Poor plastic phenomenon and solution

- Mar 06, 2018-


  Cracking is a more common plastic defects, the main reason is due to stress and deformation. Mainly residual stress, external stress and stress caused by the external environment deformation.

  (A) Cracks caused by residual stress

  Residual stress is mainly due to the following three conditions, namely overfilling, mold release and metal inserts caused. As a result of cracks in the case of excess filling, the solution can be mainly in the following aspects:

  (1) As the straight sprue pressure loss is the smallest, so if the crack occurs mainly in the vicinity of sprue, you can consider the use of multi-point distribution point gate, side gate and stalk gate way.

  (2) In the premise of ensuring that the resin does not decompose, not deteriorating, appropriately raising the resin temperature can reduce the melt viscosity and improve the fluidity, meanwhile, can also reduce the injection pressure to reduce the stress.

 (3) Under normal circumstances, when the mold temperature is low prone to stress, should be appropriate to increase the temperature. However, when the injection speed is high, even if the mold temperature is lower, can reduce the stress generation.

  (4) injection and dwell time will produce stress too long, its appropriate shortening or Th pressure relief switching effect is better.

  (5) Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, and other crystalline resins such as polyethylene and polyformaldehyde tend to generate residual stress and should be noted.

  Demoulding introduced, due to the stripping slope is small, mold-type plastic and punch rough, the launch force is too large, resulting in stress, and sometimes even in the launch of the bar around the phenomenon of bleaching or rupture. Just carefully observe the location of the crack, you can determine the cause.

  In the injection molding at the same time embedded metal parts, the most prone to stress, but also prone to cracking after a period of time, the great harm. This is mainly due to the large difference between the coefficients of thermal expansion of the metal and the resin, and the generation of cracks over time due to the stress exceeding the strength of the deteriorated resin material. In order to prevent the resulting cracking, it has been empirically known that wall thickness 7 "and the outer diameter of the insert metal are not suitable for use with inserts, and inserts have the least impact on nylon. Due to the glass fiber reinforcement Resin materials, thermal expansion coefficient smaller, more suitable for embedded parts.

  In addition, pre-molding metal inserts pre-heating, but also has good results.

  (B) external stress caused by cracking

   The external stress here is mainly due to the unreasonable design caused by stress concentration, especially in sharp corners more attention. As can be seen from Figure 2-2, it is advisable to use R / 7 "0.5-0.7.

  (C) the external environment caused by cracking

  Chemicals, water-induced moisture absorption, and excessive use of recycled materials can deteriorate physical properties and cause cracking.

  Second, insufficient filling

  The main reasons for the lack of filling the following aspects:

   i. Insufficient resin capacity.

   ii. insufficient pressure in the cavity.

   iii. Insufficient resin flowability.

   iv. Exhaust effect is not good.

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  As an improvement measure, we can start with the following aspects:

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  1) lengthen the injection time, to prevent due to the molding cycle is too short, causing the gate before curing the resin countercurrent and difficult to fill the cavity.

   2) increase injection speed.

   3) increase the mold temperature.

   4) increase the resin temperature.

   5) increase injection pressure.

   6) to expand the gate size. General gate height should be equal to the product wall thickness of 1/2 ~ l / 3.

  7) The gate is placed at the maximum thickness of the product wall.

   8) Set the exhaust groove (average depth 0.03mm, width 3 ~ smm) or exhaust rod. For smaller parts is more important.

   9) Leave a certain (about smm) buffer distance between the screw and the injection nozzle.

   10) Use low viscosity grade materials.

   11) Add lubricant.

  Third, wrinkle and ma face

  The cause of such defects is essentially the same as underfilling, but to a different degree. Therefore, the solution is basically the same as above. Especially for less liquid resin (such as polyacetal, PMMA resin, polycarbonate and PP resin, etc.) more need to pay attention to properly increase the gate and the appropriate injection time.

  Fourth, shrink pit

  The reason for shrinking the pit is also the same as that for underfilling. In principle, it can be solved by excess filling, but there is a danger of generating stress. The design should pay attention to the uniform wall thickness, and the stiffener Wall thickness.

  Five, overflow edge

  For the edge of the overflow should focus on the improvement of the mold. In the molding conditions, you can start to reduce the liquidity. Specifically, the following methods can be used:

  1) Reduce injection pressure.

  2) Reduce the resin temperature.

  4) Use high viscosity grade materials.

  5) reduce the mold temperature.

  6) grinding the surface of the mold surface occurred.

  7) The harder mold steel.

  8) increase the clamping force.

  9) Adjust the exact surface of the mold and other parts.

  10) Increase the mold support column to increase the rigidity.

  ll) According to different materials to determine the size of different exhaust slots.

  Six, weld marks

 The weld marks are generated by the fusion of the front end portions of the molten resin from different directions and incomplete fusion at the joint

of. Under normal circumstances, the main impact on the appearance of the coating, plating have an impact. Serious, have an impact on product strength

(Especially in the case of fiber-reinforced resins). Can refer to the following to be improved:

  l) adjust the molding conditions, improve liquidity. For example, increase the temperature of the resin, increase the temperature of the mold, and increase the pressure and speed of injection

Degree and so on.

  2) The addition of exhaust slots, set in the production of welding marks push rod is also conducive to exhaust.

  3) try to reduce the use of release agent.

  4) Set the process flash material and as a weld line where the formation, and then cut off to remove.

  5) If only affect the appearance, you can change the four burning position to change the location of welding marks. Or the weld lines generated from the treatment of dark shiny surface, etc., to be modified.

Seven, burn

  Depending on the cause of the burn, such as mechanical, mold or molding conditions, the solution taken is different.

 1) Mechanical reasons, for example, pyrolysis of the resin due to overheating of the cartridge due to abnormal conditions and injection into the product after burn

, Or because of the material inside the nozzle and screw thread, check valve and other parts of the resin caused by the stagnation, decomposition discoloration into products, products with a dark brown burn marks. At this time, should clean the nozzle, screw and barrel.

  2) die, mainly due to poor exhaust. This kind of burns generally occurs in fixed places, easy with the first

A situation difference. At this moment, we should pay attention to the measures such as adding exhaust vent exhaust bar.

 3) Molding conditions, the back pressure of 300MPa or more, it will overheat the barrel part, causing burns. Screw speed is too high, it will produce overheating, generally in the range of 40 ~ 90r / min as well. In the absence of exhaust groove or exhaust groove is small, the injection speed is too high will cause overheated gas burns.

  Eight, silver line

 Silver is mainly due to the hygroscopicity of the material. Therefore, the general should be lower than the resin heat distortion temperature 10 ~ 15C drying conditions. For demanding PMMA tree wax series, the need to 75t) under the conditions of drying 4 ~ 6h. Especially in the use of automatic drying hopper, you need to select a reasonable capacity according to the molding cycle (molding volume) and drying time, but also should start the boot drying several hours before the start of injection.

  In addition, the material within the Jane material stagnation will lead to silver too long. When different kinds of materials are mixed, for example, polystyrene and ABS resin, AS resin, polypropylene and polystyrene should not be mixed.

Nine, spray pattern

  Jet pattern from the gate along the flow direction, like a meandering curve. It is caused by the injection rate of resin starting from the gate too high. Therefore, the expansion of burning four cross-section or reduce the injection speed are optional measures. In addition, increasing the mold temperature also slows down the cooling rate of the resin in contact with the cavity surface, which also has a good effect of preventing the formation of a hardfaced skin during the initial stage of filling.

  Ten, warping, deformation

  Injection products warpage, deformation is a very difficult issue. Mainly from the mold design to solve, and the shaping conditions of the adjustment effect is very limited. Warpage, deformation causes and solutions can refer to the following:

  1) When residual stress causes deformation due to molding conditions, stress can be relieved by reducing injection pressure, raising the mold and making the temperature of the mold uniform, increasing the temperature of the resin, or annealing.

  2) Mold release caused by stress deformation, by increasing the number or area of putter, set the stripping slope and other methods to be resolved.

 3) As the cooling method is inappropriate, so that uneven cooling or cooling time is not enough, you can adjust the cooling method and extend the cooling time. For example, a cooling circuit can be placed as close as possible to deformation.

  4) For deformation caused by shrinkage, it is necessary to correct the design of the mold. Among them, the most important thing is to pay attention to make the product wall thickness. Sometimes, in the last resort, only by measuring the deformation of the product, in the opposite direction to trim the mold, to be corrected. Larger shrinkage resins, typically crystalline resins such as polyoxymethylene, nylon, polypropylene, polyethylene, and PET resins, are more effective than non-crystalline resins such as PMMA resins, polyvinyl chloride, polystyrene, ABS resins And AS resin, etc.) large deformation. In addition, since the glass fiber reinforced resin has fiber orientation, the deformation is also large.

  Eleven, bubbles

  According to the bubble causes, the solution to the following aspects:

  1) When the product has a large wall thickness, the outer surface of the product is cooled more rapidly than the center portion. Therefore, as the cooling progresses, the resin in the center portion expands toward the surface while shrinking, causing insufficient filling in the center portion. This is called a vacuum bubble. The solution is mainly:

  a) According to the wall thickness, determine the reasonable gate, runner size. General gate height should be 50% to 60% of the product wall thickness.

   b) until the gate seal, leaving a certain amount of supplementary injection material.

   c) injection time should be slightly longer than the gate seal time.

   d) reduce injection speed, increase injection pressure,

   e) A material of high melt viscosity grade.

   2) bubbles due to the generation of volatile gases, the solution is mainly:

   a) Fully pre-dried.

   b) Reduce resin temperature to avoid decomposition gases.

   3) bubbles caused by poor mobility, by increasing the temperature of resin and mold, injection speed to be resolved.

Twelve, albino

  The main occurrence of bleaching occurs in the introduction of ABS resin products. Poor mold release is the main reason. Of course, spray release agent is also a method, but should be careful not to follow-up process , Such as hot stamping, painting and other adverse effects.