Do you know what is injection molding process?
Mar 04, 2023
Do you know what is injection molding process?
Injection molding process refers to the process of making semi-finished products of a certain shape from molten raw materials through pressurization, injection, cooling, separation and other operations. This process is the production source of cosmetics packaging company's products. The quality of the semi-finished product determines the stability of the subsequent process and the quality of the product, and is the key process to maximize the profit of the enterprise.
(Injection mold)
Over the years, injection molding workers have overcome one difficulty after another, especially in solving the common defects (such as product shrinkage, melt, water, bubbles, bright spots, deformation, etc.) in thin and thick wall products. In general, the injection molding process mainly solves the following two problems:
1、 Process
The injection process includes four stages: filling, pressure maintaining, cooling (material storage) and Demolding. These four stages directly determine the molding quality of products, and these four stages are a complete continuous process.
(Injection molding process)
(1) Filling stage
Filling is the first step in the whole injection cycle. The time starts from the closing of the mold and the beginning of injection, and ends at about 95% of the filling of the mold cavity. In theory, the shorter the filling time, the higher the molding efficiency. But in practice, the molding time or injection speed are subject to many conditions, especially in thick-wall products.
High-speed filling: when filling at high speed, the shear rate is high, and the viscosity of plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; The local viscous heating effect will also make the curing layer thinner. Therefore, in the flow control stage, the filling method often depends on the volume to be filled, that is, in the flow control stage, due to high-speed filling, the shear thinning effect of the melt is often very large, while the cooling effect of the thin wall is not obvious, so the efficiency of the rate has the upper hand, so this filling method is suitable for thin wall products.
Low-speed filling: when the heat conduction controls the low-speed filling, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slow replenishment rate and flow of thermoplastics, the heat transfer effect is obvious, and the heat is quickly carried away by the cold mold wall. In addition to a small amount of viscous heating, the curing layer is thicker, which further increases the flow resistance at the thinner part of the wall. Therefore, this filling method is suitable for thick-wall products, even when the mold temperature is higher.
(2) Pressure maintaining stage
The role of the pressure maintaining stage is to continuously apply pressure, compact the melt and increase the density of plastic to compensate for the shrinkage behavior of plastic.
In the process of pressure maintaining, because the mold cavity has been filled with plastic, the reaction force of molten material is large. During packing compaction, the screw of the injection molding machine can only move slightly forward slowly, and the flow speed of plastic is also relatively slow. At this time, the flow is called packing flow. In the pressure maintaining stage, the cooling and solidification of the plastic by the mold wall are accelerated, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large. In the later stage of packing, the material density continues to increase, and the plastic parts gradually form. The packing state should continue until the gate is solidified and sealed, at this time, the mold cavity pressure in the packing stage reaches the highest value.
In the pressure holding stage, the plastic presents compressibility due to the high pressure. In the area with high pressure, the plastic is relatively dense and dense; In the area with low pressure, the plastic is loose and the density is low, so the density distribution changes with location and time. The plastic flow rate is very low during the pressure maintaining process, and the flow no longer plays a leading role; Pressure is the main factor affecting the pressure holding process. During the pressure maintaining process, the plastic has filled the mold cavity. At this time, the gradually solidified melt is used as the medium to transfer the pressure. The pressure in the mold cavity is transferred to the mold wall surface through the plastic, which can play a role in increasing the glossiness of the product surface.
(3) Cooling stage (storage)
In the injection molding mold, the design of cooling system is very important, because only when the plastic molecules are solidified to a certain degree of rigidity can the plastic products be prevented from deformation due to external forces during Demolding. Since the cooling time accounts for 70%~80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time and improve the injection molding productivity.
Due to the role of cooling water pipe in the mold, the heat of plastic products is transferred from the plastic in the mold cavity to the cooling water pipe through the mold base through heat conduction, and then taken away by the cooling liquid through heat convection. A few of the heat not taken away by the cooling water will continue to conduct in the mold and disperse in the air after contacting with the outside world.
The molding cycle of injection molding includes mold closing time, filling time, pressure holding time, cooling time (lower than melting time) and Demolding time, of which the cooling time accounts for the largest proportion, 70%~80%. Therefore, the cooling time will directly affect the length of plastic product molding cycle and the output.
The factors affecting the cooling rate of products include: 1. the design of plastic products; 2. The thicker the wall of plastic products, the longer the cooling time.
Mold Cooling Factors:
1. Mold material: including mold core, cavity material and mold base material, which has a great impact on the cooling rate. The higher the heat conductivity of mold material, the better the effect of heat transfer from plastic in unit time, and the shorter the cooling time.
2. Cooling water pipe configuration mode: the closer the cooling water pipe is to the mold cavity, the larger the pipe diameter, the more the number, the better the cooling effect and the shorter the cooling time.
3. Coolant flow: the greater the cooling water flow (generally the better is to achieve turbulent flow), the better is the effect of cooling water taking away heat in the form of thermal convection.
4. Properties of coolant: the viscosity and thermal conductivity of the coolant will also affect the thermal conductivity of the mold. The lower the viscosity of the coolant, the higher the thermal conductivity. The lower the temperature, the better the cooling effect.
For mold cooling, the cooling channel with three-dimensional structure produced by three-dimensional metal printing parts can be used to achieve better cooling effect.
(4) Demolding stage:
Demolding is the last link in an injection molding cycle. Although the product has been solidified, Demolding still has a great impact on the quality of the product. Improper Demolding method may lead to the product deformation and other defects due to uneven stress during Demolding.
There are two main ways of Demolding: ejector rod Demolding and push plate Demolding. When designing the mold, we should select the appropriate Demolding method according to the structural characteristics of the product to ensure the product quality.
For the mold with ejector rod for Demolding, the ejector rod shall be set as evenly as possible, and the position shall be selected at the place with the largest Demolding resistance and the largest strength and rigidity of the plastic part to avoid deformation and damage of the plastic part.
The push plate is generally used for Demolding of deep-cavity thin-walled containers and transparent products that do not allow the traces of push rod. This method is characterized by large and uniform Demolding force, smooth movement, and no obvious traces left.