Back in the mid-1960s, an iconic movie featured a recent graduate, played by Dustin Hoffman, who was attempting to find himself. One of the “old boys” came up to him and, in a solicitous voice, whispered a single word: PLASTICS. If you manufacture product, I am here to whisper: “plastics,” “glass,” and “other.” Even a small change in materials of construction can wreak havoc with the cleaning process. This means problems with assembly and coating processes, and ultimately with product performance. Here are some practical thoughts to keep manufacturing processes running profitably with new batches of plastics, glass, and other materials.
In the October issue of Clean Source, we discussed the impact of small changes in metallic substrate (1). Manufacturers may see materials compatibility problems like surface staining, discoloration, spotting, erosion, and corrosion. With non-metals, the changes can be subtle, so tried and true cleaning processes may need refinement. This may mean changing the cleaning process, but it has to be accomplished in a rational, documented manner.
I will use the term polymer instead of plastic – polymer is classier and more descriptive. Polymeric materials can show batch to batch variability in terms of cleaning, even if they meet overall specifications. Let’s consider a new batch of polymer. The new polymer may meet requirements for melting range. The batch may meet the specs for physical and structural characteristics. If the polymer has medical applications, the new batch may show the correct inertness or lack of reactivity with blood or other body fluids (in general, lack of reactivity is what we want).
Polymers are complex. Polymers are large molecules made of smaller molecules linked together like beads on a necklace. The “necklaces” may be tied together or crosslinked to provide specific properties. Polymers involve synthetic chemistry or building molecules, not just blending materials as in formulations chemistry. Formulation chemists are more like master chefs; synthetic chemists are more like builders. Manufacture of polymers involves many variables. The more complex the material and the more variables involved in producing the material, the greater the likelihood that new batches may interact unfavorably with cleaning agents and with other process fluids.
Given the number of variables, interaction of the polymer with the cleaning agent and with the cleaning process requires testing. This is because a given polymer may be used by many different companies in many and many different products. There may be many different cleaning processes. In working with any polymer, we have to be on the lookout for materials compatibility issues ranging from reversible adsorption to subtle surface damage to obvious product deformation or degradation (2).
If you are developing cleaning processes for critical or high-volume applications, it is ideal to test the proposed cleaning process using several batches of polymer. If this is not feasible, obtain a new batch of substrate before the old one is used up. Then, do a mini pilot cleaning test with the new batch. This can allow mid-course corrections in the cleaning process while avoiding production shutdown. Storing a small stock-pile of polymer that has been successfully cleaned is useful for troubleshooting.
It is not that optics manufacturers are insane; they shout and tear their hair out because they have many process variables to deal with. Many of the cleaning processes can show an inherent lack of stability and be difficult to control. Glasses are complex mixtures. Even well-defined glass, manufactured to a particular specification, varies from manufacturer to manufacture; and even for a given manufacturer, there are batch to batch variations as well.
As with polymers, the concept of testing the fabrication and cleaning processes on the new batch of material before the old one is used up can help avoid major issues.
But wait – there’s more
Even if the chemical nature of the substrate is the same, a change in the mode of initial fabrication affects cleaning. A change in tooling affects work hardening of the surface and crystalline structure. Moving to additive manufacturing (3D printing) to produce a part that had previously been constructed by traditional machining can mean changes in porosity or in surface irregularities; this may mean a modified cleaning protocol.
A product can include many different substrates including various metals, polymers, elastomers, composites, crystal, and types of glass; that combination will influence the cleaning process. Product configuration and shape or ornateness (e.g. blind holes) has an impact on cleanability. New batches of material may impact cleaning.
As one example, a cleaning process for multi-material miniature connectors was developed using a single batch of polymer. The materials of construction included metals, glasses and polymers. A new batch of polymer was introduced; it met all of the performance specifications. Unfortunately, this new batch interacted unfavorably with the cleaning agent. The dimensions of the finished product changed. Replacement with a less aggressive cleaning agent solved the problem.
To head off problems, be observant and have a procedure for orderly, documented change in the cleaning process. Keep an eye on the product and an eye on the cleaning process; and, especially with temperamental, complex materials like polymers and glasses, keep track of batch numbers.