Because the injection molding equipment and the requirements for product quality continuously develop. Higher requirements for the injection molding process are put forward. Correct selection of injection equipment, reasonable setting of molding process and optimization process conditions are the keys to improving product quality.
Selection of injection molding machine specification
When choosing the specification of injection molding machine, we must first consider the status of the production mold. Because the same injection molding machine often has to meet the production requirements of multiple molds of different sizes. And we should determine the injection molding machine specifications according to the weight of the part and the size of the mold, that is, the maximum clamping force and maximum injection volume. Then select the appropriate model according to the specifications and models provided by the injection molding machine manufacturer.
It is necessary to consider whether some special configuration is required, such as the use of special screws when producing PA, PC and other materials. And we should equip the corresponding device when forming the mold with core pulling or thread removal. Then, according to the mold structure, product quality, etc., decide whether it is necessary to choose some injection molding machines with special functions. For example, when forming thin-walled long-flow products (generally referred to as L / D300), high injection speed is needed, and precision electronic parts needs precision full-closed control injection molding machines, etc.
Clamping force setting
In theory, the clamping force can be calculated as follows:
Fcm> = K × P average × A product × 10
In the formula: Fcm-clamping force, (KN) K-safety factor, generally take 1-1.2 P average cavity pressure (MPa) A product-the maximum projection area of the product on the mold parting surface (c㎡)
In actual production, the adjustment of the clamping force should also take into account the influence of the thermal expansion of the mold during production. Generally, a margin of 0.1-0.2mm should be left. The setting principle of clamping force is :
Under the premise of ensuring product quality, low clamping force is appropriate.
Injection process parameter settings
1. Barrel temperature, mold temperature
Set the screw barrel temperature according to the performance of different plastic materials. The barrel temperature is generally set higher than the melting point of the plastic by 10 ℃ -30 ℃. It must be noted that the materials provided by different manufacturers will have different melting points and allowable residence time in the barrel due to different synthesis methods or types of additives added.
Generally, the circulating water cool the mold temperature when setting. But when producing products with high precision size or surface quality requirements, a mold temperature controller that can conduct accurate control should be used according to the process requirements.
2. Injection holding time, cooling time
Set the injection time, pressure holding time and cooling time according to product thickness, mold temperature, material properties, etc. The injection time is generally set to be slightly longer than the time for the screw to complete the injection stroke movement. Excessive injection time will not only cause negative effects such as mechanical wear and energy consumption, but also extend the molding cycle. Set the holding time according to the thickness of the product. And the thin-walled product doesn’t need the holding time when molding.
When setting the pressure holding time, make sure there is no obvious depression on the surface of the product. The weighing method can also determine the holding time. The time that the pressure holding time is gradually extended until the product quality no longer changes can be the optimal pressure holding time. Determine the cooling time according to the product thickness, mold temperature, and material properties. Generally, the cooling time amorphous polymers required is longer than that for crystalline polymers.