Definition: Burn 1. To undergo rapid combustion. 2. To undergo combustion: oxidize. 3. To become charred
Burning of plastic material within the mold may be a misnomer and or misunderstanding. We all understand that we have a burn mark on the part. The burn is typically at end of fill or an area of non-venting, or trapping of gas. This in effect should be the first clue as to what is burning. The end result is as always a black or brown mark on the plastic article itself.
Given the above statement the actual burning is in fact dieseling. Much like a diesel engine that compresses a fuel and upon compression and temperature it ignites, what happens in the mold is that we are compressing gases both air trapped in the mold and gases coming off our plastics and in doing so very rapidly this combination than ignites. The results are carbon residue (char) on our plastic article and a very pronounced mark within the tool itself. If we continue to produce products with burning in them, the tool can be damaged in that this ignition process erodes the steel (mould).
If burning is noticed the typical molders response is to slow down the fill rate of the cavity. This due to the fact that the mold is in the press and everyone is expecting parts. The results may or may not be acceptable as we may now not have a burn but only a blush or non contact of the tool face in this area, which may even be a higher gloss. Also from a quality standpoint having changed the fill rate the stress within the part may have changed thus the performance of the parts in the field, may be affected.
The real solution is to vent the tool, and or let the tool breath which is both the same. This can be easy and or difficult dependant of where the issue is, and how the tool was constructed.
Types of burns:
1- End of fill
a. Meaning that it is last pace to fill in the mold.
i. Look at vents,
ii. If slowing down speed eliminates than adding vents and or venting runner should help.
iii. Look at clamp tonnage and how much is used and how much is actually necessary
2- Within the part
a. Meaning a rib and or other detail not necessary at end of fill.
i. Watch short shot progression, are there traps
ii. Look at vents within runner
iii. Look at tool design, is area capable of accepting a vent
iv. Is a vacuum necessary for the mold cavity
3- At gate area
a. Meaning we have streak starting at the gate and than tapering off
i. Watch fill speed
ii. Is there a hang-up in the gate area
iii. Size of gate versus shear rate of material going through it.
b. Note these areas are not necessary dieseling but are degradation of the actual polymer.
4- Within runner and or part but more of a streaking issue that is not consistent but may appear similar
a. These again are similar to #3 but are created in the barrel or shot generation of the press.
b. Causes can be a cracked check ring, clearance issues within the check ring, and hang-ups in the nozzle check ring area.
5- At a knit line
a. As above do we trap gas?
b. If adding color does it happen in natural?
c. If adding color does it only occur with certain colors
i. Both b, c refer to temperature stability of colorants
The above are but a few of the issues and places to check. Additives, velocity transfer points and other factors can lead to burning in all areas of the part.
An example that I encountered too many years ago involved a clear part. Upon starting up the job we had a clear runner and parts. But when going in to optimize the process we noticed burn and than completely black parts, but not brittle parts. At first we assumed that it was contamination, but than upon slowing down injection speed all cleared up. What was happening was that the rubber component was degrading via shear through the gating system. The real clue was that the runner was clear.
SL Silvey
360-882-3183
