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Influence of injection molding process on product quality

May 30, 2022

In converting plastic particles into plastic products, plastics are often subjected to high temperature and high pressure and flow molding at high shear rates. Different molding conditions and processes will have different effects on product quality. Injection molding has plastic. It consists of four aspects: raw material, injection molding machine, mold, and injection molding process.




If the plastic raw materials are correctly selected, the technical parameters of the injection molding machine and the mold structure meet the requirements of the injection molding conditions, and the quality of the processed products is directly related to the injection molding process.




The quality of the product includes the quality of the internal material and the quality of the appearance. The quality of the internal material is mainly the mechanical strength, and the size of the internal stress directly affects the mechanical strength of the product. The main reasons for the internal stress are the crystallinity of the product and the orientation of the molecules in plastic molding.




The appearance quality of the product is the surface quality of the product. Still, the warpage and deformation of the product caused by the considerable internal stress will also affect the appearance quality. The appearance quality of the product includes the defective product, product dent, weld line, flash, bubble, silver wire, black spot, deformation, crack, delamination and discoloration, etc., which are related to molding temperature, pressure, flow rate, time and position.


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1. Molding temperature:


barrel temperature, plasticizing temperature, nozzle temperature, mold temperature, plastic drying temperature, etc.




Barrel temperature: It is the melting temperature of the plastic. If the temperature of the barrel is set too high, the viscosity of the plastic after melting is low. Under the same injection pressure and flow rate, the injection speed is fast, and the molded product is easy to flash, with silver thread, discoloration, and brittleness.




The temperature of the barrel is too low, the plasticization of the plastic is poor, the viscosity is high, the injection speed is slow under the same injection pressure and flow rate, the molded product is easy to be insufficient, and the weld line is obvious, the size is unstable, and there are cold blocks in the product.




Nozzle temperature: If the nozzle temperature is set high, the nozzle is easy to drool, resulting in cold filaments in the product. The low temperature of the nozzle caused the blockage of the mold gating system. The injection pressure must be increased to inject the plastic, but there are cold blocks in the instant moldable product.




Mold temperature: If the mold temperature is high, the injection pressure and flow rate can be set lower, but under the same pressure and flow rate, the product is prone to flash, warping deformation, and difficulty in ejecting the product from sticking to the mold. The mold temperature is low, and under the same injection pressure and flow rate, the product is insufficiently formed, such as air bubbles and weld lines.




Plastic drying temperature: Various plastics have different drying temperatures. ABS plastic is generally set to a drying temperature of 80-90 °C; otherwise, it is not easy to dry and volatilize the moisture and residual solvent. The product is prone to silver threads and air bubbles, and the strength of the product will also decrease.


latest company news about Influence of injection molding process on product quality

2. Molding process pressure:


There is pre-molding back pressure: injection pressure, clamping pressure, holding pressure, mold and neutron pressure, injection table pressure, etc.




Pre-plastic back pressure: high backpressure and high storage density, and a large amount of storage material in the same storage volume. Low back pressure, low storage material density, and less storage material. After setting the storage position and adjusting the backpressure in a large amount, you must pay attention to re-setting the storage position. Otherwise, it is easy to cause flashing or a shortage of products.




Injection pressure: Different types of plastics have different melt viscosities, and the viscosity of amorphous plastics varies significantly with the change in plasticizing temperature. The injection pressure is set according to the melt viscosity of the plastic and the flow ratio of the plastic. If the injection pressure is set too low, it will insufficiently shoot the product, and the dents, weld lines, and dimensions will be unstable. If the injection pressure is too high, it will flash the product, and it will be difficult to eject the discoloration and sticky mold.




Clamping pressure: It depends on the projected area of the mold cavity and the injection pressure. The product can quickly flash with insufficient clamping pressure and the weight increases. If the clamping force is too large, it will be difficult to open the mold. Generally, the clamping pressure setting should not exceed 120par/cm2.




Holding pressure: when the injection is over, continue to give the screw a pressure called holding pressure; at this time, the product in the mold cavity has not been frozen. The holding pressure can continue to fill the mold cavity to ensure that the product is complete. The holding pressure and pressure setting are too high, which will bring excellent resistance to the mold and the ferrule, and the top of the product is easy to be white and warped. In addition, the gate of the mold runner is quickly swelled by supplementary plastic, and the gate is broken in the runner. If the pressure is too low, the product has dents and dimensional instability.




The mold and neutron pressure, the principle of setting the mold and neutron pressure is to set the pressure according to the comprehensive situation of the size of the mold cavity area, the projected area of the core of the ferrule, and the complexity of the geometric shape of the molded product's size. Generally, the pressure of the mold and the neutron cylinder should be set to be able to push the product.




3. Flow:

It is related to the speed of the injection molding machine, including screw speed, injection speed, mold support, and neutron speed.




Screw speed: In addition to adjusting the pre-plastic flow, it is mainly affected by the pre-plastic backpressure. For example, if the pre-plastic flow is adjusted to a significant value, the pre-plastic back pressure is high. With the rotation of the screw, the sheer force of the plastic in the barrel is large, the molecular structure of the plastic is quickly cut off, and the product has black spots and black stripes, which affects the appearance, quality, and strength of the product. And the heating temperature of the barrel is not easy to control. If the pre-plastic flow is set too low, the pre-plastic storage time will be prolonged, which will affect the molding cycle.




Injection speed: The injection speed should be set rationally. Otherwise, the product quality will be affected, the injection speed will be too fast, and the product will have bubbles, scorch, discoloration, etc. If the injection speed is too slow, the molding of the product is insufficient, there are weld lines, etc.




Mould and neutron flow: it is not advisable to set too large otherwise, the ejection and core pulling action is too fast, resulting in unstable ejection and ferrule pulling, and the product is easy to top white.




4. Time setting:

In the injection molding process, there is drying time, injection and pressure holding time, cooling time, etc.




Drying time: It is the drying time for plastic raw materials. Various types of plastics have the best drying temperature and time. The drying temperature of ABS plastic is 80~90℃, and the time is 2 hours. ABS plastic generally has water absorption of 0.2 to 0.4% for 24 hours, while the water content that can be injection molded is 0.1 to 0.2%.




Injection and pressure holding time: The control method of the computer injection machine is equipped with multi-stage injection, which can adjust the pressure, speed, and injection plastic amount in stages. The speed of the plastic injected into the mold cavity reaches a constant speed, and the appearance and inner quality of the molded product are improved.




Therefore, the injection process usually uses position control instead of time control. The pressure holding is controlled by time. For example, the pressure holding time is extended, the product density is large, the weight is heavy, the internal stress is significant, the molding is complex, the top is easy to be white, and the molding cycle is prolonged. If the holding time is too short, the product is prone to dents and varying dimensions.




Cooling time: It is to ensure the stability of the product's shape. After the plastic injected into the mold cavity is molded into a product, there will be enough cooling and setting time; otherwise, the product will be easily warped and deformed when the mold is opened, and the ejection will be easy to deform and white. The cooling time is too long, and the molding cycle is prolonged, which is not economical.




5. Position control:

Some injection molding machines have mold transfer positions, mold support positions, material storage, and injection position control.




The moving position is the entire moving distance from the mold opening to the closing mold locking, called the moving mold position. The best position for the mold transfer is to take out the product smoothly. The molding cycle will belong if the mold opening distance is too large.




As long as the position of the mold is controlled, the position of the ejection from the mold can be easily removed and can remove the product.




Storage location: First of all, it is necessary to ensure the amount of plastic injected into the molded product at one time, and secondly, to control the amount of material stored in the barrel. If the storage position is controlled more than once, the injection volume is too much, and the product is easy to flash. Otherwise, the product is insufficiently formed.




Too much material in the barrel, the plastic stays in the barrel for a long time, the product is easy to fade, and the strength of the molded product is affected. On the contrary, it will affect the plasticizing quality of the plastic, and no material will be added to the mold when the pressure is maintained. The product will have dents due to insufficient molding.






The quality of injection molding products involves product design, plastic materials, mold design, and processing quality, injection molding machine selection and process adjustment, etc.




For standard production molds, to ensure and improve the quality of products, only the injection process adjustment is the adjustment of temperature, pressure, flow rate, time, and position.




Must comprehensively consider process adjustment. Suppose the size of the product changes significantly. In that case, you can start to consider the plastic raw material, whether it is a crystalline plastic or an amorphous plastic, the size of the molecular weight of the plastic, etc. It is crystalline plastic and macromolecular plastic. It is necessary to consider the crystallinity and molecular orientation of the product and the flow change of the product shape during molding, which will cause the change and size of the internal stress of the product, resulting in the shrinkage of the product.




To solve the slight change in product size from the process, the material temperature can be increased, can reduce the mold temperature, control the injection material volume, reduce the injection pressure, and increase the injection speed. It can instantly fill the mold cavity, reduce the pressure holding time, and extend the cooling time. Reduce the product's density under the condition of ensuring that the product shoots enough. It can reduce the internal stress of the product and reduce the shrinkage change of the product.




In short, the adjustment of the injection process cannot be started from a single point but must be started from the principle of the injection process. Comprehensive overall planning and consideration of problems can be adjusted from various aspects or can adjust multiple problems simultaneously. However, the adjustment method and principle depend on the quality of the products produced at that time and the technological conditions.