After the read, you will learn about 11 and more Analyses of injection molding process parameters, and how to set the injection molding process parameters.
The further development of injection molding equipment and the continuous improvement of product quality requirements have put forward higher requirements for the injection molding process.
Correct selection of injection molding equipment and reasonable setting of the injection molding process parameters and optimization of injection molding process parameters conditions are the keys to improving the quality of injection molded products.
Selection of injection molding machine specifications
When choosing injection molding machine specifications, first of all, we should consider the condition of the production mold because the same injection molding machine often has to meet the production of multiple molds of different sizes.
We should determine the injection molding machine specifications according to the weight of the parts and the size of the mold, i.e., the maximum clamping force and the maximum injection volume of the injection molding machine, and then choose a suitable model according to the specifications provided by the injection molding machine manufacturer.
Consider whether some special configurations are needed, such as special screws for the production of PA, PC, and other materials, and corresponding devices for molding molds with inlet and outlet cores or threads;
Again, determine the need to use injection molding machines with special features based on the mold structure, product quality, and other factors, such as molding thin-walled long-flow products (generally referred to as L/D300) and high injection.
For example, when molding thin-walled long-flowing products (generally L/D300), a high injection speed injection molding machine is needed, and precision electronic parts need to use a precision full closed-loop control injection molding machine, etc.
Injection molding process parameters clamping force setting
Theoretically, the clamping force can be calculated according to the following formula.
Fcm>=K×P average×A product×10
In the formula: Fcm – clamping force, (KN) K – safety factor, generally take 1-1.2 P average – the average pressure of the mold cavity (MPa) A products – products in the mold parting surface of the maximum projection area (c square meters)
In actual production, the adjustment of clamping force should also consider the impact of thermal expansion of the mold in production, generally should leave a margin of 0.1-0.2mm; the principle of setting clamping force is to ensure product quality under the premise of low clamping force is appropriate.
11 Injection molding process parameters and setting
Barrel temperature, mold temperature injection molding process parameters
According to the performance of different plastic materials to set the screw barrel temperature, the barrel set temperature is generally higher than the melting point of plastic 10 ℃ -30 ℃.
It is important to note that the melting point and the allowable residence time in the barrel may vary depending on the synthesis method or the type of additives added to the materials provided by different manufacturers.
Mold temperature is generally cooled using circulating water when setting, but when producing products with precise dimensions or high surface quality requirements, a mold temperature machine capable of accurate control should be used according to the process requirements.
Injection holding time, cooling time of injection molding process parameters
Injection molding time, injection holding time, and injection cooling time should be set according to product thickness, mold temperature, and material properties.
The injection time setting is generally slightly greater than the time for the screw to complete the injection stroke movement. Too long an injection time will produce mechanical wear and tear, increase energy consumption and other negative effects, and prolong the molding cycle.
The injection holding time setting is set according to the thickness of the product; thin-walled products can be molded without holding time.
When setting the injection holding time, as long as there is no obvious depression on the surface of the product, it can also be determined by the weighing method, and the best injection holding time can be set by gradually extending the injection holding time until the product quality no longer changes.
Injection cooling time should also be determined according to the product thickness, mold temperature, and material properties. Generally, amorphous polymers require a longer cooling time than crystalline polymers.
Injection pressure and speed of injection molding process parameters
The injection pressure setting should follow the principle of low rather than high as long as it can provide enough power to achieve the required injection speed so that the melt can fill the cavity smoothly. Too high a pressure is likely to cause internal stress in the product.
However, when molding products with high dimensional accuracy, to prevent excessive shrinkage, the high-pressure injection can be used to reduce the shrinkage of the product after demolding.
The injection speed will affect the appearance quality of the product, and its setting should be set according to the geometric structure of the mold, exhaust condition, etc. Generally, the injection speed should be increased as much as possible to reduce the filling time to ensure a good appearance.
In injection molding, when the melt flows in the mold, the mold wall will form a curing layer, thus reducing the thickness of the flowable channel.
Generally, depending on the mold structure and injection speed, the mold wall will have a curing layer of about 0.2mm. Therefore a faster injection speed is usually used in molding.
Injection stroke, multi-stage injection of injection process parameters
In injection molding, the injection stroke must be determined first. Theoretically, the injection stroke can be calculated according to the following formula.
S1 = 4 (CVp + Va) / ρDs2
Formula: injection stroke Vp – product volume ρ – resin density C – the number of cavities Va – gate volume Ds – screw diameter
In practice, if the “product + gate” total weight is known, the following formula can be used to calculate the injection stroke S1 = (M/Mmax) – Smax + (5 ~ 10) mm.
Formula S1: injection stroke, mm M – “product + gate” total weight, g Max – injection machine maximum injection volume, g / Smax – injection machine maximum injection stroke
Due to the different geometry of the sprue system and mold parts, to meet the product quality requirements, there are different requirements for the flow state of the melt in different parts of the mold (mainly refers to the pressure and speed when flowing).
In an injection molding process, when the screw pushes the melt into the mold, it is required to achieve different pressure and speed at different positions, called multi-stage injection molding.
Generally, it is more scientific to set at least three or more than four injection stages in molding, i.e., the first stage at the main flow channel, the second stage from the manifold to the gate, the third stage when the product is filled with about 90% of the cavity, and the fourth stage for the remaining part, which can be used to determine the switching position of each stage by calculating the weight method.
In actual production, the multi-stage injection process parameters should be scientifically analyzed and reasonably set according to product quality requirements, runner structure, and mold exhaust condition.
Usually, the setting can be made by debugging the observation method, setting the pressure/speed of the switching position point required for injection to 0, observing the directional position of the melt and the defective condition of the product, and gradually making adjustments until a reasonable position point is found.
In the debugging process, observation must consider the under-injection product release condition to avoid sticky mold in some recessed parts of the mold due to under-injection.
Plastic viscosity and the impact of conditions on the viscosity of injection molding process parameters
When molten plastic flows, the nature of the friction between the macromolecules is called the viscosity of plastic. And the coefficient of the size of this viscosity is called viscosity, so viscosity is a reflection of the liquidity of molten plastic.
The greater the viscosity, the dense the melt is, the worse the fluidity, and the more difficult it is to process.
In industrial applications, comparing the fluidity of plastic does not depend on its viscosity value but on the size of its melt flow index (MFI).
The MFI is the melt weight per unit time (usually 10 minutes) of a melt passing through a standard mold at a certain melting temperature and underrated pressure. To g/10min said, such as injection molding grade PP material.
In different grades, the value of MFI can vary from 2.5 to 30; the viscosity of plastic is not invariable, and changes in the characteristics of the plastic itself, external temperature, pressure, and other conditions, can contribute to changes in viscosity.
The influence of the molecular weight of injection molding process parameters
The larger the molecular weight, the narrower the molecular weight distribution, the greater the reflected viscosity.
Influence of low molecular additive of injection molding process parameters
The low molecular additives can reduce the force between large molecules. Therefore, the viscosity is reduced, some plastics molding time to add solvents or plasticizers is to reduce the viscosity so that it is easy to mold molding.
The effect of temperature on the viscosity of injection molding process parameters
The influence of temperature on the viscosity of most molten plastics is great, generally the temperature increases, the lower the viscosity reflected, but the magnitude of the reduction of the viscosity of various plastic melts varies:
PE/PP plastic, raise the temperature to improve the fluidity, reduce the melt viscosity effect is very small, the temperature is too high, the consumption increases, but the loss is not worth it
PMMA/PC/PA and other plastics, the temperature increases the viscosity drops significantly, PS ABS to increase the temperature to reduce viscosity in molding also has greater benefits.
Injection molding process parameters of the impact of shear speed
Effectively increasing the shear speed of plastic can make plastic viscosity drop, but there are some plastics, such as PC also have exceptions, its viscosity is almost not affected by the screw speed.
Injection molding process parameters of the influence of pressure
The influence of injection molding pressure on viscosity is relatively complex.
Generally, PP & PE viscosity is not greatly affected by the pressure, but the impact on PS is quite significant, the actual production, in a better-equipped machine, should pay attention to the role of high-speed injection, that is, high shear speed, rather than blindly increase the pressure.
Injection molding process parameters of the injection temperature control on the impact of the molding process
The so-called barrel temperature control refers to how the plastic in the barrel from the raw material particles has been uniformly heated into plastic viscous fluid, that is, how the barrel baking temperature configuration.
The barrel temperature should be adjusted to ensure the plastic is well-plasticized and can be injected smoothly into the mold without causing decomposition.
This requires that we cannot be subject to the temperature sensitivity of the plastic and consciously lower the plasticization temperature, injection pressure, injection speed, and other methods to force mold filling.
Plastic melting temperature mainly affects processing performance, surface quality, and color.
The control of material temperature is related to the mold of the parts, large and simple parts, the weight of the parts, and the injection volume is close to the higher baking temperature, thin-walled. Complex shapes also need to use high baking temperatures.
For thick-walled parts, for some additional operations, such as the installation of inserts, you can use a low baking temperature to identify whether the plastic melt temperature is appropriate. You can use point action in low-pressure injection by observing the empty speed. The appropriate material temperature should make the sprayed material strong, strong, without bubbles, not curled, bright and continuous.
The configuration of the material temperature is generally from the feed section to the discharge section in ascending order, but to prevent the decomposition of plastic overripe and color changes in the parts can also be slightly lower than the middle section, improper configuration of the material temperature can sometimes cause a screw failure, the screw does not turn or idle.
This can also be caused by excessive injection pressure or the failure of the screw stop collar (meson), causing the thin melt at the front of the barrel to backflow in the direction of the feed zone.
When this backflow material into the threaded end face and the barrel wall between the tiny gap and is subject to lower temperature cooling, it will be frozen into a layer of film stuck tightly between the two walls so that the screw cannot turn or slip. This affects feeding.
At this time, do not forcibly loosen the back or injection. It is recommended to turn off the cooling water at the filling port temporarily, intensify the temperature of the filling section until it is 30-50 degrees Celsius higher than the melting point of plastic, and at the same time, lower the temperature of the discharge section to near the melting temperature, wait for 10-20 minutes, then carefully turn the screw and restart the machine only when it can turn, and then slowly fill the material.
Injection molding process parameters of pressure control in the injection cycle
The actual applied pressure should be higher than the cavity-filling pressure.
During the injection molding process, the mold control pressure rises sharply and eventually reaches a peak, which is usually referred to as injection pressure. The injection pressure is higher than the cavity-filling pressure.
The role of injection holding pressure: after the cavity is filled with plastic until the gate is completely cooled to close a period, the plastic in the cavity still needs fairly high-pressure support. That is, holding pressure, its specific role is:
A: replenish the material near the gate position and stop the plastic in the cavity from hardening under residual pressure before the gate condenses to close, backflowing to the direction of the gate material source.
B: Prevent shrinkage of the part and reduce vacuum bubbles.
C: to reduce the excessive injection pressure of the parts and produce sticky mold burst or bending deformation phenomenon. So the holding pressure is usually 50%~60% of the injection pressure.
If the holding pressure or time is too long or too large, it may squeeze the cold material on the gate and runner into the part, which will add cold material bright spot near the gate position and prolong the cycle time without any benefit.
Selection of injection pressure for injection molding process parameters
According to the shape of the part
Selection for different plastic materials
If the production conditions and quality standards of the parts allow, it is recommended to use the injection molding process parameters conditions of temperature and low pressure.
Backpressure adjustment of injection molding process parameters
Backpressure represents the pressure to which the plastic is subjected during plasticization. It is also called plasticizing pressure.
- The mixing effect of color is influenced by the backpressure, and the mixing effect is enhanced by increasing the backpressure.
- The backpressure helps to remove all kinds of gas from the plastic parts and reduces the phenomenon of silver lines and bubbles.
- Proper back pressure can avoid the phenomenon of local stagnant material in the barrel, so the back pressure is often increased when cleaning the barrel.
Injection molding process parameters of injection speed control
Influence of speed: The advantage of low-speed filling is that the flow rate is smooth, the size of the part is more stable, fluctuations are small, the internal stress of the part is low, and the consistency of the internal and external stress is good,
The disadvantage is that the parts easily appear layered with bad melting point marks, water lines, etc.
High-speed mold filling can be used at a lower injection pressure, improving the gloss and smoothness of the product, eliminating the phenomenon of seam lines and layering, small shrinkage depression, and more uniform and consistent color.
The disadvantage is that it is easy to have stagnant flow or vortex flow. High-temperature rise, yellow color, poor exhaust, and sometimes difficult to release.
High-viscosity plastic may produce melt breakage and fogging on the surface of the part and also increase the tendency of internal stress caused by fin bending and thick parts cracking along the seam line.
Other injection molding process parameters
In the injection molding process parameters, in addition to the main injection molding process parameters such as molding temperature, pressure, speed, time, and multi-stage injection switching position, there are many other injection molding process parameters, such as screw speed, screw inversion anti-flow delay, and other action parameter settings, which should not be ignored.
Example of injection molding process parameters setting
Take the production of nylon cable ties as an example. The product quality requirements are as follows.
The product reaches the specified tensile strength standard; the surface is free of silver, bubbles, shrinkage marks, and other undesirable phenomena; the product has good tightness after molding and no loose phenomenon. The material used is PA66, the mold structure is hot runner type, and the gate type is point gate.
Firstly, according to the product’s characteristics and the mold’s structure, we will determine the principle of setting the process parameters.
High-speed injection molding should be selected because the product flow length is long and L/t (flow to wall thickness ratio) is 511.
The gate type is a pointed gate; a higher pressure must be used to overcome the resistance during the flow.
To ensure that the product can fill the mold smoothly, the melt must have good flowability, and the molding temperature should be appropriately high.
The high pressure and high-speed injection to the end of the end are easy to produce a flying edge. The molding machine must have low inertia pressure and speed switching.
Because of the small wall thickness of the product, no pressure-holding can be used.
Formulate the main molding injection process parameters
To formulate injection molding process parameters, we must understand the information on equipment performance, mold structure, molding materials, and product quality requirements, and set each injection molding process parameter in a scientific and reasonable way.
First, the injection molding process parameters should be adjusted gradually according to the product molding condition, and the correct order of adjustment is pressure → speed, → temperature.
The computer confirms the input parameters before the next parameter change each time the injection molding process parameters are changed. This avoids changing more than two injection molding process parameters simultaneously.
Secondly, when the product enters into stable production, the injection molding process parameters should be kept as stable as possible, and detailed records should be made.
In addition, the molding process should be fixed as much as possible every time the mold is put online to facilitate the quality control of the finished products.
Besides the 11 Analyses of Injection Molding Process Parameters article, you may also be interested in the below.
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