Classifications

• • 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
  • B29C43/00—Compression moulding, i.e. applying external pressure to flow the moulding material; Apparatus therefor
  • B29C43/02—Compression moulding, i.e. applying external pressure to flow the moulding material; Apparatus therefor of articles of definite length, i.e. discrete articles
  • B29C43/16—Forging
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
  • B29K2023/04—Polymers of ethylene
  • B29K2023/06—PE, i.e. polyethylene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2059/00—Use of polyacetals, e.g. POM, i.e. polyoxymethylene or derivatives thereof, as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2071/00—Use of polyethers, e.g. PEEK, i.e. polyether-etherketone or PEK, i.e. polyetherketone or derivatives thereof, as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2077/00—Use of PA, i.e. polyamides, e.g. polyesteramides or derivatives thereof, as moulding material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
  • B29K2105/06—Condition, form or state of moulded material or of the material to be shaped containing reinforcements, fillers or inserts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
  • B29K2105/06—Condition, form or state of moulded material or of the material to be shaped containing reinforcements, fillers or inserts
  • B29K2105/12—Condition, form or state of moulded material or of the material to be shaped containing reinforcements, fillers or inserts of short lengths, e.g. chopped filaments, staple fibres or bristles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2995/00—Properties of moulding materials, reinforcements, fillers, preformed parts or moulds
  • B29K2995/0037—Other properties
  • B29K2995/0041—Crystalline

Definitions

• the present invention relates to thermoplastic sheet composites and more particularly to the stamping of such sheet composites to make three- dimensional parts.
• Composite materials represent a fast growing segment of the plastics industry because of their capability of providing the high strength and stiffness exhibited by metals, but at a greatly reduced part weight. Composite materials additionally can display resistance to chemicals and provide a cost savings in many applications. Parts fashioned from fiber- reinforced polymeric sheet composites can exhibit high mechanical properties due to the fiber reinforcement with the polymer matrix providing the appropriate shape, resistance to chemicals, and like properties. Most currently-available composites are formed from thermosetting polymers including polyesters, epoxies, vinyl esters, polyimides, and the like. Thermosetting polymers, while providing adequate performance in many applications, do suffer from a number of disadvantages. Because thermosetting polymers are formed from reactive materials, shelf life and storage stability are a prime concern to the manufacturer and to the user. Additionally, scrap cannot be reused once the components have been reacted to form the ultimate thermosetting polymer. Often too, thermosetting polymers suffer from brittleness and insufficient impact resistance.
• thermoplastic polymers are not reactive which eliminates the storage problems encountered when dealing with thermosetting polymers. Additionally, scrap can be reused and mechanical properties, chemical resistance, and the like often can be equivalent to such properties of thermosetting polymers.
• the major disadvantage in using thermoplastic polymers is their relatively high melt viscosity which makes penetration of the polymer melt into the fiber bundles difficult. Thermosets, on the other hand, are typified by low viscosity components which readily flow among the fibers. Additionally, long term creep properties of thermoplastic matrix composites are unknown since their use in the marketplace has not found wide acceptance.
• some thermoplastic polymers have been utilized in injection molding processes to make parts; however, such parts typically are not highly reinforced.
• U.S. Pat. No. 4,014,970 proposes to solid-state stamp thermoplastic polymer composite sheets by heating the polymer sheets to a temperature just below that at which the material is no longer solid and subjecting the heated material to high compressive strain to cause an abrupt flow of the material throughout a mold cavity.
• a relative velocity of one portion of the mold to another portion of the mold of at least 6.4 cm/sec is taught in this patent.
• This patent is important in that it teaches that solid- state stamping of thermoplastic polymers is possible.
• many practical difficulties arise by practicing such process. For example, on an assembly line it is quite difficult to properly gauge accurately and reproducibly the precise temperature control required to maintain the thermoplastic material at a relatively high temperature whereat it is almost, but not quite, at its melt point.
• the present invention relates to a method for solid-state stamping of fiber-reinforced thermoplastic composites and to a method for predicting suitable, relatively broad temperature ranges whereat the thermoplastic composites can be stamped in the solid state.
• the method for solid-state stamping of fiber reinforced thermoplastic composites includes restricting the thermoplastic material to be a semi-crystalline thermoplastic polymeric material. A sheet of the thermoplastic composite is heated in an oven to a temperature which is less than the peak melting temperature of said polymeric material and such heated polymeric material then is transferred from the oven to a mold. The polymeric composite sheet is permitted to cool during the transferring of the sheet from the oven to the mold. Finally, the composite sheet is stamped in the mold.
• the temperature of the composite sheet at the commencement of the stamping operation is less than the peak melting temperature of the semi-crystalline thermoplastic material, but is at a temperature which is greater than the crystalization onset temperature of the composite.
• the peak melting temperature and the crystalization onset temperatures are determined by differential scanning calorimetry of the composite.
• Fig. 1 graphically portrays the cooling rate of several thermoplastic composite sheets which were heated prior to stamping to various .oven temperatures;
• Fig. 2 is a composite differential scanning calorimetry curve for polybutylene terephthalate;
• Fig. 3 is a composite differential scanning calorimetry curve for glass fiber-reinforced polybutylene terephthalate
• Fig. 4 is a composite differential scanning calorimetry curve for polypropylene
• Fig. 5 is a composite differential scanning calorimetry curve for glass fiber-reinforced Nylon 12;
• Fig. 6 is a composite differential scanning calorimetry curve for polyethylene terephthalate
• Fig. 7 is a composite differential scanning calorimetry curve for polyphenylene sulfide
• Fig. 8 is a composite differential scanning calorimetry curve for polyether ether ketone.
• the present invention is unique in several respects. Initially, the reinforcing fiber content limitation suffered by injection molding (about or 25% or less) thereabouts) is not a limitation suffered by the stamping process of the present invention which can function effectively at fiber contents in excess of 40% and on up to 70% or more, depending upon the fiber length, composition, weaving, aspect ratio, and like factors. A further advantage is that the stamper need not operate with a thermoplastic composite sheet which has been heated to a temperature whereat the continuous thermoplastic phase is molten. Moreover, the unexpected discoveries upon which the present invention is based even do away with the notion taught in U.S. Pat. No.
• thermoplastic composites can only take place when the composite is heated to a temperature which is just below the melt temperature of the composite.
• the thermoplastic composite can be stamped at temperatures as much as 25°- 30° C below the stamping temperature taught in the '970 patent.
• Another conventional wisdom which has been dispelled is the need for special handling procedures to convert the semi-crystalline thermoplastic material to its amorphous state in order to stamp it as taught in U.S. Pat. No. 4,263,364 and its progeny.
• the solid-state stamping process of the present invention retains conventional benefits of solid-state drawing, much like the benefits retained in analogous metal stamping processes.
• fiber orientation certainly is one such advantage retained by the solid-state stamping process of the present invention.
• the less energy that needs to be inputted in the stamping operation the less energy is required to be removed during the stamping procedure. This translates into much shorter molding times being required by the inventive process with concomitant higher throughput of parts.
• the present invention has its origins in experiments conducted whereat thermoplastic composites were heated to a temperature just short of the melt temperature of the thermoplastic composite followed by cooling of the composite to various temperatures for stamping of the composite.
• thermoplastic material exhibits an exotherm about its melt temperature when heated.
• a semi-crystalline material undergoes a transformation from solid-state to liquid state over this temperature range with the concomitant release of energy.
• semi- crystalline thermoplastic substrates when cooled from such melt temperature range exhibit a second exotherm which commences at about the onset temperature of crystalization, i.e. that temperature at which energy is released due to crystalization of the polymer.
• DSC differential scanning calorimetry
• the upper temperature limit to which the composite sheet must be heated is less than the melt point of the thermoplastic material and often is several degrees below such temperature. That is, there is a given temperature range over which the exotherm commences and prior to which it maximizes at the melt temperature of the thermoplastic material.
• the thermoplastic composites need only be heated to a temperature at about the mid point of such exotherm and even somewhat less (e.g. about 40%) in order for solid-state stamping to be efficacious.
• the melt exotherm commences at about 205° C with a peak melt temperature being recorded at about 222° .
• the PBT thermoplastic sheet need not be heated to about 220° C, but to only about 210°-215° C (and then cooled to a temperature down to 185° C) for solid-state stamping.
• reinforcing fiber typically acts to seed crystallization so that the lower temperature limit (crystallization onset temperature) will vary accordingly.
• the crystallization temperature can be lowered by controlled rapid cooling of the thermoplastic provided that the cooling rate is not so rapid as to form the amorphous form of the thermoplastic.
• the composite sheet may be stamped at such upper temperature limit or any temperature lower until the crystalization onset temperature is encountered. Over this temperature range or window properties of the ultimate part may vary so that optimization of particular properties of the part may be correlated to particular stamping temperatures within the window. Determination of such optimum stamping temperatures as correlated and related to various properties of the stamped parts is well within the skill of those in this art field.
• composite sheets of known fiber reinforcement kind and content, and known semi-crystalline thermoplastic continuous phase composition can be evaluated in accordance with another aspect of the present invention and the stamping temperature or stamping temperature window controlled for such composite sheet.
• this aspect of the present invention further enables the artisan to design an assembly line layout whereat oven heating time, placement of the oven with respect to the mold with concomitant handling times therebetween, mold closing time, and part removal from the mold can be designed with a high degree of predictability and certainty. That is, a particular stamping temperature can be determined ahead of time and correlated to a particular property or group of properties which the part desirably should possess. Once this stamping temperature or range of temperatures is known, it will be known how much the heated sheet can be cooled prior to its entry into the mold following its removal from the oven. Solid-state stamping simplifies handling procedures and the predictive power of this aspect of the present invention enables the sheet to enter the mold at the correct temperature by permitting natural cooling of the part to take place between the oven and the mold.
• thermoplastic materials find use in the present invention. Such materials include a variety of homopolymers and copolymers including, for example, olefins, polyethers, aromatic polyesters, polyamides, polyacetals, and the like.
• the thermoplastic material need only be semi-crystalline in order for use in accordance with the precepts of the present invention.
• Specific preferred semi-crystalline thermoplastic materials which have found efficacy in the present invention include, for example, polypropylene, Nylon resins, polybutylene phthalates, polyethylene terephthalates, polyphenylene sulfides, and polyether ether ketones.
• Fibrous reinforcement is not a limitation in that conventional glass, carbon, various polymers (e.g. polyamides), metal fibers, and the like find utility in the stamping process of the present invention.
• the fibers may be in the form of continuous or chopped mats, woven fabric (e.g. plain weave, twill weave, etc.), optionally surface treated with various coupling agents or wetting agents for improving impregnation, wetting, thermoplastic adhesion, and the like properties.
• fibrous reinforcement includes contents from as low as a few percent on up to about 70%.
• the composite sheets subjected to the stamping process can be formed by a variety of conventional and unconventional techniques.
• One conventional technique involves melt impregnation which may be accomplished statically or by a roll impregnation technique.
• Another conventional technique involves ply consolidation utilizing pressed prepegs as is well known in this art field.
• Wet forming processes also find utility in forming the composite sheets to be stamped and preferably that wet forming process disclosed in commonly-assigned application of Hiscock entitled Wet-Laid, Non-Woven, Fiber-Reinforced Composites Containing Stabilizing Pulp and Process of Making Same, USSN 820,485 , filed January 17, 1986 (Attorney's Docket No.
• the Hiscock wet forming process is very similar to a conventional Fourdrinier paper making process wherein a dilute water slurry of reinforcing fibers and polymer powder are filtered out on a fine mesh screen to produce a mat of fibers and polymer which then can be consolidated under heat and pressure to form a composite sheet.
• the inventive wet forming process utilizes a minor proportion of fibrous pulp in order to substantially augment the wet forming process.
• Such wet forming process can be used to advantage to form composite sheets for stamping in accordance with the precepts of the present invention.
• PBT Terephthalate
• PET Terephthalate
• Additional characterization of the polymers as used in the composites included a determination of the rate of cooling of the samples in indoor ambient air while being transferred from the oven to the mold for stamping.
• the composite samples were dried at 80° C for four hours and then placed in a small oven located near the mold for heating to a predetermined temperature.
• the actual temperature of the sample was monitored by embedding a thermocouple in the middle ply of a five-ply laminate at a point one cm. interiorly of the center of one side of the 5 cm. x 7.6 cm. sample.
• the heated sample was removed from the oven, placed in the mold, and the mold closed.
• the molds were composed of tool steel and could be closed in less that 2 seconds for stamping.
• One mold was hat-shaped (trapezoidal) with a depth of 13 mm of an angle of 60° to the horizontal plane of the part.
• the second mold was hemispherical in shape with a radius male half of 15.9 mm. Shims were used to accommodate various sheet thicknesses.
• the overall stamping cycle time from removal of the sample from the oven to opening the mold was approximately 10 seconds.
• the thermal responses of the various thermoplastic composite parts evaluated is depicted graphically in Fig. 1.
• EXAMPLE 2 Experiments to determine a temperature range of stampability of PBT-twill glass fabric composites (48 wt-% fiber loading) involved stamping temperatures ranging between 150° C and 260° C. The mold was maintained at a temperature of 87.7° C (190°F). Visual inspection of the parts revealed that acceptable parts could be stamp-formed at temperatures of between 185° C and 220° C. Below 185° C, the parts would break because the thermoplastic could not flow or deform, but could only fracture. Above 220° C, the polymer would flow excessively and thereby severely distort the fabric.
• DSC Differential Scanning Calorimetry
• the DSC curve for PBT is depicted in Fig. 2 which is a composite of two separate curves. That is, the second peak corresponds to the thermal response of the thermoplastic when heating to a temperature above its melt point. Upon cooling of the heated sample, the first peak is encountered from the high temperature side.
• the DSC curve for PBT in Fig. 2 shows an exotherm at about 205° C with a melt point of PBT determined at about 222° C. Note that the DSC melt point corresponds closely with the upper temperature limit for solid-state stamping of PBT.
• a second exotherm commencing at about 185° C occurs which exotherm corresponds to the onset of crystallization of PBT. This crystallization onset temperature also corresponds to the lower temperature limit for solid stamping of PBT.
• thermoplastic melt temperature within the temperature range bounded by the thermoplastic melt temperature and the crystallization onset temperature (as determined by DSC)
• the thermoplastic material possesses sufficient molecular mobility to deform during stamping.
• An important advantage of this solid-state stamping temperature range is that no special cool-down of the sample in the mold is required. As soon as the sample exhibits a temperature of below its crystallization onset temperature, the mold can be opened. Importantly, the crystallization onset temperature can be predicted from Fig. 1 for timing of the stamping cycle. Faster cycle times now are possible with concomitant higher throughput.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Reinforced Plastic Materials (AREA)
• Casting Or Compression Moulding Of Plastics Or The Like (AREA)
• Moulding By Coating Moulds (AREA)
• Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)

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• 1987
  • 1987-01-15 WO PCT/US1987/000094 patent/WO1987004387A1/en not_active Application Discontinuation
  • 1987-01-15 JP JP50106387A patent/JPS63502337A/ja active Pending
  • 1987-01-15 ES ES8700085A patent/ES2003655A6/es not_active Expired
  • 1987-01-15 EP EP19870901181 patent/EP0253878A1/de not_active Withdrawn

Non-Patent Citations (1)

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