Overcoming typical TPU processing problems involving moisture and temperature can be a challenge when fabricating components or extruding tubing for medical devices. This article offers several tips to address these common concerns. Additionally, it reviews specific optimization techniques medical device manufacturers can use during injection molding and extrusion.
Cynthia Edgar, CQE is sales and marketing manager for New England Urethane. She can be reached at 203-239-9629 or email at firstname.lastname@example.org.
In processing TPUs, water at elevated temperatures reacts with the isocyanateone of the building blocks of urethanes. One of the by-products of the reaction of TPUs with water is carbon dioxide. Urethanes that are processed "wet" will tend to foam or bubble during processing and that will be manifested in the final part. While many resin manufacturers recommend that the moisture content of TPUs be less than or equal to 0.05% for molding and 0.02% for extrusion, experience dictates that they be even dryer. However, when parts made from resin with a moisture content of less than 0.05% when molded and 0.02% when extruded may appear fine, their physical properties probably are not optimized. Elongation, impact strength, tensile modulus, and other properties can be negatively affected.
In addition, it is not just the percentage of moisture content that's important, but also how one gets to it. If not dried properly, TPUs will degrade. If the resin is dried too long, it will oxidize; if it is dried at too high a temperature, the polymer chains will be severed. Once the resin is dry, the material should not be exposed to air for extended periods of time as it will quickly absorb moisture again. For example, in a non-climate controlled production facility during the summer in Florida or the Philippines, the resin will absorb enough moisture to be considered "wet" in less than 30 minutes. Even if the resin is in a closed feed system, small gaps, such as holes in the tubing or poorly constructed fittings, can wreak havoc as moisture is reabsorbed.
When Melt Processing, Know the Melt Temperature
Don't heat-soak the resin. Urethanes can burn or crystallize if the temperature is too hot or the residence time is too long. It is far better to start with equipment that is too cool with the temperature then being increased than it is to start with the equipment too hot. Residence time for TPUs should not exceed three minutes; in other words, if the injection molder has a four ounce capacity and five gram parts are being fabricated, the equipment must be downsized. The same is true if a 50-mm extruder is being used to produce thin walled tubing with an I.D. of 0.25?.
Guidelines for Injection Molding TPUs
•A clamp capacity of at least 30 tons is recommended.
•For best results, the sprue should be slightly tapered in order to easily move the mold.
•All gates and runners must be tubular to allow the free flow of the resin.
•Flat walls will restrict flow and result in short shots and possibly the crystallization of the resin along the flat wall.
•Pinpoint gates should be used.
•Pressure should be maintained while the resin sets-up in the mold.
•A slight matte finish is recommended to improve release of the part from the mold. A polished finish will cause the part to stick. Mold releases compounded into the resin will, of course, improve release if a polished finish is desired.
Guidelines for Extruding TPUs
•The system should be capable of high torque at low screw speeds. High shear stress should be avoided. The polymer chains of TPUs are prone to degradation when exposed to rough handling.
•Homogenous blending must be achieved with minimal mastication.
•Depending on the durometer and melt temperature of the resin, it may be necessary to reduce the heat at the back end of the extruder. With elevated temperatures, some resins can become sticky and clog the throat of a small hopper.
•The front end of the extruder requires special considerations as well. Similar to flat walled runners in injection molding, cross-head extruders are to be avoided, if possible. The flow of the molten resin is restricted at the bend. Crystallization can occur in the section of the extruder with reduced flow, and the extrudate will have lumps. High durometer urethanes are most prone to this crystallization phenomenon.
•As with all resins, if the viscosity of the extrudate is too low, the pressure at the front end of the extruder should be modified. The most obvious way to do this is by decreasing the temperature at the die zone. The composition of the screen pack can also be changed to modify the pressure at the front end of the extruder.
•Do not suddenly quench the extrudate as it exits the die. Many processors mistakenly believe that it is better to quickly cool the semi-molten resin in order to "freeze" it into shape. This will cause tackiness and distortion. It is better to use a warm water bath. This allows phase separation of the hard and soft segments of the urethane, thereby reducing the entropy of the part, improving its stability and minimizing, if not eliminating, tackiness. If the extrudate is not allowed to phase separate at the time of manufacture, its molecular structure will realign over time and deformation of the part will be observed.
Finally, as applicable to any plastics processing, TPU should never be left in the equipment when the run is finished. Thermoplastic polyurethanes will crystallize or degrade inside the equipment, making clean-up next to impossible. And, when purging, always use purge material that is more viscous than the resin being run. A purge that is watery won't properly flush the system.
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or New England Urethane Inc. at www.neuinc.com.