The influence of injection molding process on product quality

There are two kinds of product quality, namely: the quality of internal material and the quality of appearance. The quality of the inner material mainly refers to the mechanical strength, and the size of the internal stress directly affects the mechanical strength of the product. The main cause of internal stress is the crystallinity of the product and plastic molding. The orientation of the molecules is determined. The appearance quality of the product refers to the surface quality of the product, while the internal stress causes warping and deformation of the product, which affects the appearance quality. The appearance quality of the product refers to: product deficiency, product dent, weld marks, flying edge, bubble, silver, black spot, deformation, crack, delamination and discoloration, all related to the molding temperature, pressure, flow rate, time and position.
1. Molding temperature: barrel temperature, plasticizing temperature, nozzle temperature, mold temperature and plastic drying temperature.
Barrel temperature. It is the melting temperature of the plastic. The temperature of the barrel is set too high. The viscosity of plastic after melting is very low. Under the same injection pressure and flow rate, the injection speed is fast and the molded products are easy to fly, silver, discolored and brittle.
The barrel temperature is too low, the plastic is poorly plasticized, the viscosity is high, the injection speed is slow under the same injection pressure and flow rate, the molded products are easy to be insufficient, the weld marks are obvious, the size is unstable, and the products have cold lumps.
Nozzle temperature. Nozzle temperature is set too high, the nozzle is easy to flow, resulting in cold wire in the product. Low nozzle temperature causes clogging in the mold system. Injection pressure should be increased to inject plastic, but cold lumps appear in the molded products.
Mold temperature. High mold temperature, injection pressure and flow rate can be set lower, but at the same pressure and flow rate, the product is easy to fly, warp and deform, and the product is difficult to stick to the mold. Low mold temperature, under the same injection pressure and flow rate, the product is not fully formed, bubbles, welds and other phenomena.
Drying temperature of plastic. Various plastics have different drying temperatures. ABS plastic drying temperature is generally set at 80 to 90 ℃, otherwise it is not easy to dry and evaporate water and residual solvent, the product is easy to appear silver wire and bubbles, the strength of the product will also be reduced.
2. Pressure of molding process. Pre-molding back pressure: injection pressure, mold closing pressure, holding pressure, mold neutron pressure, injection table pressure, etc.
Pre-molding back pressure: high back pressure, high storage density, high storage volume under the same storage volume. Low back pressure, low storage density and small storage volume. After setting the storage position and then making larger adjustments to the back pressure, care must be taken to reset the storage position, otherwise it is easy to cause fluffy or insufficient products.
Injection pressure: Different varieties of plastics have different melt viscosities, and the viscosity of amorphous plastics varies greatly as the plasticizing temperature rises. The injection pressure should be set according to the welding viscosity of the plastic and the flow ratio of the plastic. If the injection pressure is set too low, the product will suffer from under-injection, denting, weld lines and dimensional instability. If the injection pressure is too high, the product will have flashing edges, discoloration and sticking, and will be difficult to eject.
Clamping pressure: It depends on the projected area of the mold cavity and the injection pressure. If the clamping pressure is not enough, the product is prone to flying edge and weight increase. If the clamping force is too high, it is difficult to open the mold. Generally speaking, the clamping pressure is set to no more than 120 Pa/cm2.
Holding pressure: When the injection is finished, continue to give a pressure to the screw to hold the pressure. At this time, the product in the mold cavity is not yet frozen. You can continue to pressurize the mold cavity to keep the product full. Holding and fixing pressure is too high, which brings a lot of resistance to the mold and the collar. The product tends to turn white and warp at the top. In addition, the gate of the mold runner tends to be filled with plastic and the gate is broken in the runner. Under-pressured products have dents and dimensional instability.
The principle of setting mold and neutron pressure is based on the size of the mold cavity area and the core projection area of the ferrules, as well as the geometric complexity of the molded product. Size. In general, the pressure of the mold and neutron cylinders should be set to push the product.
3. Flow rate: It is related to the speed of injection molding machine, such as screw speed, injection speed, mold and neutron speed.
Screw speed: In addition to adjusting the preplastic flow rate, it is mainly influenced by the preplastic back pressure. If the preplastic flow rate is adjusted to a large value, the preplastic back pressure is high. With the rotation of the screw, the plastic has a large shear force in the barrel, and the plastic molecular structure is easily cut. The product appears black spots and black stripes, which affects the appearance quality and strength of the product. And the barrel heating temperature is not easy to control. The pre-molding flow setting is too low, and the pre-molding storage time is prolonged, which affects the molding cycle.
Injection speed: The injection speed is set reasonably, otherwise it will affect the product quality. If the injection speed is too fast, the product has bubbles, scorching and discoloration. If the injection speed is too slow, the product is not adequately molded and weld marks appear, etc.
Way and neutron flow: should not be set too large, otherwise the injection and core pulling action is too fast, resulting in unstable injection and lapping, and the product is easy to be white.
4. Time setting. In the injection molding process, there are drying time, injection and holding time, cooling time, etc.
Drying time. It refers to the drying time of plastic raw materials, various 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 is generally 24 hours, the water absorption rate is 0.2~0.4%, and the water content of injectable molding is 0.1~0.2%.
Injection and holding time. The computerized injection molding machine is controlled by multi-stage injection, gradable pressure adjustment, speed adjustment and adjustment of the amount of injected plastic. The plastic is injected into the mold cavity at a constant rate, and the appearance and quality of the molded product is improved.
Therefore, the injection process is commonly controlled by position, without time control, and the pressure is controlled by time. For example, with long holding time, the product is dense and heavy, with high internal stress, difficult to demold, easy to whiten and longer molding cycle. If the holding time is too short, the product is easy to have dents and unstable size.
Cooling time. It is to ensure the stable shaping of the product. After the plastic injected into the mold cavity is molded into the product, the cooling molding time is enough. Otherwise, the product will be easily warped and deformed when the mold is opened, and easily deformed and whitened when it is demolded. 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 shift position, mold position, material storage and injection position control.
The position of the mold is the whole moving distance from mold opening to mold closing and locking, which is called the mold transfer position. The best position for mold transfer is to remove the product smoothly. The mold opening distance is too large and the molding cycle is long.
The position of the mold can be easily removed by controlling the position of the mold to the mold.
Storage location. Firstly, the amount of plastic injected into the molded product at one time should be ensured, and secondly, the amount of storage in the barrel should be controlled. The storage position is controlled above the amount of one injection, the product is easy to fly the edge and the product is not fully formed.
The storage volume in the barrel is too large, the plastic stays in the barrel for too long, the product is easy to fade and affects the strength of the molded product. On the contrary, it affects the quality of plasticization, does not join the mold when holding pressure, the product is not formed and has dents.
to summarize.
The quality of injection molded products involves product design, plastic materials, mold design and processing quality, the choice of injection molding machine and process adjustment.
For a normally produced mold, the only way to ensure and improve the quality of the product is to make adjustments to the injection molding process; i.e., adjustments to temperature, pressure flow rate, time and position.
The process adjustment must be fully integrated. If the size of the product varies a lot, you can consider the plastic raw material, whether it is a crystalline plastic or non-crystalline plastic, the molecular weight of the plastic, etc. It is a crystalline plastic and a macromolecular plastic. The crystallinity and molecular orientation of the product should be considered. During the molding process, the shape of the product will change with the flow, which causes the change of stress and size of the product, resulting in the shrinkage of the product.
Solve the small size change of the product by increasing the material temperature, decreasing the mold temperature, controlling the injection volume, decreasing the injection pressure, increasing the injection speed, filling the mold cavity instantly, decreasing the holding time, and extending the cooling time. Ensure the product density when the product is injected. To achieve lower internal stress of the product, thus reducing the shrinkage rate of the product.
In short, the adjustment of injection process cannot start from a certain point, but must start from the principle of injection process. The problem can be adjusted one by one from multiple aspects or multiple problems can be adjusted at one time. However, the adjustment method and adjustment principle should be determined according to the quality of the products produced at that time and the process conditions.