Q: I tried to bond a nylon component to an SLA part, but the adhesive easily peeled off of the nylon even though I used what I thought was a very good adhesive. Any recommendations on how to get these parts bonded together?

A: Plastics have different characteristics based on whether their material structure is semi-crystalline or amorphous. Plastics are composed of long polymer chains. In an amorphous structure these polymer chains are tangled together like a plate of spaghetti noodles. Typical examples of an amorphous plastic include ABS, PVC and polycarbonate. However, in a semi-crystalline structure, the polymer chains align themselves like dry spaghetti in the box before it is cooked. A network develops of aligned “dry spaghetti” portions interspersed with tangled “cooked spaghetti” portions. Many of the engineering and high performance plastics like nylon, acetal and PEEK are semi-crystalline.

The advantages of semi-crystalline polymers are their excellent chemical resistance, low coefficient of friction properties and low melt viscosity for injection molding. However, due to the crystallization during cooling, they have high shrinkage and are usually translucent or opaque. In addition, they are difficult to paint or bond due to their low surface energy. Parts can be plasma etched in order to increase their surface energy so that standard adhesives will bond well. Also, certain specialty adhesives are made for bonding difficult plastics such as BONDiT B-45TH. Ultrasonic welding may be a good option for production parts, although it may not be the best answer for the prototypes due to the differences in materials and cost of tooling. Lastly, you can modify the design so that the parts slide together as a dovetail or equivalent, as this can greatly reduce the stress applied to the adhesive.

Q: We had a great first production run of an impeller assembly with a plastic impeller press-fit onto a metal hub. However, on the second production run, most of the impellers cracked when press-fit onto the hubs, even though the tolerance stack-up was within .0005″ of the original parts. What could be causing this issue?

A: The properties of an injection molded parts can be significantly affected by the exact processing conditions. If the molder changed the processing conditions from the first run to the second, it could be causing the differences. The cost to mold a part is tied in with the cycle time. If a molder can reduce the cycle time, it will reduce their cost to make the parts and increase their profitability. One way to decrease the cycle time is by decreasing the melt temperature so that the time for the plastic to freeze is minimized. However, this can cause the melt fronts to cool, and lead to weak knit lines. Knit lines are where the plastic flows around a feature and rejoins on the far side. If the flow is too cool, the plastic can start to solidify on the flow fronts and will not mix well when those flow fronts meet. Check the cracked parts and note whether any of the cracks originate at the knit lines. Go to Google Images and type in “knit lines” for some good visual examples of what to look for.

Q: I designed a plastic part with bosses for screws, but when installing the screws, the bosses cracked lengthwise. How do I prevent this?

A: A good rule of thumb for plastic boss design is that the overall diameter should be at least two times the major diameter of the screw. While this works well for smaller screws, for larger screw diameters, the thicker the boss wall relative to the nominal wall, the more likely for read through to become an issue. In those cases, you may have to go with a thinner wall but reinforce the boss with gussets or ribs. These reduce the likelihood that the boss will crack in the first place, but even if it does crack, they will provide something for the screw threads to hang on to.

Also, the screw type is important to consider. Thread forming screws are best for plastics with a flexural modulus of 200,000 psi or less. These plastics will deform easily around the screw threads, whereas harder plastics will not deform well and the screw threads will create high hoop stresses. For plastics with a flexural modulus between 200,000 psi and 1,000,000 psi, thread cutting screws are often a good option. However, if they must be assembled and disassembled multiple times (i.e. to access batteries, etc.), then a metal insert will be required, otherwise the thread cutting screw will cut away the threads in the hole. Several thread forming screws are available that are designed specifically to reduce stresses in plastics, and these blur the above distinctions. Good options include the Plastite fasteners and HiLo screws, some of which can even be used on materials towards the upper end of the traditional thread cutting modulus range.

About the Author:
Jeff Wickham, P.E. is the Principal at LifeHope Medical, Inc., a mechanical engineering consultancy focused on medical device design and analysis. Jeff received his bachelors in Mechanical Engineering from Tennessee Technological University, and subsequently, has worked in a wide range of industries including medical, aerospace, automotive, commercial and consumer products, and on projects ranging from DVT prevention and neurostimulation to bomb rack components and Bluetooth headsets. He has a passion for designing medical devices and finds great satisfaction in thinking up ways to help people feel better and live longer.