Method of compacting airbag cushions

Abstract

A method of thermal compacting of an airbag cushion is described where rapid heating is conducted by a microwave beam directed at a mold through a radiation transparent wall without materially heating the mold itself.

Claims

1. A method of rapid thermally forming a compressed cushion article deposited in a demountable mold, and moistened, including; heating the article to around the boiling point of water under normal pressure by a divergent microwave radiation beam, directed through at last one portion of the mold, which portion is made of microwave transparent material, and wherein the beam is formed to about the shape of sail mold portion or scanned over said mold portion, and wherein the frequency of sail radiation is selected to provide penetration depth of sail beam for substantially volumetric heating of the compressed article, and further heating said article by sail beam, while under vacuum, until the compressed article is completely dried and will substantially retain a prearranged shape after cooling and removal from the mold.

2. The method of claim 1 wherein the microwave transparent materials are selected from quartz, Teflon and nitride ceramic.

3. The method of claim 1 wherein the microwave radiation frequency is selected to be between about 28 GHz to about 100 GHz.

4. The method of claim 3, wherein the microwave beam source is selected from the group consisting of gyrotron, klystron, and traveling-wave tube.

5. The method of claim 1 wherein said mold is also ide of materials that reflect or transmit sail microwave radiation.

6. The method of claim 5, wherein said mold is made of metal, quartz or Teflon.

7. The method of claim 1, wherein said mold is at least partially lined inside by a microwave transparent material liner.

8. The method of claim 7, wherein the thickness of said liner is selected to be a multiple of about half of the radiation wavelength used in the liner material.

9. The method of claim 1, wherein the power density profile generated by the microwave beam is correspondent to the shape profile of the article.

10. The method of claim 1, wherein said article is provided with at least one additional layer selected from ceramics, rubber, polar polymers, plastics and organics, and is positioned adjacent to said article on the side opposite to the microwave transparent mold wall portion and each layer of said article is heated to its predetermined temperature range to form a composite article.

11. The method of claim 1, wherein at least one means is incorporated in said mold body to provide moistening and/or cooling of the article.

12. The method of claim 11, wherein article is moistened during heating.

13. The method of claim 11 wherein sail article is moistened shortly before creating a low vacuum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing an airbag cushion inside a mold heated by a microwave beam in accordance with the invention.

(2) FIG. 2 is a graph showing the dependency of penetration depth in nylon vs. radiation frequency.

(3) FIG. 3a. is a graph showing the diagram of the heating of the article at normal pressure.

(4) FIG. 3b. is a graph showing the diagram of the heating of the article at low vacuum.

(5) FIG. 3c. is a graph showing the diagram of the heating of the article in a regular air convection oven under normal pressure.

(6) FIG. 4 is a graph showing the dependency of the water boiling point temperature as a function of pressure.

DETAILED DESCRIPTION OF THE INVENTION

(7) A general object of the invention is to improve the method of manufacturing airbag cushions, and more specifically, to accelerate thermally forming of a cushion article.

(8) In the invention the heating of the compressed cushion article (1, FIG. 1) placed in the mold (2) to around the boiling point of water is accomplished under normal pressure by a divergent microwave radiation beam (3), through at least one portion of the mold wall (4) that is made of microwave transparent material such as Teflon, quartz, nitride ceramics, and like. Further heating is provided under low vacuum by the beam until the compressed article is completely dried and the prearranged shape is retained after cooling and removal from the mold.

(9) In the invention the approach of the microwave radiation to the side and bottom of the article (3,a; 3.b; and 3c) is improved through the at least partial lining (5) inside the mold. This promotes better uniformity of the article heating which means higher microwave power can be applied and faster heating can be provided. It is proposed to select the material for lining from the same group as selected for the microwave transparent portion of the mold wall (4).

(10) For the same reasons, the power density profile generated by microwave beam is selected to form a shape correspondent to the shape profile of the article. This approach allows achievement of additional savings in heating time to provide the heating of the thicker areas by higher power density and vice versa. This excludes unnecessary time for the heat to equalize over the article. In one embodiment of the invention the described heating profile is achieved by scanning the beam over the transparent mold wall

(11) It is further preferred to select the thickness of the liner to be about a multiple of half or more of the radiation beam wavelength used in the liner material. In this case the losses of the microwave energy is minima).

(12) In the inventive method the heating is conducted under normal pressure at the beginning until the article temperature of around the boiling point of water is achieved. Because the article is a compressed multilayer structure, the evaporation of the water/moisture from the cushion is obstructed and takes a much longer time than heating itself does (see FIG. 3a). Therefore further heating is preferred under low vacuum allowing significant reduction of the processing time (see FIG. 3b). The low pressure that surrounds the article helps the steam with air to be extracted from the article. However the low pressure reduces the boiling point of water and hence the cushion temperature is also reduces due to evaporating (see FIG. 4.) This has a negative effect on the forming of the article because it should be hot until it is completely dried. Volumetric microwave heating of the article is not affected by the low vacuum. The continuation of the microwave heating during the evaporation under low vacuum permits maintaining the article hot enough until it is dry. The article will substantially retain the prearranged shape after cooling and removal from the mold.

(13) The present invention provides the opportunity to moisten the article at any appropriate moment during the heating inside the mold by using correspondent means incorporated in the mold. It is possible, because the article is heated directly in the mold, that the metal or microwave transparent mold remains cold. This makes the processing time shorter by avoiding screening the article from microwave. Water has a higher absorption of microwave than any article material. Cooling time of the production cycle is reduced by cooling the article directly inside the mold. The provided or separate means can be used for cooling.

(14) Heating the article directly in the mold by microwave does not require heating the mold itself and significantly reduces the cooling time.

(15) In one embodiment of the present invention the mold is made of materials that reflect or transmit microwave radiation, for example, metal, quartz, Teflon, and the like and therefore the mold stays practically cold during entire forming process. This is significantly reduces the forming time.

(16) Further reduction of processing time is realized if the power density profile generated by the beam is formed to be correspondent to the profile of the article thicknesses. This provides irradiation of the thicker areas of the article with more power to heat faster. This also increases efficiency of the microwave beam heating.

(17) The present invention also provides the opportunity to form a composite article that has at least one more additional layer such as ceramics, rubber, polar polymers, plastics, organics, and the like. This additional layer is adjacent to the article from the side opposite to the mold wall transparent portion and each layer of the article is heated fast and is controllably to its predetermined temperature range by selection of the beam wavelength to provide penetration depth of the radiation greater than the thickness of the article portion that is on the top of the additional layer. At that, the wavelength selection provides appropriate absorption of the beam energy in each portion of the article to thereby heat them to predetermined temperatures.

(18) The present invention has been described in an illustrative manner. It is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the present invention may be practiced other than as specifically described.

EXAMPLES

(19) The following examples are presented to provide a more detailed explanation of the present invention and of the preferred embodiments thereof and are intended as illustrations and not limitations.

(20) Nylon fabric having surface density of 560 grams per square meter was chosen for the heating experiments. Three identical pieces of fabric, 100 gram each, were sprayed with 3 grams of water each, then each was identically folded to form a multilayer pack and placed (pressed) into a cylindrical metal mold with Teflon covering the top. Thickness of the pressed fabric was 7 mm. The temperatures inside the fabric pack was measured by a K-type thermocouple placed inside each pack and recorded by a data logger. There were three experiments:

(21) Experiment 1. Heating was conducted by a 60 GHz gyrotron beam in a metal shield chamber under normal pressure. The beam was scanned over a Teflon cover. The irradiation (heating) was conducted until water was completely evaporated (until recorded temperature inside the package started to raise over 100° C., see FIG. 3a).

(22) Experiment 2. Heating was conducted in a vacuum chamber with a microwave transparent wall. The vacuum chamber was placed in the mentioned microwave chamber. The heating to 100° C. was conducted under normal pressure (as in experiment #1) and further heating was conducted under low vacuum (100 mBr absolute pressure). The boiling point of water for the selected vacuum level is less than 50° C. Heating was stopped when recorded temperature was over 100° C. and it was witnessed that the fabric was completely dried (see FIG. 3b)

(23) Experiment 3. Heating was conducted in a regular air convection oven under normal pressure. Air temperature was 160 degree Celsius. Heating was conducted until water was completely evaporated (until recorded temperature inside the package started to raise over 100° C., see FIG. 3c).

(24) The foregoing examples illustrates that the invention heating method allows extreme acceleration for thermally forming a compressed cushion article in the production of airbags, plastic containers, sole structures, and the like.