How does the plastic molding process affect product performance?
Plastic materials in the proportion of life is getting higher and higher, and for its quality requirements are getting higher and higher, injection molding as an important means of production, the technology is also increasing more and more urgent, and the impact of injection molding products More, but the injection molding process conditions are one of the important factors, the following will introduce a variety of process parameters for the impact of product performance.
The traditional mold design and process parameter setting mainly depends on the designer's experience and skill. The rationality of the mold design can only be modified by repeated test mode, which can only be modified by repeated test mode and repair mode. Scientific basis, long production cycle, high cost, quality is also difficult to guarantee.
And the molding process simulation, mold manufacturing can be found before the design of the problem, so that mold design and process parameters set on the basis of scientific analysis, can shorten the production cycle and improve product quality. With the improvement of product quality requirements, the prediction of the forming process has become an indispensable part of the design. Therefore, it is of great significance to establish the mathematical model of the flow and heat transfer of the melt in the mold cavity during the injection molding process and to use the numerical simulation method to simulate the molding process.
As the process parameters of the molding process directly determine the flow state of the melt in the mold cavity, the quality of the product has the most direct and far-reaching impact, so find the optimal process conditions for product molding, process control process is to improve the plastic The effective way of product quality.
This is because, during the molding process, precision molding machines, reasonable mold design and excellent material properties can only be reflected in a reasonable molding process. On the other hand, defects in molding machines, mold design and material properties are sometimes The appropriate molding process set to make up. It can be seen that the injection molding process has a vital role in the quality of the product
Injection molding process conditions and their effects
1, injection pressure
Injection pressure refers to the pressure applied to the plastic melt at the top of the screw or plunger during injection. Its role is to mix and plasticize the melt, and the screw (or plunger) must provide a means to overcome the flow resistance of the solid particles and the melt in the barrel and the nozzle. So that the plastic melt to a certain speed to fill the cavity in the cavity filled with melt injection pressure after the role of compaction.
Thus making the plastic parts dense, and the melt due to cooling contraction caused by feeding, so that the plastic parts to maintain the precise shape, to obtain the required performance. The pressure of injection is mainly determined by the type of plastic, the type of injection molding machine, the temperature of mold, the structure of mold, the wall thickness of plastic parts, in which the size and structure of pouring system have great influence on injection pressure.
2, packing pressure
When the melt is filled with cavities, the effect of the injection pressure is to compact the melt in the mold, and the injection pressure is also called the packing pressure. In actual production, the packing pressure should be equal to or less than the injection The pressure used. When the pressure when the pressure and injection pressure equal to the time, often will reduce the shrinkage of plastic parts, and can ensure the stability of plastic parts and plastic parts of the mechanical properties.
But often accompanied by the release of residual stress increases, resulting in plastic parts stripping difficult, so that plastic parts prone to deformation, surface scratches, etc., but also easy to make plastic parts flash, affecting the apparent quality. Therefore, the choice of packing pressure need to consider many, careful choice.
In general, the effect of packing pressure on the quality of the product is similar to that of the injection pressure. Most of the plastic holding pressure between the injection pressure of a, and the specific pressure to determine the pressure, mainly to consider the characteristics of plastic parts and parts of the structure, and to overcome the melt from the barrel to the mold cavity flow resistance, The melt is fed into the cavity and compacted.
3, plasticization pressure
The plasticizing pressure is the pressure at which the top of the screw is subjected to retraction of the screw. Its impact on injection molding is mainly reflected in the injection molding machine on the material of plasticizing effect and its plasticizing ability. In general, increase the plasticization pressure, screw back speed decreases, the barrel of the melt by the pressure increases. So when the plasticization of the shear effect to strengthen, improve the plasticizing effect.
But to the effect is to increase the back pressure at the same time, too high plasticization pressure on the one hand because of the melt in the groove edge of the backflow and leakage and reduce the amount of plastic, may cause insufficient measurement, on the other hand Will make the shear stress is too high shear stress is too large, it may make the material degradation, resulting in bubbles or burns, affecting the quality of plastic parts.
4, mold temperature
The mold temperature refers to the temperature of the cavity surface during the molding process, the mold temperature affects the melt flow behavior of the melt, the cooling rate of the product and the product performance after molding. The setting of the mold temperature depends mainly on the viscosity of the melt. Low melt viscosity can take low mold temperature injection to shorten the cooling time and improve production efficiency. High melt viscosity should be high mold temperature injection molding.
Generally speaking, to improve the mold temperature can make the parts of the cooling rate uniformity, to prevent dents and cracks and other molding defects. Crystalline mold temperature control directly determines the cooling rate, which further determines the rate of crystallization. When the mold temperature is high, the cooling rate is small, the crystallization rate becomes larger, which is beneficial to the relaxation process of the molecule, and the molecular orientation effect is small. Mold temperature is too high, will extend the molding cycle and make the product brittle
The mold temperature is low, the cooling rate is large, the melt flow and crystallization are carried out synchronously, because the melt in the crystallization temperature + interval shortened time is not conducive to the growth of the crystal, resulting in a lower degree of molecular crystallization of the product, affecting its performance. In addition, the mold temperature is too low, the plastic melt flow resistance is very large, the flow rate becomes slow, even in the mold solidification hinder the subsequent feed, making parts short shot, forced orientation, often resulting in plastic parts shortage, Welds and other defects.
5, barrel temperature
In order to ensure the normal flow of plastic melt at the same time do not make it metamorphic decomposition, the need to choose the appropriate cylinder temperature, the average molecular weight, molecular mass distribution and more concentrated plastic and glass reinforced plastics should choose a higher temperature of the material Cylinder temperature. The cylinder temperature is generally arranged according to the principle of high to low, but when the plastic contains too much water can also be appropriate to improve the back-end temperature.
6, nozzle temperature
In order to avoid the casting phenomenon of the melt at the nozzle, it is usually necessary to make the nozzle temperature slightly lower than the maximum temperature of the barrel. In the case of low injection and low injection in the production, the effluent is not bubbled, Temperature suitable for the standard.
7, melt temperature
The melt temperature depends primarily on the temperature of the barrel and the nozzle, affecting the plasticization of the material and the filling of the melt. The increase in injection temperature is mainly beneficial to improve the melt flow, which is related to many properties of the product. Increasing the melt temperature can reduce the mechanical strength of the plastic parts, the impact strength and the deflection degree, the tensile strength and other mechanical properties in the direction of the flow direction, and the impact strength, flow length, surface roughness And other properties have improved, and can reduce the product after the contraction.
In general, increasing the melt temperature is advantageous in improving the filling condition and the transfer in the mold cavity, reducing the orientation and the like, which is advantageous for the improvement of the overall performance of the product but the excessive temperature is undesirable. When the melt temperature close to the upper limit of the injection temperature range, on the one hand prone to produce more gas, so that plastic parts produce bubbles, holes, discoloration, charring, etc., but also because of excessive liquidity to produce flash, Affect the apparent quality of products on the other hand, too high temperature will make plastic degradation, so that the strength of plastic parts to reduce the loss of flexibility, affecting the use of performance.
8, injection time
The injection time is one of the parameters that control the injection rate.
The shorter the injection time, the higher the injection rate, the size of the injection rate of plastic parts have a great impact on the performance. Increasing the injection rate can increase the filling pressure, which is beneficial to the filling process and reduce the heat dissipation in the mold. The mold cavity temperature is even and the products are uniform and compact. At the same time can reduce the product shrinkage, reduce the plastic core orientation, increase the weld strength.
The increase of injection rate is beneficial to the comprehensive performance of the product, but the high injection rate will increase the pressure loss, reduce the thickness of the curing layer, improve the surface orientation of the plastic parts, and even make the melt elastic turbulence, Surface cracks and so on.
Experiments show that: too high and low injection rate will lead to the impact of the decline in strength. On the other hand, the low injection rate makes the weld joint strength decreased, the total orientation effect increases, the internal stress increases, affecting the mechanical properties of the product.
9, holding time
The length of the holding time and the length of the cooling time also have a direct impact on the quality of the plastic parts. Shorten the holding time, will reduce the cavity pressure to accelerate, there may be backflow, so that plastic parts shrinkage, depression and other defects, and affect the stability of plastic parts size.
Increased packing time, can improve the stability of plastic parts size to avoid the occurrence of defects, get dense products. At the same time will increase the cavity pressure, change the temperature due to uneven internal stress generated. But will increase the difficulty of stripping, and sometimes easy to produce the surface of the top of the scratch or plastic bending the phenomenon of the top.
Material plasticizing time can affect the length of plastic quality, a direct impact on product performance. Time is too short can not make plastic evenly, the temperature is consistent, easy to produce lumps, silver and so on too long will make the melt due to the role of the screw decomposition, burn, etc., but also to the product quality adverse effects.
10, the impact of special technology
Vibration injection molding, in the high vibration pressure, with the vibration frequency increases, the tensile properties of products and notched impact strength was significantly improved. In addition to the addition of ultrasonic technology can also play a good effect.
Injection molding process conditions affect the apparent and mechanical properties of plastic products, each of the process conditions parameters are not independent of each other, some products defects are the result of mutual influence.