Troubleshooting in polymer processing

Styrene acrylonitrile (SAN) resins are transparent with excellent clarity, good surface gloss, dimensional stability, heat resistance, environmental stress-crack resistance, hardness, load-bearing capabilities, processing ease and good economics. Most common formulations have about 75% styrene monomer; are available in a range of grades, including glass-reinforced and high-heat distortion; common additives include plasticizers, mold-release agent, colorants, and UV stabilizers. Applications include safety glazing, cassette housings, consumer goods (dishwasher-safe mugs, tumblers, dinnerware, utensils; pen and pencil barrels, hobby-kit parts), appliances (refrigerator food trays, blender bowls, vacuum-cleaner parts), medical (syringes, IV connectors), automotive (lenses and instrument panels), packaging (cosmetics, bottles, nozzles), and industrial (battery cases and caps, instrument and meter lenses). Although the resins are commonly injection molded, they can also be extruded, compression molded and injection blow molded. As is generally the case, proper molding conditions are affected by the process chosen, the equipment used and the part to be produced and it is recommended that processors discuss particular needs with their resin supplier. SAN is hygroscopic and can absorb as much as 0.6% moisture encouraging splay, streaks and other surface defects. Therefore, the pellets should be dried prior to processing for two or more hours. Dried pellets can reabsorb moisture from humid air, so unless they are used within two to four hours, they should be protected from again picking up moisture. The resin can also experience a color shift towards yellow during its lifetime, and so the use of regrind should be monitored carefully.

 Design guidelines 

The fundamental design principles used with other engineering thermoplastics are equally valid with SAN. · Wall thickness transitions: A nominal wall thickness of 0.125 inch is commonly used. Functional wall thickness as thin as 0.020 inch may be designed. Material flow should be from thick to thick sections to avoid high stresses. Gating should be in the thickest area to allow a smooth transition to thinner sections. Gating in thinner sections can cause sinks and warping in thicker sections, and can result in excessive molded-in stresses in thin areas. · Fillets and radii: Generous radii reduce molded-in stresses and allow loads to be is distributed over a wider area. All corners should have an optimum inside radius of 50% and an outside radius of 150% of the wall thickness. · Ribs: Properly designed and located ribs allow an overall reduction in wall thickness and part, weight, and increase the load-bearing ribs are preferable to thick ones, which can result in bubbles, sink marks, and stress-concentration points. The polymer's flow length and wall thickness are the important factors in determining rib thickness. General guidelines are: radii - minimum 0.25 X wall thickness: rib height - maximum 3 X nominal wall thickness; rib thickness - maximum 0.75 X nominal wall thickness. · Undercuts: Snap-off undercuts can cause high stresses and generally are not recommended Cam-action undercuts perform satisfactorily. Concerning model design, the following should be addressed: · Tolerances: The predictable low shrinkage of SAN allows for excellent dimensional control when designing molds. Actual tolerances depend on the part design. · Gating: Gate design, size and location are determined by the type of mold, the size and shape of the part, and the resin. With SAN, the gate diameter at the part wall should beat least equivalent to the wall thickness, and located where the wall thicknesses are greater. To minimize gate blush and jetting, gates should be positioned so the melt goes against the core, core pin, or cavity wall. · Venting: Gas entrapment, which may cause brown spots, is prevented by adequate mold venting, which also allows faster filling. Vents should not exceed 0.0020 inch in depth. If possible vents should be located around the entire perimeter of the mold, and wherever the polymer flow pattern could result in air entrapment. · SAN parts can be welded to each other with several effective solvents. If a fast-drying solvent adhesive is desired, menthylene chloride is commonly used. Effective medium-drying solvent adhesives are methyl ethyl ketone, and a mixture of 30% methyl methacrylate monomer and 70% butyl acetate