Unimpregnated (top) and impregnated (bottom), post plating.
“Out, damned spot, out I say!”
What metal finisher hasn’t thought of Lady Macbeth’s famous soliloquy at some time? The incantation didn’t work for her, and it doesn’t work for metal finishers either. Spotty, flaking, bubbling, cracking or peeling finishes are seldom caused by the witches of Macbeth, but more frequently caused by a more insidious problem: porosity.
Poor plating, painting, anodizing or other coating problems costing untold dollars to metal finishers are frequently vastly reduced or eliminated by the cost-effective preprocess of impregnation.
As metal parts become end products, their value increases at every manufacturing step, especially in the finishing processes. Consequently, rejection of finished components late in the production cycle due to blemishes such as spotting, bleed out, coating bubbles and surface inclusions cost much more than rejection in an “as cast” or raw state. Why?
For castings, re-melting—while costly—allows the caster to reproduce the parts for little more than the cost of energy, generally only a small percentage of the raw part value. With weldments or brazed assemblies, the same is true: returning to the previous step is economically possible. As parts move to finishing, it’s an entirely different story. Finishing costs (polishing, plating, cleaning, coating and so on) are cumulative, and make parts progressively more valuable. They can account for half or more of the parts’ value and this investment is completely lost if parts are discarded or reworked.
A significant number of finishing problems are porosity-related. All castings and powdered metal parts are porous. Weldments or brazed fabrications can have porosity inclusions in their seams. Even the newest additive manufacturing technologies have porosity, which causes significant final finishing problems. During machining and finishing, pores can collect water, solvents, acids and oils. Later, collected contaminants can leach to the finished surface, manifesting as corrosion, pitting or spotting, ruining both the functional and decorative purposes of the part.
As metal finishers have learned, part design changes or process modifications—while helpful—do not always solve the problem. While advanced manufacturing techniques can reduce pore sizes to a nanoscale, even these size reductions do not solve the bigger problem encountered for decorative parts: surface blemishes. Indeed, for the finisher, the smaller the porosity, the more difficult it is to deal with imperfections and prepare a part for a decorative finish.
Sadly, many finishing houses have found that high rejection rates are the norm, incorporating the costs into the system and paying the price of poor finishes through lost business or low profits. However, with new developments in porosity sealing, the economics have changed. In the vast majority of cases, an inexpensive global solution to the porosity problem is available using vacuum/pressure impregnation with uniquely engineered impregnants and matching process protocols to fill porosity and provide the necessary integrity to a part’s pre-finished surface.
For those not familiar with impregnation, its primary use is to make leaking castings pressure tight. The process is simple: clean, assembly-ready parts are placed in a chamber and subjected to a vacuum of about 25 torr (29 inHg). A low dynamic viscosity thermoset resin is introduced and the chamber is pressurized, forcing the resin into the porosity. After a short period, the parts are removed from the chamber, washed to remove excess resin from the surface, and heated to 95°C (195°F) to solidify the resin in the pore. The result is a sealed pore and a leak-free, usable part at a cost of 1–5 percent of its value.
From the manufacturer’s standpoint, post-cast finishing operations—such as plating, anodizing or coating–are but one consideration in the profit cycle. Generally, primary “finishing” operations are machining and joining, using dimensional tolerances that are immediately measurable and controllable. A manufacturer knows that final finishing–the bright new look resulting from polishing, plating, anodizing, painting and so on–often harbors flaws that take time to present themselves. Bleed out, spotting, cracks, blisters and other problems can ruin a part’s looks or functionality. Early finishing successes can become costly market and reputation failures, unseen with inspections, appearing only after assembly and shipment and generally characterized as “warranty rejects,” the most expensive of any manufacturing cost.
Functional versus Decorative
Casting technologies have always focused on controlling porosity, with a mindset to eliminate porosity that would cause a part to leak when used under pressure. Functional parts for engines, pumps or many other common applications are generally unseen and therefore the “finish” is low on the manufacturing priority list. For impregnators, in their traditional role of dealing with leakers, sealing a porosity pathway somewhere along its length makes the part acceptable. With pressure tightness the measure of success, impregnators developed materials and processes to seal leaks. Similarly, for other manufactured parts, modern machining, automated design and special attention to form and fit have reduced weldment and brazing porosity leakage problems to a minimum. By combining modern impregnation processes with new casting and machining technologies, leakers have now been reduced to the level of a minor annoyance.
Enter a newly rediscovered trend in manufacturing: functional coatings, where the applied coating’s physical properties combined with the substrate’s physical characteristics produces a cost-effective, enhanced product performance. In these applications, as with decorative parts, the substrate condition plays a huge role in a coating’s success, and porosity-caused blemishes can contribute to a part’s premature functional failure.
With decorative parts, it’s all about looks. A metal finishing house that processes building hardware, for example, will have a whole different set of concerns than a valve company. Attention to surface imperfections trumps concerns about the physical integrity of the part from the start. The problems of porosity can be further complicated if machining or polishing a part opens up formerly enclosed porosity.
For a successful decorative finish, a part must have true surface integrity, and the same holds true for modern functional coatings. The real challenge with porosity is not to just plug the pore, but to fully fill it nearly up to the part’s surface, leaving no place where contaminants can remain, and yet leave no residue that can impact surface/coating interface and adhesion.
As previously mentioned, water, acids, solvents, machining fluids and other contaminants are often trapped in porosity, only to bleed out after the part has been coated. Research indicates that water, at its normal surface tension of around 70 dynes/cm², cannot easily penetrate porosity with an opening smaller than 65µm. But by adding surfactants (for washing), oils (for cutting and lubricity in machine operations) or heat (from various operations) to water, a reduction in surface tension occurs, thereby easing the contaminant’s entry into even the smallest pore. There it remains, held by capillary forces until other physical changes such as corrosion, bleed out or heat-associated eruptions occur.
To address porosity issues, some companies turned to impregnation, only to discover that its success in sealing leaks didn’t always work for coatings, primarily because of the finisher’s requirements for fully filling the porosity nearly level with the part’s surface. To solve coatings issues, a multi-step approach using unique, solvent-based cleaning techniques and reengineered impregnation materials and processing was required.
Finishers know that a clean surface is critical for high-quality coatings. Since porosity is effectively an extension of the part’s surface area, it must first be cleaned. Today, most cleaning operations are water-based, with the limitations of water playing a commanding role. Using a solvent vapor cleaning process to remove contaminants from pores was the first step. Since this process is waterless and “closed-loop,” solvents are fully recovered and reused, and the contaminants are separated for easy disposal.
Second was the use of an impregnation resin with engineered characteristics enabling it to fully fill porosity, wash entirely from the surface, and yet not wash out of the porosity. With surface tensions less than half that of water and low dynamic viscosities, traditional impregnation resins can enter and fill all voids. From an industry point of view, good penetration and surface “washability” of the resin commanded a premium, so impregnants were engineered for easy removal with water while leaving enough in the pore to seal the part. However, good washabilty also means difficulty in retaining a high volume of resin in the porosity close to the surface, leaving porosity voids that can contain contaminants. To solve this, a polyester methacrylate blend resin with slightly enhanced dynamic viscosity was chosen to allow a fuller fill.
Retaining the resin in the porosity presents a different challenge. The third step was to engineer the process to improve retention by addressing the washout issues.
Understanding and using the dynamic forces of impregnation, they developed process parameters that utilize these same forces, thus allowing resin retention in the porosity while removing excess resin that clings to the part’s surface with process’ low energy washing and curing steps. The result? Parts that have a clean surface as a base for plating, anodizing or coating, and fully filled porosity to prevent surface blemishes.
Challenges of a Porous Surface
Earlier we described the challenges faced in plating on a porous surface. For anodizing and other conversion coatings, porosity can even contribute to a thinning of the coating and cause smutting or powdering, resulting in substantially diminished corrosion protection. Since these anodic coatings result from the chemical reaction promoted by electricity between the metal surface and the acid bath, an uneven surface with deep pores requires prolonged processing to achieve the desired coating thickness. The unintended result of this lengthened time is the degradation of the oxide, resulting in a soft, powdery surface with diminished corrosion resistance or inconsistent and unacceptable visual results.
Using enhanced impregnation techniques prior to anodizing solves this problem. It fills the pores, thus preventing the entry of other corrosion-causing contamination, and it provides a better base on which to build the full anodized-coating thickness on a uniform basis, thus improving both visual and physical results.
Applied coatings also have related porosity problems. Again, contaminants—generally in the form of water-based machining fluids and cleaners—are sucked into the porosity voids. The parts are coated and progress to the heat cure stage. As the heated coatings begin to flow, level and crosslink, they effectively form a vapor-tight seal for the water entrained in the porosity. As the temperature increases to the 150–175°C (300–350°F) final cure temperature, moisture vaporizes, producing enough internal pressure 3.5–8.3 bar (50–120 psi) to erupt through the partially cured coating surface, causing a crack, flake, fissure or blister that will prevent good coating adhesion, and ultimately result in coating failure.
The bottom line is that a flawed decorative part cannot be sold and a flawed functional part cannot do its job. Either the finisher gets the blame and suffers the loss or, if it is an in-house operation, increased costs are needlessly incurred. In the end, looks suffer, parts don’t work and lots of money (and good will) is needlessly spent.
For manufacturers and finishers of decorative parts, using full-fill impregnation as an integral part of the surface finishing process saves money. By impregnating, a finishing operation can show dramatic increases in productivity and profits because part quality is ensured and the number of porosity associated rejects are reduced. Although the cost of impregnation varies by application, it usually ranges from 1–5 percent of a part’s finished value, a cost quickly paid back by reducing rejections. The same benefits can also be shown for functional coatings.
Aside from lower reject rates, FFI brings additional benefits. For example, because of FFI’s attention to cleaning the pores, surfaces are better prepared. Coating thickness and process times usually can be reduced, and functionality improves thanks to enhanced coating adhesion with no porosity interference. With better part cleanliness, liquid baths do not need to be changed as often, saving more money.
In these and other ways, proper cleaning and impregnation make for more efficient finishing operations, helping the shop save labor, time and capacity while producing parts for less cost, more quickly, and to higher quality, resulting in higher profits. In other words, to quote Ben Franklin, “A penny saved is a penny earned.”
Article From: Products Finishing, Peter T. Gebhard from Imtech