1. Field of the Invention
The present invention relates to a process for producing three-dimensional stiffening elements of a meltable plastic powder or a powder mixture comprising the meltable plastic powder in a template screen printing process and connecting the element to a substrate, particularly shoe parts, and to an apparatus for implementing the process.
2. Description of the Background
The use of thermoplastic stiffening materials in the shoe, clothing and leather accessory industries has been known for a long time. In most cases, the stiffening material is manufactured in sheet form and is used as pieces cut therefrom according to a pattern. This is done in the shoe industry, e.g., to stiffen the region of the heel, and possibly also the region of the toe.
The cut pieces also called stiffening elements or components are cut out of the sheet material by means of a cutting tool and made thinner by a skiving-process at least at the side which is not going to be folded under the "last". The cut pieces are moldable with the aid of heat, and once shaped and cooled, they should display good flexibility and shape retention.
The activation and fixation of the stiffening elements on the leather upper is effected as a function of the characteristics of the thermoplastic material either under heat or with the aid of a solvent. For toe reinforcements, a hot-melt adhesive is often spread onto one or both sides of the stiffening elements which are then pressed on the upper part of the shoe to be reinforced by means of a hot molding plug.
It is known that sheets of thermoplastic stiffening material can be manufactured in various ways, such as, for example, by coating or impregnating woven or knit or non-woven textile structures with dispersions, solutions or melted thermoplastic synthetics. According to U.S. Pat. No. 3,427,733, the sheets may also be produced by extrusion of thermoplastic synthetics or by extrusion coating of textile sheets.
DE-OS No. 2,621,195.A1 disclosed other stiffening materials and stiffening elements which are produced by melting powder mixtures of plastics, fillers and possibly other auxiliary substances onto textile substrates. It is also possible to produce molded elements withou substrate materials from these powder mixtures.
U.S. Pat. No. 3,973,285 disclosed applying stiffening material in the molten state to the upper portion of a shoe which is clamped in and held along its edges. According to this reference, the process stages of "producing the sheet material", "coating with melt adhesive", "cutting," "skiving" and "hot pressing" can be dispensed with.
The above process is used in practice for the manufacture of inexpensive shoes due to the low viscosity of the stiffening material at 190.degree. C. it has, however, a number of drawbacks such as cracking (degrading) which appears in the melt during production down times for formation of fibers, poor adhesion, and the like.
Thus, the person skilled in the art continues to operate within very close limits on the basis of various extremely critical process parameters. Applicable substances are usually not employed in practice because it is not economical.
Furthermore, DE-OS No. 2,159,226 disclosed a process in which the molten siffening material is initially poured into a mold in the first station of a machine so as to form a desired element. Then the mold is brought to the second station, in which the uncooled and still sticky part of the element is glued under pressure to the shoe upper to be stiffened. This process also lacks in ease of operation and still has considerable drawbacks.
According to DE No. 3,347,237.A1, the disadvantages of the prior art are to be overcome with a machine in which stiffening elements of a predetermined shape are produced from a meltable plastic powder instead of the molten plastics used in the past. As in the process according to DE No. 2,621,195.A1, the machine operates with three process stages; namely applying the powder in a certain shape, heating until the material melts and cooling under pressure. According to the prior art reference, the meltable powder is applied in the form of a desired stiffening element onto a receiving belt or auxiliary carrier in a screen printing process. It is then melted in a heat station of the machine, glued to the cut shoe upper to be stiffened in a press station and cooled under pressure. This is attained while keeping and receiving side of the substrate to be reinforced facing downward. This is a significant drawback.
Problems arise often during the operation of this machine for the manufacture of stiffening elements due to the difficulties observed after application of the powder, the high melting temperatures, the long periods of dwelling in the heating station and difficulties in positioning the stiffening element when it is transferred from the receiving belt or auxiliary carrier to the shoe upper to be stiffened. Up to the present time no satisfactory solution to these problems had been found. The high melting temperatures for the plastic powder (191.degree. to 260.degree.) require, for example, heating times up to about 25 seconds and more.
Processes for orienting the substrates to be reinforced during their residence time in the above machines are disclosed in DE No. 3,444,365.A1. However, construction costs for transferring the molten reinforcement material from the auxiliary carrier to the receiving side of the upper piece of the shoe to be reinforced are considerable and in many cases not economically justifiable.