4 stages determine the quality of injection molded products

September 4, 2019

4 stages determine the quality of injection molded products
 

The injection molding process of plastic parts mainly includes four stages of filling - holding pressure - cooling - demoulding. These four stages are a complete continuous process, which directly determines the molding quality of the product.


01. Filling stage
Filling is the first step in the entire injection molding cycle, from the time the mold is closed to the injection, until the mold cavity is filled to approximately 95%. In theory, the shorter the filling time, the higher the molding efficiency, but in reality, the molding time or the injection speed is subject to many conditions.

 

High speed filling
When the high-speed filling is performed, the shear rate is high, and the plastic has a viscosity drop due to shear thinning, so that the overall flow resistance is lowered; the local viscous heating effect also makes the thickness of the solidified layer thin. Therefore, in the flow control phase, the filling behavior often depends on the volume to be filled.

 

That is, in the flow control stage, due to the high-speed filling, the shear thinning effect of the melt tends to be large, and the cooling effect of the thin wall is not obvious, so the utility of the rate prevails.

 

Low speed filling
When heat conduction controls low-speed filling, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slower rate of replenishment of the hot plastic, the flow is slower, so that the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. With a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner portion of the wall.

 

Due to the flow of the fountain, the plastic polymer chains in front of

the flowing waves are directed to the almost parallel flow wavefront.

 

Therefore, when the two plastic melts meet, the polymer chains of the contact faces are parallel to each other; plus the two melts have different properties (different residence time in the cavity, temperature and pressure are different), resulting in the fusion zone of the melt. Microscopically, the structural strength is poor.

 

When the parts are placed at an appropriate angle under light, they are visually observed, and it is found that a significant bonding line is produced, which is the formation mechanism of the weld lines. The weld line not only affects the appearance of the plastic part, but also causes stress concentration due to the looseness of the microstructure, so that the strength of the portion is lowered and fracture occurs.

 

In general, the strength of the weld line which is welded in the high temperature region is better, because the polymer chain is more active at high temperatures and can be intertwined and entangled. In addition, the temperature of the two melts in the high temperature region is relatively close, and the thermal properties of the melt are almost the same, which increases the strength of the welded region; otherwise, the fusion strength is poor in the low temperature region.

 

02. Pressure maintaining stage
The role of the packing phase is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic.

 

During the pressure holding process, the back pressure is high because the cavity is filled with plastic. During the pressure-preserving compaction process, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also slow. The flow at this time is called pressure-holding flow.

 

Since the plastic is accelerated and solidified by the mold wall during the pressure holding stage, the melt viscosity is also increased rapidly, so the resistance in the mold cavity is large. In the later stage of pressure keeping, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage is continued until the gate is cured and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.

 

During the holding phase, the plastic exhibits partially compressible properties due to the relatively high pressure. In the higher pressure areas, the plastic is denser and has a higher density; in the lower pressure areas, the plastic is looser and the density is lower, thus causing the density distribution to change with position and time.

 

The plastic flow rate during the pressure holding process is extremely low, and the flow no longer plays a leading role; the pressure is the main factor affecting the pressure holding process.

 

During the packing process, the plastic has filled the cavity, and the gradually solidified melt acts as a medium for transmitting pressure. The pressure in the cavity is transferred to the surface of the mold wall by means of plastic, which has a tendency to open the mold, so that a suitable clamping force is required for the mold clamping.

 

The mold-up force will slightly open the mold under normal conditions, which will help the exhaust of the mold; however, if the mold-up force is too large, it will easily cause the molded product to burr, overflow, and even open the mold. Therefore, when selecting an injection molding machine, an injection molding machine with a large enough clamping force should be selected to prevent the mold from rising and to effectively maintain the pressure.

 

03. Cooling stage
In injection molding dies, 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:

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.

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.

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.

Coolant flow. 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.

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.

Plastic choice. 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:
The cooling channel is designed to ensure a uniform and rapid cooling effect.

 

The purpose of the design of 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.

 

When designing a cooling system, the mold designer must determine the following design parameters based on the wall thickness and volume of the plastic part—the location and size of the cooling holes, the length of the holes, the type of holes, the configuration and connection of the holes, and the flow rate of the coolant. Heat transfer properties.

 

04. Demoulding stage
Demolding is the last step in an injection molding cycle. Although the product has been cold-formed, demolding has a significant impact on the quality of the product. Improper demolding may result in uneven force on the product during demolding and defects such as product deformation when it is ejected.

 

There are two main ways of demolding: demolding of the ejector and demolding of the stripper. When designing the mold, choose the appropriate release method according to the structural characteristics of the product to ensure the product quality.

 

For the mold with demoulding demoulding, the setting of the ejector should be as uniform as possible, and the position should be selected at the place where the demoulding resistance is the largest and the strength and rigidity of the plastic parts are the largest, so as to avoid deformation and damage of the plastic parts.

 

The stripper plate is generally used for the deep cavity thin-walled container and the release of the transparent product which does not allow the trace of the putter. The mechanism is characterized in that the stripping force is large and uniform, the movement is stable, and there is no obvious residual trace.

 

Injection molding process parameters

 

1.Injection pressure

Injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure push, the plastic melt enters the vertical flow path of the mold through the nozzle of the injection molding machine (also the mainstream channel for some molds), the main flow channel, and the split flow. The process, through the gate into the mold cavity, is the injection molding process, or the filling process.

 

The pressure is present to overcome the resistance during the melt flow, or conversely, the resistance present during the flow process needs to be offset by the pressure of the injection molding machine to ensure a smooth filling process.

 

During the injection molding process, the pressure at the nozzle of the injection molding machine is highest to overcome the flow resistance throughout the melt. Thereafter, the pressure gradually decreases toward the front end of the melt along the flow length. If the internal exhaust of the cavity is good, the final pressure at the front end of the melt is atmospheric pressure.

 

There are many factors that affect the melt filling pressure. There are three categories in summary:
(1) Material factors such as the type and viscosity of the plastic;
(2) structural factors such as the type, number and location of the casting system, the shape of the cavity of the mold, and the thickness of the product;
(3) Process elements of molding.

 

2. Injection time

The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, and does not include the auxiliary time such as opening and closing of the mold. Although the injection time is short and the effect on the molding cycle is small, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Proper injection time helps the ideal filling of the melt and is very important for improving the surface quality of the product and reducing dimensional tolerances.

 

The injection time is much lower than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the overall molding time of the plastic parts. In the case of mold flow analysis, the injection time in the analysis results is equal to the injection time set in the process conditions only when the melt is completely driven by the screw rotation to fill the cavity. If the holding pressure of the screw occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.

 

3. Injection temperature

Injection temperature is an important factor affecting injection pressure. The injection molding machine barrel has 5 to 6 heating sections, each of which has its own suitable processing temperature (for detailed processing temperatures, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range. The temperature is too low, the melt plasticization is poor, affecting the quality of the molded parts, increasing the difficulty of the process; the temperature is too high, and the raw materials are easily decomposed.

 

In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher values ​​are related to the injection rate and material properties, up to 30 °C. This is caused by the high heat generated by the shearing of the melt as it passes through the injection port. In the mold flow analysis, the difference can be compensated in two ways, one is to try to measure the temperature of the melt-to-air injection molding, and the other is to include the nozzle in the modeling.

 

4. Holding pressure and time

At the end of the injection molding process, the screw stops rotating and only advances, at which point the injection molding enters the packing phase. During the pressure keeping process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume vacated due to the shrinkage of the workpiece.

If the cavity is filled without holding pressure, the part will shrink by about 25%, especially if the tendon is too large to form a shrinkage mark. The holding pressure is generally about 85% of the maximum filling pressure, of course, it should be determined according to the actual situation.


5. Back pressure

Back pressure refers to the pressure that the screw needs to overcome when reversing the backstock. The use of high back pressure facilitates the dispersion of the colorant and the melting of the plastic, but at the same time prolongs the screw retracting time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not exceeding the injection molding. 20% of the pressure.

When injection molding foam, the back pressure should be higher than the pressure created by the gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which reduces input heat and reduces temperature. However, since the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.
 

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