The impact of injection molding process on product quality

In the molding process of transforming 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. Plastic injection molding has plastics It consists of raw materials, injection molding machine, mold and injection molding process.

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

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

The appearance quality of the product is the surface quality of the product, but the warpage and deformation of the product caused by the large internal stress will also affect the appearance quality problem. The appearance quality of the products includes: insufficient products, product dents, weld marks, flash, bubbles, silver wires, black spots, deformation, cracks, delamination and peeling and discoloration, etc., which are all related to the molding temperature, pressure, flow, time and location related.

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

The barrel temperature is too low, the plasticization is not good, the viscosity is high, the injection speed is slow under the same injection pressure and flow, the molded product is easy to be insufficient, the weld mark is obvious, the size is unstable, and there are cold blocks in the product.

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

Mold temperature: 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 easy to flash, warp and deform, and it is difficult to stick the product out of the mold. The mold temperature is low. Under the same injection pressure and flow rate, the product is insufficiently formed, such as bubbles and weld marks.

Plastic drying temperature: various plastics have different drying temperatures. ABS plastics are generally set at a drying temperature of 80-90℃, otherwise it will not be easy to dry and volatilize the water and residual solvents, the products are prone to silver wires and bubbles, and the strength of the products will also decrease.

 

2. Molding process pressure: pre-plastic back pressure: injection pressure, mold clamping pressure, holding pressure, mold support and neutron pressure, injection pressure, etc.

Pre-plastic back pressure: high back pressure, high storage density, large storage capacity in the same storage volume. Low back pressure, low storage density and less storage. After setting the storage position, adjust the back pressure by a larger margin. You must pay attention to reset the storage position, otherwise it is easy to cause flashing or insufficient products.

Injection pressure: Different types of plastics have different melt viscosities, and the viscosity of amorphous plastics changes greatly with the change of plasticizing temperature. The injection pressure is set according to the melt viscosity of the plastic and the plastic flow ratio. If the injection pressure is set too low, the product will not be shot enough, dents, weld marks and size will be unstable. If the injection pressure is too high, the product flashes, discoloration and sticky mold ejection are difficult.

Clamping pressure: According to the projected area of ​​the mold cavity and the injection pressure. If the clamping pressure is insufficient, the product is easy to flash and increase the weight. If the clamping force is too large, it will be difficult to open the mold. Generally, the clamping pressure 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 products in the mold cavity have not frozen. Keep pressure can continue to fill the mold cavity to ensure full product. If the holding pressure and the pressure setting are too high, it will bring great resistance to the mold carrier and the insert core. The product is easy to top white and the top warps. In addition, the mold runner gate is easily swollen by supplementary plastic, and the gate breaks in the runner. Products with too low pressure have dents and unstable dimensions.

Supporting mold and neutron pressure, the principle of setting the supporting mold and neutron pressure is to set the pressure according to the comprehensive situation of the mold cavity area, the projected area of ​​the core, and the geometric complexity of the molded product. size. Generally, the pressure of the pallet and the neutron cylinder is set to be able to push the product.

3. Flow rate: 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 back pressure. If the pre-plastic flow rate is adjusted to be large, the pre-plastic back pressure is high, as the screw rotates, the plastic has a large shear force in the barrel, and the plastic molecular structure is easily cut. 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 must be set reasonable, otherwise the product quality will be affected, the injection speed will be too fast, the product will have bubbles, scorch, discoloration, etc. If the injection speed is too slow, the molding of the product is insufficient, and there are weld marks.

Die and neutron flow rate: It should not be set too large, otherwise the ejection and core pulling action will be too fast, which will cause the ejection and core pulling to be unstable, and the product will be easily white.

4. Time setting: There are drying time, injection and holding pressure time, cooling time, etc. in the injection molding process.

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 a water absorption capacity of 0.2 to 0.4% within 24 hours, while the water content of injection molding can be 0.1 to 0.2%.

Injection and pressure holding time: The control method of the computer injection machine is equipped with multi-stage injection, adjustable pressure, speed and plastic injection volume. The injection speed of the plastic into the mold cavity reaches a constant speed, and the appearance of the molded product and the quality of the internal material are improved.

Therefore, the injection process usually uses position control instead of time control. The holding pressure is controlled by time. For example, the holding time is longer, the product density is high, the weight is heavy, the internal stress is large, the demolding is difficult, the whitening is easy, and the molding cycle is prolonged. If the holding time is too short, the product is prone to dents and the size is unstable.

Cooling time: to ensure the stability of the product setting, it is to have enough cooling and setting time after the plastic injected into the mold cavity is formed into a product, otherwise the product will be warped and deformed when the mold is opened, and the ejection is easy to deform and white. The cooling time is too long, which prolongs the molding cycle, which is not economical.

5. Position control: Some injection molding machines have mold shift position, mold support position, material storage and injection position control.

The mold shift position is the entire distance from mold opening to mold closing and locking is called the mold shift position. The best position for moving the mold should be able to take out the product smoothly. The mold opening distance is too large, and the molding cycle is long.

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

Material storage position: First, ensure the plastic injection volume of the molded product at one time, and secondly control the storage volume 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. On the contrary, 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 affects the strength of the molded product. On the contrary, it affects the plasticization quality of the plastic, and no material is added to the mold when the pressure is maintained, and the product is insufficiently molded and has dents.

The quality of injection molded products involves product design, plastic material, mold design and processing quality, injection molding machine selection and process adjustment, etc. For normal production molds, to ensure and improve product quality, only the injection process is adjusted; that is, the temperature, pressure, flow rate, time and position are adjusted.

Process adjustment must be comprehensively planned. If the size of the product changes greatly, you can start to consider from the plastic raw materials, whether it is crystalline plastic or non-crystalline plastic, and the molecular weight of the plastic. It is a crystalline plastic and a macromolecular plastic. The crystallinity and molecular orientation of the product must be considered. The shape of the product changes to the flow during molding, which will cause the change and size of the internal stress of the product and cause the shrinkage of the product.

To solve the small change in product size, from the process, it can increase some material temperature, reduce mold temperature, control injection material quantity, reduce injection pressure, increase injection speed, and achieve instantaneous filling of mold cavity, reduce pressure holding time, and extend some cooling time. Ensure that the product density is reduced when the product is shot enough. To reduce the internal stress of the product and reduce the shrinkage of the product.

In short, the adjustment of the injection process cannot start from a single point, but must start from the principle of the injection process. Comprehensive planning and consideration of issues can be made from multiple aspects one by one or multiple issues at once. However, the adjustment method and adjustment principle should be determined from the quality of the products produced at that time and the technological status.