In order to prevent the molten material from flowing out of the nozzle automatically, resulting in the casting, the nozzle temperature is generally lower than the front barrel temperature. The friction heat generated by the molten material passing through the nozzle at high speed during injection is used to increase the material temperature.
But the nozzle temperature can not be too low, otherwise the condensate in the nozzle will plug the nozzle and the mold gating system (especially the pin-point gate). The cold material can be injected into the mold cavity, so that the plastic injection molding products are with cold spots, affecting the quality of products.
The influence of mold temperature on injection molding and product properties and the choice of nozzle temperature and barrel temperature are also related to injection pressure. When the injection pressure is lower, we should properly raise the barrel and nozzle temperature to ensure the flow of material, and vice versa.
Generally, we use "air injection" or "intuitive analysis of products" to adjust the molding process conditions and determine the best barrel and nozzle temperature before the plastic injection molding.
Mold temperature refers to the mold wall temperature in direct contact with plastic. It directly affects the fluidity of plastic mold filling, the cooling rate of products, the molding cycle and the results of crystallization, orientation, shrinkage of products. It is an important factor related to the quality of products.
For thermoplastic plastic injection molding, the mold temperature must be controlled below the plastic thermal deformation temperature or glass transition temperature to ensure that the parts have enough stiffness when they are released without deforming. Under this general principle, the specific value of mold temperature should be based on the characteristics of plastic (such as whether it is crystalline plastic, melting viscosity size, etc.), the structural characteristics of products (such as the complexity of the shape, the wall thickness, etc.), the use of products (such as the crystallinity of crystalline plastic) and other molding process conditions.
For amorphous plastics, under the condition not affecting the mold filling, the mold temperature can be lower, which is conducive to shorten the molding cycle and improve productivity. For crystalline plastics, the mold temperature not only affects the mold filling and plastic injection molding cycle, but also determines the crystal structure and crystallinity of products and then affects the properties of products.
In addition to the above factors, the choice of mold temperature should also be conducive to uniform cooling of plastic in the mold cavity, which is the reason for choosing a higher mold temperature when molding products with thick wall.