Importance of injection molding cooling
In injection molding tooling, the design of the cooling system is very important. This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and the plastic product can be prevented from being deformed by external force after demolding. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can significantly reduce molding time, increase injection productivity, and reduce costs. Improperly designed cooling systems can lengthen the molding time and increase the cost; uneven cooling will further cause warpage of plastic products.
According to the experiment, the heat from the melt entering the mold is roughly distributed in two parts, and a part of 5% is radiated and convected to the atmosphere, and the remaining 95% is conducted from the melt to the mold. In the mold, due to the action of the cooling water pipe, the heat is transferred from the plastic in the cavity to the cooling water pipe through the heat transfer through the mold frame, and then carried away by the cooling liquid through the heat convection. A small amount of heat that is not carried away by the cooling water continues to conduct in the mold, and then escapes into the air after contacting the outside.
The molding cycle of injection molding consists of clamping time, filling time, holding time, cooling time and demolding time. Among them, the cooling time accounts for the largest proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the plastic product molding cycle and the size of the production. During the demolding stage, the temperature of the plastic product should be cooled to a temperature lower than the heat distortion temperature of the plastic product to prevent the plastic product from being loosened due to residual stress or warping and deformation caused by the external force of the demolding.
Factors affecting the cooling rate of the product are:
(1) Plastic product design. Mainly the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. In general, the cooling time is approximately proportional to the square of the thickness of the plastic article or proportional to the 1.6th power of the maximum runner diameter. That is, the thickness of the plastic product is doubled, and the cooling time is increased by 4 times.
(2) Mold material and its cooling method. Mold materials, including mold cores, cavity materials, and formwork materials have a large impact on cooling rates. The higher the heat transfer coefficient of the mold material, the better the heat transfer from the plastic per unit time and the shorter the cooling time.
(3) Cooling water pipe configuration. The closer the cooling water pipe is to the cavity, the larger the pipe diameter, the greater the number, the better the cooling effect, and the shorter the cooling time.
(4) Coolant flow rate. The greater the flow rate of cooling water (generally to achieve turbulent flow), the better the effect of cooling water to remove heat by heat convection.
(5) The nature of the coolant. The viscosity and thermal conductivity of the coolant also affect the heat transfer of the mold. The lower the viscosity of the coolant, the higher the heat transfer coefficient and the lower the temperature, the better the cooling effect.
(6) Plastic selection. Plastic refers to a measure of the rate at which plastic conducts heat from a hot ground.
The higher the heat transfer coefficient of plastic, the better the heat transfer effect, or the lower specific heat of plastic, the temperature is easy to change, so the heat is easy to dissipate, the heat conduction effect is better, and the required cooling time is shorter. Processing parameter setting. The higher the material temperature, the higher the mold temperature, the lower the ejector temperature, and the longer the cooling time required.
Design rules for the cooling system:
(1) The designed cooling passage should ensure that the cooling effect is uniform and rapid.
(2) The purpose of designing the cooling system is to maintain proper and efficient cooling of the mold. Cooling holes should be of standard size for ease of processing and assembly.
(3) When designing the cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic part: the position and size of the cooling hole, the length of the hole, the type of the hole, the configuration and connection of the hole, and the coolant Flow rate and heat transfer properties.