Vehicle safety air bags are folded into a housing formed in part by a support that is attached to the vehicle, for example, the hub of the steering wheel, and to which the air bag is secured, and in part by a cover that is positioned generally between the folded air bag and the vehicle occupant. In a widely used type of air bag housing, the cover is made of a polymeric material and has weakened zones that enable it to break easily in a controlled manner when the air bag inflates, thereby enabling the air bag to emerge from the housing and fully inflate. With such a type of cover it is, of course, very important that pieces of the cover are not broken free and propelled toward the vehicle occupants, lest the occupants be injured by the flying pieces. A common way of ensuring that the cover is not broken into loose pieces is to provide reinforcing, commonly a net, to keep broken pieces of the cover from scattering. Examples of covers having reinforcing are disclosed in Japanese published patent applications Nos. 234764/1987 and 127336/1975 and Japanese published utility model applications Nos. 80928/1977, 43454/1975 and 25342/1976.
Covers with reinforcing nets are commonly made from a polymeric material with a relatively low strength, such as polyurethane foam. The net reinforcement is molded into the cover material and is interrupted to form zones of weakness so that the cover can be broken easily along those zones when the air bag inflates.
Japanese published utility model application No. 76042/1977 describes and shows an air bag cover having an external surface layer or skin of a hard polyurethane foam that provides a smooth external surface finish and a core of a low-density polyurethane foam that imparts softness (yieldability) to the cover as a whole. Zones of weakness in the form of slits or grooves are formed in the core to facilitate breaking of the cover.
The manufacture of air bags with reinforcing nets involves many steps, and it is difficult to make them with high accuracy, even when Reaction Injection Molding (RIM) is used, thus leading to a low yield rate and a high production cost.
The cover of Japanese Published Appln. No. 76042/1977 has a hard surface (Shore Hardness, 30 to 40, ASTM-2240) which is uncomfortable to the occupant if he contacts it. Also, the cover materials are of low strength, and because there is no reinforcing net, the cover is prone to being fragmented into pieces that are scattered when the air bag inflates.