For some standard applications, incorporating fillers, colors, and other additives into a resin can be as easy as pre-blending all the ingredients together and loading them into a single-screw extruder hopper for strand pelletizing. As requirements become more stringent, the compounding process becomes more interesting, involved, and specialized.
Many types of compounding machines and mixers are available, and satisfactory compounds can be produced on most of them. While individual operator preference plays a major role, certain machines are more flexible and do a better job than others on specific compounds.
Procedures for melt compounding encompass two kinds of mixing, distributive and dispersive. For example, when fibers are incorporated into a resin, good distribution is required without breaking down the fiber segments: this is characteristic of distributive mixing, a relatively gentle homogenization of the material. Dispersive mixing is associated with the breaking up of agglomerates and generally correlates to high strain and shear rates.
Machines capable of both distributive and dispersive mixing include Banbury-type batch mixers, Farrel continuous mixers, and twin-screw extruders. The twin-screw extruder offers the most flexibility, since the barrel length and screw configurations are adjustable. Downstream oil injection and powder or fiber feeding is also possible.
For materials with good melt strength that are not too soft and tacky, strand pelletizing is preferred. With this method, the pellets are cylinder-shaped and have a diameter of approximately 0.1 in. and a length of 0.125 in. Strand-pelletized compounds feed well on virtually all extruders. Soft and sticky compounds are usually underwater pelletized: the pellets are cut at the die face in a water chamber, conveyed in a pipe with the water, and then separated in a centrifugal dryer. Pellets are typically football shaped or round, with a diameter of 0.1 in. Micro-pellets with diameters as small as 0.02 in. can be produced by underwater pelletization but are generally not recommended for medical compounds because the micro-pellet dies tend to restrict flow and feed rate, causing degradation in sensitive compounds. Small pellets of about 0.06-in. diameter can be produced with a special die that prevents such degradation; these pellets will extrude on the smallest of machines.
Once the need for a new compound is determined, a development team is assembled and the compound requirements are defined. A project plan should include enough time to research existing designs and databases and to determine starting formulations. Acquiring raw materials can necessitate a lead time as long as 2 to 4 weeks. If possible, it is advantageous to schedule compounding time in advance so that materials can be compounded soon after they arrive.
Processing and physical testing come next: several iterations may be required to fine-tune both the formula and the compounding process. When the ingredients are firmly established, biocompatibility testing can begin. Several lots of compound should be produced in order to isolate the effect of control variables such as melt flow and to determine the repeatability and capability of the compounding process.
Once it is verified that the compound meets specified requirements, a production prototype run should be made and final product fabricated. Final validation should include complete biocompatibility testing, physical testing, and field testing. If the validation is successful, a formal specification is written for the compound and a product code assigned.