June 1, 2022
Compounding vs. In-house Blending
Compounding vs. In-house Blending
With the requirements for many medical device extrusions going to thinner walls and tighter tolerances, as well as smaller diameters, resin quality is critical. Many extrusions manufactured in medical device extrusion labs are a blend of a virgin resin with various additives, colors, and possibly other resins. Unless a medical extrusion lab utilizes a twin screw for its compounding and blending requirements it is at a disadvantage in maintaining product quality and consistency, especially if it depends on a single screw extrusion equipment. With these stricter requirements, any defects and/or foreign materials in the resin blends will compromise tubing performance. The quality of additives such as color concentrates and radioopacifiers must be such that there are no resulting defects in the tubing such as clumps. This blending must be done with a twin screw extruder. Though some additional filtration can be added on single screw lines to filter out clumps, this is just a band-aid and often these ‘fixes’ result in added work and potential degradation of the polymer.
Replacing color concentrate use in the extrusion lab with the base resin pre-colored by a compounding house ensures quality. Off the shelf color concentrates may not be finely enough screened to let down on a single screw and breakup any poorly distributed powder in the color concentrate pellets. Additionally, unless a color feeder is used, consistent color will be an issue (more likely lot to lot). Twin screw compounding with dedicated color feeders ensure lot-to-lot color consistency can be achieved. This is also true for radioopacifiers and other additives, which must be twin screw blended. Poorly blended color concentrate will lead to major yielding issues with thin-walled tubing like catheters, and especially in colored heat shrink tubing. During the expansion process post irradiation (aka cross linking) any small color clumps will cause bursts. This can be true of mineral-based colors that haven’t been screened well. Once the issue is identified, often the path taken is to improve the extrusion process. This is often due to the regulatory implications of changing from a color concentrate to a compounded blend. Unfortunately, it’s almost impossible to screen these clumps out with improved filtration, and why band-aid a process that most likely will require a validation when you can fix the issue at the source? Additionally, improved filtration comes with its own drawbacks, of most concern is the potential for additional opportunity to degrade the resin.
Color matching with the use of color concentrates will also be problematic lot to lot if there are color matching requirements. Though this is typically not a big issue with medical tubing, I have seen incidences where color variations from lot to lot influenced the bonding of the tubing (to other tubes) due to a shift in its laser absorbance. Spectroscopy is typically employed to confirm color matching, and this is just another complicated tool that is best left in the hands of the color compounders. The best way to set up incoming receiving for pre-blended resins is to first work with your compounding house to develop a set of Pantone color chips with the range of acceptable color variations. Be sure to consider matte chips if the tubing tends to not appear glossy.
Resin blends of two different resins also should be handled with dedicated feeders on a twin-screw extruder. Besides the fact that typically there is no way of verifying the ratio of the blend in the final tubing since the resin bases are very similar, single screws are not efficient blenders. Add to this that the two materials must be weighed separately (allowing for the occasional human error), then blended, either with a tumble blender or by hand (walking around the lab with a bag). There are many opportunities for variations with the blend that will lead to final product variations.
As noted above, often the route taken to fix FM type defects is to add filtration. One can move from the typical 50-400 mesh screen packs to more exotic weaves and as far as candle filters that can filter down to 5 microns. Though these can be effective they do come with drawbacks that include potential material degradation, set up costs and maintenance (especially with candle filters).
There are various types of single screw mixers/screw designs and often medical lab extruders will come with variations on both dispersive and distributive mixers. A few years ago, static mixers where quite popular but appear to have fallen out of favor. Another thing to consider is how the efficiency of the processes capability to provide consistent product as the screw wears, or you switch between screws (and extruders). Starting with a predictable pre-blend solves so many issues. It is also worth noting that more additives are being used in medical resins. This includes various stabilizers and fillers like nano-clays beyond the current portfolio of radioopacifiers and colors. These are either best left to the experts or investing in a twin screw capability.
Replacing color concentrate use in the extrusion lab with the base resin pre-colored by a compounding house ensures quality. Off the shelf color concentrates may not be finely enough screened to let down on a single screw and breakup any poorly distributed powder in the color concentrate pellets. Additionally, unless a color feeder is used, consistent color will be an issue (more likely lot to lot). Twin screw compounding with dedicated color feeders ensure lot-to-lot color consistency can be achieved. This is also true for radioopacifiers and other additives, which must be twin screw blended. Poorly blended color concentrate will lead to major yielding issues with thin-walled tubing like catheters, and especially in colored heat shrink tubing. During the expansion process post irradiation (aka cross linking) any small color clumps will cause bursts. This can be true of mineral-based colors that haven’t been screened well. Once the issue is identified, often the path taken is to improve the extrusion process. This is often due to the regulatory implications of changing from a color concentrate to a compounded blend. Unfortunately, it’s almost impossible to screen these clumps out with improved filtration, and why band-aid a process that most likely will require a validation when you can fix the issue at the source? Additionally, improved filtration comes with its own drawbacks, of most concern is the potential for additional opportunity to degrade the resin.
Color matching with the use of color concentrates will also be problematic lot to lot if there are color matching requirements. Though this is typically not a big issue with medical tubing, I have seen incidences where color variations from lot to lot influenced the bonding of the tubing (to other tubes) due to a shift in its laser absorbance. Spectroscopy is typically employed to confirm color matching, and this is just another complicated tool that is best left in the hands of the color compounders. The best way to set up incoming receiving for pre-blended resins is to first work with your compounding house to develop a set of Pantone color chips with the range of acceptable color variations. Be sure to consider matte chips if the tubing tends to not appear glossy.
Resin blends of two different resins also should be handled with dedicated feeders on a twin-screw extruder. Besides the fact that typically there is no way of verifying the ratio of the blend in the final tubing since the resin bases are very similar, single screws are not efficient blenders. Add to this that the two materials must be weighed separately (allowing for the occasional human error), then blended, either with a tumble blender or by hand (walking around the lab with a bag). There are many opportunities for variations with the blend that will lead to final product variations.
As noted above, often the route taken to fix FM type defects is to add filtration. One can move from the typical 50-400 mesh screen packs to more exotic weaves and as far as candle filters that can filter down to 5 microns. Though these can be effective they do come with drawbacks that include potential material degradation, set up costs and maintenance (especially with candle filters).
There are various types of single screw mixers/screw designs and often medical lab extruders will come with variations on both dispersive and distributive mixers. A few years ago, static mixers where quite popular but appear to have fallen out of favor. Another thing to consider is how the efficiency of the processes capability to provide consistent product as the screw wears, or you switch between screws (and extruders). Starting with a predictable pre-blend solves so many issues. It is also worth noting that more additives are being used in medical resins. This includes various stabilizers and fillers like nano-clays beyond the current portfolio of radioopacifiers and colors. These are either best left to the experts or investing in a twin screw capability.
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