What newcomers to injection molding process must know?

Time：2023-10-20 10:07:49 / Popularity： / Source：

Injection molding process of plastic parts mainly includes four stages: filling, pressure holding, cooling, demoulding, etc. These four stages directly determine molding quality of product, and these four stages are a complete continuous process.

Filling stage

Filling is the first step in the entire injection molding cycle, from the time mold is closed and injection is started 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.

What newcomers to injection molding process must know

High-speed filling
During high-speed filling, shear rate is high, and viscosity of plastic decreases due to effect of shear thinning, which reduces the overall flow resistance; local viscous heating effect will also reduce thickness of cured layer. Therefore, during flow control phase, 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, and cooling effect of thin wall is not obvious, so effect of velocity prevails.
Low speed filling
When heat conduction is filled at a low speed, shear rate is lower, local viscosity is higher, and flow resistance is higher. Due to slow replenishment rate and slow flow of thermoplastic, heat conduction effect is more obvious, and heat is quickly taken away by cold mold wall. Combined with a smaller amount of viscous heating, 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 aligned almost parallel to flow wave front. Therefore, when the two strands of plastic melt meet, polymer chains on contact surface are parallel to each other; in addition, properties of the two strands of melt are different (residence time in mold cavity is different,temperature and pressure are also different), resulting in poor microstructural strength in area where melts meet.
When parts are placed at an appropriate angle under light and observed with naked eye, it can be found that there are obvious bonding lines, which is formation mechanism of weld line. Weld line not only affects appearance of plastic part, but also easily causes stress concentration due to loosening of microstructure, which reduces strength of part and breaks.
Generally speaking, strength of weld line welded in high temperature area is better, because polymer chain has better mobility under high temperature conditions, can penetrate and twist each other. In addition, temperature of the two melts in high temperature area is relatively close, thermal properties of melts are almost same, which increases strength of welding region; on the contrary, in low temperature region, welding strength is poor.

Pressure holding stage

Function of packing stage is to continuously apply pressure to compact melt and increase density of plastic (densification) to compensate for shrinkage behavior of plastic. During pressure holding process, back pressure is high because mold cavity is already filled with plastic. In the process of holding pressure and compaction, 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.
In pressure holding stage, plastic is cooled and solidified faster by mold wall, and melt viscosity increases rapidly, so resistance in mold cavity is very large. In later stage of holding pressure, material density continues to increase, and plastic parts are gradually formed. Pressure holding stage continues until gate is cured and sealed. At this time, cavity pressure in pressure holding stage reaches the highest value.
During packing stage, plastic exhibits partially compressible properties due to relatively high pressure. In areas of higher pressure, plastic is denser and denser; in areas of lower pressure, plastic is looser and lower in density, resulting in a change in density distribution with location and time. During pressure-holding process, flow rate of plastic is extremely low, and flow no longer plays a leading role; pressure is main factor affecting pressure-holding process.
During pressure-holding process, plastic has filled mold cavity, and gradually solidified melt is used as medium for transmitting pressure. Pressure in mold cavity is transmitted to surface of mold wall through plastic, and there is a tendency to open mold, so an appropriate clamping force is required for mold clamping. Under normal circumstances, mold expansion force will slightly open mold, which is helpful for exhaust of mold; but if mold expansion force is too large, it is easy to cause burrs, overflow of molded product, and even open mold.
Therefore, when selecting an injection molding machine, an injection molding machine with sufficient clamping force should be selected to prevent mold expansion and effectively maintain pressure.

Cooling stage

In injection molding molds, design of cooling system is very important. This is because molded plastic product can only be cooled and solidified to a certain rigidity, and plastic product can be prevented from being deformed by external force 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 productivity, and reduce costs.
Improperly designed cooling system will prolong molding time and increase cost; uneven cooling will further cause warpage of plastic products.
According to experiment, heat from melt entering mold is generally dissipated in two parts, one part is 5% transferred to atmosphere by radiation and convection, and remaining 95% is conducted from melt to mold. Due to cooling water pipe in mold, heat is transferred from plastic in mold cavity to cooling water pipe through mold frame through heat conduction, then taken away by cooling liquid through thermal convection. A small amount of heat that is not taken away by cooling water continues to be conducted in mold, then dissipates into air after contacting outside world.
Molding cycle of injection molding consists of mold clamping time, filling time, pressure holding time, cooling time and demolding time. Among them, cooling time accounts for the largest proportion, which is about 70% to 80%. Therefore, cooling time will directly affect length of molding cycle and output of plastic products. In demolding stage, temperature of plastic product should be cooled to a temperature lower than thermal deformation temperature of plastic product to prevent plastic product from loosening due to residual stress or warping and deformation caused by external force of demolding.
Factors that affect cooling rate of product are:
1. Design of plastic products
Mainly wall thickness of plastic products. The thicker product, the longer cooling time. Generally speaking, cooling time is approximately proportional to square of thickness of plastic product, or to 1.6th power of maximum runner diameter. That is, thickness of plastic product is doubled, and cooling time is increased by 4 times.
2. Mold material and its cooling method
Mold material, including mold core, cavity material, and mold base material has a great influence on cooling rate. The higher thermal conductivity of mold material, the better effect of transferring heat from plastic per unit time, and the shorter cooling time.
3. Cooling water pipe configuration
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.
4. Coolant flow
The larger cooling water flow rate (generally better to achieve turbulent flow), the better effect of cooling water to remove heat by thermal convection.
5. Properties of coolant
Viscosity and thermal conductivity of coolant will also affect thermal conductivity of mold. The lower viscosity of coolant, the higher thermal conductivity, and the lower temperature, the better cooling effect.

6. Plastic selection
Plastic refers to a measure of how quickly plastic conducts heat from a hot place to a cold place. The higher thermal conductivity of plastic, the better thermal conduction effect, or the lower specific heat of plastic, the easier temperature is to change, so heat is easy to dissipate, thermal conduction effect is better, and cooling time required is shorter.
7. Processing parameter setting
The higher material temperature, the higher mold temperature, the lower ejection temperature, and the longer cooling time required.
Design rules of cooling system: Designed cooling channel should ensure that cooling effect is uniform and rapid.
Purpose of designing cooling system is to maintain proper and efficient cooling of mold. Cooling holes should be of standard size for ease of machining and assembly.
When designing a cooling system, mold designer must determine following design parameters according to 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.

Demolding stage

Demoulding is last link in an injection molding cycle. Although product has been cold-set, demoulding still has a very important impact on quality of product. Improper demolding method may cause uneven stress on product during demoulding, and cause defects such as product deformation during ejection. There are two main ways of demoulding: ejector demoulding and stripping plate demoulding. When designing a mold, an appropriate demoulding method should be selected according to structural characteristics of product to ensure product quality.
For mold with ejector ejector, ejector pins should be set as uniform as possible, location should be selected at place with the largest ejection resistance, the highest strength and stiffness of plastic parts, so as to avoid deformation and damage of plastic parts.
Stripper plate is generally used for demoulding of deep cavity thin-walled containers and transparent products that do not allow traces of push rods.

Gud Mould Industry Co., Ltd