In the molding process of transforming plastic particles into plastic products, plastics are often subjected to high temperature and high pressure, flow molding under high shear rate, different molding conditions and processes, will have different effects on the quality of products, injection molding has plastic raw materials, injection molding machine, mold and injection molding process four aspects.

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

The quality of the product has the quality of the inner material and the appearance quality, the quality of the inner material is mainly the mechanical strength, and the size of 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 molecule in the plastic molding.

The appearance quality of products is the surface quality of products, but the internal stress caused by warping deformation of products will also affect the appearance of quality problems. The appearance quality of the product is: insufficient products, product dents, weld marks, flash, bubbles, silver wire, black spots, deformation, cracks, delamination peeling and discoloration, etc., are related to 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: is the plastic melting temperature, barrel temperature set too high, low viscosity after plastic melting, under the same injection pressure and flow, injection speed, molding products easy to flash, silver wire, discoloration and brittle.

Barrel temperature is too low, plastic plasticization is poor, viscosity is high, injection speed is slow under the same injection pressure and flow rate, molding products are easy to be insufficient, welding marks are obvious, the size is unstable and there are cold blocks in products.

Nozzle temperature: the nozzle temperature is set high, the nozzle is easy to salivate, resulting in cold silk in the product. Low nozzle temperature causes blockage in the mold pouring system. The injection pressure must be increased to inject the plastic, but there is cold material block in the molded product.

Mold temperature: mold temperature is high, injection pressure and flow can be set a little lower, but under the same pressure flow, products are easy to flash, warping deformation, products stick mold ejection difficult. Mold temperature is low, under the same injection pressure and flow, the product molding is insufficient, bubbles, welding marks, etc.

Plastic drying temperature: various plastics have different drying temperatures. ABS plastic is generally set a drying temperature of 80 ~ 90 бу C, otherwise it is not easy to dry the moisture and residual solvent volatilization, the product is easy to have silver wire and bubbles, the product strength will also decline.

2. Molding process pressure: pre-plastic back pressure: injection pressure, mold locking pressure, pressure holding, mold and neutron pressure, shooting table pressure, etc.

Pre-plastic back pressure: high back pressure, high storage density, more storage in the equivalent storage volume. Low back pressure storage density is low, less storage. After setting the storage position, and then do a large adjustment of the back pressure, we must pay attention to re-set the storage position, otherwise it is easy to cause the product flash or insufficient.

Injection pressure: different varieties of plastics have different melting viscosity, and the viscosity of amorphous plastics changes greatly with the plasticizing temperature. Injection pressure is set according to the fusion viscosity of the plastic and the plastic flow ratio. If the injection pressure is set too low, the product is not enough, the dent, the weld mark and the size are unstable. Injection pressure is too high products flash, discoloration and sticky mold ejection difficulties.

Clamping pressure: according to the size of the projected area of the mold cavity and the injection pressure level. Insufficient clamping pressure products are easy to fly edge, weight increase. The clamping force is too large, it is difficult to open the mold. Generally, the clamping pressure is set not to exceed 120par/cm2.

Pressure holding: When the injection is over, continue to give the screw a pressure called pressure holding, then the products in the mold cavity are not frozen. Pressure holding can continue to fill the mold cavity to ensure that the product is full. The pressure holding and pressure setting are too high, which brings great resistance to the support mold and the pumping core, and the products are easy to top white and top warping. In addition, the mold runner gate is easy to be expanded tightly by the supplementary plastic, and the gate is broken in the runner. Too low pressure products have dents and size instability.

Support mold and neutron pressure, set the principle of support mold and neutron pressure is based on the size of the mold cavity area and the size of the core projection area, as well as the complexity of the geometric shape of the molded product to set the pressure size. Generally, it is necessary to set the pressure of the mold and the neutron cylinder in order to promote the product.

3. Flow: related to the speed of the injection molding machine, there are screw speed, injection speed, mold and neutron speed.

Screw speed: in addition to adjusting the pre-plastic flow rate, mainly affected by the pre-plastic back pressure. If the pre-plastic flow rate is large, the pre-plastic back pressure is high, with the rotation of the screw, the plastic shear force in the barrel is large, the plastic molecular structure is easy to be cut off, the product has black spots and black stripes, affecting the appearance quality and strength of the product, and the heating temperature of the barrel is not easy to control. The pre-plastic flow rate is set too low, and the pre-plastic storage time is extended, which will affect the molding cycle.

Injection speed: The injection speed should be set reasonably, otherwise the quality of the product will be affected, the injection speed will be too fast, and the product will have bubbles, burning, discoloration, etc. The injection speed is too slow. The product is not formed enough, and there are welding marks.

Mold and neutron flow: should not be set too large, otherwise the ejection and core pulling action is too fast, resulting in ejection and core pulling instability, the product is easy to top white.

4. Time setting: there are drying time, injection and pressure holding time, cooling time and so on in the injection molding process.

Drying time: is the drying time of plastic raw materials, all kinds of plastics have the best drying temperature and time. The drying temperature of ABS plastic is 80 ~ 90бц, and the time is 2 hours. ABS plastic is generally 24 hours, the water absorption is 0.2 ~ 0.4%, and the water content of injection molding is 0.1 ~ 0.2%.

Injection and pressure holding time: The control method of the computer injection machine is equipped with a multi-level injection can be graded pressure regulation, speed regulation and adjustment of the amount of injection plastic. The plastic speed into the mold cavity can reach a constant speed, and improve the appearance of the molded product and the quality of the interior material.

So the injection process commonly used position control, no time control, pressure is controlled by time, such as pressure holding time is longer, the product density is large, the weight is heavy, the internal stress is large, the demoulding is difficult, the easy top white, the molding cycle is extended. The pressure holding time is too short, the products are easy to produce dents, and the size is unstable.

Cooling time: It is to ensure that the product is stable, and the plastic injected into the mold cavity is formed into the product after enough cooling setting time, otherwise the product is easy to warp when opening the mold, easy to deform and top white. The cooling time is too long, extending the molding cycle, and it is not economical.

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

Shift mold position is from the open mold to the closed mold lock the entire moving distance called shift mold position. The best position for moving the mold is to be able to remove the product smoothly. The mold opening distance is too large, the molding cycle is long.

Position of the mold as long as the control of the position of the ejection from the mold can be convenient to remove the product and can be.

Storage position: First of all, to ensure the amount of plastic injection molding products, followed by control the amount of storage in the barrel. The storage position control more than one injection amount is too much, the product is easy to fly, on the contrary, the product molding is insufficient.

The amount of material stored in the barrel is too much, the plastic stays in the barrel for a long time, the products are easy to fade, and affect the strength of the molded products. On the contrary, the plasticization quality of plastic is affected, and no material is added to the mold when holding pressure, and the product is not formed enough to have a dent.

Summarize the quality of

injection molding products, involving product design, plastic material, mold design and processing quality, injection molding machine selection and process adjustment.

For the normal production of molds, to ensure and improve the quality of products, only the injection process adjustment; That is, the temperature, pressure, flow speed, time and position adjustment.

Process adjustment must be taken into account. If the size of the product changes, you can start to consider from the plastic raw materials, whether it is crystalline plastic or non-crystalline plastic, the size of the molecular weight of the plastic. Is crystalline plastics and macromolecular plastics, to consider the product crystallinity and molecular orientation, the shape of the product when the flow changes, causing the internal stress changes and size of the product, resulting in product shrinkage changes.

To solve the small size change of the product, from the process can improve some material temperature, reduce the mold temperature, control the injection amount, reduce the injection pressure, improve the injection speed, to achieve an instant full of mold cavity, reduce the pressure holding time, extend the cooling time, in the case of ensuring that the product shot foot to reduce the product density. To reduce the internal stress of the product, and reduce the shrinkage of the product change.

In short, the adjustment of the injection process can not be started from a single point, must be started from the principle of the injection process. Comprehensive overall consideration of the problem, can be adjusted from many aspects one by one or a number of problems once adjusted. However, the adjustment method and the adjustment principle should be determined from the quality and process status of the products produced at that time.

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