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Influence of the injection molding process on the quality of injection molded products

Table of Contents

In the molding process of transforming plastic particles into plastic products, plastic is often subjected to high temperatures and pressure and flows under a high shear rate.

Different molding conditions and processes will have different effects on the quality of products, and injection molding has four components: plastic raw materials, injection molding machine, injection mold, and plastic injection molding process.

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

The quality of the product has the internal material quality and appearance quality, the internal material quality is mainly 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 molecules in plastic molding, etc.

The appearance quality of the products is the surface quality of the products, but the warpage and deformation caused by the internal stress will also affect the appearance quality problems.

The appearance quality of products includes product shortage, product dent, fusion mark, flying edge, bubble, silver wire, black spot, deformation, crack, delamination and discoloration, etc., all related to injection molding temperature, pressure, flow time, and position.

Molding temperature

Barrel temperature, plasticizing temperature, nozzle temperature, mold temperature, and plastic drying temperature, etc.

Barrel temperature: it is the melt temperature of the plastic. If the barrel temperature is set too high, the viscosity of plastic will be low after melting, and the injection speed will be fast under the same injection pressure and flow rate, and the molded products will be easy to fly, silver, discolored, and brittle.

If the barrel temperature is too low, the plastic will be poorly plasticized, the viscosity will be high, the injection speed will be slow under the same injection pressure and flow rate, and the molded products will be easily deficient, with obvious melt marks, unstable size and cold lumps in the products.

Nozzle temperature: high nozzle temperature setting, nozzle salivation, resulting in products with cold material wire. Low nozzle temperature causes clogging in the mold casting system. Injection pressure has to be increased to inject plastic, but there are cold material lumps in the instantly molded products.

Mold temperature: high mold temperature, injection pressure, and flow rate can be set lower, but under the same pressure and flow rate, the products are easy to fly, warp and deform, and the products are difficult to stick to the mold and eject. Mold temperature is low, in the same injection pressure and flow rate, the products are not enough, bubbles, fusion marks, etc.

Plastic drying temperature: various plastics have different drying temperatures. ABS plastic is generally set a drying temperature of 80 ~ 90 ℃, otherwise, it is not easy to dry and evaporate the water and residual solvent, the product is easy to have silver and bubbles, and the strength of the product will also be reduced.

Molding process pressure

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

Pre-molding back pressure: high back pressure storage density, more storage in the same storage volume. Low back pressure storage density is low and less material is stored.

After setting the storage position, and then do a larger adjustment of the back pressure, you must pay attention to resetting the storage position, otherwise, it is easy to cause products to fly to the edge or insufficient.

Injection pressure: Different varieties of plastics have different melt viscosity, and the viscosity of non-crystalline plastics varies greatly with the change in plasticizing temperature.

Injection pressure is set according to the melt viscosity of plastic and the plastic process ratio. If the injection pressure is set too low, the product will not be shot enough, dent, fusion marks, and size instability. If the injection pressure is too high, the product will have a flying edge, discoloration, and difficulty in ejecting the injection mold.

Clamping pressure: according to the size of the projection area of the mold cavity and the injection pressure is high or low.

Insufficient clamping pressure makes the product easy to fly and increases its weight. If the clamping force is too large, it will be difficult to open the mold. General clamping pressure is set at no more than 120par/cm2.

Pressure-holding: When the injection is completed, that is, continue to give the screw a pressure called pressure-holding, at this time the mold cavity products have not frozen. Pressure-holding can continue to fill the mold cavity to ensure that the product is full.

If the pressure is set too high, it will bring resistance to the mold and the core, and the products will be easily topped with white and warped, in addition, the mold runner gate will be easily expanded by the plastic and the gate will be broken in the runner. The pressure is too low and the product has dents and is unstable in size.

The principle of setting the pressure of the mold and neutron is based on the size of the mold cavity and the size of the core projection area of the inserted core, as well as the complexity of the geometry of the molded product. Generally speaking, the pressure of the die and neutron cylinders should be set at a level that can push the product.

Flow rate

Related to the injection molding machine speed, there are screw speed, injection speed, tray mold, and neutron speed, etc.

Screw speed: In addition to adjusting the pre-molding flow, it is mainly affected by the pre-molding back pressure.

If the pre-molding flow rate is adjusted to large, pre-molding back pressure and high, with the screw rotation, plastic in the barrel shear force, the plastic molecular structure is easy to cut, and the product has black spots and black stripes, affecting the appearance of product quality and strength, and the barrel heating temperature is not easy to control.

The pre-molding flow rate is set too low, and the pre-molding storage time is prolonged, which will affect the molding cycle.

Injection speed: The injection speed should be set reasonably, otherwise it will affect the product quality. If the injection speed is too fast, the products will have bubbles, burning, and discoloration. If the injection speed is too slow, the product will not be formed enough, and there will be fusion marks, etc.

Bracket mold and neutron flow: should not be set too large, otherwise, the ejection and extraction of the core activities are too fast, resulting in the ejection and extraction of the core is not stable, and the product is easy to top white.


Time setting

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

Drying time: It is the drying time of plastic raw materials, various categories of plastics have the best drying temperature and time. 80~90℃ and 2 hours for ABS plastics. 24 hours for ABS plastics in general, 0.2~0.4% of water absorption, and 0.1~0.2% of water content that can be injection molded.

Injection and holding time: computer injection machine control methods are equipped with multi-stage injection that can be graded to regulate pressure, and speed and adjust the amount of injected plastic.

So that the injection into the mold cavity plastic speed achieves a constant speed, and improves the appearance of molded products and the quality of the inner material.

Therefore, the injection molding process is commonly used in position control, not time control, pressure is controlled by time, such as holding time is long, the product density and weight are heavy, internal stress is large, difficult to release, easy to top white, molding cycle time is extended. The holding time is too short, the products are easy to produce dents, and the size is not stable.

Cooling time: This is to ensure the stability of the product shaping, is injected into the mold cavity plastic molding into products after sufficient cooling and shaping time, otherwise the products are easy to warp and deform when opening the mold, easy to deformation, and the top is white. Cooling time is too long, extending the molding cycle, and not economic.

Position control

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

Mold shift position: the whole moving distance from mold opening to mold closing and locking is called mold shift position. The best position for mold transfer is to be able to take out the products smoothly. If the mold opening distance is too large, the molding cycle should be long.

The position of the mold support: As long as the control of the ejected position from the mold can be convenient to take down the products and can.

Storage position: first of all, to ensure the amount of plastic injected at one time for the molding products, and second, to control the amount of material stored in the barrel. Storage location control over the amount of a single injection is too much, the product is easy to fly edge, and vice versa, the product molding is not enough.

Too much stock in the barrel, the plastic stays in the barrel for a long time, the products are easy to lose color, and affects the strength of the molded products. On the contrary, it affects the plasticizing quality of plastic, and no material will be added to the mold when holding pressure, and the products will be under-molded with dents.

Conclusion

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

For normal production molds, the only way to ensure and improve the quality of products is to adjust the injection process; that is, to adjust the temperature, pressure, flow rate, time and position. Process adjustment must be comprehensive and integrated.

If the size of the product changes a lot, you can start to consider the plastic raw materials, is crystalline plastic or non-crystalline plastic, the size of the plastic molecular weight, etc.

Is crystalline plastic and macromolecular plastic, to consider the crystallinity of the product and molecular orientation, the shape of the product on the flow of molding changes, and cause the product internal stress changes and size, resulting in product shrinkage changes.

To solve the problem of small changes in product size, we can increase the material temperature, reduce the mold temperature, control the injection volume, reduce the injection pressure, increase the injection speed, to instantly fill the mold cavity, reduce the holding time, extend the cooling time, and reduce the density of the injection molded product under the condition of ensuring that the product is injected. To reduce the internal stress of the product, and reduce the shrinkage of the product changes.

In short, injection process adjustment cannot be done from a single point, but the principle of the injection process. Comprehensive and balanced consideration of the problem can be adjusted from a variety of aspects one by one or several issues at a time to adjust.

However, the adjustment method and adjustment principle should be determined from the quality of the products produced at that time and the injection moulding process condition.

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