What Is Injection Molding And Extrusion Molding?
What is injection molding?
The molten plastic is injected into the mold. Then, the liquid plastic is cooled and solidified in the mold. Finally, the cooled plastic product is taken out of the mold.
Injection molding is the most commonly used method in plastic molding. The reason is that as long as with the mold, even complex shapes can be mass-produced. In injection molding, a variety of products can be produced by only changing the mold. From small to large, from simple to complex, the mold reduces the production cycle and also saves costs.
In plastic molding, the molding methods of many products (second only to injection molding) include extrusion molding and blow molding. In the case of extrusion molding, the equipment is usually large. And due to the kneading with various additives, they are usually manufactured by large companies (such as material manufacturers).
The plastic is melted at a high temperature of about 200°C, inject into the mold, and then cooled and solidified. The injection molding machine consists of two main parts. Melt the plastic and realize the injection, lock the “injection device” and “clamping device” to prevent the mold from opening.
Injection molding machine
The injection device is composed of an injection mechanism such as a “syringe” and a “plasticizing mechanism” that melts the plastic. The mold clamping device has a mold clamping mechanism for opening and closing the mold and an ejection mechanism for taking out the finished product.
Schematic diagram of injection molding process
Screw is often used to melt the plastics. A heater is wound on the outside of the metal round barrel (cylinder) to heat it. The plastic resin are put into the barrel. After rotating the screw, the resins are conveyed to the front end along the screw groove, and melted after heating. When the screw is turned counterclockwise, molten plastic will accumulate on the front of the barrel.
Next, tighten the mold with a mold clamping device. Then, the part of the injection outlet connected to the top of the cylinder (called the nozzle) is firmly pressed against the hole of the mold inlet. When push the screw, the accumulated plastic is pushed into the mold, cooled and solidified. Plastic shrinks as it hardens, so add a little more molten plastic after all filling is complete.
Mold opening and closing
The pressure during injection molding is 2,000kgf/cm2, and the pressure in the mold is 200-500kgf/cm2. The mold requires a clamping force of about 150 tons, so it cannot be opened easily. The large mold weighs 50 tons, and the clamping force of the molding machine is about 3,000 tons. The characteristic of injection molding is the efficient production of high-precision products.
However, the machines and molds are very expensive. Therefore, this method is suitable for mass production.
For mixed metal and plastic products, first put the metal part into the mold, and then inject the plastic around the mold. This method is called insert molding.
It is called extrusion because the plastic is pushed out and out of the extrusion mold part. The typical product is vinyl chloride pipe. According to the shape of the product, there are various cross-sectional shapes, such as round and square.
Most of the raw materials for extrusion are not resin but plastic powder. The molded plastic that has just come out of the extrusion port has not been cooled and it is easily deformed. Therefore, in order to suppress deformation, it is circulated after extrusion.
The corrugated tube on the washing machine can also be made by extrusion molding. After extrusion, compressed air is delivered for molding. In addition, by designing extrusion holes, perforating the inside and outside of the mold, rotating the inside and outside molds and pushing them apart, a grid can be formed.
In GB, a sheet with a thickness of 0.25 mm or more is called a sheet. While a sheet with a thickness of less than 0.25 mm is called a film. The extrusion die has an elongated shape. It is called T-die extrusion because it expands in a T-shape in the extrusion direction.
The process of increasing the winding speed and stretching the sheet in the length direction is called stretching. The plastic is reinforced by stretching the plastic in the longitudinal direction, and by performing a process called biaxial stretching that stretches not only in one direction of flow but also in the left and right directions. In this way, such a film is manufactured, which is very strong regardless of the length direction and the direction perpendicular to it.