Plastic thermoforming technology
The injection molding method of thermoplastics is to melt the plastic material and then inject it into the film cavity. Once the molten plastic enters the mold, it is cooled into a certain shape according to the cavity.
The resulting shape is often the final product, and no further processing is required before installation or use as the final product. Many details, such as raised parts. Ribs and threads can be formed in one step of injection molding.
The injection molding machine has two basic components: an injection device and a mold clamping device for melting and feeding plastic into the mold.
The function of mold clamping device lies in:
(1) Make the mold closed under the injection pressure;
(2) Take out the product.
The injection device melts the plastic before it is injected into the mold, and then controls the pressure and speed to inject the melt into the mold. There are two designs of injection devices currently used: screw-type preplasticizers or two-stage devices, and reciprocating screws. The screw-type pre-plasticizer uses the pre-plasticized screw (first stage) to send molten plastic to the injection rod (second stage).
The advantages of the screw preplasticizer are constant melt quality, high pressure and high speed, and precise injection volume control (using mechanical thrust devices at both ends of the piston stroke). These advantages are required for transparent, thin-walled products and high production rates. The disadvantages include uneven residence time (causing material degradation), higher equipment costs and maintenance costs.
The most commonly used reciprocating screw injection device does not require the plunger to melt and inject the plastic. The powdered or granular plastic in the hopper is melted and sent to the check space at the front end of the screw through the rotating screw, and the plastic fluid flows through the front end of the screw and accumulates in front of the screw.
The accumulation of molten plastic in front of the screw pushes the screw towards the rear of the injection device. The rotation of the screw, the accumulation of melt and the movement to the rear continue until a certain injection volume is formed. In the next working cycle of the equipment, the tip of the screw is closed in the reverse direction to prevent the material from returning along the screw. The function of the screw tip and the feed screw is like plunger plunging, pressing the plastic into the mold.
The advantages of reciprocating screw include reduced residence time of plastic, self-cleaning screw and screw tip. These advantages are the key when processing heat-sensitive materials and when using colored raw materials or when the variety of resin is changed, and the screw and barrel must be cleaned.
Currently widely used mold clamping device design includes: toggle-type mold clamping device, hydraulic mold clamping device and hydraulic-mechanical mold clamping device. The toggle-type mold clamping device is suitable for small-tonnage equipment due to its low cost during manufacturing. Its features include high mechanical efficiency of the locking operation, built-in mold locking slowing device, slow mold damage and fast mold clamping operation.
The mold clamping cylinder pushes the horizontal top plate forward to extend the elbow and move the pressure plate forward. When the mold clamping device is closed, the mechanical benefit is reduced and the pressing plate is moved quickly. When the pressure plate reaches the position where the mold is closed, the elbow is changed from high speed-low mechanical benefit to low speed-high mechanical benefit.
Low speed is the key to protect the mold, and high mechanical benefits are needed to form large tonnage. Once the ribs are fully extended, hydraulic pressure is no longer necessary to maintain tonnage. In order to open the mold clamping device, hydraulic pressure is applied to the opposite side of the mold clamping plunger. In order to prevent the molded product from being damaged, the mold must be opened slowly.
Through the movement of the entire elbow joint device and the movement of the pressure plate device along the lever (moved to the position where the elbow device is fully extended before the mold is closed), the mold clamping device is adjusted to adapt to different mold heights.
