Competition amongst device companies in the institutional market has energized the use of colors for brand recognition amongst physicians, nurses, and other healthcare providers. With the rise in home healthcare, many devices are now entering consumer markets where branding and aesthetics help influence the buying decision.
At the same time, minimally invasive procedures are being introduced to reduce patient trauma and increase comfort and outcome. These procedures rely on successfully smaller devices with thinner walls and less material volume. In order to achieve consistent, vibrant colors within thin sections, pigment loading levels and pigment dispersion is critical.
Current economic challenges are also placing greater focus on polymer pigmenting options that provide the best cost/performance solution for a given application volume. With short production runs needed for smaller, highly specialized devices, process efficiencies and production yields can have a greater effect on cost than material price alone.
Color Concentrates — The Traditional Approach
Product or component colors have always been used to define brands, differentiate critical features or improve aesthetics of medical devices. A wider range of colors are frequently finding their way into medical components as macro trends continue to redefine the medical market.
Color concentrates, or masterbatches, are generally accepted by the plastics industry as an economical means for pigmenting polymers. Masterbatches consist of a polymer matrix and high loadings of pigments and/or other additives. The polymer matrix is either the same or compatible with the polymer to which it will be added. Common carriers for masterbatches include polyethylene, polypropylene, polystyrene, and ethyl vinyl acetate (EVA).
Masterbatch pellets are combined with natural resin prior to processing, creating a ‘salt-and-pepper mix’ that is then melt blended in the extruder or injection molding machine. Master batches are blended in a specified ratio, known as the let-down ratio (LDR). Let-down ratios are typically 20:1 up to 50:1 (virgin resin to masterbatch), or five percent to two percent loading percentage. (See Figure 1) The designed use level for a given masterbatch depends on the desired color, pigment types, pigment loading, and anticipated processing machine characteristics.
Although masterbatches are relatively expensive due to high loadings of costly pigments, economies can be achieved where longer production runs use high volumes of resin. This approach can help keep inventory costs low because the bulk of material is natural polymer and can be used with multiple colors as needed.
Masterbatches rely on injection or extrusion machines to provide sufficient mixing with the natural resin. Machines with small screw diameters, shallow flight profiles and short screw lengths are less reliable when it comes to dispersing masterbatch concentrates. In such machines, the use of masterbatches is technically challenging or not economical compared to pre-colored polymers when working with high pigment loadings or difficult color matches.
Since the amount of a masterbatch is relatively small in comparison to the base polymer, consideration of key processing parameters is required to achieve consistent color matches. Precise metering of both the masterbatch and polymer is required. “Dry blending” the ingredients in a mixer prior to adding them to the hopper is frequently used. Concentrated masterbatch pellets are denser than the natural polymer pellets and tend to sift downward in the hopper due to vibration and drying air movement. This can lead to variable loading levels and color drifts throughout a production run.
Color metering weigh feeders can help to minimize ratio and color variation. Also, loss-in-weight feed hoppers helps to ensure precise metering of the materials into the injection or extrusion machine. Hopper feed screws must be designed to properly meter and mix the masterbatch pellets with virgin pellets at the right ratio, prior to entering the machine screw. These capital investments are often more difficult to justify for limited production runs of specialty medical devices.
Sufficient set-up time and engineering oversight is also required to ensure proper blending of the concentrate and achieving the desired color consistency from part-to-part. Lost production time and material due to extended set-up time can be offset by material savings in long manufacturing runs of material intensive parts. This can more significantly impact the cost of parts in short runs with limited and costly materials.