US3562383A - Method for preventing edge defects in compression stretched acrylic sheets - Google Patents

Method for preventing edge defects in compression stretched acrylic sheets Download PDF

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Publication number
US3562383A
US3562383A US758395A US3562383DA US3562383A US 3562383 A US3562383 A US 3562383A US 758395 A US758395 A US 758395A US 3562383D A US3562383D A US 3562383DA US 3562383 A US3562383 A US 3562383A
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Prior art keywords
acrylic
blank
stretched
groove
compression
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Expired - Lifetime
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US758395A
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English (en)
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Ronald L Ayres
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Fortin Plastics Inc
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Fortin Plastics Inc
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C67/00—Shaping techniques not covered by groups B29C39/00 - B29C65/00, B29C70/00 or B29C73/00
    • B29C67/0011—Shaping techniques not covered by groups B29C39/00 - B29C65/00, B29C70/00 or B29C73/00 for shaping plates or sheets
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/24—Structurally defined web or sheet [e.g., overall dimension, etc.]
    • Y10T428/24777—Edge feature

Definitions

  • This invention relates to stretching of acrylic sheets and, more particularly, to the prevention of edge defects in stretched acrylic sheets.
  • acrylic material Because of its strength, formability, and light transmission characteristics, acrylic material has a variety of uses and particularly has been used to form windows and canopies for aircraft. Manufacture of the acrylic part often involves a stretching step in which the acrylic material is heated and stretched to enhance its physical characteristics and/or to provide the acrylic part with a required curvature.
  • stretching of acrylic blanks has been accomplished by either pulling the edges of the acrylic blanks in opposing directions such as is described in U.S. Pat. Re. 24,978 of Bottoms et al. or of compression stretching acrylic blanks as described in U.S. patent application Ser. No. 691,294 of Terry D. Fortin, tiled on Dec. 18, 1967.
  • the method of this invention comprises providing the circumferential end surfaces of acrylic blanks with a substantially continuous groove therein.
  • the depth of the groove is at least about 5% and, preferably, is greater than about 10% of the initial thickness of the acrylic blank.
  • the groove may have a variety of configurations such as, for example, V- and U-shaped configurations and the centerline of the groove may be offset from the centerline of the circumferential end surfaces. However, it is preferable to position the groove so that its center-line corresponds with the center-line of the circumferential end surfaces.
  • FIG. l is a perspective view of an unstretched acrylic blank showing one form of circumferential end groove of this invention formed therein;
  • FIG. 2 is a partial perspective view of a compressionstretched acrylic sheet which was stretched from an acrylic blank having no circumferential end groove therein (i.e., prior art);
  • FIG. 3 is a partial perspective view of a compressionstretched acrylic sheet stretched from the acrylic blank of FIG. l;
  • FIG. 4 is a partial sectional view of the unstretched acrylic blank of FIG. 1 taken along the lines 4-4 of FIG. l;
  • FIG. 4a is a sectional view of the stretched acrylic blank of FIG. 3 taken along the lines 4a-4a of FIG. 3;
  • FIG. 5 is a sectional view of an acrylic blank provided with another form of circumferential groove of this invention but different in shape from the groove of FIG. 4;
  • FIG. 5a is a sectional view of that portion of the acrylic blank shown in FIG. 5 after being compression-stretched;
  • FIGS. 6, 7 and 8 are sectional views of unstretched acrylic blanks having variously shaped and positioned I circumferential end grooves of this invention.
  • FIG. 9 is a partial elevational view of a stretched acrylic sheet showing edge roll produced from compression stretching.
  • the method of this invention comprises providing the circumferential end surfaces of acrylic blanks with a groove which is substantially continuous therewith.
  • the depth of the groove is at least about 5% and, preferably is at least about 10% of the initial thickness of the acrylic blank.
  • Various congurations e.g., V- and U-shaped and rectangular configurations, may be used.
  • the center line of the groove lies along the center line of the acrylic blank taken in the thickness direction.
  • Acrylic blanks having the aforedescribed groove may be heated and compression stretched as described in the U.S. application Ser. No. 691,294 of Terry D. Fortin, filed Dec. 18, 1967 (hereafter referred to as said Fortin application).
  • said Fortin application When acrylic blanks are grooved and stretched in this manner, the resultant stretched acrylic sheets exhibit substantially no edge defects. Therefore, the loss of acrylic material is substantially eliminated.
  • the edges of the acrylic blanks after being stretched by the method described in said Fortin application, often are highly stressed resulting in weekened or cracked edges (FIG. 2).
  • Acrylic blanks stretched according to said Fortin application are heated to their softening temperature range and, thereafter, compressed to cause the acrylic material to ow or squeeze radially outwardly to form a stretched acrylic sheet of reduced thickness. It is believed that the undesirable edge defects result from the partially molten edges of the acrylic blanks rolling non-uniformly outwardly from the central portion of the acrylic blanks. This rolling of the outer edges apparently takes place at a rate faster than the radially outward movement of the remainder of the acrylic blank. This differential rate of movement is believed to be the cause of the stresses and cracks which are introduced into the edges of the stretched acrylic sheet.
  • the numeral 10 designates an as-cast acrylic blank which has a circumferential end surface 11.
  • the term acrylic blank will be employed herein, and in the claims, to denote unstretched acrylic material.
  • the end surface 11 of the blank 10 is provided with a continuous groove 12.
  • edge defects refers to a random, uneven stressing, cracking and/or breaking of the edges of stretched sheets as shown, by way of example, in FIG. 2. Such edge defects can result in substantial loss of acrylic material. While the shallow circumferential grooves which may be present around the edges of acrylic material after it has been stretched by the herein-described method, such as shown in FIG. 3, and constitutes a loss of material, such loss is only a minor fraction of that due to uneven stresses and other edge cracking. This loss is minimal because the shallow grooves extend a uniform distance into the material and are not accompanied by cracking, stressing, etc., which would necessitate the removal of good material in order to remove stressed or cracked as in the case with stretched sheets exhibiting edge defects.
  • FIG. 3 A section 13 of an acrylic sheet originally having a shape and edge groove 12 such as is shown in FIG. 1, is shown in FIG. 3 after being compression-stretched.
  • the stretched sheet 13 has a much smaller edge groove 14.
  • the smaller groove 14 is produced by the filling in of the original groove 12 as the acrylic blank 10 is compression-stretched.
  • the latter is further shown by comparing the grooves 12 of FIGS. 4 and 5 with the grooves 14 of FIGS. 4a and 5a, respectively.
  • the stretched sheets 13 of FIGS. 4a and 5a have substantially smaller grooves 14 than the acrylic blanks 10 of FIGS. 4 and 5.
  • the shape of the groove does not appear to be critical. Grooves of various cross-sectional shapes, c g., V-shaped, U-shaped, circular and rectangular (FIGS. 4, 5 and 6) have been successfully employed. Additionally, the mouth of the groove may be as wide as, or narrower than, the width of the circumferential end surfaces, as shown in FIGS. 5 and 8. The groove is substantially coextensive with the length of the circumferential end surface (FIG. l) since edge defects may be introduced into the stretched sheet at any point along the end surface if there is no grooving at that point.
  • a test blank with a groove having a particular depth (greater than 10% of the thickness of the blank) and, thereafter, to compression stretch the test blank to the desired stretched size.
  • depth of the groove in the stretched part it can be determined whether to use shallower or deeper grooves in the production blanks. For example, if a relatively deep groove remains in the stretched sheet, a shallower groove than initially present in the test blank can be formed or cut in the production blanks so that edge losses from the stretched part will be further minimized.
  • Edge roll As shown in FIG. 9.
  • edge roll is present along the edges of a stretched acrylic sheet, a border-line condition is indicated and the groove placed in the acrylic blank should be deepened.
  • the acrylic blank is provided with a groove having a depth of at least about 5% and preferably greater than 10% of the original blank thickness and, further, which will be substantially eliminated in the stretched acrylic sheet. That is, optimally the depth of the acrylic blank groove is selected so that the flow of material during compression stretching substantially fills y the groove. Such groove selection, of course, minimizes or eliminates any losses of material from the stretched sheet.
  • the production acrylic blanks are provided with edge groove by any method, e.g., sawing, which will not critically stress the blank.
  • the blanks are then ready to be compression stretched. Compression stretching of the grooved blanks may be performed by any desired technique. However, it is presently preferable to employ the compression stretching method described in said Fortin application, the description of which is incorporated herein by reference to said Fortin application.
  • EXAMPLE 1 An as-cast Plexiglas 55 blank (manufactured by Rohm & Haas) having initial dimensions of 6 in. X 6 in. x 0.751 in. thick was provided with a circumferential groove having a configuration substantially as shown in FIG. 8. The maximum depth of the groove was 0.125 inch, or about 16% of the initial acrylic blank thickness.
  • This blank was coated with Mold-Wiz F-57 lubricant (Teflon colloidal dispersion manufactured by Axel Plastics Research Lab., Inc.) and placed between a pair of polished glass plates, and isothermally heated to a temperature of 295 F.
  • Mold-Wiz F-57 lubricant Teflon colloidal dispersion manufactured by Axel Plastics Research Lab., Inc.
  • the blank was thereafter compressed at a thickness reduction rate of about 0.10 inch per minute to produce a stretched acrylic sheet measuring 9.5 in. X 9.5 in. x 0.270 in. thick. Thereafter, the temperature of the acrylic sheet was reduced at a rate of 13 F. per minute to a temperature below about F.
  • EXAMPLE 2 A blank of the same material and of the same size as used in Example 1 was provided with a V-shaped circumferential groove similar to that shown in FIG. 4 and having a maximum depth of 0.1 inch or about 13% of the acrylic blank thickness. This blank was stretched under substantially the same conditions as described in Example 1 to a stretched acrylic sheet having dimensions of 9 in. x 9 in. x 0.350 in. thick.
  • EXAMPLE 3 An acrylic blank formed from the same material and having the same dimensions as that described in Example 2 was provided with a V-shaped groove similar to that used in ⁇ Example 2 except that the maximum depth of the groove was 0.046 inch or about 6% of the acrylic blank thickness. This blank was stretched under the same conditions as employed in Example 2 to a stretched acrylic sheet having dimensions of 9.5 in. x 9.5 in. x 0.300 in. thick.
  • EXAMPLE 4 An 'as-cast acrylic blank of Plexiglas 55 having dimensions of 12.5 in. x 12.5 in. x 0.732 in. was provided with a circumferential groove of square cross-section measuring 0.187 inch in depth (about 25% of the original blank thickness) and positioned substantially as shown in FIG. 5. This acrylic blank was compression stretched employing the method of Example 1 to a stretched acrylic sheet having dimensions of 21.6 in. x 21.6 in. x 0.245 in.
  • EXAMPLE 5 An acrylic blank of Plexiglas 55 having dimensions of 12.5 in. x 12.5 in. x 0.740 in. was provided with a continuous circumferential groove similar to that shown in FIG. 7 and having a maximum depth of about 0.375 inch (about 50% of the initial blank thickness). This blank was compression stretched by the method of Example 1 to a stretched acrylic sheet having dimensions of 22.2 in. x 22.2 in. x 0.233 in.
  • EXAMPLE 6 An 'as-cast acrylic blank of Plexiglas 55 having dimensions of 12.5 in. x 12.5 in. x 0.735 in. was provided with a continuous circumferential V-shaped groove as shown in FIG. 4 and having a maximum depth of about 0.312 inch (about 40% of the initial blank thickness). This acrylic blank was stretched by the method of Example 1 to a stretched acrylic sheet having dimensions of 21 in. x 22 in. x 0.235 in.
  • a method for compression stretching an acrylic blank having a top and a bottom surface and circumferential end surfaces extending therebetween comprising the steps of:
  • said acrylic blank is selected from the group consisting of methyl methacrylate and modified methyl methacrylate.
  • a method of compression stretching an acrylic blank having a top and a bottom surface and a circumferential end surface therebetween comprising the steps of providing said circumferential end surfaces with a groove substantially coextensive therewith, said groove having a depth greater than about 5% of the initial thickness of said blank and positioned symmetrically about a plane passing through the centerline of said groove;

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Shaping By String And By Release Of Stress In Plastics And The Like (AREA)
  • Casting Or Compression Moulding Of Plastics Or The Like (AREA)
US758395A 1968-09-09 1968-09-09 Method for preventing edge defects in compression stretched acrylic sheets Expired - Lifetime US3562383A (en)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4340557A (en) * 1980-12-16 1982-07-20 Ball Corporation Method of making unfestooned plastic containers from polygonal blanks
US4360483A (en) * 1981-08-10 1982-11-23 Ppg Industries, Inc. Apparatus for and method of pressing plastic sheets
US4361527A (en) * 1981-10-21 1982-11-30 Ppg Industries, Inc. Method of forming stretched acrylic sheets
US4377611A (en) * 1980-12-29 1983-03-22 Swedlow, Inc. Plastic transparencies having improved resistance to cracking
US4419412A (en) * 1980-12-16 1983-12-06 Ball Corporation Plastic blank structure
US4454203A (en) * 1980-11-24 1984-06-12 Ppg Industries, Inc. Pressing plastic with extensible coating
US4470945A (en) * 1982-03-25 1984-09-11 United Technologies Corporation Method and apparatus for applying an electron beam drilling backer to a metal sheet
US4921646A (en) * 1988-08-31 1990-05-01 Shell Oil Company Forming laminated billets from individual billets
US4966803A (en) * 1987-07-23 1990-10-30 Stamicarbon, B.V. Polymer films partially provided with stiffened segments, process for the production thereof, and the use thereof
US20220063134A1 (en) * 2020-08-26 2022-03-03 Disco Corporation Inspection substrate

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4454203A (en) * 1980-11-24 1984-06-12 Ppg Industries, Inc. Pressing plastic with extensible coating
US4340557A (en) * 1980-12-16 1982-07-20 Ball Corporation Method of making unfestooned plastic containers from polygonal blanks
US4419412A (en) * 1980-12-16 1983-12-06 Ball Corporation Plastic blank structure
US4377611A (en) * 1980-12-29 1983-03-22 Swedlow, Inc. Plastic transparencies having improved resistance to cracking
US4360483A (en) * 1981-08-10 1982-11-23 Ppg Industries, Inc. Apparatus for and method of pressing plastic sheets
US4361527A (en) * 1981-10-21 1982-11-30 Ppg Industries, Inc. Method of forming stretched acrylic sheets
US4470945A (en) * 1982-03-25 1984-09-11 United Technologies Corporation Method and apparatus for applying an electron beam drilling backer to a metal sheet
US4966803A (en) * 1987-07-23 1990-10-30 Stamicarbon, B.V. Polymer films partially provided with stiffened segments, process for the production thereof, and the use thereof
US4921646A (en) * 1988-08-31 1990-05-01 Shell Oil Company Forming laminated billets from individual billets
US20220063134A1 (en) * 2020-08-26 2022-03-03 Disco Corporation Inspection substrate
US12220839B2 (en) * 2020-08-26 2025-02-11 Disco Corporation Inspection substrate

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GB1244627A (en) 1971-09-02
FR1594237A (enExample) 1970-06-01

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