Reason Analysis of Crack, Microcrack and Whitening in injection Molding

May 29, 2018

 

 

Cracking, including surface filamentary cracks, microcracks, top white, cracking and risk of injury caused by die and channel adhesion, demoulding and application cracking according to cracking time. Description: injection molded parts break when ejected, or easy to break or crack in treatment. The analysis is as follows:

 

1.The plasticizing capacity of injection molding machine should be appropriate, too small plasticizing is not fully mixed and brittle, and too large will degrade.

 

2, die:

1)to balance the ejection, such as the number of rod, cross-sectional area to be sufficient, mold slant enough, cavity surface must be smooth enough, so as to prevent the force caused by the top out of residual stress concentration and open. crack.

 

2)the structure should not be too thin, the transition part should be arc transition, avoid sharp angle, chamfer cause stress concentration.

 

3) Use less metal inserts to prevent internal stress from increasing due to different shrinkage rates.

 

4)In order to prevent the formation of vacuum negative pressure, an appropriate inlet air channel should be set for deep bottom parts.

   5)the main channel is sufficient to release the mold before curing, so it is easy to demoulder.

   6)Main-way bushing and nozzle joint shall prevent cold-hardened materials from dragging and the parts to be stuck to the die.

   7)the runner and gate are too small to produce excessive shear rates in the filling of the die, Use a full circle runner and increase the size of the runner and gate to provide an acceptable shear rate of 3 during the filling stage of the die.

 

Technical aspects:

1)too high injection pressure, too fast speed, more filling, too long injection time, Will cause excessive internal stress and cracking.

2)Adjust the opening speed and pressure to prevent the rapid drawing parts from cracking.

3)properly raise the mold temperature, limit the speed of the cooling body flowing through the mould, make the parts easy to unmold, the plastic degrades in the ejection cylinder, causes the plastic molecular structure to break down, reduces the temperature of the projectile cylinder and reduces the pressure back in all districts, Use exhaust cartridges to ensure discharge The holes run correctly and each hole sets the correct temperature.

4) Prevent cracking due to weld marks, degradation of plastics resulting in lower mechanical strength.

5)Appropriate use of release agent, pay attention to the elimination of mold surface adhere to the aerosol and other substances.

6)The residual stress of the workpiece can be reduced by annealing and heat treatment immediately after forming.

7)The melt glue temperature is too low, the rear zone and nozzle are heated on the ejection cylinder, and the screw speed or high speed rotation is reduced to obtain the correct screw surface speed.

8)The filling speed of the mold is too slow to increase the injection speed; the stable pad is maintained on the injection molding machine.

9)The pressure of the injection parts is not properly released or untreated, the injection molded parts are annealed, if the nylon plastic products are immersed in warm water

 

4.the material is:

 

   1.the recycled material content is too high and the strength of the parts is too low, Reduce the amount of recycled materials mixed with new plastics.

  2.excessive humidity causes chemical reaction between some plastics and water vapor, which reduces the strength and results in cracking at the top.

 3.Material itself is not suitable for processing environment or poor quality, contamination can cause cracking, heterogeneous plastic impurities, check plastic impurities; thoroughly clean the ejection cylinder; the hopper or material loading machine to remove and through Some of the moulded parts of the bottom are not partially broken, but the surface produces fine cracks. Depending on the degree and appearance of the cracks, the more serious ones are called cracks or cracks, and the lesser ones are called microcracks or cracks. Crack looks very much like a crack, but it is essentially two different meanings, that is, crack is not a void like defect, because the added stress is in parallel direction of the polymer itself, so if it is heated, it can return to the state of no crack. Crack and crack can be distinguished by this method. Microcracks occur not only after forming, but also when placed or in contact with solvent vapors. ABS and impact resistance Polystyrene does not produce this phenomenon at all, but it appears in the form of whitening at the top part of the top push, and the phenomenon of whitening can be eliminated by using hot air heating. There are two reasons for this phenomenon. Whitening refers to the whitening of stress. It is known from the process of stress yield curve and crack whitening on the graph that it really takes energy. After whitening, cracks will occur if only a small amount of energy is added.

  1. When the mold is not unsmooth, the die-drawing inclination of the die is insufficient, which becomes the inclination of the backward drawing die, or the rough polishing of the die cavity convex die is extremely poor, so it is difficult for the parts to be demoulded, so that the push force is too large and the stress is produced, which leads to the breakage or whitening of the parts. Due to the master It is easier to produce this phenomenon when the sprue polishing is not good enough for the parts to stick to the static die or the moving die, when the side wall is convex and concave and the hard demoulding is adopted. In short, when this kind of defect occurs, we should first pay attention to the polishing of the die, and add the top rod near the crack place of the forming parts at the same time to increase the angle of drawing die, so that the parts can not bend and get off the mould reasonably. For polymethyl methacrylate (PMMA) forming parts, the plastic itself is brittle and the surface needs luster, so most of them are molded by chromium plating mould. However, electroplating has the property of not easy to be coated on the plane, but it is easy to be overlaid to the corner, so it becomes the inclination of the reverse drawing die, so it must be paid special attention to. Besides, Since polycarbonate PC-PVC and other materials are easily glued to the chrome plating layer of the mould, especially in the corner, it is necessary to pay attention to the inverted cone. Polycarbonate PCs are prone to crack in die inserts. In cases like this, it is best to add reinforced fiberglass to the material. Products with residual stress will produce cracks due to solvent, oil, and medicine. The angle of demoulding should be sufficient, the pin should be distributed evenly, the product should not be designed with sharp angle, and the thickness difference of the product should be avoided as far as possible. Just take a closer look at the crack. To determine the cause.

  2. Over-filling is caused by too much concern about the forming shrinkage, resulting in too much molten material in the injection cavity, resulting in a great strain inside the forming parts. The shrinkage is very small, not only easy to crack, but also easy to cause microcracks after a period of time. In order to eliminate overfilling cracking, the melting material temperature can be raised, the injection pressure can be reduced, and the mold temperature can be raised, but only if the melt material is easy to be injected into the cavity can be guaranteed. According to the appearance of forming parts and other related reasons, in order to avoid microcracks when forming parts must be overfilled, post-treatment should be carried out after forming (such as heat treatment, which can eliminate internal cracks). Change is effective.

  3. When the cooling is not sufficient, the parts will be ejected at the end of complete hardening, sometimes cracking or whitening around the top. This defect can be eliminated by adequate cooling or by changing the cooling mode of the mould itself.

   4. The stress is concentrated around the insert because of the shrinkage of plastic. Although this force can hold the insert firmly, the plastic around the insert tends to crack when the stress is too high. It is easy to produce stress and crack only after a period of time when the metal is embedded in the injection molding. This is mainly due to the thermal expansion of metals and resins The difference of expansion coefficient produces stress, and over time, the strength of resin material should exceed that of the gradually deteriorating resin material, resulting in cracks. To reduce the cracks around the inserts, it is effective to preheat the inserts or minimize the shrinkage difference. Plasticizing is more effective (I. e., pre-heating the metal inserts, which alleviates the residual stress in the molding, the same principle, It is also a method to replace the block heating with the annealing after forming.

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