Receptacles – Table dish
Reexamination Certificate
2000-08-31
2002-04-16
Moy, Joseph M. (Department: 3727)
Receptacles
Table dish
C220S575000, C425S384000, C425S388000, C264S549000
Reexamination Certificate
active
06371327
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to disposable food service articles and more particularly to compartmented food service articles thermoformed from plastic sheet by way of plug-assist thermoforming.
BACKGROUND
Plug-assist thermoforming is often desirable, especially to aid in the redistribution of material into a mold. Typically, a mechanically driven plug is used to prestretch the plastic sheet into a female mold cavity or around mold features. Vacuum and/or pressure is then applied to pull and/or push the sheet against the mold surface where it is cooled to set the shape. The heat softened plastic sheet can either be formed up (plug on bottom/mold on top) or formed down (plug on top/mold on bottom). Numerous other variations of plug-assisted thermoforming are also possible as discussed by Professor Throne. A detailed discussion of plug-assist thermoforming is set forth in a book entitled
Thermoforming
by James L. Throne (1987).
As noted on pg. 207 of
Thermoforming:
“The plug design parameters include the shape of the plug tip, the plug penetration depth (relative to the cavity depth) and the plug diameter (relative to the cavity diameter). The plug surface temperature and coefficient of friction (between the plug surface and the stretching sheet) are also considered design factors.”
Plugs can be produced from wood, syntactic foam, aluminum, etc. Plugs are frequently heated or cooled. Syntactic foam plugs typically are not heated or cooled since they are insulators. Syntactic foam is commonly used since it is easily machined and does not instantaneously chill the plastic sheet upon contact. Plugs may be designed with a hollow center bottom so they do not cool the bottom material and can more easily redistribute the material towards the corners where sheet thinning will often occur. Internal mold corners and edges will typically be substantially thinner than the original plastic sheet thickness from the sheet stretching process as detailed in chapters 4 and 7 of
Thermoforming.
In order to overcome the problems of sheet thinning and to assist the formation of shaped articles, a variety of plug-assist techniques have been employed. See, for example, U.S. Pat. No. 5,641,524 of Rush et al. where an apparatus useful for thermoforming a disposable cup is illustrated.
Some compartmented plastic articles can be produced without any plug-assist. Large generous internal corners and edges, gentling sloping side angles and shallow depths are often features designed into the products to minimize sheet thinning. Higher caliper sheet can also be used to compensate for sheet thinning. This increases the cost for the disposable plastic products and may not be an acceptable alternative.
Plug-assist forming can be applied to aid in the stretching and material distribution for the compartmented plastic product. One option for a plug design may be an offset of the mold profile and rib geometry (angle, height, etc.) stroked to within ⅛ to ¼ inch from the mold bottom and/or rib sidewall & top. Another option is to employ a plug with larger external radii between the plug bottom and relieved offset rib profile. Such alternatives do not provide the advantages of the present invention as discussed hereinafter.
SUMMARY OF INVENTION
The plug design of the present invention typically has a near vertical rib relief sidewall (~90 degrees to plug bottom), and a near sharp/small external radii between the plug bottom and near vertically relieved rib sidewall. It was determined through experimentation/prototype production, that this design provided the least amount of rib to plate bottom corner thinning compared to two other plug designs. The preferred compartmented plate design has near sharp rib to bottom & rib to profile junctions where excessive thinning can occur. The excessive thinning may allow food fluids to seep through due to plastic “pinholing”.
Specific improvements of the present invention include positioning the near sharp external radii between the plug bottom and vertical relief areas near vertically in-line with the mold rib to bottom & rib to profile near sharp junctions. It is possible to extend the plug along the plate mold sidewall profile to reduce sheet thinning at the transition of the rib to sidewall profile. The bottom portions of the plug may be concave hollowed such as not to contact/cool the sheet and to allow more stretching into the corner near sharp junctions to further reduce bottom thinning.
The plug design of the present invention provides utility for compartmented disposable food serving containers regardless of their shape. The overall container shape could be round, oval, square, rectangular or polyhexal with rounded corners. Containers with any number of compartments with ribbed dividers greater than two can be formed with this plug design. This design is most useful when the rib to bottom & rib to profile junctions are near sharp. Any form of plug-assisted thermoforming can be practiced with the described plug design and positioning technique. The unique plug design allows the use of low caliper, lighter weight plastic sheet to produce containers with near sharp rib edge designs and minimizes the severe corner thinning that often occurs.
Salient features of the plug design which provide the thermoforming and container benefits in particular embodiments are:
1. Near Vertical Rib Relief
2. Near Sharp/Small Radii External Rib Relief Edges
3. Rib Relief Edges Positioned Near Vertically Aligned Above Mold Rib to Bottom and Rib to Profile Junctions
4. Plug Closure to Approximately {fraction (1/16)}″ From Product Profile and/or Rib
In another aspect of the invention, there is provided compartmented disposable food serving article having a substantially planar bottom portion and a plurality of rib portions configured so as to segment the article into a plurality of food serving compartments, wherein the rib portions are substantially angularly joined to the bottom portion and wherein the food serving article is thermoformed utilizing a plug-assisted thermoforming process. The process includes: positioning a softened plastic sheet to be in close proximity to a thermoforming mold having a substantially planar mold surface and a plurality of rib portions projecting axially from the substantially planar mold surface. Typically, the heat softened plastic sheet is perimeter or individually clamped around the thermoforming mold or molds to create a seal. The rib portions define a plurality of angular junctions with the substantially planar mold surface. The second step of the process is thermoforming the disposable compartmented food service container, wherein a plug-assist member is applied to the softened plastic sheet to urge the sheet toward conformity with the thermoforming mold. The plug-assist member includes a plurality of rib relief portions positioned, configured and dimensioned to fit about the rib portions of the mold in a forming operation at a forming clearance distance of about 0.125 inches or less from the mold. The plug is also provided with a plurality of substantially vertical wall rib relief portions configured and dimensioned to be substantially vertically aligned with the angular junctions of the rib portions of the mold with the substantially planar mold surface. The vertical wall portions are adjacent a plurality of angular corner portions transitioning between the vertical wall rib relief portions of the plug and a bottom surface of the plug.
The compartmented disposable food serving article generally has a wall thickness of from about 10 to about 80 thousandths of an inch (“mils”) with a wall thickness of from about 15 to about 25 mils being more typical.
The articles may be formed from a variety of filled or unfilled polymers such as addition polymers or condensation polymers. Polymers employed may be those listed on page 53 of
Thermoforming
noted above, particularly ABS; Acronitrile; Acetate, PMMA; Acrylic/PVC; Butyrate; PC; PET; Polyethersulfone; 20% GR PES; HDPE; Propionate; PP; 40% GR PP; P-Sulfone; P-St
Lamson Wayne W.
Littlejohn Mark B.
Swiontek Anthony J.
Ferrell Michael W.
Fort James Corporation
Moy Joseph M.
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