The blanket is made of natural rubber. It is nominally 25 mm (1 in) thick and contains textile reinforcing located approximately 3mm from the inside surface. This reinforcing extends across the blanket width to within about 12 mm of the edges. The 12 mm of unreinforced blanket edges act as seals to prevent moisture absorption by the textile material.
The reinforcing has a high modulus of elasticity and therefore exhibits virtually no stretching under normal operating conditions. Further, the reinforcing’s non-extensibility and its location within the blanket controls rubber flow and displacement during passage through the nip. This maintains blanket integrity and performance during use. Hardness of the rubber blanket is usually 55 ± S Shore A Durometer (107 ± 17 P&J Plastometer). However, blankets can be ordered and are sometimes used with rubber as soft as 45 Shore A Durometer (143 P&J Plastometer) or as hard as 65 Shore A (75 P&J) but their use should be discussed with Clupak, Inc. before ordering because hard or soft blankets often produce unusual results and sometimes cause operating problems.
Blankets are usually black, but when processing certain special nonwoven materials a white rubber blanket may be required. White blankets must be special ordered, and are usually a little more expensive. Further, because white rubber is compounded without using carbon black as a strengthening agent, blankets made with it have about 10% shorter life then black ones in similar service. Clearly, white rubber blankets should be purchased and used only when the nonwoven material being softened is discolored on contact with black rubber.
Blankets weigh approximately 1.19 g/cm3, about 30 kg per square meter. A blanket with a face width of 5 m and 7620 mm inside circumference therefore weighs around 1150 kg.
Blankets are made by a technique of wrapping uncured rubber and textile reinforcing around the polished surface of a steel mandrel. This forms a blanket with an inside circumference corresponding to the mandrel’s outside circumference. Next, the rubber is vulcanized by inserting the blanket wrapped mandrel into an autoclave and curing at elevated temperatures for a predetermined time. The blanket is then ground, trimmed to length, and removed from the mandrel ready for shipment. Correct storage and use of the blanket, as discussed later in this manual, is extremely important because further vulcanizing, i.e. hardening, can occur if the blanket is stored in a hot area or touches a hot surface for extended periods of time.
The large capital cost of mandrels and autoclaves used for blanket manufacturing tend to limit the number of blanket manufacturers as well as the blanket sizes available. Although only four inside blanket circumferences are manufactured because of limited mandrel diameters available, blankets can be made to almost any desired face width up to at least the 9000 mm length of the longest mandrels now in use.