Tech Talk Blog

We’ve all been touched by the recent earthquake destruction in Haiti.  And our team is now seeing a renewed interest in surface treatments that can help building materials (like cement) resist seismic activity and sudden impact.

By adding specially treated polyethylene fibers to concrete mixtures, contractors are able to enhance the strength and durability of preformed structures.  With plasma treatment, our team can select the optimum gas chemistry and operating condition to improve the bond strength and interface toughness of ethylene fibers.  Treated fibers have a significantly improved bond strength compared to virgin, non-treated fibers.  With our plasma process, we can enhance the structural integrity of fiber cement mixtures used in buildings, bridges, and other superstructures.

If you have been machining metal for a while the change to plastics can be a little daunting. There are some tricks to the trade and some basic things you need to know about thermal expansion, speeds and feeds and the use of coolant.  The biggest thing to remember is that most plastics, especially those that are thermoplastic, will melt when they get hot enough. Thermosets won’t melt but can be brittle to machine so they are a totally different problem. Since heat is the culprit you must machine each plastic with the knowledge that it will grow, sometimes very rapidly, and then shrink again after machining. Some materials have to be machined once, normalize at room temperature, and then go back for final cuts. Sharp tooling, properly designed tools, speeds and feeds are all critical so there will be a learning curve. TriStar offers a “Machining Plastics” seminar and design manual through their website. For even more tips, Ask The Experts – that’s what they are there for! It’s not rocket science but there are tricks to the trade!

UHMW Polyethylene is an interesting material in the polymer world.  It’s one of the lowest cost plastics on the market yet it offers some properties that few other polymers do. UHMW is best known for it’s abrasive wear resistance and impact strength but even within this small polymer family there are variations. UHMW stands for ultra high molecular weight and under this description UHMW is produced with a molecular weight range of 3.5 to 6 million. The molecular weight has a fairly dramatic impact on several key properties. For instance, a 4 million molecular weight grade has an abrasion resistance of 100 when measured using a sand slurry test. A 6 million molecular weight is 75 which is a further 25% improvement! Compare this with steel which has a resistance of 160 and you can see why UHMW is the material of choice for abrasive wear environments.

There are several other variations on UHMW including cross linked, glass and moly enhanced and even a high temperature grade.

Check out our Materials Resources Guide to see the entire selection.

More and more companies are looking to save weight and costs by changing metal parts to plastic. Processing is always a big question since plastics can be molded, machined, extruded, etc. One method of making large plastic parts is through casting. This process is applied to parts that would be too costly to even machine out of plastic sheet or rod. Some cast nylons can be made up to 6 feet in diameter and weight hundreds of pounds. Several grades of nylon can be used in this process and the cost savings in both tooling and machining is phenomenol. Examples of cast nylon parts include large sheaves used on cranes, railroad locomotive components,  outrigger plates on trucks or trailers and large wear pads for boom cranes.

Next time you consider cost and weight reductions and need a durable, strong plastic, remember Tri Star’s cast nylon process.

Manufacturer’s of plain bearing materials will sometimes reference K factor which is a way of measuring the actual wear of the material over time. What is left out of the equation is how that K factor varies depending on the PV of the application. As an example, Delrin AF is a popular material for plain bearings. If you have an application wher your load is 45 psi and your speed is 100 feet per minute, your approximate K factor is 51. Take that same material and decrease the load to 20 psi and increase the speed to 350 feet per minute and your K factor increases to 70. That’s almost 40% greater wear. Another example is Ertalyte TX, a bearing grade PET from Quadrant. The K factor at 10 psi and 100 feet per minute is approximately 21. At 45 psi and 360 feet per minute the K factor increases to 464. So consider K factor as you do PV and coefficient of friction. These are numbers that are always relative to all of the surrounding factors in the bearing design.

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.

There are many options when it comes to cleaning parts; solvent wiping, soaking, ultrasonics, abrasion. But, more and more are considering plasma treatments, specifically plasma cleaning. Granted that plasma cleaning is horrible at removing bulk or globed on gunk from a device simply because it would take a very long time for all organics to be eliminated. But, to use plasma as the final step in complete organic removal, nothing is better.

Plasma cleaning of critical parts like medical implants, electronics, or diagnostics is one of the best methods. The operational cost and chemical use is very low making this technique environmentally friendly. The plasma technique is completely 360 degree cleaning organic residue from the smallest feature. The surface will be hyper-clean, uniform and ready for packaging or to some other critical step where pristine surfaces are necessary. Learn more about plasma treatments in our Shooting Star eLetter and be sure to check out the archives!