JPS6155451B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a textured mold. More particularly, the present invention relates to a mold that rolls over during the flow of the plastic to be molded so that the bubbles are brought to the invisible back side of the sheet to be molded.

When molding viscous plastic materials into sheets for use in body panels and the like, it has become standard practice to polish the die to the highest possible gloss. This high degree of polish was provided to reduce surface irregularities. The polished die is made of sheet molding compound (SMC) that precisely reproduces the exact surface of the mold.
It was a byproduct of development. Unfortunately, the highly polished die did not achieve the desired end result. Sheets made using highly polished dies will still contain significant porosity, noticeable corrugations and divots, visible fiber patterns on the surface, tangerine skin on finished areas, and visible run lines. It was.

The most commonly used material for compression molding of body panels has been a low profile sheet molding compound (SMC). SMC
is a compound consisting of a polyester resin, a filler, a catalyst, chopped glass strands, a mold release agent and additives for a low profile that expand during the curing reaction.

SMC U.S. Patent Nos. 3701748 and 3772241
No., No. 3674893, No. 3577478, No. 3548030,
and No. 3466259. SMC is widely used in the automotive industry as a molding compound for external vehicle parts because of its (a) low cost, (b) ability to mold large parts, and (c) ability to mold ribs and bosses for fixation. I ended up being killed.

However, visible pores and protruding fibers on the surface were persistent problems. These defects appear after the first application of primer and result in extensive rework and repainting of the part. To obtain a completely smooth, flat surface, SMC molding tools require polishing the die to the highest possible gloss.
It had become standard practice in the prior art.

Recently, the automobile industry has made a major transition to body panels made from plastic sheets, in order to reduce weight and thereby improve fuel efficiency. One advantage for consumers is that in northern climates where roads are salty, body panels do not rust in a year or two. Another advantage is the elimination of rattling noises caused by metal-to-metal contact. One difficulty with plastic body panels was that it was difficult to create a surface free of visible imperfections without a subsequent sintering operation.

Creating such surfaces on car panels used to require sandpapering the defective surfaces of the panels to make them extremely smooth. This created dust, and workers were often required to wear breathing masks. The mask is quite uncomfortable on a hot day near a hot curing press.

Prior to the development of the present invention, a raised surface was welded to the vertical back side of a grill opening panel mold, and panels made were sold for more than a year prior to the date of this application.

Abrasion of the mold surface that forms the invisible (not intended to be visible) surface of the part that is made, creating a visible surface of this part that is more flow resistant than the polished surface of the mold. It has also been discovered that the rolling action of a viscous plastic mass can occur if a surface is created. This rolling action facilitates the release of air bubbles trapped in the SMC sheet, allowing the air to escape through the cutting blades of the mold and
The air trapped in the viscous plastic is transported to the non-visible surfaces of the part and directed away from the visible surfaces as described below.

Applicants have theorized, but do not intend to be bound by this theory, that the rolling action of the flowing plastic transports air bubbles to the less visible side of the part. However, while it is known that the molds and methods of the present invention are viable in overcoming the problems of the prior art, it is possible that the scientific reasons for this success may not be clearly understood. There are a lot of things. It is known that surface porosity, a major problem in the prior art, has been greatly reduced by the present invention. Besides, (1) corrugations and depressions become shallower and less noticeable;

(2) There were no visible fiber patterns on the surface, and the finish after painting was smooth.

(3) Surface irregularities of the paint are eliminated due to the formation of a rough surface that tends to create valleys and ridges that retain the paint during solvent evaporation separation.

(4) Due to the flow of material on the surface of the mold and the subsequent delay in interlaminar flow, the flow streaks are less noticeable and the part is stronger at the flow streaks.

The size and depth of the abrasion will depend on the shape of the mold, how the die is loaded (filled core or void), and whether the riffs and bosses are located to impede flow on one surface more than the other. It depends on whether the The treatment consists of scraping the mold surfaces with an abrasive material that imparts the desired texture to the individual mold surfaces or to a portion of each mold surface. Materials that can be used to provide the desired texture include aluminum oxide, glass beads, sand, and metal grit. The particles of textured material may be 40 to 250 mesh.

Each mold surface is treated such that the flow of material across the mold surface is faster on the more visible surfaces of the part than on the less visible surfaces of the part. This difference in flow rate causes rolling motion of the glass fibers on the rough surface, slowing the flow and allowing air to escape from the material during flow and become trapped on the surface where it is less visible.

A glass-smooth mold surface results in the material tending to slide in clumps over the hot mold surface, allowing air to become trapped at the top surface and causing the molten plastic to flow through the front surface. Make sure the flow lines are clear. When the mold is textured as above, the small depressions are filled with resin, which prevents fibers from showing on the surface and ensures homogeneous mixing of the material in the flow lines, resulting in a stronger part. arise.

Parts with visible convex surfaces have a coarse texture on the concave side and a fine texture on the convex side (see Figure 1). If the concave side is the exposed side, both sides of the part preferably have the same texture (see Figure 2). In practice, the texture is used to reverse the flow pattern of the material so that the resistance on one side of the part causes all the air enclosed in the part to exit to the back side (see Figure 3).

If intermittent bosses or ribs are on a flat part with a straight cutting edge, the bosses or ribs should be placed in such a way that the entire front surface of the material stream reaches the cutting edge at the same time to avoid air entrapment. A smoother passage between the ribs and the material feed should be created immediately than in the surrounding area, so that the flow into this thicker area is accelerated.

FIG. 1 is a drawing of a body panel with a coarse texture on the inside surface and a fine texture on the outside surface. This texture was created by corresponding coarse and fine textures on the mold surface.

FIG. 2 shows a body panel where the length (area) of the invisible surface is greater than the length (area) of the interior surface. Both surfaces have the same fine texture as 2.

Figures 1 and 2 are simplified from the actual manner merely to illustrate the invention.

FIG. 3 is a cross section through the power dome at the front end of the Chrysler B body. Again, the outer surface has a fine texture so that the plastic flows faster along the surface of the fine texture, while the rolling over releases the trapped air and carries it to the back side of the part with the coarse texture. I understand.

FIG. 3 shows a corresponding mold with corresponding fine and coarse textured surfaces to eliminate porosity.

FIG. 4 shows the bottom side of a mold for forming the interior of a Corvette roof panel. Coarse and fine patterns are shown combined with boss indentations to retard flow on the surface of the viscous plastic.

FIG. 5 shows the uniformly textured top side (visible side) of a Corvette roof panel.

Details of the materials or methods used to make the sheet molding compounds do not form part of this invention and may be found in the SPI Handbook of J. Gilbert Mohr et al.
Technical and Engineering of Reinforced
Published in Plastics/Composites, 2nd edition, Copyright 1973, Van Nostrand. See especially pages 175-242.

Generally, SMC contains a mixture of unsaturated polyester dissolved in styrene, peroxides, lubricants, fiberglass low profile additives, hardeners, and fillers. The fiberglass constitutes 15-60% by weight of the mixture, preferably 22-35%. Fiberglass is 12 to 50 mm long and has a very small diameter. The polyester and styrene together usually constitute about 15% by weight of the mixture. Lubricants such as zinc stearate and peroxide are present in small amounts. Thickening agents such as magnesium oxide may also be present in small amounts. Impact modifiers such as methacrylic acid or copolymers of acrylic acid and polymethyl methacrylate may also be used. There may also be combinations of impact modifiers and low profile additives such as butadiene-styrene block copolymers.

Treating the mold surface includes any method of roughening the mold surface by removing portions of the surface to create an irregularly roughened surface. These treatments include mechanical scratching as described above, chemical and electrochemical etching as known in the art, and removing material from portions of the mold surface as also known in the art. Except for denting, including electric arc and plasma treatments. Although it is possible to provide the desired texture on the mold surface by casting the mold from scratch, this procedure is not preferred.

The method of the invention can be applied to the molding of articles from viscous plastics. This kind of molding technology is
Includes SMC technology, bulk molding compound technology, and injection molding, including reaction injection molding. The use of SMC is particularly suitable for the production of automobile body panels.

Viscous plastics can have a viscosity of 1000 to 10 x ¹⁰⁷ centipoise. Preferred SMCs have a viscosity of 15 x 10 ⁶ to 50 x 10 ⁶ centipoise at room temperature.

Molds that can be used to practice the present invention include any of the well-known molds of the prior art. These are usually two-part molds with a bottom section and an upper joining section. The void space between the two sides of the mold is usually 3 mm to 12 mm, without considering the reinforcing material.
is within the range of The area of the mold surface is in square centimeters
It ranges from 5000 to 1×10 ¹² cm ² .

The following procedure was used to create the removable roof section of the Corvette automobile. The bottom half of the mold used is shown in FIG. 4, and the top half of the mold used is shown in FIG. A quantity of sheet molding compound sufficient to completely fill the mold is placed in charge area 1 of FIG. Areas 3, 5, 7, 9, 11 and 13 are relatively smooth areas that allow the sheet molding compound to flow freely into the mold recesses forming the bosses. Bosses are 15, 17, 19, 21, 23, 2
5, 27, 29, 31 and 33. The dotted area 35 indicates the mold area that has been textured by the impact of the glass beads on the mold.
After the ingredients are added, the mold is closed.

The area behind the boss indicated by 37 has a roughness of 60 to 90 to create a rougher surface in this area than 35.
It is textured with grit aluminum oxide to slow the flow of the sheet forming compound after it enters the boss.
This slowing of SMC movement on the rough surface allows the sheet molding compound at the upper mold surface to increase its velocity relative to the lower surface, rolling and carrying porosity-causing air bubbles to the lower surface.

FIG. 5 shows the upper mold surface with a smoother texture 39 compared to the texture on the lower mold surface. The upper surface is textured by impact with glass beads. Both the upper and lower mold surfaces are heated during operation to cure the sheet molding compound. The mold surface in Figure 5 is
It has a homogeneous texture 39 that is less rough than the average texture of the mold surface in FIG. Both the upper and lower mold surfaces are heated to cure the sheet molding compound and form a cured roof panel.

In operation of the mold, the upper mold half is lowered hydraulically relative to the sheet forming charge in the lower mold half. This pressure causes the charge to roll into the textured area, trapping porosity in the lower surface of the panel. The charge flows relatively freely across the untextured area of the lower mold surface to fill the void forming the boss. After filling the bosses, the remaining sheet molding material flows to the corner areas behind the bosses and is also subjected to rolling action there, so that the porosity is trapped on the bottom side of the panel.

The appropriate times, temperatures, and other conditions utilized in the molding technique are known to the art and are available from two of the manufacturers of the resins used to make sheet molding compounds: the Dow Chemical Company and the Rohm and Haas Company. Probably available from.

In addition to Corvette roof panels, the invention has been used to make grill opening panels, camper roofs, automobile hoods, spoilers, and many other body parts.

[Brief explanation of the drawing]

Figure 1 is a drawing of a body panel with a rough texture on the inner surface and a rough texture on the outer surface. Figure 2 shows the case where the length (area) of the visible surface is larger than the length (area) of the inner surface. Figure 3 is a drawing of the body panel, and Figure 4 is a cross section passing through the power dome at the front end of the Chrysler B body.
Figure 5 shows the bottom side of the mold for forming the interior of the corvette loop panel; Figure 5 shows the top side of the uniform texture of the corvette loop panel.

Claims (1)

Claims: 1. Heat curing in a mold having two heated dies having at least one area with a rougher surface on one of the dies than opposing areas on the other die. The process consists of a first step in which a thermosetting liquid is introduced, and a second step in which the two dies are brought close together to form a cavity so that the thermosetting liquid flows and fills the cavity prior to curing, resulting in rotational movement of the glass fibers. Similarly, the flow of thermosetting liquid is retarded over a die that has a rougher surface compared to the other die, thereby separating the rougher surface from the smoother surface that is intended to be viewed. A method of forming a thermoset liquid having an initial viscosity at room temperature between 1000 centipoise and 7 x 10 ⁷ centipoise and containing chopped glass fibers to form a sheet having: 2. The method of claim 1, wherein the rougher surface of one die is formed when the die is first cast. 3. The method of claim 1, wherein the rougher surface of one of the dies is formed by removing material from the surface by scraping, etching, electric arc or plasma treatment. 4 The initial viscosity of the thermosetting liquid is 15×10 ⁶ and 50×10 ⁶
Claim 1 between centipoise;
Method 2 or 3.