Thermoformed articles having an improved insert therein

Details Thermoformed articles having an improved insert therein Thermoformed articles having an improved insert therein

1999-03-03

2001-07-24

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Plural parts with edges or temporary joining means each...

C428S076000, C428S520000, C428S522000, C428S053000, C004S593000, C004S612000, C027S007000

Reexamination Certificate

active

06265036

ABSTRACT:

TECHNICAL FIELD
This invention relates to reinforcing inserts used in combination with thermoformable sheet material. These inserts serve two functions. First, they provide at least one rigid surface over which the sheet is thermoformed. That is, they act as a mold for the heated sheet during the thermoforming process. Second, these inserts provide reinforcement to the finished product such as acrylic spas, shower stall floors, burial caskets, and the like.
BACKGROUND OF THE INVENTION
Thermoformable products such as acrylic spas, shower stalls, burial caskets, and the like inherently suffer from the same drawback. Economic and manufacturing limitations make the finished product susceptible to fracturing at stress bearing points. For instance, the steps and bottom of spas, shower stall floors, and any point of curvature of an acrylic sheet are all prone to cracking. These stress points develop because the acrylic sheet flexes under load and the stresses concentrate in specific areas to a level high enough to cause the part to fail.
Thermoformable sheets are typically heated above their heat distortion temperature (HDT) in the initial stages of a thermoforming process. This causes the sheet to expand proportional to the material's coefficient of thermal expansion (COTE). After the forming stage of the process, the sheet is allowed to cool. The cooling stage causes the sheet to shrink unless the shrinkage is inhibited by a foreign object such as a typical prior art mold or insert. As the sheet cools around a rigid insert, it cannot shrink easily, if at all. Therefore, stress accumulates in some areas, such as in comers. Thinning of the material also occurs in these areas. Usually, the inserts will restrict the contraction of the sheet in areas of curvature and the material becomes susceptible to fracturing. The thermoformed product may even crack during cooling to room temperature. This deficiency is characteristic of all plastic sheet materials and is particularly evident with acrylic sheets, such as poly(methyl) methacrylate (PMMA). It may also occur in composite sheets made of PMMA and a thermoplastic substrate layer such as acryonitrile butadiene styrene (ABS), or other materials.
Attempts have been made to reinforce acrylic and similar type products. Favaron, in U.S. Pat. No. 5,400,556 discloses a step system for swimming pools comprising a unitary stair module including a stair shell formed of plastic sheet material and a plurality of elongated rigidizing members which are encapsulated within the plastic sheet material of the shell, and support braces for supporting the unitary stair module.
While the above reference discloses a method for providing support to a thermoplastic sheet material, the difficulties associated with stress build up in and near areas of curvature have not been avoided. These difficulties are not as critical in products such as a modular step systems wherein there are few areas of curvature greater than 90°. That is, where the thermoplastic material has been deformed by more than 90°. Additionally, the above reference does not address the difficulties encountered with sheet material thermoformed over an area greater than a few square feet.
The present invention alleviates the stress associated with deformation of an acrylic or other thermoplastic sheet by more than about a 90° angle. The present invention also alleviates the stress associated with thermoforming large areas of sheet material over male type mold configurations. By male type configurations, we mean thermoformed sheets placed over a shaped profile, wherein the shape raises from the horizontal plane and the thermoforming process allows the sheet to be draped over it. Inserts are positioned on top of the male type configuration or the horizontal surface. The inserts will be enveloped by the formed sheet, so that they will be trapped by it. In the present invention, “removable plug” section(s) are positioned within the inserts, and can be removed when the thermoformed part is removed from the mold. In doing so, the inserts are allowed to become shorter, thus the thermoformed material is not subjected to the same level of stress, that a thermoformed material would be exposed to with a typical molded insert. This technique allows for a stronger, more resilient finished product that is not as susceptible to fracturing.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a new and improved type of insert for use with thermoformable plastic and acrylic sheets. These inserts provide rigidity and stability to the thermoformed product, and become an integral part of the finished product.
Another object of the present invention is to provide a new and improved type of insert having at least one removable portion that, prior to complete cooling of the sheet material, is removed thereby allowing the insert to, in effect, shrink from its original size.
A further object of the present invention is to provide a new and improved type of insert wherein stress fracturing in areas of deformation of the original sheet material is either eliminated, or is greatly reduced. This is believed to be attributable, to a large extent, to the ability of the insert of the present invention to shrink from its original size after the removable portion is removed. The insert of the present invention has the ability to reduce, or eliminate stress, and the accompanying tendency to fracture, but its reinforcing properties are not compromised to any appreciable extent.
Yet another object of the present invention is to provide a method of reducing stress in thermoformed products having a permanent insert incorporated therein by pre-heating the mold insert prior to thermoforming the plastic or acrylic material, thereby reducing the difference in the COTE between the thermoformable material and the mold insert. That is, a heated insert will also shrink to some extent, depending on its own COTE, upon cooling. This shrinkage creates space for the sheet material to contract into, which thereby reducing stress in the shrinking acrylic or thermoplastic material.
The new and improved inserts of the present invention find use in such applications as the spa, burial casket, and plumbing-ware industries. These inserts possess the unique feature of having at least one removable portion. The insert allows the thermoplastic or acrylic (both referred to herein as sheet material) to be thermoformed around the insert thereby providing rigidity and support to the final product. Removal of the removable plug section of the insert allows the insert structure to, in effect, shrink or constrict from its original size.


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patent: 5525290 (1996-06-01), Carpenter
patent: 5716581 (1998-02-01), Tirrell et al.
patent: 5834126 (1998-11-01), Sheu

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