CA1211262A - High current low voltage heating method and apparatus for producing a smooth finish surface on injection molded foamed resin products - Google Patents
High current low voltage heating method and apparatus for producing a smooth finish surface on injection molded foamed resin productsInfo
- Publication number
- CA1211262A CA1211262A CA000431132A CA431132A CA1211262A CA 1211262 A CA1211262 A CA 1211262A CA 000431132 A CA000431132 A CA 000431132A CA 431132 A CA431132 A CA 431132A CA 1211262 A CA1211262 A CA 1211262A
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- metallic sheet
- metal mold
- mold
- electric power
- bulk
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Abstract
HIGH CURRENT LOW VOLTAGE HEATING METHOD AND APPARATUS FOR
PRODUCING A SMOOTH FINISH SURFACE ON INJECTION MOLDED FOAMED
RESIN PRODUCTS
ABSTRACT OF THE INVENTION
The invention comprises a method and apparatus to quickly heat a mold cavity surface or portions thereof just prior to the injection of a foamable plastic resin. The apparatus comprises one or more thin metal surface sheets or bands of high electrical resistivity in the mold cavity covering those portions of the mold where a smooth surface finish on the product is desired. In an alternative form the surface sheets may be plated or sprayed metal of high resistivity on a layer of thin electrically non-conducting plastic in turn adhered to the mold cavity surface.
The thin metal surface sheet is resistance heated approximately 300°F in a few seconds or less by a low voltage high current source of electric power just prior to the injection of the foamable plastic.
The high current passing through the metal of high resistivity causes conversion of electrical energy into sensible heat very quickly to accomplish quick heating of the mold cavity surface sheet. The low voltage eliminates the need for substantial electrical insulation between the thin metal surface sheet and the bulk metal of the mold. Where steel molds and steel surface sheets are employed, the normal oxide coating on the steel serves as the insulation at the interface of the surface sheet and the mold.
To reduce or minimize arcing in the electrical switch gear employed to momentarily connect the high current low voltage source to the surface sheet, a water cooled electric heating pipe bypass is employed as an electrical energy sink during the portion of the molding process that does not employ heating of the surface sheet.
PRODUCING A SMOOTH FINISH SURFACE ON INJECTION MOLDED FOAMED
RESIN PRODUCTS
ABSTRACT OF THE INVENTION
The invention comprises a method and apparatus to quickly heat a mold cavity surface or portions thereof just prior to the injection of a foamable plastic resin. The apparatus comprises one or more thin metal surface sheets or bands of high electrical resistivity in the mold cavity covering those portions of the mold where a smooth surface finish on the product is desired. In an alternative form the surface sheets may be plated or sprayed metal of high resistivity on a layer of thin electrically non-conducting plastic in turn adhered to the mold cavity surface.
The thin metal surface sheet is resistance heated approximately 300°F in a few seconds or less by a low voltage high current source of electric power just prior to the injection of the foamable plastic.
The high current passing through the metal of high resistivity causes conversion of electrical energy into sensible heat very quickly to accomplish quick heating of the mold cavity surface sheet. The low voltage eliminates the need for substantial electrical insulation between the thin metal surface sheet and the bulk metal of the mold. Where steel molds and steel surface sheets are employed, the normal oxide coating on the steel serves as the insulation at the interface of the surface sheet and the mold.
To reduce or minimize arcing in the electrical switch gear employed to momentarily connect the high current low voltage source to the surface sheet, a water cooled electric heating pipe bypass is employed as an electrical energy sink during the portion of the molding process that does not employ heating of the surface sheet.
Description
HIGH CURRENT LOW VOLTAGE ~I~ATING M~THOD A~D ~PPAR~TUS FOR
PRODUCING A S~lO~TI~ FINISH SURFAC~ O~ INJECTIO~ MOLDED FOAM~D
RESIN PRODUCTS
BACKGROUND OF THE INV~TION
The field of the invention pertains to injection molding of foamable plastic products and, in particular, to the provision of a smooth surface finish on the foamed plastic products.
For aesthetic and functional reasons a smooth, shiny and blemish free surface finish is desired on many foamed plastic products. Such a smooth and blemish free surface finish can be provided by molding with dies heated to a relatively high temperature (300F - 550F) prior to injecting the foamable plastic into the die cavity. The upper temperature is typically selected to be above the heat dis-tortion temperature of the foamable plastic. The dies must then be cooled to approximately 140F before the mold is opened and the product ejected. The repeated heating and cooling of the relatively massive bulk of the mold requires considerable time and energy.
In order to substantially lessen the cycle time necessary to heat and cool the mold, means for heating only the cavity surface just prior to injection of the foamable plastic have been developedO
Exemplifying this approach in the prior art is U.S. Pat. 4,201,742.
Disclosed is a method and apparatus for heating the mold cavity surface by means of condensing steam thereon and draining the condensate from the cavity just prior to injection of the foamable plastic resin. This method provides a smooth surface finish on the product as desired, however, despite much effort to overcome water marks and streaks caused by condensate near the bo-ttom of the mold cavity, such defects continue to be a problem.
A method and apparatus for dry heating the cavi-ty surface is shown in U.S. ~at. 2,979,773 wherein a semiconductive film is coated on the mold surface and an aluminum electrode plated onto the semiconductive film. The mold body constitutes the other electrode. A heating !., " .t current passes through the entire mold with the greatest temperature rise occurring in the semiconductive film. Such a construction is limited to relatively short production runs because the aluminum and semiconductive film coatings are subject to rapid wear with repeated molding cycles.
U.S. Pat. 3,173,175 discloses a thin metal resistance element on or embedded in a glass or ceramic insulative layer. Glass and ceramic coatings, however, severely llmit the mold steels that can be used to support the glass and ceramic coatin~s and are very difficult to apply only to those areas of the mold surface to be heated.
U.S. Pat. 2,984,887 discloses an electrically heated copper or silver coating or preformed cup inserted in a ceramic mold. The mold is heated for a time sufficient to dry the skin on ceramic green ware such that the green ware shrinks away from the mold and can be removed easily without damage. The cup or liner is porous or only covers part of the mold cavity surface to permit the absortion of vapors from the drying ceramic mix. The temperatures are limited and the heating time on the order of a half a minute to drive out the moisture without damage to the part.
S~ qMAR'~ OF TT~E INVENTION
The invention comprises a method and apparatus to quickly heat an injection mold surface or portions thereof just prior to the injection of a foamable plastic resin or molding compound. The apparatus comprises one or more thin metal surface sheets or bands in the mold cavity covering those portions of the rnold where a smooth surface finish on the product is desired. In an alternate form the electrically heated surface sheet may be plated or sprayed (sputtered) metal of a high resistivity on a layer of thin electrically non-con-ducting plastic in turn adhered to the mold cavity surface. A suitable plastic for the non-conducting layer may be selected from the polylmide group.
The thin metal surface shee~ is resistance heated abou-t 300F in a few seconds or less by a low voltaye high current source of electric power just prior to injection of the foamable plastic. The high current passing through the metal of high resistivity causes conversion of electrical energy into sensible heat very quickly to accomplish quick heating of the mold cavity surface sheet. The low voltage eliminates the need for substantial electrical insulation between the thin metal surface sheet and the bulk metal of the mold or die. Where steel molds and steel surface sheets are employed, the normal oxide coating on the steel serves as the electrical insulation at the interface of the surface sheet and the mold.
The thin adhesive layer of plastic in the alternate form acts as an electrical insulator also without substantially reducing the heat transfer through the layer during the co~ling cycle of the molding process. A second advantage to the oxide layer or thin adhesive layer is the elimination of damage and wear due to the repeated cycling pressures and temperatures. Crushing of the insu-lative layer is essentially eliminated. The metal surface sheek is typically steel two millimeters or less in thickness backed by the bulk steel mold. The sturdy construction thereby results in a more serviceable and longer lived mold cavity construction than the prior art.
To reduce or minimize arclng in the electrlcal switch gear employed to momentarily connect the high current low voltage source of electric power to the surface sheet, a water cooled electric heating pipe bypass is employed as an electrical energy sink during the portion of the molding cycle -that does not employ heating of the surface sheet.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section of a mold modified in accordance with the invention; and, FIG. 2 is a partial cross section of the mold of FIG. 1 with a second modification according to the invention.
.V~
DESCRIPTION OF T~-IE P~ FERP~ED EMBODI~E~;l'rS
In FIG. 1 the cross section of a mold in closed position is shown including male 10 and female 12 mold halves enclosing a mold cavity 14. The female mold half 12 is attached to a fixed plate 16 and the male mold half 10 attached to a traveling plate 18. The fixed plate 16 and traveling plate 18 are aligned by spaced guide bars 20 and the traveling plate 18 moved by the actuator 22. Also movable on the gu~de bars 20 is an ejector plate 24 and a plurality of ejector pins 26 for detaching the plastic molded product from the male mold half 10 upon completion of the molding cycle.
Extending into the female mold half 12 are an injection nozzle 28 and shut off pin 30. The nozzle 28 co~municates with a sprue 32 in turn in communication with the mold cavity 14. The holes 34 and 36 indicate a plurality of cooling fluid passages formed in the male and female mold halves for carrying cooling fluid during the molding process. The remaining elements of an injection molding machine are well known in the art and the method and apparatus for mechanically actuating the mold and injecting the foamable plastic resin is not shown or described in detail.
Forming the inside surface of the mold cavity 14 is a thin metallic sheet 38 conforming to the shape of the female mold half 12.
The thin metallic sheet 38 includes an aperture 40 to allow communica-tion between the sprue 32 and the mold cavity 14. The metal].ic sheet 38 extends outside the mold on opposite sides as indicated at 42 and 44 where electrical cable attachments 46 and 48 are attached to the sheet 38. Low voltage high current electric cables 50 and 52 are fastened to the cable attachments 46 and 48 respectively.
Electric cable 52 is connected through a switch 5~ to a low voltage high current source of electric power 56. The cable clamp 58 attaching cable 52 to the power source 56 also attaches a third electric cable 60 thereto. A metalic heating pipe 62 is electrically connected at 64 to cable 60. Electric cable 50 is attached at 66 to the heating pipe 62 at a location spaced from the at-tachment at 64.
A fourth electric cab~e 6~ is attached to the heating pipe 62 at 70 substantially adjacent attachment 66 and extends to a cable clamp 72 providing electrical connection with the low voltage high current source 56. The attachments 66 ~nd 70 may be a common attachment to the heating pipe 62. During operation of the injection molding machine according to the process described below a cooling fluid is passed through the heating pipe 62.
A modification of the apparatus is illustrated in FIG. 2 wherein both the emale 12' and male 10' mold halves are covered wi~h thin metallic sheets 38' and 38" respecti~ely. The ends of the ejector pins 26l extend through the sheet 38" as shown. To enable the mold to be opened and closed, separate cable attachments 48' and 48" and separate cables 52' and 52" are attached to the sheets 38' and 38" respectivelyO
similar arrangement is provided on the opposite side of the mold for cable 50.
The metallic sheet is preferably made of low carbon steel, silicon steel or stainless steel to provide a relatively high resistence to the flow of electric current. The voltage is typically limited to less than 50, however, the current can range up to and if required, exceed 2000 amperes. By limiting the voltage, the natural oxide coating 74 on the metal1iC sheet 38 and the underlying steel mold 12 acts as an adequate electrical insulator at the interface contact between the mold 12 surface and the ~heet 38. Alterna~ively, a thin adhesive layer, also designated by 74, may be applied between the mold 12 and the sheet 38 to act as both an electrical insulator and means to secure the sheet to the mold surface.
Such an adhesive layer is formed as thin as possible to minimize the resistance to heat transfer during the cooling portion of the molding cycle.
In another alternative construction, the metallic sheet may be formed by plating or spraying (sputtering) a metal of high resistivity onto a layer of thin electrically non-conducting plastic in turn adhered to the mold 12 surface.
z~
Typical foamed plastic rnolding compounds require that the surface of the mold cavity be in excess of the heat distortion temperature of the plastic to obtain a Class A smooth surface finish.
Such a temperature is between 350F and 550F for most common foamable polymersA By utilizing a metallic sheet 38 of substantially
PRODUCING A S~lO~TI~ FINISH SURFAC~ O~ INJECTIO~ MOLDED FOAM~D
RESIN PRODUCTS
BACKGROUND OF THE INV~TION
The field of the invention pertains to injection molding of foamable plastic products and, in particular, to the provision of a smooth surface finish on the foamed plastic products.
For aesthetic and functional reasons a smooth, shiny and blemish free surface finish is desired on many foamed plastic products. Such a smooth and blemish free surface finish can be provided by molding with dies heated to a relatively high temperature (300F - 550F) prior to injecting the foamable plastic into the die cavity. The upper temperature is typically selected to be above the heat dis-tortion temperature of the foamable plastic. The dies must then be cooled to approximately 140F before the mold is opened and the product ejected. The repeated heating and cooling of the relatively massive bulk of the mold requires considerable time and energy.
In order to substantially lessen the cycle time necessary to heat and cool the mold, means for heating only the cavity surface just prior to injection of the foamable plastic have been developedO
Exemplifying this approach in the prior art is U.S. Pat. 4,201,742.
Disclosed is a method and apparatus for heating the mold cavity surface by means of condensing steam thereon and draining the condensate from the cavity just prior to injection of the foamable plastic resin. This method provides a smooth surface finish on the product as desired, however, despite much effort to overcome water marks and streaks caused by condensate near the bo-ttom of the mold cavity, such defects continue to be a problem.
A method and apparatus for dry heating the cavi-ty surface is shown in U.S. ~at. 2,979,773 wherein a semiconductive film is coated on the mold surface and an aluminum electrode plated onto the semiconductive film. The mold body constitutes the other electrode. A heating !., " .t current passes through the entire mold with the greatest temperature rise occurring in the semiconductive film. Such a construction is limited to relatively short production runs because the aluminum and semiconductive film coatings are subject to rapid wear with repeated molding cycles.
U.S. Pat. 3,173,175 discloses a thin metal resistance element on or embedded in a glass or ceramic insulative layer. Glass and ceramic coatings, however, severely llmit the mold steels that can be used to support the glass and ceramic coatin~s and are very difficult to apply only to those areas of the mold surface to be heated.
U.S. Pat. 2,984,887 discloses an electrically heated copper or silver coating or preformed cup inserted in a ceramic mold. The mold is heated for a time sufficient to dry the skin on ceramic green ware such that the green ware shrinks away from the mold and can be removed easily without damage. The cup or liner is porous or only covers part of the mold cavity surface to permit the absortion of vapors from the drying ceramic mix. The temperatures are limited and the heating time on the order of a half a minute to drive out the moisture without damage to the part.
S~ qMAR'~ OF TT~E INVENTION
The invention comprises a method and apparatus to quickly heat an injection mold surface or portions thereof just prior to the injection of a foamable plastic resin or molding compound. The apparatus comprises one or more thin metal surface sheets or bands in the mold cavity covering those portions of the rnold where a smooth surface finish on the product is desired. In an alternate form the electrically heated surface sheet may be plated or sprayed (sputtered) metal of a high resistivity on a layer of thin electrically non-con-ducting plastic in turn adhered to the mold cavity surface. A suitable plastic for the non-conducting layer may be selected from the polylmide group.
The thin metal surface shee~ is resistance heated abou-t 300F in a few seconds or less by a low voltaye high current source of electric power just prior to injection of the foamable plastic. The high current passing through the metal of high resistivity causes conversion of electrical energy into sensible heat very quickly to accomplish quick heating of the mold cavity surface sheet. The low voltage eliminates the need for substantial electrical insulation between the thin metal surface sheet and the bulk metal of the mold or die. Where steel molds and steel surface sheets are employed, the normal oxide coating on the steel serves as the electrical insulation at the interface of the surface sheet and the mold.
The thin adhesive layer of plastic in the alternate form acts as an electrical insulator also without substantially reducing the heat transfer through the layer during the co~ling cycle of the molding process. A second advantage to the oxide layer or thin adhesive layer is the elimination of damage and wear due to the repeated cycling pressures and temperatures. Crushing of the insu-lative layer is essentially eliminated. The metal surface sheek is typically steel two millimeters or less in thickness backed by the bulk steel mold. The sturdy construction thereby results in a more serviceable and longer lived mold cavity construction than the prior art.
To reduce or minimize arclng in the electrlcal switch gear employed to momentarily connect the high current low voltage source of electric power to the surface sheet, a water cooled electric heating pipe bypass is employed as an electrical energy sink during the portion of the molding cycle -that does not employ heating of the surface sheet.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section of a mold modified in accordance with the invention; and, FIG. 2 is a partial cross section of the mold of FIG. 1 with a second modification according to the invention.
.V~
DESCRIPTION OF T~-IE P~ FERP~ED EMBODI~E~;l'rS
In FIG. 1 the cross section of a mold in closed position is shown including male 10 and female 12 mold halves enclosing a mold cavity 14. The female mold half 12 is attached to a fixed plate 16 and the male mold half 10 attached to a traveling plate 18. The fixed plate 16 and traveling plate 18 are aligned by spaced guide bars 20 and the traveling plate 18 moved by the actuator 22. Also movable on the gu~de bars 20 is an ejector plate 24 and a plurality of ejector pins 26 for detaching the plastic molded product from the male mold half 10 upon completion of the molding cycle.
Extending into the female mold half 12 are an injection nozzle 28 and shut off pin 30. The nozzle 28 co~municates with a sprue 32 in turn in communication with the mold cavity 14. The holes 34 and 36 indicate a plurality of cooling fluid passages formed in the male and female mold halves for carrying cooling fluid during the molding process. The remaining elements of an injection molding machine are well known in the art and the method and apparatus for mechanically actuating the mold and injecting the foamable plastic resin is not shown or described in detail.
Forming the inside surface of the mold cavity 14 is a thin metallic sheet 38 conforming to the shape of the female mold half 12.
The thin metallic sheet 38 includes an aperture 40 to allow communica-tion between the sprue 32 and the mold cavity 14. The metal].ic sheet 38 extends outside the mold on opposite sides as indicated at 42 and 44 where electrical cable attachments 46 and 48 are attached to the sheet 38. Low voltage high current electric cables 50 and 52 are fastened to the cable attachments 46 and 48 respectively.
Electric cable 52 is connected through a switch 5~ to a low voltage high current source of electric power 56. The cable clamp 58 attaching cable 52 to the power source 56 also attaches a third electric cable 60 thereto. A metalic heating pipe 62 is electrically connected at 64 to cable 60. Electric cable 50 is attached at 66 to the heating pipe 62 at a location spaced from the at-tachment at 64.
A fourth electric cab~e 6~ is attached to the heating pipe 62 at 70 substantially adjacent attachment 66 and extends to a cable clamp 72 providing electrical connection with the low voltage high current source 56. The attachments 66 ~nd 70 may be a common attachment to the heating pipe 62. During operation of the injection molding machine according to the process described below a cooling fluid is passed through the heating pipe 62.
A modification of the apparatus is illustrated in FIG. 2 wherein both the emale 12' and male 10' mold halves are covered wi~h thin metallic sheets 38' and 38" respecti~ely. The ends of the ejector pins 26l extend through the sheet 38" as shown. To enable the mold to be opened and closed, separate cable attachments 48' and 48" and separate cables 52' and 52" are attached to the sheets 38' and 38" respectivelyO
similar arrangement is provided on the opposite side of the mold for cable 50.
The metallic sheet is preferably made of low carbon steel, silicon steel or stainless steel to provide a relatively high resistence to the flow of electric current. The voltage is typically limited to less than 50, however, the current can range up to and if required, exceed 2000 amperes. By limiting the voltage, the natural oxide coating 74 on the metal1iC sheet 38 and the underlying steel mold 12 acts as an adequate electrical insulator at the interface contact between the mold 12 surface and the ~heet 38. Alterna~ively, a thin adhesive layer, also designated by 74, may be applied between the mold 12 and the sheet 38 to act as both an electrical insulator and means to secure the sheet to the mold surface.
Such an adhesive layer is formed as thin as possible to minimize the resistance to heat transfer during the cooling portion of the molding cycle.
In another alternative construction, the metallic sheet may be formed by plating or spraying (sputtering) a metal of high resistivity onto a layer of thin electrically non-conducting plastic in turn adhered to the mold 12 surface.
z~
Typical foamed plastic rnolding compounds require that the surface of the mold cavity be in excess of the heat distortion temperature of the plastic to obtain a Class A smooth surface finish.
Such a temperature is between 350F and 550F for most common foamable polymersA By utilizing a metallic sheet 38 of substantially
2 millimeters thickness, the surface temperature of the sheet can be raised 300F in a fraction of a second. Thus, the mold surface temperature subsequent to -the ejection of the previous part, about 140F, can be raised above the heat distortion temperature in a fraction of a second after the mold is reclosed just prior to injection of the plastic moldlng compound. The tlme required to reheat the mold surface is therefore a small fraction of the cycle time for the part and the bulk material of the mold halves 10 and 12 is not substantially reheated and recooled with each part cycle. The actual heating time for a particular part will depend upon the size of the part, the required temperature increase and the amperage applied to the metallic sheet.
Because a very high current load is applied for a short period of time durlng each cycle and then disconnected by switch 54, a bypass load ln the form of heating pipe 62 is provided to minimize arcing in the switch and current surges applied to the current source 56. The electrical resistance of the pipe 62 is high relative to the resistance of the s~eet 38, thus closure of the switch 54 effectively creates a short circuit path through the sheet 38. The bulk of the current from source 56 passes through the sheet 38 with the switch 54 closed. Because of the electrically insulating layer of oxide between the sheet 38 and the mold half 12, the bulk oE the elec-tric current passes through the sheet in preference to the mold half 12 despite use of steel for both the mold half and the sheet.
A suitable source of electric power 56 can be a continuous duty arc welder supply or an alternating or direct current continuous duty motor generator set. The capacity of the power source and the amperage capacity of the switch 54 will be deterrnined by the volume of the metallic sheet 38 and the -temperature ln~nease required. The switch 54 is closed and opened by the machine control circuitry.
The adhesive plastic layer in the alternative form of the invention may be a polyimide or other plastic suitable for high temperatures (600F) and repeated cyclic heating.
The surface heating method is very quick and therefore imposes a negligible time penalty on the product molding cycle -time. The electric energy utilized to heat the bypass pipe 62 may he recycled as hot water to maintain the mold temperature or to preheat the foamable plastic resin prior to injection.
'~Ihere the particular product shape requires a mold cavity su~face with relatively small radii of curvature or sharp corners, local-ized overheating o~ the metallic sheet or heater band can occur. To prevent such localized overheating, the thickness or cross section of the sheet is adjusted by locally thickening the sheet. Thickening can be accomplished effectively by spraying or plating on additional metal at the localized areas.
As an example of the process and apparatus, a heater band or metallic sheet is constructed with the following dimensions and property:
Length - 30 cm.
Width = 10 cm.
Thickness = 0.2 cm.
Resistivity = 30 micro-ohm-centimeters at 20C.
For a temperature increase of 200C or 392F, the following times are typically required with the following amperages:
1000 amp. requires 0.09 sec.
500 amp. requires 0.37 sec.
100 amp. requires 0.90 sec.
Thus, the mold cavity surface can be heated ver~ quickly above the heat distortion temperature of the resin just before or as the injection of foamable resin begins. The bulk of the surface sheet is minimal in comparison with the bulk of the mold halves, thus the newly formed foamed plastic product can be cooled quickly below the heat distortion temperature. Very little mass is heated and 7~
cooled cyclically in the process. The heating time is so short as to not appreciably lengthen the cycle time for the entire moldi.ng process cycle. The layer of oxide or thin adhesi.ve plastic between the metallic sheet and the bulk of the mold minimizes resistance to heat transfer during the cooling portion of the process cycle.
Because a very high current load is applied for a short period of time durlng each cycle and then disconnected by switch 54, a bypass load ln the form of heating pipe 62 is provided to minimize arcing in the switch and current surges applied to the current source 56. The electrical resistance of the pipe 62 is high relative to the resistance of the s~eet 38, thus closure of the switch 54 effectively creates a short circuit path through the sheet 38. The bulk of the current from source 56 passes through the sheet 38 with the switch 54 closed. Because of the electrically insulating layer of oxide between the sheet 38 and the mold half 12, the bulk oE the elec-tric current passes through the sheet in preference to the mold half 12 despite use of steel for both the mold half and the sheet.
A suitable source of electric power 56 can be a continuous duty arc welder supply or an alternating or direct current continuous duty motor generator set. The capacity of the power source and the amperage capacity of the switch 54 will be deterrnined by the volume of the metallic sheet 38 and the -temperature ln~nease required. The switch 54 is closed and opened by the machine control circuitry.
The adhesive plastic layer in the alternative form of the invention may be a polyimide or other plastic suitable for high temperatures (600F) and repeated cyclic heating.
The surface heating method is very quick and therefore imposes a negligible time penalty on the product molding cycle -time. The electric energy utilized to heat the bypass pipe 62 may he recycled as hot water to maintain the mold temperature or to preheat the foamable plastic resin prior to injection.
'~Ihere the particular product shape requires a mold cavity su~face with relatively small radii of curvature or sharp corners, local-ized overheating o~ the metallic sheet or heater band can occur. To prevent such localized overheating, the thickness or cross section of the sheet is adjusted by locally thickening the sheet. Thickening can be accomplished effectively by spraying or plating on additional metal at the localized areas.
As an example of the process and apparatus, a heater band or metallic sheet is constructed with the following dimensions and property:
Length - 30 cm.
Width = 10 cm.
Thickness = 0.2 cm.
Resistivity = 30 micro-ohm-centimeters at 20C.
For a temperature increase of 200C or 392F, the following times are typically required with the following amperages:
1000 amp. requires 0.09 sec.
500 amp. requires 0.37 sec.
100 amp. requires 0.90 sec.
Thus, the mold cavity surface can be heated ver~ quickly above the heat distortion temperature of the resin just before or as the injection of foamable resin begins. The bulk of the surface sheet is minimal in comparison with the bulk of the mold halves, thus the newly formed foamed plastic product can be cooled quickly below the heat distortion temperature. Very little mass is heated and 7~
cooled cyclically in the process. The heating time is so short as to not appreciably lengthen the cycle time for the entire moldi.ng process cycle. The layer of oxide or thin adhesi.ve plastic between the metallic sheet and the bulk of the mold minimizes resistance to heat transfer during the cooling portion of the process cycle.
Claims (26)
1. Apparatus for heating several hundred degrees Fahrenheit within a matter of seconds or less at least a portion of the interior surface of a metal mold without substantially heating the bulk of the metal mold, the apparatus comprising, a relatively thin metallic sheet of substantially high electric resistivity covering the portions of the metal mold surface to be heated, said metallic sheet electri-cally insulated from the bulk metal mold surface at the interface therebetween by a relatively thin non-structural electrically insulative coating, said electrically insulative coating providing a negligible increase in thermal resistance between the metallic sheet and the bulk metal mold, a source of high current low voltage electric power, said metallic sheet electrically connected to the source of electric power so as to heat the metallic sheet upon the application of the electric power, said electrically insulative coating of sufficient electrical resistivity in combination with said low voltage to cause the bulk of said high current to pass through said metallic sheet and only a small portion of said high current to pass through the bulk metal mold.
2. The apparatus of claim 1 wherein the resistivity of the metallic sheet substantially equals the resistivity of the bulk metal mold.
3. The apparatus of claim 1 wherein the electrically insulative coating comprises a naturally occurring oxide layer on the metallic sheet.
4. The apparatus of claim 1 wherein the electrically insulative coating comprises the naturally occurring oxide layers on the metallic sheet and the bulk metal mold surface.
5. The apparatus of claim 1 including means to attach electric cables to opposite sides of the metallic sheet and means to supply a current of greater than 100 amperes at a voltage of less than 50 volts to the metallic sheet.
6. The apparatus of claim 1 wherein the electrically insulative coating comprises a layer of plastic adhered to the bulk mold surface and the thin metallic sheet comprises a metal coating applied to the layer of plastic .
7. The apparatus of claim 6 wherein the plastic layer is formed of polyimide.
8. The apparatus of claim 1 wherein the electrical resistivity of the metallic sheet is from 10 to 100 micro-ohm-centimeters.
9. Apparatus for heating several hundred degrees Fahrenheit within a matter of seconds or less at least a portion of the interior surface of a metal mold without substantially heating the bulk of the metal mold, the apparatus comprising, a relatively thin metallic sheet of substantially high electrical resistivity covering the portions of the metal mold surface to be heated, said metallic sheet electrically insulated from the bulk metal mold surface at the interface therebetween by a relatively thin non-structural electrically insulative coating, said electrically insulative coating providing a negligible increase in thermal resistance between the metallic sheet and the bulk metal mold, a source of high current low voltage electric power, said metallic sheet electrically connected to the source of electric power so as to heat the metallic sheet upon the application of the electric power, an electrical energy sink comprising a fluid cooled electrical heating pipe, said heating pipe electri-cally connected to the source of electric power, and, switch means in the circuit connecting the metallic sheet electrically in parallel with the heating pipe and to the source of electric power.
10. The apparatus of claim 9 wherein the metallic sheet and the heating pipe are electrically connected in parallel to the source of electric power.
11. The apparatus of claim 10 wherein the switch means is momentarily operable to short circuit the bulk of the high current through the metallic sheet in preference to the heating pipe.
12. The apparatus of claim 9 wherein the electrical resistance of the metallic sheet is sufficiently less than that of the heating pipe such that closure of the switch means effectively short circuits the electric current through the metallic sheet.
13. The apparatus of claim 9 wherein the resistivity of the metallic sheet substantially equals the resistivity of the bulk metal mold.
14. The apparatus of claim 9 wherein the electrically insulative coating comprises a naturally occurring oxide layer on the metallic sheet.
15. The apparatus of claim 9 wherein the electrically insulative coating comprises the naturally occurring oxide layers on the metallic sheet and the bulk metal mold surface.
16. The apparatus of claim 9 including means to attach electric cables to opposite sides of the metallic sheet and means to supply a current of greater than 100 amperes at a voltage of less than 50 volts to the metallic sheet.
17. The apparatus of claim 9 wherein the electrically insulative coating comprises a layer of plastic adhered to the bulk metal mold surface and the thin metallic sheet comprises a metal coating applied to the layer of plastic.
18. The apparatus of claim 17 wherein the plastic layer is formed of polyimide.
19. The apparatus of claim 9 wherein the electrical resisitivity of the metallic sheet is from 10 to 100 micro-ohm-centimeters.
20. Apparatus for heating several hundred degrees Fahrenheit within a matter of seconds or less at least a portion of the interior surface of a metal mold without substantially heating the bulk of the metal mold, the apparatus comprising, a relatively thin metallic sheet of substantially high electrical resistivity covering the portions of the metal mold surface to be heated, said metallic sheet electrically insulated from and in thermal communication with the bulk metal mold at the interface therebetween by means of a non-structural electrically insulative coating sufficiently thin to provide negligible thermal resistance, a source of high current low voltage electric power,said metallic sheet electrically connected to the source of electric power so as to heat the metallic sheet rapidly upon the application of the electric power for a few seconds or less, said electrically insulative coating of sufficient electrical resistivity in combination with said low voltage to cause the bulk of said high current to pass through said metallic sheet and only a small portion of said current to pass through the bulk metal mold.
21. A process for quickly manufacturing injection molded foamed resin products with a smooth surface finish comprising the steps of:
closing the injection mold, applying high current low voltage electric power to a heater band on the surface of the mold cavity to substantially increase the mold surface temperature within a matter of seconds or less in those portions of the mold cavity covered by the heater band, disconnecting the high current low voltage electric power from the heater band, injecting a molten foamable resin into the mold cavity, the foamable resin thereupon expanding against the hot heater band, reducing the temperature of the resin in the mold cavity below the heat distortion temperature of the resin by cooling the mold and heater band, and, opening the injection mold and removing the plastic product.
closing the injection mold, applying high current low voltage electric power to a heater band on the surface of the mold cavity to substantially increase the mold surface temperature within a matter of seconds or less in those portions of the mold cavity covered by the heater band, disconnecting the high current low voltage electric power from the heater band, injecting a molten foamable resin into the mold cavity, the foamable resin thereupon expanding against the hot heater band, reducing the temperature of the resin in the mold cavity below the heat distortion temperature of the resin by cooling the mold and heater band, and, opening the injection mold and removing the plastic product.
22. The process of claim 21 wherein the heater band temperature is raised from approximately 150° F. to a maximun of 600°F.
23. The process of claim 21 wherein the current applied to the heater band ranges from 100 to 3000 amperes.
24. The process of claim 21 wherein the voltage applied to the heater band is limited to less than 50 volts and the current exceeds 100 amperes.
25. The process of claim 21 wherein at least a portion of the resin is injected prior to disconnecting the electric power to the heater band.
26. The process of claim 21 wherein the heater band is supplied with electric power for less than five seconds per molding cycle.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000431132A CA1211262A (en) | 1983-06-24 | 1983-06-24 | High current low voltage heating method and apparatus for producing a smooth finish surface on injection molded foamed resin products |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000431132A CA1211262A (en) | 1983-06-24 | 1983-06-24 | High current low voltage heating method and apparatus for producing a smooth finish surface on injection molded foamed resin products |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1211262A true CA1211262A (en) | 1986-09-16 |
Family
ID=4125546
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000431132A Expired CA1211262A (en) | 1983-06-24 | 1983-06-24 | High current low voltage heating method and apparatus for producing a smooth finish surface on injection molded foamed resin products |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1211262A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2647479A1 (en) * | 2012-04-04 | 2013-10-09 | Plastiques du Val de Loire | Method for injection moulding of a foam plastic material |
-
1983
- 1983-06-24 CA CA000431132A patent/CA1211262A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2647479A1 (en) * | 2012-04-04 | 2013-10-09 | Plastiques du Val de Loire | Method for injection moulding of a foam plastic material |
| FR2989019A1 (en) * | 2012-04-04 | 2013-10-11 | Plastiques Du Val De Loire | INJECTION MOLDING PROCESS OF PLASTIC FOAM MATERIAL |
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