It looks simple, but high gloss black is a very difficult surface finish to achieve. Those Xbox or PS4 cases require a lot of work, time, and money. To be fair, we could find a defect or two in most parts if we looked long or hard enough. The key to making a medical device look great is to minimize obvious defects that catch the eye.

Here are some common causes and solutions to minimize colour variations in your medical device. 

Texture and Light
It’s surprising how much of an impact surface texture has on the perception of color. Two parts (one with a slight texture and the other one smooth) will appear to be a different color (or at least a shade), even though they were painted at the same time with the exact same paint.

Try as we may, different processes create different textures that, in turn, can cause a color perception difference. We can try to achieve similar surface finishes, but often we get close, yet not close enough.

Solutions: 

• Create intentional color breaks—where there is a different surface finish, choose a different color.
• Create intentional surface finish breaks—rather than try to match surface finish, intentionally make them different, so the difference looks intentional, rather than a mistake.
• Create color gaps—two parts, the same color but different textures can appear the same if they are separated by distance, such as an additional component in a different color.

Batch Differences
Lower volume manufacture has the added problem that custom colors (matching company logos, for example) can have large minimum order quantities (MOQ) and not make financial sense. The alternative is to mix at the time of painting. Because this is a manual process, successive batches may have color variations. Most of the time, these variations go unnoticed until the devices placed are side-by-side or parts from different batches are used on a single device.

Furthermore, different parts made at different facilities may have some slight variations—all within specification, but nonetheless different enough for the eye to detect.

Solutions:

• Ship all parts to be painted to the same factory if possible. Many companies will paint parts they haven’t produced (for a fee).
• Where company colors are not present, choose standard paint colors (use Pantone Color Palette as a guide) available from the paint supplier. Standard paints have higher production controls.
• Consider the MOQ—it may be viable if the paint is consumed by multiple parts. Quantities of the batch can be shipped to different locations.
• Add a yield to the components in case you need to use parts from different batches on the same device. Scrapping a part may be the better option, rather than a color mismatch.

Injection Molding
One of the many advantages of injection molding plastics is color can be molded directly into the part. The need to paint disappears, reducing variability and costs. Scratches, scuffs, or other defects are less visible on injection molded plastic than on paint surfaces. (Lighting plays a large role.)

How color is introduced into the part can have variations. Some methods will have more variation than others. At the high end, fully compounded material can be purchased. Compounded material is fully premixed, with colored pellets.

Masterbatching is another method. Here, the colorant is encapsulated within a carrier resin. The colorant is well mixed within the resin material, which makes it easier for molders to mix the masterbatch with the natural plastic. A large quantity of the masterbatch can be mixed to cover several production runs with better control.

Mixing defects can still occur, but the opportunity for variation is much less than compounding the material in house, where complete dispersion and batch to batch variation may be present.

Solutions:

• Masterbatch material. Color variations may still occur, but these should only appear over multiple injection mold runs when a new masterbatch is ordered.
• Run similar parts on the same machine, so the material doesn’t have to be swapped. If parts are close in size, they can be run on the same machine, using the same material and mix. If parts are too disparate in size, smaller parts may be grouped in a family tool and run on the same machine as a single, large part.
• Use color breaks, as previously noted.

In summary, all processes have some variations that need to be addressed. By carefully designing components, we can minimize color variations in a medical device and mitigate the impact that different processes or different surface finishes can have on its appearance.

---

Dana Trousil is the Medical NPI Team Lead at StarFish Medical.