely determined by the efficiency of the heating and cooling of plastic. In blow molding process, the heat of melting plastic is taken away by the mold, the product is cooled and set, and the cooling time of the product is more than 60% of the molding cycle, and the large and thick wall products are as high as 90%. Therefore, improve the cooling efficiency of blow molding products, can shorten the molding cycle, reduce energy consumption.
1.the influence of mold temperature on the blow molding process
High die temperature: the cooling rate decreases and heat exchange time becomes longer, is not conducive to the melting material molecular crystals, formed crystal ball and continue to grow, will produce size of crystal, plastic impact strength decreased, resulting in short service life. Low mold temperature: when the mold temperature is lower than the temperature of 4 DEG C, due to the room temperature and air humidity effect, the mold surface very easy to produce cold and hot attached phenomenon, attached to a large number of drops, drops of water will affect plastic parts surface is bright and clean degree and easy to mold rust corrosion phenomenon, so blow mold temperature general control in about 10 DEG C, prevent to produce obvious cold and hot attached phenomenon. Cooling system water is not arranged, it will lead to the uneven cooling of the parts of the various parts of the shrinkage rate is different, resulting in product warp, deformation of the problem. Therefore, a good cooling system for blow molding is essential, it can shorten the blow molding cycle, size stability, shrinkage deformation of small products, but also to extend the life of products and molds.
2.Structure design of cooling system
The cooling method of blow molding products can be cooled, with three kinds of internal cooling and cooling. The main cooling system of our company is composed of external cooling and post cooling. 1 external cooling method is to open the cooling system inside the mold wall, through the outer wall heat transfer to cooling blow molding products. In our company the most common mold wall open cooling system is in the in the mould wall from longitudinal and transverse direction drilled cooling channels.
1 Cooling fluid inlet;
2 Cooling fluid outlet;
3 Tube plug;
4 The bar with a tap；
5 Spiral steel sheet
The cooling system of the method can be used for manufacturing the water by mechanical processing, and the manufacture is simple and convenient, and the cost is low. The spiral copper set in the pores increase the flow rate, can reduce the scale. The cooling aperture is generally 10 ~ 15mm, for large mold can reach 30mm. Larger pore size requires cooling fluid has high flow rate, pump configuration of large power transmission device, the distance between the center between two adjacent channels is 3 ~ 5 times of the hole diameter, the distance between the hole wall and a cavity for 1 ~ 2 times of the hole diameter, with uniform and sufficient cooling of the product. The hole type cooling passage is easy to be cleaned, and is easy to be modified. When some of the mold cavity boss is relatively high, the heat is more concentrated, and the mold can not do vertical and horizontal direction of the cooling channel, the cooling rod can be used to make the mold cooling.
D1 = D + 0． 1 ～ 0． 2mm（Convenient cooling rod assembly and disassembly and not easy to fall off）D2 = Dx2（Ensure heat conduction speed and stability) cooling rods have good heat transfer performance, can be one end of the rapid heat transfer to the other end, mounted cooling rods, in the right position to connect with the cooling water, on the realization of the a thermal conversion process.When placed in the cooling rod, the rod must be coated with a special heat conductive lubricant, its main function: 1, temperature resistance, good thermal conductivity; 2, rust, lubrication, late loading and convenient. The bottom of the cooling rod is used as a plug for plugging, which can bring great convenience to the subsequent maintenance.）
2.1 Internal cooling method
Blow molding products in the cooling, the outer wall due to contact with the low temperature die cavity was faster cooling, the inner wall and blowing air contact, the heat transfer is very little, so the cooling is slow. The difference in cooling rate between the inner and outer walls may cause the product to warp. One solution is to keep the pressure in the mold a little longer, so that the product is completely finalized and then removed, so the whole cycle is extended, the impact of production efficiency. Therefore. Research and development of the various cooling method, namely the spray, liquid nitrogen or carbon dioxide, air refrigeration, circulating air or mixed media injection has been blowing expansion type billet inside, inside the rapid cooling of the product. In the internal cooling, the most commonly used liquid nitrogen and carbon dioxide, because they are clean, do not play the role of chemical, applicable to a variety of blow molding products.
Germany Kautex Krupp blow molding machine using liquid N2 to cool the compressed air to – 185 degrees after the injection of green, can improve the productivity of about 30%. The initial temperature of N2 is at the temperature of the heat exchanger, and the heat of the compressed air is absorbed in the heat exchanger to close to the room temperature as the gas leaving the cooler.
2.2 Post cooling method
The internal and external cooling methods are aimed at the products in the blow mould. If at the higher temperature, as soon as possible from the mold to take out the products, in the post station for cooling, can obviously shorten the blow molding cycle. In the release, the temperature is not too high, generally in the mold after half of the normal cooling time, to keep the product shape.
1 Take out mechanism clamping products;
2 First working position residual material;
3 Second station cooling products;
4 Delivery products;
5 Product removal mechanism;
6 Removal device;
7 Clamping sleeve;
8 Central cooling chamber;
9 Cooling plate
Mechanism for automatically taking out articles of the system,the clamping barrel residual material in the template before opening the top,and sent to the first station,In the upper and lower ends of the clamping sleeve is used for clamping the clamping sleeve,The remainder is cut up and down.After the clamping sleeve of the VAT is sent to the second station,Here, the central cooling chamber and the upper and lower cooling plate close and tight,to make it cool.The bucket opening down, can be injected into the compressed air (pressure up to 0.3Mpa),In this way, and could be excluded in the vat of the heat transfer effect.The inner barrel wall and the cooling chamber close contact cooling, cooling was sent to the conveyor belt.After this cooling method, greatly reduces the product in the mold the default type of time, can effectively reduce the production cycle time, improve the productivity.The good or bad of the product depends on whether the mold is reasonable design and processing, which is an important factor in the mold cooling system,It has a direct impact on the life of the mold, the quality of products and production efficiency, etc., so the design of the cooling system must be considered in the design of the mold.