The further development of injection molding equipment and the continuous improvement of product quality requirements all place higher requirements on the injection molding process. The correct selection of injection equipment, and the rational setting of the molding process, and the optimization of process conditions are the keys to improving product quality.
Injection molding machine specifications
When selecting the specifications of the injection molding machine, it is necessary to first consider the situation of the production of the mold, because the same injection molding machine often needs to meet the production of multiple pairs of different molds. The size of the injection molding machine should be determined according to the weight of the parts, the size of the mold, etc., that is, the injection molding machine. Maximum clamping force and maximum injection volume, then select the appropriate model according to the specifications provided by the injection molding machine manufacturer.
It is necessary to consider whether special configurations are required, such as the use of special screws for the production of materials such as PA and PC, and the corresponding devices for the molding of molds with core withdrawal or thread removal; again, according to the structure of the mold, product quality, etc. The factors to determine whether to use some injection molding machines with special functions, such as molding thin-walled long flow products (generally referred to as L/D300), high injection speed injection molding machines are required, precision electronic parts need to use precision full closed-loop control injection molding machine Wait.
Clamping force setting
In theory, the clamping force can be calculated as follows:
Fcm>=K*P average*A product*10
In the formula: Fcm - clamping force, (KN)K - safety factor, generally take 1-1.2 P average - cavity average pressure (MPa) A product - the maximum projected area of the product on the mold parting surface (cm2)
In the actual production, the adjustment of the clamping force should also take into account the impact of thermal expansion of the mold during production. Generally, 0.1-0.2mm margin should be left; the setting principle of clamping force is the prerequisite for ensuring product quality. It is appropriate to use low clamping force.
Injection molding process parameter setting
Barrel temperature, mold temperature
According to the performance of different plastic materials to set the screw barrel temperature, the barrel set temperature is generally higher than the plastic melting point of 10 °C -30 °C. It must be noted that different manufacturers of materials due to the synthesis method or the type of additive added, Their melting point and allowable residence time in the barrel also vary.
The mold temperature is usually set using circulating water cooling. However, in the production of products with high precision or high surface quality, a mold temperature controller capable of accurate control should be used according to the process requirements.
Injection pressure holding time, cooling time
The injection time, dwell time and cooling time must be set according to the product thickness, mold temperature, material properties, etc. The injection time setting is generally slightly larger than the time for the screw to complete the injection stroke. Long injection time will not only produce Mechanical wear, increased energy consumption and other negative effects, but also will extend the molding cycle. Set the dwell time according to the thickness of the product, thin-walled products do not need to hold time in the molding time;
When setting the pressure holding time, as long as there is no obvious depression on the surface of the product, it can also be determined by weighing method, and gradually increase the pressure holding time until the time when the product quality no longer changes can be set as the optimal holding pressure time. Also need to determine according to the product thickness, mold temperature, material properties, the general amorphous polymer cooling time is longer than the crystalline polymer.
Injection pressure, speed
The injection pressure should be set in accordance with the principle of appropriate low and high, as long as it can provide enough power to achieve the required injection speed, so that the melt can smoothly fill the cavity. Excessive pressure can easily cause internal stress in the product; but When molding high dimensional accuracy products, in order to prevent the product from shrinking excessively, high pressure injection can be used to reduce the shrinkage of the product after demoulding.
The injection speed will affect the appearance quality of the product. The settings should be set according to the mold's geometry, exhaust conditions, etc. Generally, under the premise of ensuring a good appearance, the injection speed should be increased as much as possible to reduce the filling time.
In injection molding, when the melt flows in the mold, a solidified layer is formed on the mold wall, thereby reducing the thickness of the flowable passage. Generally, according to the mold structure and the injection speed, the mold wall has a solidified layer of about 0.2 mm. Rapid injection speed is usually used in forming.
Injection stroke, multi-stage injection parameters
In molding, the injection stroke must first be determined. In theory, the injection stroke can be calculated as follows:
S1=4 (CVp+Va) /ρDs2
In the formula: Injection stroke Vp - Product volume ρ - Resin density C - Number of cavities Va - Gate volume Ds - Screw diameter
In actual production, if the total weight of 'product + gate' 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 - Total weight of product + gate, g Mmax - Maximum injection volume of injection molding machine, g/Smax - Maximum injection stroke of injection molding machine
Due to the different geometry of the runner system and various parts of the mold, in order to meet the product quality requirements, there are different requirements for the flow state of the mold-filled melt (mainly the pressure and velocity during the flow) in different parts. In an injection process, the screw direction When the mold advances the melt, it is required to realize different pressures and speeds at different positions, which is called multi-stage injection molding.
It is more scientific to set at least three or more injections of plastic parts during molding. That is, the main section is the first section, and the runner-to-gate section is the second section. The product is filled with about 90% of the cavity. In the third paragraph, the remaining part is the fourth paragraph, and a calculation weight method can be used to determine the switching position points of each segment;
In actual production, the multi-stage injection process parameters should be scientifically analyzed according to product quality requirements, flow channel structure, and mold exhaust conditions, and reasonably set. Normally, debugging observation method can be used to set and the injection needs to be switched. The pressure/speed at the position is set to 0, the position of the melt and the defect status of the product are observed, and adjustments are made gradually until a reasonable point is found. However, attention must be paid to the release of the undershot product during the commissioning observation. , so as not to die in some depressions of the mold due to lack of injection.
Other process parameters
In injection molding, in addition to the setting of several major parameters such as molding temperature, pressure, speed, time, and multi-stage injection switching position, there are many other process parameters, such as back pressure, screw speed, and screw anti-flow. Delays and other action parameter settings, etc., cannot be ignored.
Process parameter setting example
Take Nylon cable ties for example, product quality requirements:
The product meets the specified tensile standards; surface is free of silver, air bubbles, sink marks, and other undesirable phenomena; the product has good tightness after molding and no loosening occurs. The material used is PA66, the mold structure is hot runner, and the gate type is Point the gate.
First of all, according to product features and mold structure to determine the principle of setting the process parameters:
Due to the long product flow length, L/t (flow to wall thickness ratio) is 511, high-speed injection molding should be selected;
Gate type is point gate, higher pressure must be used to overcome the flow resistance;
In order to ensure the smooth filling of the product, the frit must have good fluidity, and the molding temperature should be appropriately high;
When the high-pressure high-speed injection to the end is easy to produce flash, the molding machine must have a low inertia pressure, speed switching;
Due to the small wall thickness of the product, it is not necessary to use pressure retention; formulate the main molding process parameters
To formulate the injection process parameters, it is necessary to understand the information of equipment performance, mold structure, molding material and product quality requirements, and scientifically and reasonably set the molding parameters.
Firstly, according to the molding conditions of the products, the process parameters should be adjusted step by step. The correct adjustment sequence is pressure → speed → temperature. Each time the parameters are changed, the input parameters are confirmed by the computer before the next parameter change, and simultaneous changes should be avoided. More than two parameters
Secondly, when the product enters into stable production, it is necessary to keep the stability of each parameter as far as possible, and detailed records should be kept. If the change range is too large, the cause should be promptly searched. In addition, the molding process should be fixed as much as possible when the mold goes online, which is convenient for the quality control of the finished product. .