13mm Flange Plug Caps, Polyethylene, Natural - 4000/Case Made of polyethylene. Plug type push-in caps are made of polyethylene for test tubes with outside diameters from 12 to 17mm. 17mmplug type caps (Series T401-10) are also for use with the Simport 15ml Centrifuge Tube series T408. Product photo may not exactly match the product offered for sale. Please refer to the product description.