How to deal with the whitening and shrinkage of plastic injection molded parts

For injection molded parts, shrinkage and whitening are generally a pair of conflicting problems. Usually, part shrinkage is caused by low cavity pressure and insufficient injection of parts. And top whitening is usually caused by high cavity pressure and over-injection. Logically, this pair of happy enemies should be mortal enemies and never get along with each other. But such a strange thing happened.

Because the mold will produce spray at the gate, the technician uses a slow speed to pass through the gate. The actual time for the plastic to pass through the runner and gate is 2.8 seconds, and the total injection time is 5.7 seconds. The filling speed is very slow.

The mold fixed mold uses mold temperature, the actual temperature is 60℃, and the dynamic mold uses cooling water, the actual temperature is 28℃.

I tried the following:

Method 1: Increase the holding pressure of the first stage and speed up the holding speed to improve the peripheral shrinkage, and reduce the holding pressure of the second stage to improve the top whitening problem, but there is no effect.

Method 2: Delete the 4th and 5th speeds of the injection, and directly use the 3rd speed to quickly fill to the 20mm holding pressure switching point, while increasing the holding pressure and slowing down the holding speed. This allows the plastic to shrink slowly, so that the pressure does not rise sharply. But it has no effect.

Method 3: Based on method 2, the cooling water of the movable mold is changed to mold temperature, and the actual temperature is about 40℃. The improvement effect is obvious, there is no white top and no shrinkage. And the molding cycle is not extended.

Cause analysis:

The diameter of the part is 180mm, but the mold gate thickness is only 0.9mm, the width is 3.5mm, and the speed through the gate is very slow, and the total filling time is 5.7 seconds. In addition, the movable mold is cooling water, which leads to the melt solidifying at the gate in advance during the shrinkage and holding stage before the plastic pressure is effectively transmitted to the periphery of the part.

In addition, because the movable mold is connected to cooling water, the plastic cools very quickly, and the rapidly shrinking plastic holds the mold core with greater force, making demolding more difficult. In addition, the ribs in several places of the movable mold also increase the resistance to demolding.

The thin gate, slow filling and relatively low mold temperature cause a large loss of plastic pressure, which leads to a very uneven distribution of pressure in the mold cavity, which is larger around the gate and smaller around the periphery. The superposition of these situations causes the parts to have relatively conflicting defects such as shrinkage and whitening.

Improvement measures:

Once the cause of the problem is figured out, it is not difficult to formulate improvement measures.

Properly increasing the temperature of the movable mold can reduce pressure loss and make the cavity pressure distribution more uniform. This not only prolongs the freezing time of the gate, effectively transmits the plastic pressure to the periphery of the part, but also slows down the cooling speed of the plastic, reduces the shrinkage holding force, and makes demolding easier.

In addition, the gate thickness and width should be appropriately increased (the gate thickness is usually 80% of the wall thickness) to increase the cross-sectional area through which the plastic passes.

Of course, increasing the draft angle of the ribs and polishing them is also helpful.