Tech Talk Blog

Q: Quick question for you in regards to a plasma system cleaning parts. Is there such thing as a part being too dirty? What happens if we have a part that has been soaked in CNC lube or even worse dipped in oil? Can you give me a rough idea of the limitations that a plasma system in regards to dirty parts?

A: Yes. A part can be too dirty for plasma to be effective. A good rule-of-thumb is ; If you can wipe if off with your thumb, it is too dirty. In other words… if a part has so much oil or contaminate that it can be moved with your finger, the part needs to be, at least, wiped down. When a part has excessive contamination it takes a long time for the plasma to completely remove all the contamination. The plasma can certainly remove all the contaminate, but it is doing it one molecule at a time. So, the thicker the oil layer the longer the time.

So, it is a good idea to wash the parts before plasma. But one may ask, if I have to wash them, why do I need plasma? The answer is: the wash cannot remove all the contaminate, because the bath always has some level of contamination. The plasma chamber never can have contamination, because all the contaminates are converted to gas and pumped away.

Have a look at our Materials Resource Guide and our TriStar site, too.

Question: How do I decide whether I should injection mold my plastic parts or machine them?

Answer: The fast answer is usually based on volumes. If you can justify the cost of an injection molding tool, which can run from a few thousand dollars to tens of thousands of dollars, then the decision comes down to some other key points.

1. Are the tolerances moldable? Depending on the material, types of fillers and geometry of the part  it may not be a part that is conducive to molding. Holding tight tolerances is difficult at best and may require machining.

2. The geometry of a part may require variations in wall thickness, i.e. heavy and thin sections across the part. Wall variations that are not properly blended could lead to internal stresses, distortion and eventual cracking.

3. Draft or taper is generally required in molded parts due to part ejection needs. If your part requires close straightness or parallelism it may not be attainable by molding but is through machining.

4. Internal stress is much less prevalent in machined parts since the stock shapes are stress relieved prior to machining. Additional stress relief can be done in mid or post machining if so needed. Stresses from molding are more prone to warpage, especially high temperature applications with high end materials.

5. Surface requirements of a part are better suited to a machined part over a molded part. Sprue/gate marks could leave a blemish or a flat spot that can’t be repaired. Molded parts could also have sink areas and weld lines that infringe on the part finish.

6. Design flexibility is much greater with machined parts versus molded. Once the tooling is made, changes to the design and subsequent tool modifications can get very expensive. Machining parts can also give you the freedom to change materials based on performance requirements. Since tools are designed for specific shrink rates it could also lead to major costs should a material change be needed on the part.

Machined versus molded? Definitely issues that need to be reviewed and TriStar is available to help.

Q: I have a question for you. We are anodizing some aluminum parts per Mil A 8625F, Type 2, Class I and it appears to be getting somewhat poor results. I wanted to get your take on this surface chemistry that we are trying to bond to. Here are some brief notes. Hopefully this is enough info for you to comment on.

Mil A 8625F, Type 2, Class 1Type II: Sulfuric Acid anodizing Class 1: This means that the anodize is not dyed or pigmented.

There is also a secondary operation of sealing:

per Mil A 8625F, Type 2, class 1, Section 3.8.1:

“When class 1 is specified, sealing shall be accomplished by immersion in a sealing medium such as a 5 percent aqueous solution of sodium or potassium dichromate (ph of 5.0 to 6.0) for 15 minutes a 90C to 100C, in boiling deionized water, cobalt or nickel acetate, or other suitable chemical solutions…”

We are trying to bond a peroxide cured silicone to this treated aluminum surface. I am not familiar with the sealing process as described above. Can you give me some insight on this?

A: Sulfuric anodize, commonly referred to as Type II anodizing, is formed by using an electrolytic solution of sulfuric acid at room temperature and a current density of 15 to 22 Amps per square foot. The process will run for 30 to 60 minutes depending on the alloy used. This will produce a generally clear coating, depending on sealing, a minimum of 8µm thick. One third of the coating thickness will build up per surface and 2/3 will be penetration. Sulfuric anodize coatings are often sealed to enhance corrosion resistance, lock in dyes, or both. Hot water seals produce the clearest sulfuric anodize while sodium dichromate yields a yellow-green appearance but is generally a better seal. Sulfuric anodizing is rather tolerant of aluminum alloys for anodizing with the exception of high-silicon die-cast alloys such as 380. The less alloying elements there are the higher the clarity and depth of color of the anodize coating.

The best adhesion is reached when each bonding aluminum atom carries a single reactive group. Even at room temperature, acetic acid or fatty acids can easily pass through a coating infiltrating chemical bonds by advanced attack and leading to adhesion failure. The aluminum atom progressive binds with reactive groups which then leave and the bond fails.

Hope that helps! If you want your questions answer – Ask The Experts!