Definition: Cracking, formation of narrow separation in surface or wall. Usually result of rupture of chemical bonds.
Cracking within a part can be explained as a defect, and product failure. In many cases all aspects of the part production may be at fault. What is meant by this is that everyone from the designer of the part, to the tool maker to the molder and even material supplier contributed to the issue. Though this is not the case every single time it is in more than a few.
To explain cracking or more specifically breakage of plastic product one must study failures, and in so doing one needs to understand the process. An example of a material that is brittle is glass. As children many have broken windows, or at least put a hole through one, when one looks at the results of this broken glass one sees that we have fracture breaks or what is referred to as hackles. These hackles are the curved lines in the fracture zone which tend to radiate out. They tend to curve back to the origin. Thus if one were to look at a piece of broken plastic these same type of lines are evident. By tracing and following these lines back it is possible to determine where the breakage started.
This is not to say that much force was needed to start the break, it is just to state that the breakage or cracking in the part started at this point.
What this now does is focus attention to this point of the break or crack, and what one is looking for is a defect, which can or could be any of the following: sharp corners, inclusions, voids, contamination, scratches, flash, weld lines, flow lines etc.
Now in order to get though a lot of issues one may want to examine many parts and to establish if there is a pattern to the breakage? If so than we may have a tooling and or design defect within our part. Is the breakage random, not consistent in breakage, then we may need to look at degradation of materials.
If in fact the breakage is not consistent but tends to always be in same general area two of more things may be occurring, such as a minor design flaw and degradation of materials, or even wrong melt flow of materials.
A good example is Polycarbonate, a normally tough unbreakable material. Yet Polycarbonate does not like, sharp corners, likes to be dried properly, and does not like long residence time in barrels. Also as in one case when changing melt flow of the material there is drop off in impact when one goes to higher flow materials. In one case changing from an 18 melt flow to a 24 melt flow material grade caused parts to break consistently. Though molding and processing was easier, due to part design and function this grade with that melt flow could not be used.
From a design stand point sharp corners should be avoided, flow of materials into the part should be such that we are always flowing from thick to thin, most with a uniform wall. From the tool making side of things all inside corners in the plastic part should have a minimum of 0.005" radius, and if radiuses are placed into the tool for the part the junction between the radius and straight wall should be blended and no corners created.
From the processing the materials should be properly handled, dried properly to spec, and processed properly, this includes looking at residence time, pressures, and speeds. If coloring the proper carrier should be used.
While this may seem basic many times the processor, tool maker and designer can get away with things, and all well point to this fact that it did not break prior. But in some cases when everyone is trying to get away with things the part well fail. It is the total of all the little things.
In some materials such as HDPE over packing of the gate area can lead to cracking and brittle parts due to changing the density of the material in this area.
Questions to ask:
1- Do all the cavities break?
- Should be review on continuous shots, for a period of possibly 100 shots
- Map out results, it may be only a few that are consistent
2- If they do they break is it random or in same area?
- If random and no pattern look to a material degradation issue
- If always in the same area look to a tooling or design issue.
3- If adding color or additives run natural and note results.
- Problem goes away than is issue with additives/color.
- Problem stays same, design, tooling, and material.
Only a short list but a start for your own troubleshooting list.
SL SILVEY
01012012.01
