What causes an injection molded part to develop a crack? As you know by now (if you’re reading our blog regularly), we need to analyze the problem “from the plastics point of view”. And you also know that the plastic knows just 4 conditions: heat, pressure, flow and cooling.
The location of the crack can tell us a lot about its specific cause. Here are some of the most common scenarios for cracks in injection molded parts:
Cracks in the Direction of Flow – If the crack develops in the direction of the plastic’s flow into the mold cavity, the cause is molecular orientation. In other words, the long-chain polymer molecules are “too aligned” for that particular part. So from the plastic’s point of view, they need less alignment to create strength in the direction of flow. How can we achieve this? Well, we can either change the fill rate (and the change could be increase or decrease), or we can reduce the cooling rate of the plastic. That would require an increase in mold temperature. In all cases, what you’re trying to do is allow the plastic molecules that have become highly oriented during the mold filling stage to lose some of their orientation. Slower cooling of the part does this by allowing them to become less oriented as they take longer to cool.
Internal Cracks – Another type of part cracking problem is internal cracks that initially don’t propagate to the surface of the molded part. This problem only occurs when using rigid plastics. In transparent parts, you will usually see these internal cracks before the molded part fails. The cause of this type of part cracking is internal stress caused by high plastic pressure during molding. This stress can be caused by a cavity pressure that is either too high or too low. The solution to an internal part cracking problem is to change the packing pressure.
Cracks in Corners – When cracks appear in the corners of plastic parts, the cause is too much plastic shrinkage onto the mold core in that corner. In many cases, you will also see that the part is hanging up on the core during ejection. The solution from the plastics point of view is to reduce shrinkage by increasing cavity pressure (pack more plastic into the cavity). Another possible solution is to eject the part sooner, before too much shrinkage takes place. This would require some trial and error to find the correct amount of plastic part cooling.
Surface Cracks – Surface cracks are tiny hairline cracks on the surface of a molded part that usually occur days, months or sometimes even years after the parts are molded. This type of cracking is caused by molecular orientation at the surface of the plastic part. There is always some stress on the surface of a plastic part. Over time, chemical attack or sunlight will reduce the strength of the plastic until the internal stress causes these cracks. Surface cracks that do not appear for a while after the part is molded are problematic. You have no way of knowing about them until they actually appear.
Cracking at the Gate – Cracks that appear near or at the gate location of a molded part are usually caused by molecular orientation. The gate area is an area where the highest fill rates occur. High fill rates results in more molecular orientation. The solution, again the way the plastic see’s things, is to reduce the retained molecular orientation. The mold designer may put a slight recess in the mold directly under the gate to allow the plastic under the gate to cool a little more slowly, giving this plastic more time to reduce its retained orientation as it cools. One other possible cause of plastic part cracking at the gate is high plastic pressure in the gate region. Allowing carefully controlled discharge will relieve that pressure.
Our Injection Molding Part Problems and Solutions course teaches your production personnel how to solve cracks and many other common injection molded part defects.