Method of making rotatable rolls of plastic material

Details Method of making rotatable rolls of plastic material Method of making rotatable rolls of plastic material

2000-07-03

2002-09-03

Ball, Michael W. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S242000, C156S245000, C264S242000, C264S255000, C264S264000, C264S328700, C029S895000, C492S049000, C384S052000, C384S587000

Reexamination Certificate

active

06444065

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for producing rotatable plastic rollers comprising a spindle, an outer running roller layer and an inner running roller layer.
Plastic rollers of any type, in particular those with a metal shaft as the spindle, are customarily produced by mounting onto a spindle a separately produced plastic roller. In order that the plastic roller does not slip off the spindle after mounting, it must either have an undercut, for example an annular bead which grips in an annular depression on the spindle, or be secured by laterally fitted disks.
A major disadvantage of these processes is that the mounting of the plastic roller is time-consuming and often labor-intensive. What is more, the undercuts must be produced in such a way that the mounting of the plastic roller onto the spindle can still be accomplished. This generally leads to considerable backlash between the plastic roller and the spindle, which can have the effect of imprecise running, running noises or greater wear. If the plastic roller is secured by lateral disks, further mounting work is required and additional material costs arise.
It is also known to construct plastic rollers from a plurality of layers of different plastics. For example, in DE 93 19 307 U1 there is a description of castors for rollers which comprise a rim body and a hard outer shell as the tire, respectively of thermoplastic polymer, and also a soft, sound-absorbing intermediate layer. In this case, the intermediate layer is optionally formed by an injected elastomer, which becomes solidly bonded to the rim body and the tire by contact fusion in the surface regions of the latter, or by a polyurethane injection molding which is mechanically connected securely to the rim body and the tire, for example by undercuts or profiled grooves.
SUMMARY OF THE INVENTION
The object consequently was to provide a more cost-effective and environmentally friendly process for producing rotatable plastic rollers.
This object is achieved according to the invention by a process for producing rotatable plastic rollers comprising a roller shaft (=spindle of the plastic roller), an outer running roller layer and an inner running roller layer, in which process the inner running roller layer is produced from a thermoplastic material between the outer running roller layer and the roller shaft, the inner running roller layer bonding solidly with the roller shaft and a sliding gap forming between the inner running roller layer and the outer running roller layer.
In this process, use is made of the property of thermoplastic materials usually considered to be a disadvantage, that they undergo a greater or lesser contraction in volume (=shrinkage) during cooling down from the melt. According to the invention, this often undesired property is utilized by filling the cavity between the roller shaft and the outer running roller layer with the melt of a thermoplastic material in the production of a plastic roller. During cooling down, the thermoplastic material shrinks onto the roller shaft on account of the contraction in volume and is consequently solidly bonded to the latter. At the same time, the plastic shrinks away from the outer running roller layer, whereby a sliding gap is formed and the outer running roller layer becomes rotationally movable around the inner running roller layer. Further advantages of the process according to the invention consist in that neither a subsequent mounting step nor any securement of the plastic roller by additional components is required.
The process according to the invention can be carried out in an extremely simple way by initially placing the roller shaft and the outer running roller layer in an injection mold, referred to hereafter as mold for short, and injecting the thermoplastic material into the remaining cavity between the roller shaft and the outer running roller layer.
However, the process according to the invention is preferably carried out by the multi-component injection-molding process, in that only the roller shaft is initially placed in the mold and both the outer running roller layer and the inner running roller layer are produced by injection molding in the mold. For this purpose, a mold core, also referred to as a slide, is introduced into the mold around the roller shaft, so that there remains a cavity in the form of the outer running roller layer to be produced, into which a plastic is then injected. Once this plastic has cured sufficiently, the slide is removed, i.e. opened, and a thermoplastic material is injected into the cavity thus formed around the hollow shaft. During cooling down, this plastic then shrinks, as described, onto the roller shaft and away from the outer running roller layer.
In particular in the case of plastic rollers with a metal roller shaft, to dampen the running noises it is appropriate to sheath the roller shaft with a thermoplastic elastomer. This can take place on the one hand by a corresponding sheathing already having been applied to the roller shaft before it is encapsulated in the mold. On the other hand, however, it is also possible to choose the dimensions of the slide such that there remains between the slide, which in principle has the form of a cylinder jacket, and the roller shaft a cavity into which the thermoplastic elastomer can be injected. During cooling down, this plastic then shrinks onto the roller shaft and consequently provides a secure hold. After opening the slide, the thermoplastic material is then injected into the remaining cavity between the outer running roller layer and the sheathing comprising the thermoplastic elastomer. The thermoplastic material then shrinks onto the layer comprising the thermoplastic elastomer. The thermoplastic material and the thermoplastic elastomer are preferably chosen such that these two plastics enter into a chemical or physical bond with each other at their interface.
Rotatable plastic rollers which are constructed from any number of layers can be produced by the multi-component injection-molding process according to the invention. However, there must be at least two layers, the outer running roller layer and the inner running roller layer. During the production of plastic rollers by the process according to the invention, furthermore, it must be ensured that good adhesion is produced between the roller shaft and the inner running roller layer and that the materials from which the outer and inner running roller layers are made cannot enter into a bond with each other. The roller shaft and the outer running roller layer may generally consist of a plastics material, for example a thermosetting material or a reinforced or partially crystalline thermoplastic material, or of a metallic material. The roller shaft is preferably made of steel, while friction-modified plastics, for example friction-modified polyacetals, polyesters or polyamides, are suitable in particular for the outer running roller layer. Friction-modified plastics are understood here to mean those plastics which contain lubricants, i.e. additives for improving the sliding and dry-running properties. Suitable additives are generally known for all plastics.
In principle, all known thermoplastic materials are suitable as the thermoplastic material for the production of the inner running roller layer, for example polyacetals, e.g. polyoxymethylene (POM), polyester, e.g. polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyamides (PA), in particular nylon
6
, nylon
46
or nylon
66
, polycarbonates (PC), polyvinyl chloride (PVC) or polyolefins, e.g. polyethylene (PE) or polypropylene (PP). During cooling down from the melt, these plastics should have a contraction in volume of 0.1 to 5%, preferably from 0.3 to 3.5% and particularly preferably from 0.5 to 2.5%. In addition, they may contain additives customary for thermoplastics, as well as fillers and reinforcing agents. Depending on the size of the plastic rollers to be produced, the thermoplastic material is optimized, for example by the addit

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