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How to avoid Fissure and Burn Mark in your Plastic Production

Oct 2, 2018 9:22:00 PM

After analyzing the Short Shot and 10 other plastic production issues, we decided to come back to some other problems you can encounter during your injection molding plastic production. Specifically, we'll talk about fissures and burn marks in this article.

The fissure

After injection molding, the surface of the plastic parts can get cracked in a number of different ways. These are called fissures.  Some of the results can be seen in the picture below

fissure on white plasticfissure on black plastic

Now let's have a look to the reasons of that fissure and the solutions:

1. High residual stress.

In the case of high residual stress, the direct gate with minimum pressure loss and high injection pressure can be adopted in mold design and manufacture. The forward gate can be changed into multiple pinpoint gates or side gates while also the gate diameter can be reduced. When a side gate is designed, a protruding gate that can be removed after forming can be used. In the process operation, reducing the residual stress by lowering the injection pressure is the most simple method because the injection pressure is proportional to the residual stress. The temperature in feed barrel and mold should be increased appropriately while the temperature of melt and mold should be reduced, the cooling time and speed of billet in mold should be controlled, and the recovery time of oriented molecular chain should be longer.

2. External forces lead to the concentration of residual stress.

In general, such defects always occur around the top bar. After the occurrence of such defects, the ejection device should be carefully checked and calibrated, and the ejection bar should be set at the place with the maximum demolding resistance such as at the convex platform or the reinforcing bar. If the number of jack sets is impossible to be expanded due to the constraint of the pushing area, the method of multiple jacks with a small area can be adopted. If the draft angle of the mold cavity is not enough, the surface of the mold may also scratch and form a folding pattern.

3. The thermal expansion coefficient of forming material is different from that of the metal insert.

The metal insert should be preheated, especially when the cracks on the surface of the plastic parts are released at the beginning of an operation. Most of these are caused by the low temperature of the insert. In addition, in the selection of insert material and the line expansion coefficient should be as close as possible to plastic characteristics of the material. When selecting molding materials, high molecular weight plastics should be used as much as possible. If low molecular weight molding materials must be used, the thickness of plastics around the inserts should be designed to be thicker.

4. Improper or impure material selection.

Our experience shows that loose plastics with low viscosity do not crack easily. Therefore, in the production process, we should focus on choosing the appropriate molding raw materials. In the process of operation, special care should be taken not to mix polyethylene and polypropylene and other plastics since this makes it easier to create cracks. During the molding process, the mold release agent is also a foreign body for the melt. If the dosage is not appropriate, it will also cause cracks, so the dosage should be reduced as far as possible.

5. Poor structural design of plastic parts.

Stress concentration is most easily generated in the sharp angle and the gap in the plastic part structure. These designs lead to cracks and cracks on the surface of the plastic part. Therefore, the outer angle and inner angle of the plastic part structure should use the maximum radius as far as possible to make a circular arc. Our testing shows that the ratio of the optimal arc radius to the wall thickness of the corner is 1:1.7.

6. The cracks in the mold are reflected onto the surface of the plastic part.

In the process of injection molding, due to repeated injection pressure on the mold, the edge of the cavity with an acute angle will produce fatigue cracks -- especially near the cooling hole. When the cracks on the surface of the mold cavity are reflected on the surface of the plastic parts, the cracks on the plastic parts will always appear in the same shape in the same place in each shot. When this crack appears, it should be immediately checked for the same crack on the surface of the cavity corresponding to the crack. If the crack is caused by the resetting effect, the mold should be repaired by mechanical processing.

Experience shows that the products of PS and PC are easy to crack. The crack caused by excessive internal stress can be eliminated by annealing.

The causes of cracks in the plastic parts and the improved methods are shown in the following table

Reason analysis Improving method
Too high injection pressure or too fast terminal injection speed Reduce injection pressure or terminal injection speed
The pressure is too high or the pressure is too long to Reduce pressure keeping pressure or shorten time keeping the pressure
Melting temperature or mold temperature is too low/uneven Raise melt temperature or die temperature (can be formed with less injection pressure) and make mold temperature uniform
The gate is too small and the shape and position are not suitable Increase gate size, change gate shape and position
The parting Angle is not enough, the mold is not smooth or inverted Increase draft Angle, polish mold, eliminate backoff
Thimble is too small or not enough Increase thimble or increase thimble number
It's going too fast Slow down the ejection speed
The temperature of the metal insert is low Preheat the metal insert
Excessive proportion of water outlet material reuse Reduce the proportion of feed water or do not recycle
Too much stress on the mold Control or improve internal stress, annealing treatment
Poor mold exhaust (trapped air) Improve the effect of mold exhaust, reduce burning

 

The Burn Mark

What is Burn Mark

If, during the process of injection molding, any of the following occurs :

  1. the injection mold venting is not good or too fast,
  2. there is air in the mold before discharge,
  3. the air is under instantaneous high-pressure compression,
  4. the plastic rapidly expanding (extreme temperature can be as high as 300 °C) or
  5. will melt burning yellow,
a burning phenomenon in some places will occur as in the the picture following:

burn mark on plastic 

The reasons and The solutions

Reason analysis Improving method
Too fast terminal injection Slow down the final injection rate
Poor mold exhaust Increase or increase the vent (vacuum injection molding)
Excessive injection pressure Reduction of injection pressure (reduction of compression)
Excessive melting temperature (viscosity reduction) Reduce melt temperature and fluidity
The gate is too small or in the wrong place Change gate or position (change exhaust)
Poor thermal stability of plastics (easy to decompose) Switch to plastic with better thermal stability
Excessive clamping force (the exhaust seam becomes smaller) Reduce the clamping force or edge - clamping injection
The vent or discharge needle is blocked Clean the stain or clean the thimble in the vent

 

 

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More content on plastic production issues and how to avoid them:  

How to avoid deformation in your plastic production

How to avoid silver mark and splay mark in your plastic production 

How to avoid short shot in your plastic production

How to avoid color streaks and gloss difference in your plastic production

How to avoid bulges and vacuum vesicles in your plastic production

How to avoid a weld line in your plastic production

How to avoid flashing and bubbles in your plastic production

How to avoid jetting and air lines in your plastic production

 

 

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