Injection molding process and characteristics of each stage

Time:2024-11-26 08:04:16 / Popularity: / Source:

Injection molding process mainly includes six stages: mold closing-filling-pressure holding-cooling-opening-demolding. These six stages directly determine molding quality of product, and these six stages are a complete continuous process.
injection molding mold 

1. Filling stage

Filling is the first step in the entire injection molding cycle. Time starts from mold closing and injection molding until mold cavity is filled to about 95%. In theory, the shorter filling time, the higher molding efficiency; but in actual production, molding time (or injection speed) is subject to many conditions.
High-speed filling. Shear rate is high during high-speed filling, and plastic has a decrease in viscosity due to effect of shear thinning, which reduces the overall flow resistance; local viscous heating effect will also make solidified layer thinner. Therefore, in flow control stage, filling behavior often depends on size of volume to be filled. That is, in flow control stage, due to high-speed filling, shear thinning effect of melt is often large, while cooling effect of thin wall is not obvious, so effect of rate prevails.
Low-speed filling. When heat conduction controls low-speed filling, shear rate is low, local viscosity is high, and flow resistance is large. Since hot plastic replenishment rate is slow and flow is slow, heat conduction effect is more obvious, and heat is quickly taken away by cold mold wall. In addition, due to small amount of viscous heating phenomenon, thickness of solidified layer is thicker, which further increases flow resistance at thinner wall.
Due to fountain flow, plastic polymer chains in front of flow wave are arranged almost parallel to flow wave front. Therefore, when two plastic melts meet, polymer chains on contact surface are parallel to each other; coupled with different properties of two melts (different residence time in mold cavity, temperature and pressure), microscopic structural strength of melt intersection area is poor. When parts are placed at an appropriate angle under light and observed with naked eye, it can be found that there are obvious joint lines, which is formation mechanism of weld mark. Weld mark not only affects appearance of plastic part, but also its microstructure is loose, which is easy to cause stress concentration, thereby reducing strength of part and causing fracture.
Generally speaking, weld mark strength produced in high temperature zone is better. Because under high temperature conditions, polymer chain is relatively active, can penetrate and entangle each other. In addition, temperature of the two melts in high temperature zone is relatively close, and thermal properties of melts are almost same, which increases strength of weld area; on the contrary, in low temperature zone, weld strength is poor.

2. Holding pressure stage

Function of holding pressure stage is to continuously apply pressure, compact melt, increase density of plastic (densification), and compensate for shrinkage behavior of plastic. During holding pressure process, back pressure is high because mold cavity is already filled with plastic. During holding pressure compaction process, screw of injection molding machine can only move forward slowly and slightly, and flow speed of plastic is also relatively slow. Flow at this time is called holding pressure flow. During holding stage, plastic is cooled and solidified by mold wall, and melt viscosity increases rapidly, so resistance in mold cavity is very large. In the later stage of holding, material density continues to increase, and plastic part is gradually formed. Holding stage should continue until gate is solidified and sealed. At this time, mold cavity pressure in holding stage reaches the highest value.
During holding stage, due to high pressure, plastic shows some compressible characteristics. In high pressure area, plastic is denser and has a higher density; in low pressure area, plastic is looser and has a lower density, so density distribution changes with position and time. During holding process, plastic flow rate is extremely low, and flow no longer plays a leading role; pressure is main factor affecting holding process. During holding process, plastic has filled mold cavity, and gradually solidified melt at this time serves as a medium for transmitting pressure. Pressure in mold cavity is transmitted to mold wall surface with help of plastic, and there is a tendency to open mold, so an appropriate clamping force is required for clamping. Under normal circumstances, mold expansion force will slightly expand mold, which is helpful for exhaust of mold; but if mold expansion force is too large, it is easy to cause burrs, overflow, and even expand mold. Therefore, when choosing an injection molding machine, you should choose an injection molding machine with a sufficiently large clamping force to prevent mold expansion and effectively maintain pressure.
Under new injection molding environment conditions, we need to consider some new injection molding processes, such as gas-assisted molding, water-assisted molding, foam injection molding, etc.

3. Cooling stage

In injection molding mold, design of cooling system is very important. This is because molded plastic products can only be cooled and solidified to a certain rigidity, and then plastic products can be prevented from being deformed by external forces after demolding. Since cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly shorten molding time, improve injection molding productivity, and reduce costs. An improperly designed cooling system will prolong molding time and increase cost; uneven cooling will further cause warping and deformation of plastic products.
According to experiments, heat from melt entering mold is generally dissipated in two parts. One part is 5% transferred to atmosphere through radiation and convection, and remaining 95% is conducted from melt to mold. Due to cooling water pipe in mold, heat of plastic product is transferred from plastic in mold cavity to cooling water pipe through mold frame through heat conduction, and then taken away by coolant through heat convection. A small amount of heat that is not taken away by cooling water continues to be conducted in mold, and dissipates into air after contacting outside world.
Molding cycle of injection molding consists of mold closing time, filling time, holding time, cooling time and demolding time. Among them, cooling time accounts for the largest proportion, about 70% to 80%. Therefore, cooling time will directly affect length of molding cycle and output of plastic products. Temperature of plastic product should be cooled to a temperature lower than thermal deformation temperature of plastic product during demolding stage to prevent plastic product from being relaxed due to residual stress or warping and deformation caused by external forces of demolding.
injection molding mold 
Factors affecting cooling rate of product are:
Plastic product design. Mainly wall thickness of plastic product. The thicker product, the longer cooling time. Generally speaking, cooling time is approximately proportional to square of thickness of plastic product, or proportional to 1.6th power of maximum flow channel diameter. That is, when thickness of plastic product doubles, cooling time increases by 4 times.
Mold materials and their cooling methods. Mold materials, including mold core, cavity materials and mold frame materials, have a great influence on cooling speed. The higher thermal conductivity of mold material, the better effect of transferring heat from plastic per unit time, and the shorter cooling time.
Cooling water pipe configuration method. The closer cooling water pipe is to mold cavity, the larger pipe diameter, and the more number, the better cooling effect and the shorter cooling time.
Coolant flow rate. The larger cooling water flow rate (generally better to achieve turbulence), the better effect of cooling water taking away heat by heat convection.
Coolant properties. Viscosity and thermal conductivity of coolant will also affect thermal conductivity of mold. The lower viscosity of coolant, the higher thermal conductivity, the lower temperature, and the better cooling effect.
Plastic selection. Plastic refers to measure of speed at which plastic transfers heat from hot places to cold places. The higher thermal conductivity of plastic, the better heat conduction effect, or the lower specific heat of plastic, the easier it is to change temperature, so heat is easy to dissipate, the better heat conduction effect, and the shorter cooling time required.
Processing parameter setting. The higher material temperature, the higher mold temperature, the lower ejection temperature, and the longer required cooling time.
Cooling system design rules:
Designed cooling channel must ensure uniform and rapid cooling.
Purpose of designing a cooling system is to maintain proper and efficient cooling of mold. Cooling holes should use standard sizes to facilitate processing and assembly.
When designing a cooling system, mold designer must determine following design parameters based on wall thickness and volume of plastic part - location and size of cooling hole, length of hole, type of hole, configuration and connection of hole, flow rate and heat transfer properties of coolant.

4. Demolding stage

Demolding is last link in an injection molding cycle. Although product has been cold-formed, demolding still has a very important impact on quality of product. Improper demolding methods may cause uneven force on product during demolding and product deformation during ejection. There are two main ways of demolding: ejector demolding and stripper demolding. When designing a mold, appropriate demolding method should be selected according to structural characteristics of product to ensure product quality.
For molds that use ejector demolding, ejector should be set as evenly as possible, position should be selected where demolding resistance is the largest, strength and rigidity of plastic part are the largest to prevent deformation and damage of plastic part.

5. Process parameters

Injection pressure

Injection pressure is provided by hydraulic system of injection molding system. Pressure of hydraulic cylinder is transmitted to plastic melt through injection molding machine screw. Under pressure, plastic melt enters vertical flow channel (also main flow channel for some molds), main flow channel, branch flow channel of mold through nozzle of injection molding machine, and enters mold cavity through gate. This process is injection molding process, or filling process. Existence of pressure is to overcome resistance in flow process of melt, or conversely, resistance in flow process needs to be offset by pressure of injection molding machine to ensure smooth progress of filling process.
During injection molding process, pressure at nozzle of injection molding machine is the highest to overcome flow resistance of melt throughout process. Afterwards, pressure gradually decreases along flow length toward front wave of melt. If exhaust inside cavity is good, final pressure at the front of melt is atmospheric pressure.
There are many factors that affect melt filling pressure, which can be summarized into three categories:
(1) Material factors, such as type and viscosity of plastic;
(2) Structural factors, such as type, number and position of gating system, shape of mold cavity and thickness of product;
(3) Process elements of molding.

Injection time

Injection time here refers to time required for plastic melt to fill cavity, excluding auxiliary time such as mold opening and closing. Although injection time is very short and has little effect on molding cycle, adjustment of injection time has a great effect on pressure control of gate, runner and cavity. Reasonable injection time helps melt to fill ideally, is of great significance for improving the surface quality of product and reducing dimensional tolerance.
Injection time should be much lower than cooling time, about 1/10 to 1/15 of cooling time. This rule can be used as a basis for predicting total molding time of plastic parts. When performing mold flow analysis, injection time in analysis result is equal to injection time set in process conditions only when melt is completely pushed by screw to fill cavity. If screw pressure holding switch occurs before cavity is filled, analysis result will be greater than process condition setting.
injection molding mold 

Injection temperature

Injection temperature is an important factor affecting injection pressure. Barrel of injection molding machine has 5 to 6 heating sections, and each raw material has its appropriate processing temperature (for detailed processing temperatures, please refer to data provided by material supplier). Injection temperature must be controlled within a certain range. If temperature is too low, melt will not be plasticized well, affecting quality of molded part and increasing difficulty of process; if temperature is too high, raw material is easy to decompose. In actual injection molding process, injection temperature is often higher than barrel temperature. Higher value is related to injection rate and performance of material, and can be as high as 30℃. This is caused by high heat generated by shear when melt passes through injection port. There are two ways to compensate for this difference when performing mold flow analysis. One is to try to measure temperature of melt when it is injected into air, and the other is to include nozzle when modeling.

Holding pressure and time

When injection molding process is about to end, injection molding enters holding pressure stage. During holding pressure process, nozzle of injection molding machine continuously adds material to cavity to fill volume vacated by shrinkage of part. If cavity is filled without holding pressure, part will shrink by about 25%, especially ribs will shrink too much and form shrinkage marks. Holding pressure is generally about 85% of maximum filling pressure, which should be determined according to actual situation.

Back pressure

Back pressure refers to pressure that needs to be overcome when screw rotates and retreats to store material. Use of high back pressure is conducive to dispersion of colorants and melting of plastics, but it also prolongs screw retraction time, reduces length of plastic fibers, and increases pressure of injection molding machine. Therefore, back pressure should be lower, generally not exceeding 20% of injection pressure.

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