1. Technical Field
The present disclosure relates to a resin energy absorber and a method for forming the resin energy absorber. In particular, the disclosure relates to a resin energy absorber that ensures sufficient deformation stroke not only in the case where a uniform impact load is applied to the whole energy absorber but also in the case where an impact load is applied from an oblique direction. This resin energy absorber reduces local variation in energy-absorbing property. In particular, the disclosure also relates to a method for forming the resin energy absorber that has: a low-cost, lightweight, and simplified structure; and a satisfactory energy-absorbing property. This method forms the resin energy absorber with satisfactory formability.
2. Related Art
An impact energy absorber for vehicle protects an occupant inside the vehicle by absorbing impact energy caused by external impact load due to vehicle crash. The impact energy absorber for vehicle is installed, for example, inside a door panel or a ceiling panel.
An impact load by an unexpected vehicle crash makes it difficult to predict a position and a direction of the impact applied to the impact energy absorber for vehicle. Enlarging a surface for receiving this impact load in the impact energy absorber for vehicle to deal with the unpredictable impact load makes it difficult to reduce the weight of the vehicle.
The impact energy absorber for vehicle includes one of structures that are classified broadly into a grid-shaped rib type, a truncated-cone-shaped rib type, and a long-groove-shaped rib type.
Japanese Patent No. 2775146 discloses the grid-shaped rib type. The grid-shaped rib type is made from synthetic resin material. The grid-shaped rib type has, on its one surface of a planar top panel, plate-shaped ribs with predetermined height disposed upright in a grid pattern. The plate-shaped ribs are coupled to one another in the grid pattern and are integrally deformed upon receiving an impact load. This reduces local variation in absorbing property of the impact energy absorber for vehicle. This structure increases a repulsion force in response to the impact load, thus making it difficult to sufficiently deform the impact energy absorber for vehicle by the impact load. In view of this, this structure has difficulty in ensuring a desired impact energy-absorbing property.
Further, the grid-shaped rib type is formed by injection forming. The injection forming requires the grid-shaped rib disposed upright relative to a top panel. In view of this, in the case where an impact load is obliquely applied to the load receiving surface of the impact energy absorber for vehicle, the grid-shaped rib tends to fall down. This makes it difficult to ensure a sufficient deformation stroke. Due to the grid-shaped rib, on the other hand, there is a limit to thinning the impact energy absorber for vehicle. This increases the repulsion force of the impact energy absorber for vehicle. This makes it difficult to sufficiently deform the impact energy absorber for vehicle under the impact load.
Japanese Patent No. 3186563 discloses the grid-shaped rib type, similarly to Japanese Patent No. 2775146. The grid-shaped rib type includes a plurality of ribs, which are oriented in a predetermined direction and disposed in a grid pattern, and a base plate that is integrally formed with respective ends of the plurality of ribs. The plurality of ribs is installed on the base plate along the direction of the impact load. The injection forming requires a draft angle. The draft angle thickens a root portion of the rib, thus making it difficult to deform the rib. The rib includes a cross-sectionally reduced portion made by forming a cut-out portion. The cross-sectionally reduced portion reduces an increase of the deformation load when the deformation, which is caused by the impact load, of the ribs advances toward the base plate of the ribs. This actively promotes the deformation of the ribs.
However, the grid-shaped ribs are formed by injection forming similarly to Japanese Patent No. 2775146. This requires the grid-shaped rib disposed upright relative to the base plate. In view of this, the grid-shaped ribs easily fall down in the case where the impact load is obliquely applied to the load receiving surface of the impact energy absorber for vehicle. This makes it difficult to ensure a sufficient deformation stroke.
Japanese Patent No. 4597832 discloses the truncated-cone-shaped rib structure. The truncated-cone-shaped rib type is made from synthetic resin. The truncated-cone-shaped rib type includes a plurality of truncated-cone-shaped ribs arranged at required intervals, and surface connecting portions, which are located in areas where the truncated-cone-shaped rib is missing. The surface connecting portion is coupled to bottom portions of these truncated-cone-shaped ribs to be supported. The truncated-cone-shaped ribs include a plurality of slits around the truncated-cone-shaped ribs, the slits allowing outer peripheral side faces of the truncated-cone-shaped ribs to deform outward when the truncated-cone-shaped ribs are crushed and deformed. This structure increases the repulsion force since the truncated-cone-shaped ribs are each independent. This structure sufficiently deforms the impact energy absorber for vehicle under the impact load. However, this structure is likely to cause local variation in absorbing property of the impact energy absorber for vehicle.
More specifically, the truncated-cone-shaped rib type has a plurality of slits around the truncated-cone-shaped ribs. The truncated-cone-shaped ribs tend to cause circular apical surfaces of the truncated-cone-shaped ribs to bottom out in the case where the impact load is squarely applied to the load receiving surface of the impact energy absorber for vehicle. This makes it difficult to ensure the sufficient deformation stroke of the impact energy absorber for vehicle. On the other hand, the truncated-cone-shaped ribs tend to fall down in the case where the impact load is obliquely applied to the load receiving surface of the impact energy absorber for vehicle. This makes it difficult to ensure the sufficient deformation stroke of the impact energy absorber for vehicle.
WO 2008/105517 A discloses the long-groove-shaped rib type. The long-groove-shaped rib type includes a first wall on the impact-receiving side, a second wall facing the first wall through a hollow portion at a distance, a deep groove portion, and a plurality of impact absorbing ribs. The deep groove portion includes a melt-bonding surface that is formed by hollowing each of the first wall and the second wall to make long groove shapes and integrally bonding respective apical surfaces of the long groove shapes. The plurality of impact absorbing ribs includes a shallow groove portion that faces the apical surface of the deep groove portion at a distance. The long-groove-shaped rib type is different from a configuration disclosed in Japanese Patent No. 4597832 in the following points. The long-groove-shaped rib type includes a long groove formed by “vacuuming respective mold-side surfaces of two sheets of molten thermoplastic resin toward corresponding molds” or “applying pressure to the respective other surfaces of the two sheets of molten thermoplastic resin sheets toward the corresponding molds”. The long-groove-shaped rib type is integrally formed by melt-bonding with the apical surfaces of the long grooves by clamping split mold blocks. The long-groove-shaped rib type includes a hollow portion that is formed through formation of a ring-shaped parting line.
This configuration ensures reduced local variation in absorbing property of the plurality of impact absorbing ribs in a long groove shape, compared with the independent truncated-cone-shaped rib disclosed in Japanese Patent No. 4597832. However, the long-groove-shaped rib type lacks a rib oriented across the plurality of impact absorbing ribs. This is likely to cause the impact absorbing ribs to fall down in the case where the impact load is obliquely applied to the load receiving surface of the impact energy absorber for vehicle. This makes it difficult to ensure a sufficient deformation stroke of the impact energy absorber for vehicle.
Additionally, the long-groove-shaped rib type increases air pressure in a hermetic hollow portion in the case where the impact load is squarely applied to the load receiving surface of the impact energy absorber for vehicle. This increase in the air pressure increases a repulsion force, which makes it difficult to ensure the sufficient deformation stroke of the impact energy absorber for vehicle under the impact load. This makes it difficult for the impact energy absorber for vehicle to have a desired energy-absorbing property. This requires an extra process such as providing an opening on the load receiving surface.