How does the injection molding process affect product performance?

The proportion of plastic materials in life is getting higher and higher, and the requirements for their quality are also getting higher and higher, and injection molding is an important means of production, and the improvement of technology is more and more urgent.

However, injection molding process conditions are one of the important influencing factors, the following will introduce the influence of individual process parameters on the performance of products.

The traditional mold design and process parameter setting mainly depend on the experience and skills of the designer, the reasonableness of the mold design only depends on repeated mold trials and mold repair, and the process parameter setting can only be modified by repeated mold trials, which lacks scientific basis, long production cycle, high cost, and difficult to guarantee the quality.

The simulation of the injection molding process can find out the problems in the design before mold manufacturing so that the mold design and process parameter setting can be established based on scientific analysis, which can shorten the production cycle and improve the quality of products.

With the improvement of product quality, the prediction of the molding process has become an indispensable part of the design. Therefore, it is important to establish a mathematical model of melt flow and heat transfer in the mold cavity during the injection molding process and use numerical simulation to realize the simulation of the molding process.

Since the process parameters of the molding process directly determine the flow state of the melt in the mold cavity, which has the most direct and far-reaching impact on the quality of the products, it is an effective way to improve the quality of plastic products by finding the optimal process conditions for the molding process and controlling the molding process.

On the other hand, the defects of molding machinery, mold design, and material performance can sometimes be compensated by suitable molding process settings. Thus, the injection molding process plays a vital role in the quality of products.

Injection molding process conditions and their effects

1. Injection pressure

Injection pressure refers to the pressure loaded by the top of the screw or plunger on the plastic melt during the injection molding process. Its role is to make the melt mixed and plasticized, the screw (or plunger) must provide to overcome the resistance to the flow of solid particles and melt in the barrel and nozzle.

The injection pressure acts as compaction after the cavity is filled with melt. This makes the plastic part dense and makes up for the shrinkage of the melt due to cooling so that the plastic part keeps its precise shape and obtains the required properties.

The injection pressure is mainly determined by the type of plastic, the type of injection machine, the temperature of the mold, the structure of the mold, and the wall thickness of the plastic part, among which the size and structure of the pouring system have a great influence on the injection pressure.

2. Pressure holding pressure

When the melt is filled with the cavity, the injection pressure plays a role in compacting the melt in the mold, at this time, we call the injection pressure also the holding pressure.

When the holding pressure is equal to the injection pressure, the shrinkage rate of the injection molded part is often reduced, and the stability of the molded part and the mechanical properties of the molded part can be guaranteed.

However, it is often accompanied by an increase in residual stress during demolding, which makes it difficult to demold the part, makes it easy to produce deformation and surface scratches, and also makes it easy to produce flying edges, which affects the apparent quality of the part. Therefore, it is necessary to consider various aspects when choosing the holding pressure and choose carefully.

Generally speaking, the impact of holding pressure on the quality of products is similar to the impact of injection pressure. The specific holding pressure is determined by considering the characteristics of the plastic material and the structure of the part, and overcoming the resistance to the flow of the melt from the barrel to the mold cavity, feeding the melt into the cavity and compacting it.

3. Plasticizing pressure

Plasticizing pressure refers to the pressure on the top of the screw when the melt is backed off by the screw. Its influence on injection molding is mainly reflected in the plasticizing effect of the injector on the material and its plasticizing ability.

Generally speaking, if the plasticizing pressure is increased, the receding speed of the screw decreases, and the pressure on the melt in the barrel increases. As a result, the shearing effect of plasticization is enhanced and the plasticizing effect is improved.

However, it is important to note that, while increasing the back pressure, too high plasticizing pressure on the one hand because of the melt in the edge of the screw groove backflow and leakage and reduce the amount of plasticization, may cause under-measurement, on the other hand, the shear heat is too high shear stress is too large, it is possible to make the material degradation, bubbles or burns, affecting the quality of plastic parts.

4. Mold temperature

Mold temperature refers to the temperature of the surface of the mold cavity during the molding process. Mold temperature affects the mold-filling flow behavior of the melt, the cooling rate of the product, and the performance of the product after molding.

The setting of mold temperature mainly depends on the viscosity of the melt. The low viscosity of the melt can be injected at low mold temperature to shorten the cooling time and improve production efficiency. Higher melt viscosity should be injected at high mold temperatures.

Generally speaking, increasing the mold temperature can make the cooling rate of the parts even and consistent, and prevent molding defects such as dents and cracks. The mold temperature control of crystalline plastics directly determines the cooling rate, which further determines the rate of crystallization.

The cooling rate is small when the mold temperature is high, and the crystallization rate becomes large, which is conducive to the relaxation process of molecules and the molecular orientation effect is small. If the mold temperature is too high, it will prolong the molding cycle and make the product brittle.

Low mold temperature, the cooling rate is large, the flow of melt and crystallization synchronization, due to the melt in the crystallization temperature + zone residence time is shortened, is not conducive to the growth of crystals, 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, and even in the mold filling solidification to prevent the subsequent feeding, making the parts short shot, forced orientation is large, often resulting in plastic parts lack of material, dents, fusion joints, and other defects.

5. Barrel temperature

To ensure the normal flow of the plastic melt and not to make it degenerate and decompose, you need to choose the right barrel temperature, the average molecular mass of a large, concentrated molecular mass distribution of plastics, and glass-reinforced plastics should choose a higher temperature barrel temperature.

Barrel temperature is generally arranged according to the principle of high front and low rear, but when the plastic contains too much water fee can also be appropriate to increase the temperature of the rear end.

6. Nozzle temperature

To avoid the melt at the nozzle to produce the flow phenomenon, usually need to make the nozzle temperature slightly lower than the highest temperature of the barrel, in production will generally be the case of low-speed injection into the air, the spray stream without bubbles, smooth as the temperature of the appropriate standard.

7. Melt temperature

Melt temperature mainly depends on the barrel and nozzle temperature of the two parts, affecting the plasticization of materials and melt injection filling. The increase in injection temperature is mainly conducive to improving the fluidity of the melt, which is related to many characteristics of the product.

Increasing the melt temperature can reduce the mechanical properties such as internal stress, impact strength and deflection in the flow direction, tensile strength, etc., while improving the impact strength, flow length, surface roughness, and other properties perpendicular to the flow direction, and reducing the post-shrinkage of the product.

From a general point of view, increasing the melt temperature is conducive to improving the mold filling condition and the transfer in the mold cavity, reducing the orientation, etc., which is conducive to the improvement of the overall performance of the product.

When the melt temperature is close to the upper limit of the injection molding temperature range, on the one hand, it is easy to produce more gas, so that the plastic parts produce bubbles, cavities, discoloration, scorching, etc., but also too much to improve the fluidity and produce flying edge, affecting the apparent quality of the product on the other hand.

Too high a temperature will make the plastic degradation, so that the strength of plastic parts reduces the loss of elasticity, etc., affecting the use of performance.

8. Injection time

Injection time is one of the parameters to control the injection rate. The shorter the injection time, the higher the injection rate, and the size of the injection rate has a great impact on the performance of the plastic parts.

Increasing the injection rate can make the mold pressure increase, which is conducive to the process of filling the mold, and make the heat loss in the mold filling reduced, the mold cavity temperature is more uniform, and the product is uniform and dense.

At the same time, it can reduce the shrinkage rate of the product, reduce the core orientation of the plastic part, and increase the strength of the fusion joint.

The increase in injection rate is beneficial to the overall performance of the products, but too high an 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, so that the plastic parts are easy to form flying edge, surface cracks, etc.

It has been proved experimentally that too high or too low an injection rate will lead to a decrease in impact strength. On the other hand, the injection rate is too low, so the strength of the melt joint decreases, the total orientation increases, the internal stress increases, etc., affecting the mechanical properties of the product.

9. Pressure-holding time

The length of holding time and cooling time also has a direct impact on the quality of plastic parts. Shortening the holding time will make the mold cavity pressure decrease faster, may produce backflow, so that the plastic parts produce shrinkage, depression, and other defects, and affect the stability of the size of the plastic parts.

Longer holding time can improve the stability of the size of the molded part to avoid the above defects and get dense products. At the same time, it will increase the pressure of the mold cavity and change the internal stress caused by uneven temperature. However, it will increase the difficulty of demolding, and sometimes it is easy to produce surface scratching or bending of the plastic part.

The length of the plasticizing time of the material can affect the plasticizing quality and directly affect the product performance. Too short a time can not make the plasticization of uniform, consistent temperature, easy to produce hard blocks, silver wire, etc. And too long will make the melt due to the role of the screw and decomposition, burns, etc., but also the quality of the product has a negative impact.

10. Special process impact

In vibration injection molding, under high vibration pressure, with the increase in vibration frequency, the tensile properties and notch impact strength of the product is significantly improved. In addition, the addition of an ultrasonic process can also play a good effect.

Conclusion

Plastic Injection molding process conditions on the plastic products of the apparent and mechanical properties caused by the impact of each process conditions parameters are mutually influenced and not independent, some of the defects of the product are the result of mutual influence.