CLPEX Chat Board

Extrusion or Sprue Marks

When I started out we sometimes received marks from plastic items that we called extrusion marks. I have learned recently that some LPE call these ‘Sprue Marks’. It really does not matter what they are called as they are still interesting.

I was told and believe to some extent that they were formed when the plastic item is forced into the mold that either the mold was not the proper temperature or the plastic material was not the optimum temperature and the wave form is produced.

The following were recovered from light switches, water faucet knobs and shelf brackets.

Would be interested in hearing anyone’s thoughts or ideas on ‘Extrusion or Sprue Marks’ or even some additional photos that could illustrate the same!

A colleague was kind enough to give me this piece of milk crate showing these sprue marks.



If anyone needs to see a larger image, flip me a message and I'll send it along via e-mail.

Shane Turnidge

Sprue is the wrong word. A "sprue" is the passage through which the material is poured or injected to reach the void that will be filled to form the object. And I don't think that "extrusion" marks does it, since these objects aren't formed by extrusion but are injection molded. The "wings," the flat parts from flows out of the mold void are "flash."

See if this description of an injection molding flaw and its cause sounds right"

Orange peel’s origin
“Orange peel” or a rippled surface defect typically occurs at the end of the flow path in thick-walled parts molded of high-viscosity materials. During injection at low velocity, solidification occurs on the surface too quickly. The high resistance to flow produces uneven frontal flow, and the solidified outer layer will not fully contact the cavity wall. The result is ripples. These ripples will freeze and holding pressure will no longer be able to smooth them out. The solution is to raise the melt temperature and increase the injection speed. Sounds right

Another writer calls them surface wrinkles. Same cause. Another calls them "orange peel or surface ripples." So you can calls 'em orange peel, or you can calls 'em ripples, but you can't calls 'em sprue.

Gerald, I did not like ‘Sprue’ either but that is just what I was told. As for ‘Extrusion’ that is just the word the ole Sarge used to call them but he often got words mixed up. He pronounced ‘Aluminum’ like the British do ‘Al-li-min-e-um’ and I guarantee that he had never been to England. So dropping those two words is no big deal.

I personally like “ARF” Anomalous Ridge Feature as it would also go well with Anomalous Ripple Feature. We have an ARF over here an ARF over there, everywhere there is an ARF ARF. Kind of comes after ACE too.

For those that are more adventurous it could be called a BASIC ANOMALOUS RIDGE FEATURE (BARF).

As for ‘Orange Peel’ I already use that one in rock tumbling when some soft rock is tumbled and flakes peel off of it. It is said to be orange peeling.

I also want to say you are the best researcher I know.

More ARF’s are to be presented in the future.

`Plastic which has been cooled by sprues and runners is further cooled in the cavity and filling occurs at high viscosity; consequently, plastic in contact with the mold surface is pressurized in a semi-solidified condition and stripes perpendicular to the flow direction are formed on the surface of the molded product.'

The website did not have clear and distinct illustrations, but they were representative of what is being discussed.

BTW, these 'friction ridge' features were described as 'surface stress patterns' in an article (very brief in content) by Mark Wojcik in Fingerprint Whorld (20:76), 1994.

The 'Orange Peel Effect", while a form of molding distortion, is not in this category.

I believe the marks seen in electrical tape (which has polymer components) are not 'flow marks' (as described above), but perhaps caused by stress factors when the tape is manipulated. While not being "Schallamach" in the original context as being formed by stress from friction, they could be closely related in being formed '90 degrees' to the direction of force/stress.

Call them what you want, but not friction ridges.

Perhaps Sprue is the wrong word but I too don't think "Orange Peel" is appropriate either.
If Ernie is right and these features form 90 degrees to the direction of the force then they more than likely are some form of compression wave.
The thing that interests me most about these features is their prevalence in nature. I've been trying to find a name that applies to phenomenon such as sand ripples, certain cloud formations and indeed, these marks found on plastics.

If anyone has any thoughts in this regard I'd love to hear them.

Shane

While the plastics people seem to understand the mechanics of it and the nature as forming as a mold surface cools the moving front of molten plastic, their term "orange peel" was poorly chosen, since nothing about it really resembles an orange skin. The "flow" of the ripples and their symmetry where the shape of the object suggest a symmetrical mold look very much like the kind of temperature distributions seen when a hot irregular object it heat-imaged. They also remind me of the flow of stress lines I see in poorly annealed glass when viewed by transmitted polarized light and a polarized viewer. And that, too, is a matter of heat distribution, uneven cooling. A concentric circular pattern would be consistent with injection perpendicular to the opposite mold surface. While it's true that they will form 90 degrees to the direction of flow, that flow will be distorted as different portions of it's front strike cool portions of a mold at different times. And of course, depending on the point at which the melt is injected, the flow front itself may be expand radially from that point. So, these patterns can be complex.

I guess if I had to start from nothing and coin a term, I'd try "injection ripple" or "thermal error ripple." It would be interesting if a plastic surface bearing this ripple defect left its own impression and you had to argue its formation and uniqueness.

(How much do we have to talk about this here before we would be qualified to testify to that as "experts?" )

I brought this one back to the top, because 'I like it'. Also it might help with the other thread on the glove prints.

Since this thread and the other on 'S-Waves' several people have sent me some additional examples. I need to drop them on photo bucket and get them posted.

We shall see.

Any more comments on wave forms other than friction ridges----would be welcome-----either pro or con-----I will take them both.

Come On----Tell The World What You Think!