Injection molding process generally includes four stages of filling, holding, cooling, and demoulding. These four injection molding process stages directly determine molding quality of product, and the four injection molding process stages are a complete continuous process.

 

(1) Filling is the first step in entire injection molding cycle, from the time that mold is closed to injection molding, until mold cavity is filled to approximately 95%. In theory, the shorter filling time, the higher molding efficiency, but in practice, molding time or injection speed is subject to many conditions.
(2) High speed filling. When high-speed filling is performed, shear rate is high. Plastic has a viscosity drop due to shear thinning, so that overall flow resistance is lowered. Localized viscous heating effect also makes thickness of solidified layer thin. Therefore, in flow control phase, filling behavior often depends on the volume to be filled. That is, in the flow control stage, due to high-speed filling, shear thinning effect of melt tends to be large, and cooling effect of thin wall is not obvious, so utility of rate prevails.
(3) Low speed filling. When heat conduction controls low-speed filling, shear rate is low, local viscosity is high, and flow resistance is large. Due to slower rate of replenishment of hot plastic, flow is slower, so that heat conduction effect is more obvious, and heat is quickly taken away by cold mold wall. With a small amount of viscous heating, thickness of solidified layer is thicker, which further increases flow resistance at the thinner portion of wall.
Due to flow of fountain, plastic polymer chains in front of flowing waves are directed to almost parallel flow wavefront. Therefore, when two plastic melts meet, polymer chains of contact faces are parallel to each other; and two melts have different properties (residence time in cavity is different, temperature and pressure are different), resulting in poor microstructural strength of fusion zone. Place parts at an appropriate angle under light and observe with naked eye that a significant bond line can be found, which is formation mechanism of weld line. Weld line not only affects appearance of plastic part, but also causes stress concentration due to looseness of microstructure, so that strength of this portion is lowered and fracture occurs.
In general, strength of weld line which is welded in high temperature region is better, because polymer chain is better in high temperature and can be entangled with each other. Temperature of two melts in high temperature region is relatively close, and thermal properties of melt are almost same, which increases strength of welded region. Conversely, fusion strength is poor in the low temperature region.

 

Role of pressure preserving stage is to continuously apply pressure, compact melt, and increase density (densification) of plastic to compensate for shrinkage behavior of plastic.
During holding pressure process, back pressure is high because cavity is filled with plastic. During pressure preserving compaction process, screw of injection molding machine can only slowly move forward slightly, and flow speed of plastic is also slow. Flow at this time is called pressure-holding flow. Since plastic is accelerated and solidified by mold wall during pressure preserving stage, melt viscosity is also increased rapidly, so resistance in mold cavity is large.
In later stage of pressure keeping, material density continues to increase, and plastic parts are gradually formed. Pressure preserving stage is continued until gate is cured and sealed. At this time, cavity pressure in pressure preserving stage reaches the highest value.
During holding phase, plastic exhibits partially compressible properties due to relatively high pressure. In higher pressure areas, plastic is denser and has a higher density; in lower pressure areas, plastic is looser and density is lower, thus causing density distribution to change with position and time.
Plastic flow rate during holding pressure process is extremely low, and flow no longer plays a leading role; pressure is the main factor affecting holding pressure process. During packing process, plastic has filled cavity, and gradually solidified melt acts as a medium for transmitting pressure. Pressure in cavity is transferred to surface of mold wall by means of plastic, which has a tendency to open mold, so that a suitable clamping force is required for mold clamping.
Mold-up force will slightly open mold under normal conditions, which will help exhaust of mold. However, if mold-up force is too large, it will easily cause molded product to burr, overflow, and even open mold. Therefore, when selecting an injection molding machine, an injection molding machine with a large enough clamping force should be selected to prevent mold from rising and to effectively maintain pressure.

 

In injection molding dies, design of cooling system is very important. This is because molded plastic product can be prevented from being deformed by an external force only after being cooled and solidified to a certain rigidity and then demoulding. Since cooling time accounts for about 70% to 80% of entire injection molding cycle, a well-designed cooling system can significantly reduce molding time, increase injection productivity, and reduce costs. Improperly designed cooling systems can lengthen molding time and increase cost. Uneven cooling will further cause warpage of plastic products.
According to experiment, heat from melt entering mold is roughly distributed in two parts, and a part of 5% is radiated and convected to atmosphere, remaining 95% is conducted from melt to mold. In mold, due to action of cooling water pipe, heat is transferred from plastic in cavity to cooling water pipe through heat transfer through mold frame, and then carried away by cooling liquid through heat convection. A small amount of heat that is not carried away by cooling water continues to conduct in mold, and then escapes into air after contacting outside.
Molding cycle of injection molding consists of clamping 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 plastic product injection molding cycle and size of production. During demolding stage, temperature of plastic product should be cooled to a temperature lower than heat distortion temperature of plastic product to prevent plastic product from being loosened due to residual stress, warping and deformation caused by external force of demolding.

Mainly wall thickness of plastic products. The greater thickness of product, the longer cooling time. In general, cooling time is approximately proportional to square of thickness of plastic product or proportional to 1.6th power of maximum runner diameter. That is, thickness of plastic product is doubled, and cooling time is increased by 4 times.

Mold materials, including mold cores, cavity materials, and formwork materials have a large impact on cooling rates. The higher heat transfer coefficient of mold material, the better heat transfer from plastic per unit time and the shorter cooling time.

The closer cooling water pipe is to cavity, the larger pipe diameter, the greater quantity, the better cooling effect, and the shorter cooling time.

The greater flow rate of cooling water (generally to achieve turbulent flow), the better effect of cooling water to remove heat by heat convection.

Viscosity and thermal conductivity of coolant also affect heat transfer of mold. The lower viscosity of coolant, the higher heat transfer coefficient, the lower temperature, and the better cooling effect.

Heat transfer coefficient of plastic refers to a measure of rate at which plastic conducts heat from a hot ground.
The higher heat transfer coefficient of plastic, the better heat transfer effect, or specific heat of plastic is low, temperature is easy to change, so heat is easy to dissipate, heat conduction effect is better, and required cooling time is shorter. Processing parameter setting. The higher material temperature, the higher mold temperature, the lower ejector temperature, and the longer cooling time required.

(1) Designed cooling passage should ensure that the cooling effect is uniform and rapid.

(2) Purpose of designing cooling system is to maintain proper and efficient cooling of mold. Cooling holes should be of standard size for ease of processing and assembly.
(3) When designing cooling system, mold designer must determine following design parameters according to wall thickness and volume of plastic part: position and size of cooling hole, length of hole, type of hole, configuration and connection of hole, flow rate and heat transfer properties of coolant.

 

Demolding is the last step in an injection molding cycle. Although product has been cold-formed, demoulding has a very important influence on the quality of product. Improper demolding may result in uneven force during demolding and deformation of product when it is ejected. There are two main ways of demolding: ejector demoulding and stripper demolding. When designing mold, choose appropriate demoulding method according to structural characteristics of product to ensure product quality.
For mold with ejector demoulding, setting of ejector should be as uniform as possible, and position should be selected at the place where demoulding resistance is the largest, strength and rigidity of plastic parts are the largest, so as to avoid deformation and damage of plastic parts. Stripper plate is generally used for deep cavity thin-walled container and demoulding of transparent product which does not allow trace of putter. Mechanism is characterized by large and uniform release force, stable movement, and no obvious traces.