With conical bottoms Available in your choice of clear polypropylene (PP) or polyethylene tetraphthalate (PET) Sterile, certified RNase-/DNase-free and nonpyrogenic High speed RCF rating up to 15,500xG CentriStar Caps provide a revolutionary plug feature, wider knurls, and roll-over edge design to improve grip; easy-on, easy-off thread design; and a flat top with matte finish for easy marking Original plug-seal caps also available Large white marking spot and black printed graduations, 500 tubes per case.