Plastic molds are no longer afraid of failure!

Common defects and control methods in plastic molds


One of the common defects of plastic products is cracking, which is mainly caused by too much pressure, and the main deformation is reflected in residual, external and external environmental stress.

Second, the main reasons for insufficient filling and filling are as follows: i. Insufficient resin capacity. Ii. Insufficient pressurization in the cavity. Iii. Insufficient fluidity of the resin. Iv. The exhaust effect is not good.

plastic molds

As an improvement measure, we can mainly start from the following aspects:
1) Lengthen the injection time to prevent the resin from flowing back before the gate is solidified due to the short molding cycle, which makes it difficult to fill the cavity.

2) Increase the injection speed.
3) Increase the plastic molds temperature.
4) Increase the resin temperature.
5) Increase the injection pressure.
6) Expand the gate size. Generally, the height of the gate should be equal to 1/2 to l/3 of the wall thickness of the product.7) The gate is placed at the maximum wall thickness of the product.
8) Set the exhaust groove (average depth 0.03mm, width 3~smm) or the exhaust rod. It is more important for smaller workpieces.
9) A certain (about smm) buffer distance is left between the screw and the injection nozzle.
10) Use low viscosity grade materials.
11) Add lubricant.

Third, wrinkle and pockmark
The cause of this defect is essentially the same as the filling, but to a different extent. Therefore, the solution is basically the same as the above method. Moldman magazine WeChat is worth watching! In particular, it is more necessary to pay attention to appropriately increasing the gate and the proper injection time for resins with poor fluidity (such as polyoxymethylene, PMMA resin, polycarbonate, and PP resin).
Fourth, shrink pit
The reason for the shrinkage pit is also the same as the filling deficiency. In principle, it can be solved by excess filling, but there is a danger of stress. The design should pay attention to the uniform wall thickness, and the ribs and studs should be reduced as much as possible. Wall thickness.
Five, overflow
The focus of processing on the overflow should be mainly on the improvement of the mold. In terms of molding conditions, it is possible to reduce the fluidity. Specifically, the following methods can be used:
1) Reduce the injection pressure.
2) Lower the resin temperature.
4) Use high viscosity grade materials.
5) Reduce the mold temperature.
6) Grinding the mold surface where the overflow occurs.
7) Use a harder mold steel.
8) Improve the clamping force.
9) Adjust the joint surface of the accurate mold and so on.
10) Increase the mold support column to increase rigidity.
Ll) Determine the size of different exhaust slots according to different materials

plastic molds

Six, weld line
The weld line is caused by the front end portion of the molten resin from different directions being cooled and not being completely fused at the joint. Under normal circumstances, it mainly affects the appearance and affects the coating and plating. In severe cases, it has an effect on the strength of the product (especially in the case of fiber reinforced resins). Can be improved by referring to the following items:
l) Adjust molding conditions to improve liquidity. For example, increase the resin temperature, increase the mold temperature, increase the injection pressure and speed.
2) Adding a venting groove, and providing a push-out rod at the place where the weld line is generated is also advantageous for exhausting.
3) Minimize the use of release agents.
4) Set the process flash as the place where the weld line is formed, and then cut and remove after molding.
5) If only the appearance is affected, the four positions can be changed to change the position of the weld line. Alternatively, the portion where the weld mark is generated may be treated as a dark glossy surface or the like.
Seven, burns
The solutions adopted are different depending on the burn caused by mechanical, mold or molding conditions.
1) Mechanical reasons, for example, due to abnormal conditions, the barrel is overheated, the resin is pyrolyzed, burned, injected into the product, or the resin is stagnant due to the nozzle and screw thread, check valve, etc. in the material. Disintegrated and discolored, brought into the product, with dark brown burn marks in the product. At this time, the nozzle, screw and barrel should be cleaned.
2) The reason for the mold is mainly due to poor exhaust. This type of burn usually occurs in a fixed place and is easily distinguished from the first case. At this time, attention should be paid to measures such as adding a venting groove to the exhaust rod.
3) In terms of molding conditions, when the back pressure is above 300 MPa, the barrel is partially overheated, causing burns. When the screw speed is too high, overheating will also occur, generally in the range of 40 to 90 r/min. When there is no venting groove or the venting groove is too small, the injection speed is too high, which may cause burnt of the overheated gas.
Eight, silver line
The silver wire is mainly caused by the hygroscopicity of the material. Therefore, it should generally be dried under conditions of 10 to 15 C lower than the heat distortion temperature of the resin. For the more demanding PMMA tree wax series, it needs to be dried for 4-6 hours under the condition of about 75t). Especially when using the automatic drying hopper, it is necessary to select a reasonable capacity according to the molding cycle (forming amount) and drying time, and it is also necessary to start the baking before a few hours before the start of the injection.
In addition, too long a material stagnation time in the material will produce silver lines. When mixing different kinds of materials, such as polystyrene and ABS resin, AS resin, polypropylene and polystyrene, etc., it is not suitable to mix.
Nine, spray pattern
The jet pattern is a trace that curves like a meander from the gate along the flow direction. It is caused by the high injection speed of the resin starting from the gate. Therefore, expanding the cross section of the four burners or lowering the injection speed is an optional measure. Further, by increasing the temperature of the mold, the cooling rate of the resin in contact with the surface of the cavity can be slowed down, which also has a good effect in preventing the formation of the surface-hardened skin at the initial stage of filling.
Ten, warping, deformation
The warpage and deformation of the injection product is a very difficult problem. The main problem should be solved from the mold design, and the adjustment effect of the molding conditions is very limited. The causes and solutions for warpage and deformation can be referred to the following:
1) When deformation is caused by residual stress caused by molding conditions, stress can be relieved by reducing the injection pressure, increasing the mold and making the mold temperature uniform, and increasing the resin temperature or by annealing.
2) When the stress is deformed due to poor mold release, it can be solved by increasing the number or area of ​​the push rod and setting the draft angle.
3) Since the cooling method is not suitable, if the cooling is not uniform or the cooling time is insufficient, the cooling method and the cooling time can be adjusted. For example, a cooling circuit can be placed as close as possible to the deformation.
4) For the deformation caused by the molding shrinkage, it is necessary to correct the design of the mold. Among them, the most important thing is to make the wall thickness of the product consistent. Sometimes, in the case of a last resort, the mold must be trimmed in the opposite direction by measuring the deformation of the product. A resin with a large shrinkage ratio is generally a crystalline resin (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin) than a non-crystalline resin (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin). And AS resin, etc.) have large deformation. In addition, since the glass fiber reinforced resin has fiber orientation, the deformation is also large.
Eleven, whitening
The whitening phenomenon mainly occurs in the introduction part of ABS resin products. The poor release effect is the main reason. It can be improved by reducing the injection pressure, increasing the draft angle, increasing the number or area of ​​the push rod, and reducing the surface roughness of the mold. Of course, the spray release agent is also a method, but care should be taken not for the subsequent process. Such as hot stamping, painting, etc. have adverse effects.
Twelve, air bubbles

plastic molds
According to the cause of the bubble, the solution to the problem is as follows:
1) When the wall thickness of the product is large, the outer surface is cooled faster than the center portion. Therefore, as the cooling progresses, the resin at the center portion is expanded toward the surface while being shrunk, so that the center portion is insufficiently filled. This condition is called a vacuum bubble. The main solutions are:
a) Determine the proper gate and runner size based on wall thickness. Generally, the height of the gate should be 50% to 60% of the wall thickness of the product.
b) Until the gate is sealed, a certain amount of supplementary injection material is left. C) The injection time should be slightly longer than the gate sealing time. d) reduce the injection speed, increase the injection pressure, e) use a material with a high melt viscosity grade.
2) The bubbles caused by the generation of volatile gases are mainly solved by:
a) Adequate pre-drying.
b) Lower the resin temperature to avoid decomposition gases.
3) Bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and the mold and increasing the injection speed.
Plastic molds are no longer afraid of failure!

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