What is the impact of the injection molding process on product performance?

The traditional mold design and process parameter setting mainly depend on the experience and skill of the designer. The rationality of the mold design depends only on repeated trial and repair, and the setting of process parameters can only be modified by repeated trials. Scientific basis, long production cycle, high cost, and quality is difficult to guarantee.

By simulating the molding process, problems in the design can be found before the mold is manufactured, so that the mold design and process parameter settings are based on scientific analysis, which can shorten the production cycle and improve the product quality. With the improvement of the quality requirements of the products, the prediction of the molding process has become an indispensable part of the design. Therefore, it is of great significance to establish a mathematical model for the flow and heat transfer of the melt in the cavity during the injection molding process and to simulate the molding process using numerical simulation methods.

Since the process parameters of the molding process directly determine the flow state of the melt in the cavity, it has the most direct and far-reaching influence on the quality of the product. Therefore, the optimal process conditions for the molding of the product are found, and the process control of the molding process is to improve the plastic. An effective way of product quality. This is because in the molding process, precision molding machinery, reasonable mold design and excellent material properties can only be reflected in a reasonable molding process. On the other hand, defects in molding machinery, mold design and material properties can sometimes pass. Proper molding process settings to compensate. It can be seen that the injection molding process plays 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 exerted by the top of the screw or the plunger on the plastic melt during the injection process. Its role is to mix and plasticize the melt, and the screw (or plunger) must provide resistance to the flow of solid particles and melt in the barrel and nozzle. The plastic melt is filled with the cavity at a certain speed, and the injection pressure acts as a compaction after the cavity is filled with the melt. Thereby, the plastic part is densified, and the shrinkage of the melt due to cooling is fed, so that the plastic part maintains a precise shape and obtains the required performance. The pressure of injection is mainly determined by the type of plastic, the type of injection molding machine, the temperature of the mold, the structure of the mold, and the wall thickness of the plastic part. The size and structure of the casting system have a great influence on the injection pressure.

2, pressure holding pressure

When the melt fills the cavity, the injection pressure acts to compact the melt in the mold. At this time, we also call the injection pressure as the holding pressure. In actual production, the holding pressure should be equal to or less than the injection. The pressure used. When the pressure at the time of pressure keeping is equal to the pressure at the time of injection, the shrinkage rate of the plastic part is often lowered, and the stability of the plastic part and the mechanical properties of the plastic part can be ensured. However, it is often accompanied by an increase in residual stress during demolding, which makes it difficult to demold the plastic parts, cause deformation of the plastic parts, scratches on the surface, etc., and it is easy to cause flashing of the plastic parts and affect the apparent quality. Therefore, when choosing a holding pressure, you need to consider it in many ways and choose carefully.

In general, the effect of holding pressure on the quality of the product is similar to the effect of injection pressure. The holding pressure of most plastics is between one injection pressure, and the specific holding pressure is determined by considering the characteristics of the plastic material and the structure of the workpiece, and overcoming the flow resistance of the melt from the barrel to the cavity. The melt is fed into the cavity and compacted.

3. Plasticizing pressure

Plasticizing pressure refers to the pressure that the top melt of the screw is subjected to when the screw is retracted. Its influence on injection molding is mainly reflected in the plasticizing effect and plasticizing ability of the injection machine. In general, increasing the plasticizing pressure, the screw retreating speed is reduced, and the pressure experienced by the melt in the barrel is increased. Therefore, the shearing action is strengthened during plasticization, and the plasticizing effect is improved.

However, it should be noted that while increasing the back pressure, the excessive plasticizing pressure reduces the amount of plasticization due to the backflow and leakage of the melt at the edge of the groove, which may cause insufficient measurement. The shearing heat is too high and the shear stress is too large, which may cause the material to degrade, generate bubbles or burns, and affect the quality of the plastic parts.

4, mold temperature

The mold temperature refers to the temperature of the cavity surface during the molding process, and the mold temperature affects the mold filling flow behavior of the melt, the cooling rate of the product, and the product properties after molding. The setting of the mold temperature depends mainly on the viscosity of the melt. For low melt viscosity, low mold temperature injection can be used to shorten the cooling time and increase production efficiency. High melt viscosity should be injection molded at high mold temperature.

Generally speaking, increasing the mold temperature can make the cooling rate of the workpiece uniform, and prevent molding defects such as dents and cracks. The mold temperature control of the crystalline plastic directly determines the cooling rate, which further determines the rate of crystallization. When the mold temperature is high, the cooling rate is small, and the crystallization rate becomes large, which is favorable for the relaxation process of the molecule, and the molecular orientation effect is small. The mold temperature is too high, which will lengthen the molding cycle and make the product brittle. The mold temperature is low, the cooling rate is large, and the flow of the melt is synchronized with the crystallization. Since the residence time of the melt in the crystallization temperature + interval is shortened, it is not conducive to the growth of the crystal, resulting in a low degree of molecular crystallization of the product, which affects its performance. In addition, if the mold temperature is too low, the flow resistance of the plastic melt is large, the flow rate becomes slow, and even the solidification in the filling mold hinders the subsequent feeding, so that the short shot of the workpiece and the forced orientation are large, which often causes the plastic parts to be short of material, depression, Defects such as weld seams.

5, barrel temperature

In order to ensure the normal flow of the plastic melt without deteriorating and decomposing, it is necessary to select a suitable temperature for the barrel, a large average molecular mass, a plastic with a relatively concentrated molecular mass distribution, and a glass reinforced plastic. Cartridge temperature. The barrel temperature is generally arranged according to the principle of high height and low front, but the back end temperature can be appropriately increased when the plastic contains too much water.

6, nozzle temperature

In order to avoid the casting 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-speed air injection in production, the sprayed stream is not bubbled, and smooth The right temperature standard.

7, melt temperature

The melt temperature mainly depends on the temperature of the barrel and the nozzle, affecting the plasticization of the material and the injection molding of the melt. The increase in injection temperature is primarily beneficial for improving the fluidity of the melt, which is related to many properties of the article. Increasing the melt temperature can reduce the internal mechanical stress, the impact strength in the flow direction, the mechanical properties such as deflection and tensile strength, and the impact strength, flow length and surface roughness perpendicular to the flow direction. The performance is improved and the post-shrinkage of the product can be reduced.

In general, increasing the melt temperature is beneficial to improve the filling condition and the transfer in the cavity, and to reduce the orientation, etc., which is advantageous for the improvement of the overall performance of the product but an excessively high temperature is not preferable. When the melt temperature is close to the upper limit of the injection temperature range, on the one hand, more gas is easily generated, causing bubbles, voids, discoloration, charring, etc. of the plastic part, and burrs are generated by excessively improving fluidity. On the other hand, the excessive quality of the product will degrade the plastic, reduce the strength of the plastic parts, lose the elasticity, etc., affecting the performance.

8, injection time

Injection time is one of the parameters controlling the injection rate.

The shorter the injection time, the higher the injection rate, and the injection rate has a large effect on the performance of the plastic part. Increasing the injection rate can increase the filling pressure, which is beneficial to the filling process, and reduces the heat loss in the filling, the cavity temperature is relatively uniform, and the product is uniform and compact. At the same time, the shrinkage rate of the product can be reduced, the core orientation of the plastic part can be reduced, and the strength of the welded joint can be increased. The increase of injection rate is beneficial to the comprehensive performance of the product, but the excessive injection rate will increase the pressure loss, reduce the thickness of the solidified layer, improve the surface orientation of the plastic parts, and even cause elastic turbulence of the melt, so that the plastic parts are easy to form flash edges, Surface cracks, etc.

It has been experimentally proven that an injection rate that is too high or too low will result in a decrease in impact strength. On the other hand, the too low injection rate causes the strength of the welded joint of the plastic part to decrease, the total orientation effect increases, and the internal stress increases, which affects the mechanical properties of the product.

9, pressure holding time

The length of the holding time and the cooling time also have a direct impact on the quality of the plastic part. Shortening the holding time will accelerate the pressure drop in the cavity, and it may cause backflow, which will cause defects such as shrinkage holes and depressions in the plastic parts, and affect the dimensional stability of the plastic parts.

The longer the holding time, the stability of the plastic part can be improved to avoid the above defects, and a dense product can be obtained. At the same time, the cavity pressure is increased, and the internal stress due to temperature unevenness is changed. However, it will increase the difficulty of demolding, and sometimes it is easy to cause the surface to be scratched or the plastic part is bent. The length of plasticization time of the material can affect the quality of plasticization and directly affect the performance of the product. When the time is too short, the plasticization is uniform, the temperature is uniform, and the hard block, the silver wire, etc. are easily generated, and the melt is decomposed, burned, etc. due to the action of the screw, and the product quality is adversely affected.

10, special process impact

Vibration injection molding, under high vibration pressure, with the increase of vibration frequency, the tensile properties and notched impact strength of the product are obviously improved. In addition, the addition of ultrasonic technology can also achieve good results.

in conclusion:

The injection molding process conditions have an impact on the appearance and mechanical properties of plastic products. Each process condition parameter is mutually independent and not independent. Some product defects are the result of mutual influence.