CA1108819A - Method and mold for forming sheets having reduced surface imperfections - Google Patents
Method and mold for forming sheets having reduced surface imperfectionsInfo
- Publication number
- CA1108819A CA1108819A CA304,391A CA304391A CA1108819A CA 1108819 A CA1108819 A CA 1108819A CA 304391 A CA304391 A CA 304391A CA 1108819 A CA1108819 A CA 1108819A
- Authority
- CA
- Canada
- Prior art keywords
- mold
- plastic
- flow
- sheet
- texture
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
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
- B29C33/00—Moulds or cores; Details thereof or accessories therefor
- B29C33/42—Moulds or cores; Details thereof or accessories therefor characterised by the shape of the moulding surface, e.g. ribs or grooves
- B29C33/424—Moulding surfaces provided with means for marking or patterning
-
- 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
- B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
- B29C45/17—Component parts, details or accessories; Auxiliary operations
- B29C45/26—Moulds
- B29C45/37—Mould cavity walls, i.e. the inner surface forming the mould cavity, e.g. linings
- B29C45/372—Mould cavity walls, i.e. the inner surface forming the mould cavity, e.g. linings provided with means for marking or patterning, e.g. numbering articles
-
- 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
- B29K2105/0094—Condition, form or state of moulded material or of the material to be shaped having particular viscosity
-
- 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
- B29K2995/0072—Roughness, e.g. anti-slip
-
- 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
- B29K2995/0072—Roughness, e.g. anti-slip
- B29K2995/0073—Roughness, e.g. anti-slip smooth
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
- B29L2031/00—Other particular articles
- B29L2031/30—Vehicles, e.g. ships or aircraft, or body parts thereof
- B29L2031/3055—Cars
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Moulds For Moulding Plastics Or The Like (AREA)
- Moulding By Coating Moulds (AREA)
- Casting Or Compression Moulding Of Plastics Or The Like (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Reinforced Plastic Materials (AREA)
Abstract
A METHOD AND MOLD FOR FORMING
SHEETS HAVING REDUCED SURFACE IMPERFECTIONS
Abstract of the Disclosure A mold for compression molding a plastic sheet from a viscous fluid provides more resistance to flow on the side of the sheet designed to be the underside than on the side of the sheet designed to be seen. This re-sistance to flow can be imparted by roughening the under-side of the mold more than the top surface of the mold.
The purpose for roughening one surface of the mold more than the other is so that the plastic mass will flow faster on the smoother side of the mold and carry bubbles to the rough side of the mold via a rolling action.
By this method employing textured molds, the surface of automotive panels can be made having reduced surface imperfections.
SHEETS HAVING REDUCED SURFACE IMPERFECTIONS
Abstract of the Disclosure A mold for compression molding a plastic sheet from a viscous fluid provides more resistance to flow on the side of the sheet designed to be the underside than on the side of the sheet designed to be seen. This re-sistance to flow can be imparted by roughening the under-side of the mold more than the top surface of the mold.
The purpose for roughening one surface of the mold more than the other is so that the plastic mass will flow faster on the smoother side of the mold and carry bubbles to the rough side of the mold via a rolling action.
By this method employing textured molds, the surface of automotive panels can be made having reduced surface imperfections.
Description
BACKGROUND OF ~HE INVEN~ION
Field of the Invention : .
This invention is directed to textured molds, more particularly to molds which cause a rolling-over during ~low of khe plastic being molded 80 that bubbles are carried to the back non-visable side o~ the sheet being molded.
DESCRIPTION OF THE PRIOR ART
In the molding of viscous plaskic material to form sheets used as body panels and the like, lt has become standard practlce to polish dies to the highest glos~
obtainable. The hlgh polish was imparted to minlmize surface irregularities. The polished die was an out growth of the development of SMC that more accurately reproduced the exact surface of the mold. Unfortunately, the highly polished dies did not accomplish the deslred end result. Sheets produced uslng the highly polished dies still contained signi~icant porosity, notlceable waves and sinks, fiber patterns visable on the sur~ace, . 2b orange peel on the ~inished part and visable flow lines.
The material most commonly used in the compression molding of body panels has been low profile sheet molding compound (SMC). SMC is a compound consisting of poly-ester resin, fillers, catalysts, chopped glass strands, release agents and a low profile additive that expandsduring the curlng reaction.
SMC has been described in U.S. Patents 3,701,748;
3,772,241; 3~674,893~ 3,577,478, 3,548,o30 and 3,466,259.
SMC has gained widespread use ln the automotive indu~try as a molding compound ~or exterior vehicle part~ due to (a? low cost, (b) ability to ~orm large parts, and (c~
ability to mold in ribs and bosses for fastening.
A persistent problem, however, has been apparent porosity and fiber prominence on the surface. These defects show up after the first applicatlon of primer and result in extensive reworking and repainting of the part. It has become a prior art standard practice in tooling for SMC to polish dies to the highest gloss obtainable to obt~in an absolutely smooth flat surface.
The reason that the automotive industry has switched to body panels made from plastic ~heet recently on a massive scale, has been to reduce weight3 and thus, im-prove fuel efficiency. One benefit to the consumer was body panels that dld not rust ~hrough in a year or two in northern climates where roads are salted. Other benefits included elimination of rattling nol~es gener-ated by metal-to-metal contact. One dlfficulty with the plastic body panels has been the problem o~ producing a surface free of visable imperfections without a subse-quent sanding operatlon.
To produce such a surface on an automotive panel it has been necessary1 ln the past, to extensively smooth the surface of panels having lmperfections with sandpaper.
This generates dust, often requiring the laborers doing the ~ob to wear respirator masks. This ls quite uncom-fortable on hot days near the hot curing presses~
~81~
Before the development of the present invention, raised areas were welded onto the vertical backside of a grill opening panel mold and the panels produced were sold more than one year prior to the present application date.
SUMMARY OF THE INVENTION
The invention provides a method of preventing visible surface imperfections in a sheet of fiberglass relnforced plastic formed in a compression mold which comprises providing the surface of the mold forming the non-visible plastic surface with a rougher surface than the surface of the mold forming the visible plastic surface, flowing plastic between the two mold surfaces and allowing the plastic to harden.
It has been unexpectedly discovered that if the surface of the mold forming the non-visible surface of the part produced is abraded to form a surface more resistant to flow than the non-abraded surface of the mold forming the visible surface of the part, a rolling action of the viscous plastic mass occurs. The rolling action facilitates the release of air bubbles trapped in SMC sheet allowing said air to escape out through the ~
shear edge and carries the air trapped in the viscous plastic to the non- !,~"
visible surface of the part and away from the visible surface as on page 5.
The applicant has theorized that the rolling action of the flow-ing plastic carries air bubbles to the less visible side of the part, but does not intend to be bound by this theory. It is only known that the method of the present invention is operative to overcome the problems of the prior art and it is quite possible that the scientific reasons for the success, is not clearly understood. It is known that surface porosity, the big problem of the prior art, has been greatly reduced by the present invention. In `~
addition:
1) waves and sinks are shallower and less noticeable
Field of the Invention : .
This invention is directed to textured molds, more particularly to molds which cause a rolling-over during ~low of khe plastic being molded 80 that bubbles are carried to the back non-visable side o~ the sheet being molded.
DESCRIPTION OF THE PRIOR ART
In the molding of viscous plaskic material to form sheets used as body panels and the like, lt has become standard practlce to polish dies to the highest glos~
obtainable. The hlgh polish was imparted to minlmize surface irregularities. The polished die was an out growth of the development of SMC that more accurately reproduced the exact surface of the mold. Unfortunately, the highly polished dies did not accomplish the deslred end result. Sheets produced uslng the highly polished dies still contained signi~icant porosity, notlceable waves and sinks, fiber patterns visable on the sur~ace, . 2b orange peel on the ~inished part and visable flow lines.
The material most commonly used in the compression molding of body panels has been low profile sheet molding compound (SMC). SMC is a compound consisting of poly-ester resin, fillers, catalysts, chopped glass strands, release agents and a low profile additive that expandsduring the curlng reaction.
SMC has been described in U.S. Patents 3,701,748;
3,772,241; 3~674,893~ 3,577,478, 3,548,o30 and 3,466,259.
SMC has gained widespread use ln the automotive indu~try as a molding compound ~or exterior vehicle part~ due to (a? low cost, (b) ability to ~orm large parts, and (c~
ability to mold in ribs and bosses for fastening.
A persistent problem, however, has been apparent porosity and fiber prominence on the surface. These defects show up after the first applicatlon of primer and result in extensive reworking and repainting of the part. It has become a prior art standard practice in tooling for SMC to polish dies to the highest gloss obtainable to obt~in an absolutely smooth flat surface.
The reason that the automotive industry has switched to body panels made from plastic ~heet recently on a massive scale, has been to reduce weight3 and thus, im-prove fuel efficiency. One benefit to the consumer was body panels that dld not rust ~hrough in a year or two in northern climates where roads are salted. Other benefits included elimination of rattling nol~es gener-ated by metal-to-metal contact. One dlfficulty with the plastic body panels has been the problem o~ producing a surface free of visable imperfections without a subse-quent sanding operatlon.
To produce such a surface on an automotive panel it has been necessary1 ln the past, to extensively smooth the surface of panels having lmperfections with sandpaper.
This generates dust, often requiring the laborers doing the ~ob to wear respirator masks. This ls quite uncom-fortable on hot days near the hot curing presses~
~81~
Before the development of the present invention, raised areas were welded onto the vertical backside of a grill opening panel mold and the panels produced were sold more than one year prior to the present application date.
SUMMARY OF THE INVENTION
The invention provides a method of preventing visible surface imperfections in a sheet of fiberglass relnforced plastic formed in a compression mold which comprises providing the surface of the mold forming the non-visible plastic surface with a rougher surface than the surface of the mold forming the visible plastic surface, flowing plastic between the two mold surfaces and allowing the plastic to harden.
It has been unexpectedly discovered that if the surface of the mold forming the non-visible surface of the part produced is abraded to form a surface more resistant to flow than the non-abraded surface of the mold forming the visible surface of the part, a rolling action of the viscous plastic mass occurs. The rolling action facilitates the release of air bubbles trapped in SMC sheet allowing said air to escape out through the ~
shear edge and carries the air trapped in the viscous plastic to the non- !,~"
visible surface of the part and away from the visible surface as on page 5.
The applicant has theorized that the rolling action of the flow-ing plastic carries air bubbles to the less visible side of the part, but does not intend to be bound by this theory. It is only known that the method of the present invention is operative to overcome the problems of the prior art and it is quite possible that the scientific reasons for the success, is not clearly understood. It is known that surface porosity, the big problem of the prior art, has been greatly reduced by the present invention. In `~
addition:
1) waves and sinks are shallower and less noticeable
2) no fiber pattern is apparent on the surface, resulting in a smoother finish after painting;
., ... . .: :
., ... . .: :
3) surface irregularities of the paint i~ elimin-ated, due to the formatlon o~ a rough ~ur~ace that tends to create valleys and rldge~, that hold paint during flash-off of the solvent; and
4) due to retardation Or material flow at the sur~aces of the mold and subsequent inter-laminous flow3 flo~ llnes are less noticeable and parts are stronger at the ~low line.
The slze and depth of the abrasion i8 dependent on mold configuration, placement of charge in die (core or cavity charged) and whether ribs and bos~es are situated so as to lmpede the ~low on one sur~ace more than the other. Treatmènt conslsts of abradlng the sur-faces o~ the molds with an abraslve that wlll impart the deslred texture to each individual mold surface or part of each mold surface. Materials which may be uæed to lmpart the desired texture includes aluminum oxlde, glass beads, sand and metal grit. The particles of the texturing material can be from 40 to 250 mesh.
Each mold surface is treated so that speed of materlal flow across the mold surfaoe will be faster on the more visable surface of the part relative to ~he less ~isable surface of the part. This di~ference in ; rate of flow causes a rolling motion of the glas~ fibers over the rough sur~ace,retarding flow and allowlng air to escape out of material during flow and to be entrapped on the less visable surface.
A glass smooth mold surface results in material g tending to slide in globs o~er the hot mold sur~ace allowing air to be entrapped at the top surface and ~low lines to become apparent due to a front Or melted resin.
When molds are textured as above, the climples flll with resin preventing fibers from being prominent on the sur~ace and ensuring a homogenous mix o~ materials at the flow lines, resulting in stronger parts.
A part having a convex ourved visable surface will have a coarse texture on the concave side and a flne texture on the convex side (see Fig. l). If the concave side is the exposed side, both sides of the part pre-ferably have the same texture (see Fig. 2). In e~rect, the texture is used to reverse, through resistance on one side of the part, the M ow pattern of the material so that whatever air is enclosed in the part wlll be on the backside tsee Fig. 3).
I~ an intermittent boss or rib is on a flat part that has a straight shear edge, a smoother path than the surrounding area should be created immedlately between the boss or rib and the material supply (charge), to accelerate Plow into t~is thicker area so that ~he entire ~ront of the material flow reaches the shear edge simul- ~-taneously to avoid enclosing air (see Fig. 4).
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a drawing o~ a body panel having a coarse texture on the interior surface and a ~lne texture on the exterior surface. This texture was generated by the corresponding coarse and fine textures of the mold 8~g :
surfaces.
Flgure 2 shows a body panel wherein the length (area~ of the visable surface is greater than the length ~area~ of the interior surface. Both surfaces have a fine texture as do the two surfaces of the mold.
Figures 1, and 2 are simplified from actual practice merely to exemplify the present invention.
Figure 3 is a cross-section through the power dome of a Chrysler B-body front end. Again, it is seen that the exterior surface has a fine texture so that the plastic will flow faster along the fine textured surface while rolling over releasing entrapped air and carrying bubbles to the backside of the part which has a coarse texture.
Figure 4 shows the bottom side of a mold which forms the inside of a Corvette roof panel. Coarse patterns ;~
` and smooth patterns in combination with boss recesses to slow down the surface flow of the viscous plastic are shown.
Figure 5 shows the uniformly textured top ~visable~
side of a Corvette roof panel.
Details of the materials or process used in making sheet molding compounds form no part of the present invention and are disclosed in SPI ~andbook of Technical and Engineering of Reinforced Plastics/Composites by J. Gilbert Mohr, et al, Second Edition, Copyright 1973 Van Nostrandt.
. .
g In particular, see pages 175 th~ough 242.
Generally speaking, SMC contains a mixture of an unsaturated polyester dissolved in styrene, peroxide, lubricant, fiberglass low profile additive, ¢urlng agent and ~iller. The fiberglass constitutes 15 to 60% of the mix by weight and preferably 22 to 35%. The fiber-glass is 12 to 50 millimeters long and haæ a very small diameter. The polyester plus styrene usually constitutes about 15% by welght of the mix. The lubricant~ e.g., zinc stearate~and the peroxide are present in small amounts. A small amount of a thiakener such as magnesium oxide may also be present. An lmpact modi~ier such as pol.ymethyl mekhacrylate copolymer wlth methacrylic or acrylic acid can also be used. A combination impact modifier and low profile additive such as a butadiene-styrene block copolymer may also be present.
DESCRIPTIO~ OF THE PREFERRED EMBODIMENTS
Treatment of the mold surface includes any process which will roughen the mold surface by removing portlons of the surface to form an irregular, rough sur~ace. ~;
Such treatment includes the mechanlcal abrasion treat-ments discussed above, chemical and electrochemical etching as ls known in the art, and electrical treatment such as electric arc or plasma treatment whereby material is removed ~rom portions of the mold surface to form pits as is also known in the art. The deslred texture can also be lmparted to the mold surface by the initial casting of the mold~ but this procedure ls not pre~erred.
. . / ~
The proces~ of the present invention is appllcable to the molding of artlcles f'rom vi~cous pla~tics. Such molding technlques include SMC techniques~ ~ulk molding compound techniques and in~ection moldin~ technique~
includlng reaction inJection moldlngO Xt 18 particularly suitable ~or the use of SMC in the manufacture of automo-tive body panel~.
The viscous plastics can have a viscosity of ~rom 1,000 to 10 x 107 centipolses. The prererre~ SMC has a viscoslty of from 15 x 106 to 50 x 106 cenklpoi~e~ at room temperature.
The mold that can be used in the prac~ice o~ the present invention lncludes any o~ the well-known rnolds of the prior art. These are usually two-piece molds having a bottom sectlon and a top mating section. The space between the two faces o~ khe mold usually ranges from 3 milllme~ers to 12 milllmeters disregarding rein-~orclng memberæ. The area o~ mold sur~ace in square centlmeters ranges from 5,000 to 1 x 10l2 cm2.
The ~ollowing procedure has been used ln manufac-turing the removable roof sectlon of the Corvette auto-moblle. The bottom half of the mold used ls shown in Figure 4 and the top half of the mold used is shown ln Figure 5. A quantity of sheet molding compounds suffi-cient to completely flll the mold ls placed ln charge placement area 1 of Figure 4. Areas 3, 5, 7, 9 3 11 and 13 are relatively smooth areas whlch allow the sheet moldlng compound to flow freely to the mold recesse~
which form bosses. Bosses are shown at 15, 17, 19, 21, g 23, 25, 27, 29~ 31 and 33. The dotted area~ 35 repreaent areas of the mold whlch have been textured by impingement~
o~ glass beads against ~he mold. A~ter the charge ls placed, the mold is closed.
The area behind the bosses indicated by 37 are textured by 60 to 90 grit aluminum oxide to produce even a rougher sur~ace in the area than 35 so as to slow down the ~low of the sheet molding compound a~ter it has entered the bosses.
This slowing down of the travel of the SMC at the rougher surface allows the sheet moldin~ compound at the upper mold 3ur~ace to lncrease its speed relative to the lower sur~ace, roll over and ¢arry the bubbles which causes poroslty to the lower surface.
Figure 5 show3 the upper mold sur~ace which has a smoother texture 39 compared to the texture Or khe lower ~old sur~ace. The upper surface is textured by lmplngement with glass beads. Both the upper mold surface and the lower m~ld surface are heated in operatlon so as to cure the sheet molding compound. The surface of the mold of ~igure 5 has a uniform texture 39 which is less rough than the average texture of the mold surface`or Figure 4. Both ; the upper and lower mold surfaces are heated so as to cure the sheet molding compound to form the cured roo~ panel.
In the operation o~ the mold the upper mold half ls `
brought down by hydraulic pressure against the sheet moldlng charge in the lower mold half. The pressure causes the charge to roll in the textured areas trapping poroslty in the lower sur~ace o~ the panel. The charge -flows relati~ely free acros the areas o~ the lower mold .
. .. . .
surface which is not textured to flll the cavity whlch ~orm the bosses. A~ter the filling of the bosses the remaining sheet molding material rlows to the corner areas behlnd the bosses where, again, it is æub~ected to the rolling action which causes the porosity to be trapped on the bottom side of the panel.
The appropriate times and temperatures and other conditlons utilized in the moldlng techniques are known in the art and also probably available from the Dow Chemical Company and the Rohm & Haas Co~pany, two o~ the manurac-turers o~ the resins used to make ~heet molding compounds. ~, In additlon to Corvette roo~ panels, the present invention has al~o been used to manu~acture grill openlng panels 9 camper roo~s, automobile hoods, spoiler~, and many other body parts.
The slze and depth of the abrasion i8 dependent on mold configuration, placement of charge in die (core or cavity charged) and whether ribs and bos~es are situated so as to lmpede the ~low on one sur~ace more than the other. Treatmènt conslsts of abradlng the sur-faces o~ the molds with an abraslve that wlll impart the deslred texture to each individual mold surface or part of each mold surface. Materials which may be uæed to lmpart the desired texture includes aluminum oxlde, glass beads, sand and metal grit. The particles of the texturing material can be from 40 to 250 mesh.
Each mold surface is treated so that speed of materlal flow across the mold surfaoe will be faster on the more visable surface of the part relative to ~he less ~isable surface of the part. This di~ference in ; rate of flow causes a rolling motion of the glas~ fibers over the rough sur~ace,retarding flow and allowlng air to escape out of material during flow and to be entrapped on the less visable surface.
A glass smooth mold surface results in material g tending to slide in globs o~er the hot mold sur~ace allowing air to be entrapped at the top surface and ~low lines to become apparent due to a front Or melted resin.
When molds are textured as above, the climples flll with resin preventing fibers from being prominent on the sur~ace and ensuring a homogenous mix o~ materials at the flow lines, resulting in stronger parts.
A part having a convex ourved visable surface will have a coarse texture on the concave side and a flne texture on the convex side (see Fig. l). If the concave side is the exposed side, both sides of the part pre-ferably have the same texture (see Fig. 2). In e~rect, the texture is used to reverse, through resistance on one side of the part, the M ow pattern of the material so that whatever air is enclosed in the part wlll be on the backside tsee Fig. 3).
I~ an intermittent boss or rib is on a flat part that has a straight shear edge, a smoother path than the surrounding area should be created immedlately between the boss or rib and the material supply (charge), to accelerate Plow into t~is thicker area so that ~he entire ~ront of the material flow reaches the shear edge simul- ~-taneously to avoid enclosing air (see Fig. 4).
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a drawing o~ a body panel having a coarse texture on the interior surface and a ~lne texture on the exterior surface. This texture was generated by the corresponding coarse and fine textures of the mold 8~g :
surfaces.
Flgure 2 shows a body panel wherein the length (area~ of the visable surface is greater than the length ~area~ of the interior surface. Both surfaces have a fine texture as do the two surfaces of the mold.
Figures 1, and 2 are simplified from actual practice merely to exemplify the present invention.
Figure 3 is a cross-section through the power dome of a Chrysler B-body front end. Again, it is seen that the exterior surface has a fine texture so that the plastic will flow faster along the fine textured surface while rolling over releasing entrapped air and carrying bubbles to the backside of the part which has a coarse texture.
Figure 4 shows the bottom side of a mold which forms the inside of a Corvette roof panel. Coarse patterns ;~
` and smooth patterns in combination with boss recesses to slow down the surface flow of the viscous plastic are shown.
Figure 5 shows the uniformly textured top ~visable~
side of a Corvette roof panel.
Details of the materials or process used in making sheet molding compounds form no part of the present invention and are disclosed in SPI ~andbook of Technical and Engineering of Reinforced Plastics/Composites by J. Gilbert Mohr, et al, Second Edition, Copyright 1973 Van Nostrandt.
. .
g In particular, see pages 175 th~ough 242.
Generally speaking, SMC contains a mixture of an unsaturated polyester dissolved in styrene, peroxide, lubricant, fiberglass low profile additive, ¢urlng agent and ~iller. The fiberglass constitutes 15 to 60% of the mix by weight and preferably 22 to 35%. The fiber-glass is 12 to 50 millimeters long and haæ a very small diameter. The polyester plus styrene usually constitutes about 15% by welght of the mix. The lubricant~ e.g., zinc stearate~and the peroxide are present in small amounts. A small amount of a thiakener such as magnesium oxide may also be present. An lmpact modi~ier such as pol.ymethyl mekhacrylate copolymer wlth methacrylic or acrylic acid can also be used. A combination impact modifier and low profile additive such as a butadiene-styrene block copolymer may also be present.
DESCRIPTIO~ OF THE PREFERRED EMBODIMENTS
Treatment of the mold surface includes any process which will roughen the mold surface by removing portlons of the surface to form an irregular, rough sur~ace. ~;
Such treatment includes the mechanlcal abrasion treat-ments discussed above, chemical and electrochemical etching as ls known in the art, and electrical treatment such as electric arc or plasma treatment whereby material is removed ~rom portions of the mold surface to form pits as is also known in the art. The deslred texture can also be lmparted to the mold surface by the initial casting of the mold~ but this procedure ls not pre~erred.
. . / ~
The proces~ of the present invention is appllcable to the molding of artlcles f'rom vi~cous pla~tics. Such molding technlques include SMC techniques~ ~ulk molding compound techniques and in~ection moldin~ technique~
includlng reaction inJection moldlngO Xt 18 particularly suitable ~or the use of SMC in the manufacture of automo-tive body panel~.
The viscous plastics can have a viscosity of ~rom 1,000 to 10 x 107 centipolses. The prererre~ SMC has a viscoslty of from 15 x 106 to 50 x 106 cenklpoi~e~ at room temperature.
The mold that can be used in the prac~ice o~ the present invention lncludes any o~ the well-known rnolds of the prior art. These are usually two-piece molds having a bottom sectlon and a top mating section. The space between the two faces o~ khe mold usually ranges from 3 milllme~ers to 12 milllmeters disregarding rein-~orclng memberæ. The area o~ mold sur~ace in square centlmeters ranges from 5,000 to 1 x 10l2 cm2.
The ~ollowing procedure has been used ln manufac-turing the removable roof sectlon of the Corvette auto-moblle. The bottom half of the mold used ls shown in Figure 4 and the top half of the mold used is shown ln Figure 5. A quantity of sheet molding compounds suffi-cient to completely flll the mold ls placed ln charge placement area 1 of Figure 4. Areas 3, 5, 7, 9 3 11 and 13 are relatively smooth areas whlch allow the sheet moldlng compound to flow freely to the mold recesse~
which form bosses. Bosses are shown at 15, 17, 19, 21, g 23, 25, 27, 29~ 31 and 33. The dotted area~ 35 repreaent areas of the mold whlch have been textured by impingement~
o~ glass beads against ~he mold. A~ter the charge ls placed, the mold is closed.
The area behind the bosses indicated by 37 are textured by 60 to 90 grit aluminum oxide to produce even a rougher sur~ace in the area than 35 so as to slow down the ~low of the sheet molding compound a~ter it has entered the bosses.
This slowing down of the travel of the SMC at the rougher surface allows the sheet moldin~ compound at the upper mold 3ur~ace to lncrease its speed relative to the lower sur~ace, roll over and ¢arry the bubbles which causes poroslty to the lower surface.
Figure 5 show3 the upper mold sur~ace which has a smoother texture 39 compared to the texture Or khe lower ~old sur~ace. The upper surface is textured by lmplngement with glass beads. Both the upper mold surface and the lower m~ld surface are heated in operatlon so as to cure the sheet molding compound. The surface of the mold of ~igure 5 has a uniform texture 39 which is less rough than the average texture of the mold surface`or Figure 4. Both ; the upper and lower mold surfaces are heated so as to cure the sheet molding compound to form the cured roo~ panel.
In the operation o~ the mold the upper mold half ls `
brought down by hydraulic pressure against the sheet moldlng charge in the lower mold half. The pressure causes the charge to roll in the textured areas trapping poroslty in the lower sur~ace o~ the panel. The charge -flows relati~ely free acros the areas o~ the lower mold .
. .. . .
surface which is not textured to flll the cavity whlch ~orm the bosses. A~ter the filling of the bosses the remaining sheet molding material rlows to the corner areas behlnd the bosses where, again, it is æub~ected to the rolling action which causes the porosity to be trapped on the bottom side of the panel.
The appropriate times and temperatures and other conditlons utilized in the moldlng techniques are known in the art and also probably available from the Dow Chemical Company and the Rohm & Haas Co~pany, two o~ the manurac-turers o~ the resins used to make ~heet molding compounds. ~, In additlon to Corvette roo~ panels, the present invention has al~o been used to manu~acture grill openlng panels 9 camper roo~s, automobile hoods, spoiler~, and many other body parts.
Claims
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of preventing visible surface imperfections in a sheet of fiberglass reinforced plastic formed in a compression mold which comprises providing a surface of the mold forming a non-visible plastic surface with a rougher surface than a surface of the mold forming a visible plastic surface, flowing plastic between the two mold surfaces and allowing the plastic to harden.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US80170977A | 1977-05-31 | 1977-05-31 | |
| US801,709 | 1977-05-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1108819A true CA1108819A (en) | 1981-09-15 |
Family
ID=25181855
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA304,391A Expired CA1108819A (en) | 1977-05-31 | 1978-05-30 | Method and mold for forming sheets having reduced surface imperfections |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS53149273A (en) |
| CA (1) | CA1108819A (en) |
| DE (1) | DE2802236C3 (en) |
| FR (1) | FR2392794A1 (en) |
| GB (1) | GB1598480A (en) |
| IT (1) | IT1095072B (en) |
| SE (1) | SE426562B (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5943120U (en) * | 1982-09-13 | 1984-03-21 | ミサワホ−ム株式会社 | FRP molded product with grain |
| JPS634145U (en) * | 1986-06-26 | 1988-01-12 | ||
| JPS63112120A (en) * | 1986-10-31 | 1988-05-17 | Ube Nitto Kasei Kk | Method for molding reinforced thermoplastic resin molding |
| JP2519985B2 (en) * | 1988-08-25 | 1996-07-31 | ダイセル化学工業株式会社 | Injection molding stamper |
| JPH033157U (en) * | 1989-05-30 | 1991-01-14 |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1404387A1 (en) * | 1958-11-22 | 1968-11-21 | Alfred Kepka | Process for the production of objects from liquid synthetic resins reinforced with fiber material and hardened by the action of catalysts |
| JPS5119802B2 (en) * | 1971-08-21 | 1976-06-21 | ||
| DE2649267C2 (en) * | 1976-10-29 | 1984-09-20 | Bayer Ag, 5090 Leverkusen | Process for the production of metal fiber reinforced plastic semi-finished and finished products |
-
1978
- 1978-01-19 DE DE2802236A patent/DE2802236C3/en not_active Expired
- 1978-02-15 JP JP1549478A patent/JPS53149273A/en active Granted
- 1978-05-30 CA CA304,391A patent/CA1108819A/en not_active Expired
- 1978-05-30 IT IT24004/78A patent/IT1095072B/en active
- 1978-05-30 SE SE7806259A patent/SE426562B/en unknown
- 1978-05-31 GB GB25767/78A patent/GB1598480A/en not_active Expired
- 1978-05-31 FR FR7816215A patent/FR2392794A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6155451B2 (en) | 1986-11-27 |
| SE426562B (en) | 1983-01-31 |
| IT1095072B (en) | 1985-08-10 |
| GB1598480A (en) | 1981-09-23 |
| DE2802236C3 (en) | 1981-12-24 |
| FR2392794B1 (en) | 1980-07-11 |
| IT7824004A0 (en) | 1978-05-30 |
| FR2392794A1 (en) | 1978-12-29 |
| SE7806259L (en) | 1978-12-01 |
| JPS53149273A (en) | 1978-12-26 |
| DE2802236B2 (en) | 1981-05-07 |
| DE2802236A1 (en) | 1978-12-07 |
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