What is the Impact of Injection Molding Process on Product Performance?

Abstract: The proportion of plastic materials in daily life is increasing, and the requirements for their quality are also increasing. Injection molding, as an important production method, urgently needs to improve technology. There are many factors that affect injection molded products, but the processing conditions of injection molding are one of the important influencing factors. The following will introduce the impact of each process parameter on product performance.
Traditional mold design and process parameter setting mainly rely on the experience and skills of designers. The rationality of mold design can only be achieved through repeated trial and repair, and the setting of process parameters can only be modified through repeated trial and repair, lacking scientific basis. The production cycle is long, the cost is high, and the quality is difficult to guarantee.
By simulating the forming process, problems in the design can be identified before mold manufacturing, and mold design and process parameter settings can be based on scientific analysis, which can shorten the production cycle and improve product quality. With the increasing demand for product quality, predicting the forming process has become an indispensable part of design. Therefore, establishing a mathematical model for the flow and heat transfer of molten metal in the mold cavity during injection molding and using numerical simulation methods to simulate the molding process is of great significance.
Due to the fact that the process parameters of the molding process directly determine the flow state of the melt in the mold cavity, it has the most direct and far-reaching impact on the quality of the product. Therefore, finding the optimal process conditions for product molding and controlling the molding process is an effective way to improve the quality of plastic products.
This is because during the molding process, precise molding machinery, reasonable mold design, and excellent material performance can only be reflected under reasonable molding process settings. On the other hand, defects in molding machinery, mold design, and material performance can sometimes be compensated for through appropriate molding process settings. It can be seen that the injection molding process plays a crucial role in the quality of products

Injection molding process conditions and their effects
1. Injection pressure
Injection pressure refers to the pressure exerted by the top of the screw or plunger on the plastic melt during the injection process. Its function is to mix and plasticize the molten material, and the screw (or plunger) must provide overcoming the flow resistance of solid particles and molten material in the barrel and nozzle. Make the plastic melt fill the mold cavity at a certain speed, and the injection pressure plays a compaction role after the mold cavity is filled with the melt.
This makes the plastic part dense and replenishes the shrinkage caused by cooling of the molten material, thereby maintaining the precise shape of the plastic part and achieving the required performance. The injection pressure is mainly determined by the type of plastic, the type of injection molding machine, the temperature of the mold, the mold structure, and the wall thickness of the plastic part. The size and structure of the pouring system have a significant impact on the injection pressure.
2. Maintaining pressure
When the melt fills the mold cavity, the injection pressure plays a role in compacting the melt inside the mold. At this time, we call the injection pressure the holding pressure. In actual production, the holding pressure should be equal to or less than the pressure used during injection. When the pressure during holding is equal to the pressure during injection, it often reduces the shrinkage rate of the plastic part, and can ensure the stability and mechanical properties of the plastic part.
However, it is often accompanied by an increase in residual stress during demolding, which makes it difficult to demold plastic parts, makes them prone to deformation, surface scratches, and also easily causes flash edges, affecting the apparent quality. Therefore, when choosing the holding pressure, it is necessary to consider multiple factors and choose carefully.
Generally speaking, the impact of holding pressure on the quality of products is similar to that of injection pressure. The holding pressure of most plastics is between the injection pressure, and the determination of the specific holding pressure mainly considers the characteristics of the plastic material and the structure of the workpiece, and overcomes the flow resistance of the melt from the barrel to the mold cavity, feeding the melt into the mold cavity and compacting it.
3. Plasticization pressure
Plasticization pressure refers to the pressure exerted on the top of the screw when the molten material moves backwards. Its impact on injection molding is mainly reflected in the plasticizing effect and plasticizing ability of the injection machine on materials. Generally speaking, increasing the plasticizing pressure results in a decrease in the backward speed of the screw, and the pressure on the melt inside the barrel increases accordingly. So the shear effect is strengthened during plasticization, and the plasticizing effect is improved.
However, it should be noted that while increasing the back pressure, excessive plasticization pressure can reduce the amount of plasticization due to the reflux and leakage of the melt at the edge of the screw groove, which may cause insufficient measurement. On the other hand, it can cause excessive shear heat and shear stress, which may cause material degradation, produce bubbles or burns, and affect the quality of the plastic parts. 

4. Mold temperature
Mold temperature refers to the temperature on the surface of the mold cavity during the molding process. Mold temperature affects the flow behavior of the melt during mold filling, the cooling rate of the product, and the performance of the formed product. The setting of mold temperature mainly depends on the viscosity of the molten material. Low viscosity molten materials can be injected at low mold temperatures to shorten cooling time and improve production efficiency. High mold temperature injection molding should be used for materials with high viscosity.
Generally speaking, increasing the mold temperature can ensure a uniform and consistent cooling rate of the workpiece, preventing the formation of defects such as dents and cracks. The mold temperature control of crystalline plastics directly determines the cooling rate, thereby further determining the rate of crystallization. When the mold temperature is high, the cooling rate decreases and the crystallization rate increase, which is conducive to the relaxation process of molecules and reduces the molecular orientation effect. If the mold temperature is too high, it will prolong the molding cycle and make the product brittle.
Low mold temperature and high cooling rate result in synchronous flow and crystallization of the molten material. Due to the shortened residence time of the molten material in the crystallization temperature+range, it is not conducive to crystal growth, resulting in a lower degree of molecular crystallization of the product and affecting its performance. In addition, if the mold temperature is too low, the flow resistance of the plastic melt is high, the flow rate slows down, and even solidifies during mold filling, hindering subsequent feeding, resulting in short shots and forced orientation of the parts, often causing defects such as material shortage, depression, and fusion joints in the plastic parts.
5. Barrel temperature
In order to ensure the normal flow of plastic melt without causing it to deteriorate or decompose, it is necessary to choose a suitable barrel temperature. For plastics with high average molecular weight and concentrated molecular weight distribution, as well as glass reinforced plastics, higher barrel temperatures should be chosen. The temperature of the material barrel is generally arranged according to the principle of high in the front and low in the back, but when there is too much water cost in the plastic, the temperature at the back can also be appropriately increased.
6. Nozzle temperature
In order to avoid the phenomenon of melt spreading at the nozzle, it is usually necessary to make the nozzle temperature slightly lower than the highest temperature of the barrel. In production, in the case of low-speed injection into the air, the sprayed solution without bubbles and smooth is generally regarded as the standard for suitable temperature.
7. Melt temperature
The temperature of the melt mainly depends on the temperature of the barrel and nozzle, which affects the plasticization of the material and the injection molding of the melt. The increase in injection temperature is mainly beneficial for improving the fluidity of the melt, which is related to many characteristics of the product. Raising the melt temperature can reduce the mechanical properties such as internal stress, impact strength in the streamline direction, deflection, and tensile strength of the plastic part, while improving the impact strength, flow length, surface roughness perpendicular to the streamline direction, and reducing the post shrinkage of the product.
Overall, increasing the melt temperature is beneficial for improving the filling condition and transfer within the mold cavity, reducing orientation, and enhancing the overall performance of the product. However, excessively high temperatures are also not advisable. When the temperature of the melt approaches the upper limit of the injection molding temperature range, on the one hand, it is easy to generate a lot of gas, causing bubbles, voids, discoloration, burning, etc. in the plastic parts. On the other hand, excessive improvement in flow ability also leads to flash, affecting the apparent quality of the products. On the other hand, excessive temperature can cause plastic degradation, reduce the strength of the plastic parts, lose elasticity, etc., and affect their performance.

8. Injection time
Injection time is one of the parameters that controls the injection rate. The shorter the injection time, the higher the injection rate, and the magnitude of the injection rate has a significant impact on the performance of the plastic part. Increasing the injection rate can increase the mold filling pressure, which is beneficial for the mold filling process and reduces the heat loss during mold filling. The mold cavity temperature is relatively uniform, and the product is uniform and dense. At the same time, it can reduce the shrinkage rate of the product, reduce the orientation of the plastic core, and increase the strength of the fusion joint.
The increase in injection rate is beneficial for the overall performance of the product, but excessive injection rate can increase pressure loss, reduce the thickness of the curing layer, improve the surface orientation of the plastic part, and even cause elastic turbulence in the melt, making the plastic part prone to forming burrs, surface cracks, etc.
Experimental results have shown that injection rates that are too high or too low can lead to a decrease in impact strength. On the other hand, a low injection rate leads to a decrease in the strength of the plastic melt joint, an increase in the overall orientation effect, and an increase in internal stress, which affects the mechanical properties of the product.
9. Holding time
The length of holding time and cooling time also directly affects the quality of plastic parts. Shortening the holding time will accelerate the decrease in mold cavity pressure, which may lead to backflow, causing defects such as shrinkage and depression in the plastic part, and affecting the stability of the plastic part size.
Extending the holding time can improve the stability of the plastic part size and avoid the occurrence of the above-mentioned defects, resulting in dense products. At the same time, it will increase the pressure in the mold cavity and change the internal stress caused by uneven temperature. But it will increase the difficulty of demolding, and sometimes it is easy to cause surface scratches or bending of plastic parts.
The length of material plasticization time can affect the quality of plasticization and directly affect product performance. If the time is too short, it cannot make the plasticization uniform and the temperature consistent, which can easily produce hard blocks, silver wires, etc. However, if it is too long, it can cause the molten material to decompose, burn, etc. due to the action of the screw, and also have a negative impact on product quality.
10. Special process impact
Vibration injection molding, under high vibration pressure, significantly improves the tensile performance and notch impact strength of the product with the increase of vibration frequency. In addition, the addition of ultrasonic technology can also have a good effect.
Conclusion
The processing conditions of injection molding have an impact on the appearance and mechanical properties of plastic products. Each process parameter affects each other and is not independent. The defects of some products are the result of mutual influence.