Injection molding products refer to products that are formed by heating and plasticizing plastics with an injection molding machine, and then injecting them into the cavity of the molding mold to form them. After cooling and cooling, the melt solidifies and then demolding.

Application

Plastic injection molding products are of various varieties and have a wide range of applications, especially in textile equipment and automobile manufacturing industries, where there are various shapes of injection molding products as accessories. Medical equipment, cultural and educational supplies, and various containers, turnover boxes, and shoes that can be seen everywhere in people's daily lives. There are also various complex injection molding structural parts, functional parts, and precision parts for special purposes. Injection molding products are widely used in all fields of the national economy such as transportation, packaging, post and telecommunications, communications, construction, home appliances, computers, aerospace, and defense, and have become indispensable means of production and consumer goods.

Features

1) Fewer molds are required to produce injection molding products. 2) The labor required is relatively low. 3) The production efficiency of injection molding products is high. 4) There is little waste of raw materials during injection molding. 5) The design, manufacturing and mold trial cycle is very long, and the production is slow. 6) The startup investment is large, so it is not suitable for the production of small batches of plastic parts. 7) The quality of molded products is limited by many factors, so the technical requirements are high and it is difficult to master.

Common Problems

1. Dents 2. Plastic products are short of material 3. Silver streaks are produced 4. Deformation occurs 5. Cracks appear 6. Stress cracks 7. Net cracks appear 8. Whitening 9. Welding marks 10. Burning spots, etc.

Increase the injection pressure, extend the injection holding time, reduce the barrel temperature and mold temperature, and force cooling where the dents are produced. Fill the flow edge where the dents are produced. When there is a narrow space on the material side where the dents are produced, make this part thicker. The difference in the thickness of the designed products should be completely avoided. The ribs that are prone to dents should be as short as possible in a narrow shape. Increase the mold temperature, increase the barrel temperature, increase the injection pressure, and add a gas escape groove (depth 0.02~0.04mm) with a width of 5~10mm on the parting surface. Increase the gate and flow edge. In the case of a large number of molds per mold, the gate of the cavity that is short of material should be enlarged. Also, change the configuration of the flow edge, add a gas escape pin, and improve the finish of the mold. Avoid designing different product thicknesses, add a gate in the thick part of the product, understand the occasions where the product is used, and try to use materials with good fluidity if appropriate. Dry the material completely. (High temperature and short time drying are not effective, generally 85.C temperature is used for 4 hours) Increase the mold temperature, reduce the heating barrel temperature, and keep the barrel injection nozzle warm. Make the flow edge thicker. Avoid designing different product thicknesses, and add a gate in the thick part of the product. Fully cool and solidify in the mold (extend the cooling time timer), increase the barrel temperature, and reduce the injection pressure. Make the mold cooling uniform. To avoid the difference in product thickness, set the gate (1-1) where the product thickness is large. Since straight lines are prone to warping, make a large R curve, make the product reversibly bendable, increase the number of ejector rods, and increase the demolding slope.

Warping deformation

refers to the shape of the injection molded product deviating from the shape of the mold cavity. It is one of the common defects of plastic products. With the development of the plastics industry, people have higher and higher requirements on the appearance and performance of plastic products. The degree of warping deformation, as one of the important indicators for evaluating product quality, has also received more and more attention and attention from mold designers. Mold designers hope to predict the reasons why plastic parts may warp during the design stage so as to optimize the design, thereby improving the efficiency and quality of injection molding production, shortening the mold design cycle, and reducing costs.

1. The influence of mold structure on the warping deformation of injection molded products

In terms of mold design, the factors that affect the deformation of plastic parts mainly include the pouring system, cooling system and ejection system.

1. Design of the gating system

The position, form and number of the injection mold gate will affect the filling state of the plastic in the mold cavity, thus causing deformation of the plastic part.

The longer the flow distance, the greater the internal stress caused by the flow and shrinkage compensation between the frozen layer and the central flow layer; conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the flow of the part, the thinner the thickness of the frozen layer during mold filling, the lower the internal stress, and the warping deformation will be greatly reduced. Figure 1 shows a large flat plastic part. If only one central gate or one side gate is used, the plastic part will be distorted after molding because the shrinkage rate in the diameter direction is greater than the shrinkage rate in the circumferential direction; if multiple point gates or film gates are used instead, warping deformation can be effectively prevented.

When point casting is used for molding, the position and number of the gates also have a great influence on the degree of deformation of the plastic part due to the anisotropy of plastic shrinkage.

Since 30% glass fiber reinforced PA6 is used, a large injection molded part weighing 4.95kg is obtained. Therefore, many reinforcing ribs are set along the flow direction of the four walls, so that each gate can be fully balanced. The experimental results show that setting the gate according to Figure f has a better effect. But the more gates, the better.

In addition, the use of multiple gates can also shorten the flow ratio (L/t) of the plastic, so that the material density in the mold cavity is more uniform and the shrinkage is more uniform. At the same time, the entire plastic part can be filled under a smaller injection pressure. The smaller injection pressure can reduce the molecular orientation tendency of the plastic and reduce its internal stress, thereby reducing the deformation of the plastic part.

2. Design of the cooling system

During the injection process, the uneven cooling speed of the plastic part will also form an uneven shrinkage of the plastic part. This shrinkage difference leads to the generation of bending moment and causes the plastic part to warp.

If the temperature difference between the mold cavity and the core used in injection molding of flat plastic parts is too large, as shown in Figure 3, the melt close to the cold mold cavity surface cools down quickly, while the material layer close to the hot mold cavity surface will continue to shrink. The uneven shrinkage will cause the plastic part to warp.

In addition to considering the temperature balance of the inner and outer surfaces of the plastic part, the temperature consistency of each side of the plastic part should also be considered, that is, when the mold is cooled, the temperature of the cavity and the core should be kept uniform as much as possible, so that the cooling speed of each part of the plastic part is balanced, so that the shrinkage of each part is more uniform, and deformation is effectively prevented. Therefore, the arrangement of cooling water holes on the mold is very important. After the distance from the tube wall to the cavity surface is determined, the distance between the cooling water holes should be as small as possible to ensure that the temperature of the cavity wall is uniform. At the same time, since the temperature of the cooling medium rises with the increase of the length of the cooling water channel, the cavity and core of the mold will have a temperature difference along the water channel. Therefore, the water channel length of each cooling circuit is required to be less than 2m. Several cooling circuits should be set up in large molds, and the inlet of one circuit is located near the outlet of another circuit. For long plastic parts, the cooling circuit shown in Figure 4 should be used to reduce the length of the cooling circuit, that is, to reduce the temperature difference of the mold, so as to ensure uniform cooling of the plastic parts.

3. Design of the ejector system

The design of the ejector system also directly affects the deformation of the plastic parts. If the ejector system is arranged unbalanced, it will cause an imbalance in the ejection force and cause the plastic parts to deform. Therefore, when designing the ejector system, efforts should be made to balance it with the demolding resistance. In addition, the cross-sectional area of the ejector rod should not be too small to prevent the plastic parts from being deformed due to excessive force per unit area (especially when the demolding temperature is too high). The ejector rod should be arranged as close as possible to the part with large demolding resistance. Under the premise of not affecting the quality of the plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejector rods as possible should be set to reduce the overall deformation of the plastic parts.

When using soft plastics to produce large, deep-cavity, thin-walled plastic parts, due to the large demoulding resistance and the soft material, if a single mechanical ejection method is used, the plastic parts will be deformed, even penetrated or folded, causing the plastic parts to be scrapped. If a combination of multiple components or gas (liquid) pressure and mechanical ejection is used, the effect will be better.

2. The influence of the plasticization stage on the warpage of the product

The plasticization stage is the transformation of glassy particles into a viscous flow state to provide the melt required for filling the mold. In this process, the temperature difference of the polymer in the axial and radial directions (relative to the screw) will cause stress in the plastic; in addition, the injection pressure, rate and other parameters of the injection machine will greatly affect the orientation of the molecules during filling, thereby causing warpage.

3. The influence of the filling and cooling stages on the warpage of the product

The process of molten plastic being filled into the mold cavity under the action of injection pressure and cooling and solidifying in the cavity is the key link of injection molding. In this process, the temperature, pressure and speed are coupled to each other, which has a great impact on the quality and production efficiency of plastic parts. Higher pressure and flow rate will produce high shear rate, which will cause differences in molecular orientation parallel to the flow direction and perpendicular to the flow direction, and produce a "freezing effect". The "freezing effect" will produce freezing stress and form internal stress of the plastic part. The influence of temperature on warpage is reflected in the following aspects.

(1) The temperature difference between the upper and lower surfaces of the plastic part will cause thermal stress and thermal deformation;

(2) The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas;

(3) Different temperature conditions will affect the shrinkage rate of the plastic part.

IV. The influence of demolding stage on product warpage

Plastic parts are mostly glassy polymers in the process of leaving the cavity and cooling to room temperature. Unbalanced demolding force, unstable movement of the ejection mechanism or improper demolding ejection area can easily cause product deformation. At the same time, the stress frozen in the plastic part during the mold filling and cooling stages will be released in the form of deformation due to the loss of external constraints, resulting in warping.

5. The influence of shrinkage of injection molded products on warping

The direct cause of warping of injection molded products is the uneven shrinkage of plastic parts. If the influence of shrinkage during the filling process is not considered in the mold design stage, the geometric shape of the product will be very different from the design requirements, and serious deformation will cause the product to be scrapped. In addition to the deformation caused by the filling stage, the temperature difference between the upper and lower walls of the mold will also cause the difference in shrinkage between the upper and lower surfaces of the plastic part, resulting in warping.

For warping analysis, shrinkage itself is not important, but the difference in shrinkage is important. In the injection molding process, the shrinkage rate of the plastic in the flow direction is greater than the shrinkage rate in the vertical direction due to the arrangement of polymer molecules along the flow direction during the injection and filling stage, which causes the injection molded part to warp. Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage will cause warping. The difference between the shrinkage rates of crystalline plastics in the flow direction and the vertical direction is greater than that of non-crystalline plastics, and their shrinkage rate is also greater than that of non-crystalline plastics. The large shrinkage rate of crystalline plastics and the anisotropy of their shrinkage lead to a much greater tendency for crystalline plastic parts to warp and deform than non-crystalline plastics.

Sixth, the influence of residual thermal stress on product warpage

During the injection molding process, residual thermal stress is an important factor causing warpage and has a great influence on the quality of injection molded products. Since the influence of residual thermal stress on product warpage is very complex, mold designers can use injection molding CAE software for analysis and prediction.

There are many factors that affect the warpage of injection molded products. The structure of the mold, the thermophysical properties of the plastic material, and the conditions and parameters of the injection molding process all have different degrees of influence on the warpage of the product. Therefore, the study of the warpage mechanism of injection molded products must comprehensively consider many factors such as the entire molding process and material properties.

Weld seams

When the molten plastic in the cavity meets the holes of the insert, the area with discontinuous flow rate, and the area where the filling material flow is interrupted, it cannot be completely fused and a linear weld seam is generated. In addition, weld seams will also be generated when the gate injection molding occurs. The strength and other properties at the weld seam are very poor. The main reasons are analyzed as follows:

1. Processing:

(1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are too low, causing the molten material entering the mold to cool prematurely and form a weld seam.

(2) When the injection pressure and speed are too high, injection will occur and a weld seam will appear.

(3) The rotation speed should be increased, and the back pressure should be increased to reduce the viscosity of the plastic and increase the density.

(4) The plastic should be dried well, and recycled materials should be used less. Weld seams will also appear if the release agent is too much or of poor quality.

(5) Reduce the clamping force to facilitate exhaust.

2. Mold:

(1) There are too many gates in the same cavity. The number of gates should be reduced or set symmetrically, or as close to the weld as possible.

(2) The exhaust at the weld is poor, and an exhaust system should be set.

(3) The runner is too large or the size of the pouring system is inappropriate. The gate should be set to avoid the melt flowing around the insert hole, or inserts should be used as little as possible.

(4) The wall thickness varies too much or is too thin. The wall thickness of the part should be made uniform.

(5) If necessary, a fusion well should be opened at the weld to separate the weld from the part.

3. Plastic:

(1) Lubricants and stabilizers should be added appropriately to plastics with poor fluidity or heat sensitivity.

(2) The plastic contains many impurities. If necessary, replace it with a better quality plastic.

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