The depressions and cavities on the surface of plastic products are called shrinkage. Shrinkage not only affects the appearance of products, but also reduces the quality and strength of products. Generally speaking, shrinkage is also the most likely to occur in the molding process. The reason of shrinkage is related to molding technology, mold design and the use of plastic.

1. Different plastics have their own shrinkage rate. The following table is for reference. Usually, raw materials that are easy to shrink are crystalline, such as nylon and pleated rubber. During the injection process, the crystalline plastic is in a fluid state when heated, and the molecules are irregularly arranged. When they are injected into the cavity of the cold mold, the plastic molecules are arranged neatly into crystals, and as a result, the volume shrinkage is less than the specified size range, that is, the shrinkage rate.

2. Injection technology

In the control of injection process, shrinkage occurs: insufficient pressure, too slow injection speed, too small or too long gate, resulting in uneven flow of plastic melt during injection. Therefore, when using the injection machine, we must pay attention to the molding conditions and whether the pressure is enough to prevent shrinkage.

3, mold and product design:

The runner design and cooling device of the mold also have great influence on the finished product. Because of the poor heat transfer ability of plastics, the farther away from the mold wall, the thicker the solidification and the slower the cooling. There should be enough plastic to fill the mold cavity, so that when the screw of the injection machine is injected or maintained, the plastic will not flow back and reduce the pressure. On the other hand, the nozzle can't coagulate too fast, so as to avoid the pressure drop caused by the semi-cured plastic blocking the flow passage, which will lead to the product shrinkage. The shrinkage rate of plastic in different molds is different, so the barrel temperature should be properly controlled to prevent the plastic parts from overheating; Extending the cycle can ensure that the product has enough time to cool down.

If the shrinkage problem is solved, the quality of finished products can be improved, the secondary waste can be reduced and the production efficiency can be improved.

Second, the finished product is difficult to demould (mucous membrane)

Mucosa will appear in the finished product during injection molding. First of all, we should consider whether the injection pressure and holding pressure are too high. Excessive injection pressure will cause the finished product to be supersaturated, make the plastic fill into other gaps, and cause the finished product to get stuck in the mold cavity, which makes it difficult to demould, and it is easy to have mucous membrane when taking out the product.

When the feed pipe temperature is too high, two things will happen. First, the plastic will decompose and deteriorate when heated, making it lose its original characteristics, and it will crack or break during demoulding, resulting in mucous membranes. Second, the rubber compound is not easy to cool after it is filled into the mold cavity, which needs to prolong the cycle time and reduce the economic benefit. Therefore, it is necessary to adjust the working temperature of plastic according to its characteristics. If the inlet and outlet of the mold are unbalanced, the cooling rate of the finished product will be different. Therefore, there is mucosal phenomenon when the finished product is demoulded.

Third, rubber catheter mucosa

Generally speaking, the characteristic of the runner mucosa is that the nozzle sticks to the runner of the upper mold or breaks easily on the runner after opening the mold, resulting in the defects of the lower mold products.

Fourth, the finished product has pores.

In the process of injection molding, sometimes many small bubbles appear in the finished product, which not only affects the strength and mechanical properties of the product, but also greatly reduces the appearance value of the finished product. Therefore, when bubbles appear in the finished product, you can check the following factors and deal with them accordingly.

Usually, when the mold has ribs, bubbles are easy to form because of the different thickness of the finished product or the different cooling rate and shrinkage degree of the plastic in the mold, so the design of the mold should pay special attention.

In the use of raw materials, if there is water vapor in the plastic, the plastic will decompose when heated, and the water vapor will form gas when heated, which will form bubbles in the product because it is too late to eliminate it. If the tolerance of the glue injection screw is too small, air will easily enter the mold cavity to form bubbles.

Five, finished product deformation

Warpage or large dimensional deviation of plastic products belongs to finished product deformation, such as too fast ejection, too high mold temperature, uneven mold temperature, asymmetric runner system and so on. The two biggest possibilities are: 1, the plastic parts are uneven in thickness or the corners are not smooth enough to cool and shrink evenly, resulting in warping and deformation. 2. For some flat plastics, the runner gate must be located at the corner of the product for the sake of beautiful surface. When injecting glue, the melted plastic can only be injected into the mold cavity at a high speed from one side, so the plastic molecules solidified in the mold cavity are all pulled in the same direction (called orientation, at this time, the internal stress of the plastic part is large: it is pulled back to the original state when demoulding, resulting in deformation).

In order to make molten plastic fill the mold cavity smoothly, the following points should be avoided as far as possible in its design:

1, the thickness difference in the same plastic part is too large.

2, there is an excessive acute angle.

3. The creep zone is too short, which makes the thickness change very different.

From the analysis of the gate, the design of the mold should ensure that the plastic can enter the mold cavity smoothly, so the diversion should avoid right-angle turning, and if there is a turning angle, there should be an arc transition area as far as possible. Therefore, short and coarse shunt is the most ideal, which is helpful to reduce fluid resistance and fluid orientation. It should also be considered that too large a runner will increase waste and affect the appearance of the product.

In addition, in order to avoid the deformation caused by the difficulty of demoulding due to the different density of plastic during filling, the cross-sectional area and shape of the shunt should be changed according to the injection amount and product shape. After the parts that are difficult to form are thickened, the main channel should be enlarged accordingly, so that the cross-sectional area of the main channel is equal to or greater than the sum of the cross-sectional areas of the main channel.

In addition, there are two noteworthy problems. One is the form of ejection device for plastic parts. Too few thimbles are easy to cause deformation and warping, but too many thimbles will affect the appearance of finished products. At this time, you can consider pushing the plate. Second, the design of cooling runner of mold cavity should make the whole plastic part shrink evenly and improve the product quality.

Six, silver stripes, bubbles

The formation of shooting pattern is generally due to the rapid start of injection, so that the air in front of the mold cavity can not be discharged by the rubber melt, and the air is mixed into the rubber, which makes the surface gloss and color of the product uneven, which is called shooting pattern. Shooting not only affects the appearance, but also reduces the mechanical strength of the finished product. Therefore, in order to avoid this defect, we must find out the reason and improve it.

Because the injection mold is formed by the gas contained in the molten plastic, the main sources of this gas are as follows:

(1) Plastic itself contains water or oil;

Because the plastic is exposed to the air in the manufacturing process, water vapor or oil agent is inhaled, or some components with incorrect proportions are mixed, these volatile substances are heated to produce gas when melting glue.

(2) heating to decompose raw materials:

If the temperature, back pressure and melting speed of screw barrel are adjusted too high, or the molding cycle is too long, PVC, plug steel, PC and other heat-sensitive materials. It is easy to generate gas due to high temperature heating decomposition.

(3) Air

There is air between plastic particles. If the temperature of the screw barrel at the glue inlet is adjusted too high, so that the plastic surfaces melt and stick together without being completely compressed, the air between plastic particles cannot be completely eliminated.

Therefore, ideal plastic products can be obtained by drying plastic, adopting appropriate melting temperature and speed and matching with appropriate back pressure. In addition, mold design is also a very important link. Generally, a mold with a large runner and a small nozzle will reduce the chance of gas entering the mold cavity.

In injection molding technology, there is a way to prevent the appearance of radiation. In the structure of the mold, there are a pressurizing device and a compressed air inlet. After mold locking, compressed air enters the mold to increase the air pressure in the mold cavity. When the molten plastic enters the high-pressure mold, the pores of the mold begin to exhaust. At this time, maintaining a certain pressure in the mold cavity and increasing the air pressure in the mold can avoid the probability of shooting. For example, when the water content of ABS is 0. 1%, injection lines will appear. The ABS with high moisture content can be treated by gradually increasing the air pressure in the mold, and there will be no projectile.

Seven, Birna other peaks.

Burr (commonly known as pen tip) is a common injection problem in injection molding. When the pressure of plastic in the mold cavity is too high, the parting force is greater than the clamping force, which forces the mold to open and the plastic is extruded, forming a peak on the surface of plastic parts. However, there are many reasons for this problem, including plastic, damaged injection molding machine and improper adjustment. It is not easy to find out its solution. Because the viscosity of plastic will affect its flow speed and pressure loss, too high or too low viscosity may produce burrs. If the viscosity of plastic is too low, its fluidity is high, and it is easy to flow into the tiny gap between the clamping surfaces of the mold, increasing the parting force until another peak appears.

Generally speaking, plastic temperature has a great influence on viscosity, and pressure and shear rate also have an influence on pressure. If the temperature of plastic increases, its viscosity will decrease, while if the temperature decreases, its viscosity will increase.

Another problem with plastics is its drying conditions and whether it is mixed with impurities. Some plastics, such as nylon or ABS, have properties that affect plastics. As for polycarbonate, although it has no water absorption, its performance is also very sensitive to surface moisture. Therefore, during molding, many plastics must be dried to correctly control their properties. If plastics are mixed with impurities or different kinds of plastics, the performance of plastics is more difficult to predict.

The pressure of plastic in the mold cavity will change with the filling of the mold cavity. Before the cavity is filled, the pressure at the front end of the melt is almost equal to zero, while the pressure at the injection port is higher than that at other positions in the cavity. When the mold cavity is completely filled, the pressure loss of plastic flow no longer exists, and the pressure in the whole mold cavity becomes the same static pressure, so the force that forces the mold to open will greatly increase, resulting in burrs. To avoid this phenomenon, once the cavity is filled, the injection pressure must be adjusted to a lower holding pressure. In addition to correctly adjusting the pressure control system of the injection machine, another auxiliary method is to reduce the injection speed first. In this way, the plastic at the front end of the melt has time to cool and partially solidify, thus avoiding the appearance of burrs. Because the injection speed is too slow, it will also slow down the production and cause some defects such as envelope. It is best to adjust the injection speed to match the adopted clamping force. Otherwise, burrs may also appear.

If there is a problem with the mechanical structure of the ejector, it is more complicated and it is difficult to find the cause of the problem. For example, there is a deviation in the parallelism between the templates, or the stress of the template pull rod is uneven, which will also cause the clamping force between the molds to be unbalanced, resulting in the peak of the weak clamping force. On the other hand, if the screw or glue melting cylinder is worn heavily, the melt may slide and flow backwards between the outer diameter of the thread and the cylinder, thus the pressure position switching point is incorrect, resulting in local burr and insufficient glue injection.

In addition to the above factors, if there is a problem with the mold, there will be burrs. For example, if the mold is used for a long time and some parts are worn, it is easy to have burrs. Even some small defects, such as the blockage of exhaust holes, will cause the pressure in the mold cavity to rise, and burrs will appear if the pressure is too high. In some multi-cavity molds, if the runner design is unbalanced, the plastic flow will be asymmetric. In order to avoid insufficient pressure of a single mold, other molds may have burrs.

Eight, short shots of finished products

Underfilling refers to the phenomenon that the molten material does not completely flow through all corners of the molding space.

The reasons for insufficient filling are unreasonable setting of molding conditions, imperfect mold design and manufacture, and too thin meat thickness of molded products. The countermeasures of molding conditions are to increase the molding temperature and mold temperature of materials, increase the injection pressure and speed, and improve the fluidity of materials. The size of the main runner or runner, or the position, size, position and number of gates can be increased in the mold to make the molten material flow easily. In order to make the gas in the molding space exhaust smoothly, exhaust holes should be set at appropriate positions.

Nine, combination line

A bonding line is a thin line formed by the confluence of two or more molten glue materials. The reasons for the adhesion line are as follows:

(1) The material flow mode caused by the shape of the molded product (mold structure);

(2) the fluidity of molten materials is poor;

(3) Foreign substances, such as air, volatiles or release agents, participate in the merging of molten materials;

The bonding line is caused by the extremely low temperature of the front part of the flowing material when it is fused, and the fused part is not completely fused. Inevitably, the materials will fuse around the windows and holes of the molded products, resulting in adhesive lines. When the fluidity of the material is particularly good, the bonding line can be almost invisible, and at the same time, the temperature of the material and the mold can be increased, and the position of the bonding line can also be moved to other places. Or a vent hole is arranged in the melting part to quickly exhaust the air and volatiles in this part. Or setting an overflow pool at the fusion place and cutting it off afterwards are all effective countermeasures.

Adhesive lines not only hinder the appearance of molded products, but also are detrimental to the strength of molded products. The adhesive strength of non-reinforced plastics without glass fiber and other fillers is almost the same as that of other parts. However, glass fiber reinforced plastics (FRTP) do not melt at the melting part, and the strength of this part is usually low.

Ten, the molding surface gloss is not good.

The molding surface loses the original luster of the material, forms a milky white film and becomes a fuzzy state, which can all become poor surface luster.

The poor gloss of the molding surface is mostly caused by the state of the mold surface. When the surface of the mold is poorly polished, the surface of the molded product will definitely not get good gloss. However, when the mold surface is in good condition, increasing the material temperature and mold temperature can improve the surface gloss of the product. Excessive use of release agent or oily release agent is also one of the reasons for poor surface gloss.

XI. Black Line

Black stripes are black stripes on molded products, which are mainly caused by thermal decomposition of materials and are common in materials with poor thermal stability.

The effective countermeasure to prevent black line is to prevent the material temperature in the barrel from being too high and slow down the injection speed. If there are scars or gaps on the inner wall of the barrel or the screw, the materials attached to this part will overheat and cause thermal decomposition. The decomposition of plastic in the retaining ring can also cause thermal decomposition, so special attention should be paid to prevent black lines from appearing in materials with high viscosity and easy decomposition. Iron-iron friction caused by eccentricity of feed pipe and screw will lead to the blackening of materials in the barrel and the appearance of black lines.

XII. Flow patterns

Traces of molten material flowing when flowing, stripe pattern centered on gate.

The plastic flowing into the molding space first cools too fast when flowing, and forms a boundary with the material flowing later. In order to prevent flow marks, the temperature of plastic can be increased, the fluidity of plastic can be improved and the injection speed can be adjusted.

If the cold plastic left at the front end of the nozzle of the injection molding machine directly enters the molding space, it will leave traces, so a sufficient retention part (cold material well) is designed at the joint of injection and runner or at the joint of runner. It can effectively prevent the generation of traces, and at the same time, it can also increase the size of the gate to prevent it.

Thirteen. When the mold is opened or ejected, the finished product is broken.

Hair-like cracks will appear on the surface of the finished product when it is broken, and when the molded product has sharp corners, small cracks that are not easy to see will often appear in this part, and finally cracks will occur due to the interference of external forces when ejecting or opening the mold, which is one of the fatal defects of the finished product.

(1) not easy to demould.

(2) Overfilling.

(3) The mold temperature is too low.

(4) Structural defects of molded products.

If you want to avoid cracks caused by poor demoulding. The mold space must have enough demoulding inclination, and check the size, position and shape of the ejector pin. When ejecting, the demoulding resistance of each part of the finished product should be uniform.

Overflow refers to excessive injection pressure or excessive material metering during injection molding, which leads to cracks during demoulding. In this state, the deformation of die parts also increases, which makes demoulding more difficult and leads to cracks. At this time, reduce the injection pressure appropriately to prevent overfilling.

Too much internal stress often remains in the gate, and it is easy to crack near the gate, especially the straight gate, which is easy to break due to internal stress, such as cup-shaped or bowl-shaped molded products, and radial cracks are easy to appear around the gate.