GB1598480A - Method of moulding a thermosetting liquid setting into a sheet - Google Patents

Description

(54) A METHOD OF MOULDING A THERMOSETTING LIQUID SETTING INTO A SHEET (71) We, THE GENERAL TIRE & RUBBER COMPANY, a corporation organized and existing under the laws of the State of Ohio, of One General Street, Akron, Ohio, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a method of molding a thermosetting liquid into a sheet of material, and more particularly, to a method of molding a thermosetting liquid having an initial viscosity of between 1000 and 7 x 107 centipoises at room temperature, which contains chopped glass fibres.

In the molding of viscous plastic material to form sheets used, for example, as body panels, it has become standard practice to polish dies to the highest gloss obtainable.

A high polish is imparted to minimize surface irregularities. The polished die is an out-growth of the development of SMC (defined below) that more accurately reproduced the exact surface of the mold. Unfortunately, the highly polished dies do not accomplish the desired end result. Sheets produced using the highly polished dies still contain significant porosity, noticeable waves and sinks, fiber patterns visible on the surface, orange peel on the finished part and visible 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 polyester resin, fillers, catalysts, chopped glass strands, release agents and a low profile additive that expands during the curing reaction.

SMC has been described in U.S. Patent Specification Nos. 3,701,748; 3,772,241; 3,674,893; 3,577,478; 3,548,030; and 3,466,259.

SMC has gained widespread use in the automotive industry as a moulding compound for exterior vehicle parts due to (a) low cost, (b) ability to form large parts, and (c) ability to mold in ribs and bosses for fastening.

A persistent problem, however, is apparent porosity and fiber prominence on the surface. These defects show up after the first application of primer and result in extensive reworking and repainting of the part. It has become a standard practice in tooling for SMC to polish dies to the highest gloss obtainable to obtain an absolutely smooth fiat surface.

The reason that the automotive industry has switched to body panels made from plastic sheet recently on a massive scale, is to reduce weight, and thus, improve fuel efficiency. One benefit to the consumer is that body panels are produced that do not rust through in a year or two in nortliern climates where roads are salted.

Other benefits include elimination of rattling noises generated by metal-to-metal contact.

One difficulty with the plastic body panels is the problem of producing a surface free of visible imperfections without a subsequent sanding operation.

To produce such a surface on an automotive panel it has been necessary, in the past, extensively to smooth the surface of panels having imperfections with sandpaper.

This generates dust, often requiring the labourers doing the job to wear respirator masks. This is quite uncomfortable on hot days near the hot curing presses.

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.

According to the present invention, there is provided a method of molding a thermosetting liquid having an initial viscosity between 1000 and 7x 107 centpoises at room temperature and containing chopped glass fiber, comprising a first step of placing the thermosetting liquid in a mold having two heated dies, one of the dies having at least one region with a rougher surface than the opposed region of the other die, and a second step of bringing the two dies Ito- gether thereby to form a cavity, so that the thermosetting liquid flows to fill the cavity prior to setting, the flow of the thermosetting liquid being retarded over the die with the rougher surface as compared with the other die, such that a rolling motion of the glass fibers is produced, thereby to form a sheet with a rough surface and a smoother surface, the smoother surface being intended to be the viewed surface of the sheet. Preferably, in the method according to the present invention, the rougher surface is formed either by the initial casting of the die or by removal of material from the surface by abrasion, etching, electric arc or plasma treatment.

It has been unexpectedly discovered that if the surface of the mold forming the less visible surface of the part produced is rough, e.g. is abraded to form a surface more resistant to flow than the opposite 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 less visible surface of the part and away from the visible surface.

The applicants have theorized that the rolling action of the kowing plastic carries air bubbles to the less visible side of the part, but do not intend to be bound by this theory. It is only known that the method of the present invention can overcome the problems of the prior art and it is quite possible that the scientific reason for the success is not clearly understood. It is known that the surface porosity, the big problem in the prior art, can be greatly reduced by the present invention. In addition, the following advantages are obtainable: 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 are eliminated, due to the formation of a rough surface that tends to create valley and ridges, that hold paint during flash-off of the solvent; and 4) due to retardation of material flow at the surfaces of the mold and subse quent interlaminous flow, flow lines are less noticeable and parts are stronger at the flow line.

The size and depth of the abrasion is dependent on mold configuration, the positioning of a charge in the die (core or cavity charged) and whether ribs and bosses are situated so as to impede the flow on one surface more than the other. Treatment consists of abrading the surfaces of the molds with an abrasive that imparts the desired texture to each individual mold surface or to part of each mold surface.

Materials which may be used to impart the desired texture include aluminium oxide, 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 material flow across the mould surface will be faster on the more visible surface of the part relative to the less visible surface of the part. This difference in rate of flow causes a rolling motion of the glass fibers over the rough surface, retarding flow and allowing air to escape out of the material during flow and to be entrapped on the less visible surface.

A glass smooth mold surface may result in material tending to slide in globs over the hot mold surface allowing air to be entrapped at the top surface and flow lines to become apparent due to a front of melted resin. When molds are textured as above, the dimples fill with resin preventing fibers from being prominent on the surface and ensuring a homogeneous mix of materials at the flow lines, resulting in stronger parts.

Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings wherein: Figure 1 shows a body panel having a coarse texture on the interior surface and a fine texture on the exterior surface. This texture was generated by the corresponding coarse and fine textures of the mold surface.

Figure 2 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 rolloing over releasing entrapped air ana carrying bubbles to the backside of the part which has a coarse texture.

Figure 3 shows the bottom side of a mold which forms the inside of a Corvette (Registered Trade Mark) 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 4 shows the uniformly textured top (visible) side of a Corvette (Registered Trade Mark) roof panel mold.

Details of the materials or process used in making sheet molding compounds form no part of the present invention and are disclosed in SPI Handbook of Technical and Engineering of Reinforced Plastics Composites by J. Gilbert Mohr, et al, Second Edition, Copyright 1973 Van Nostrandt. In particular, see pages 175 to 242.

Generally speaking, SMC contains a mix ture of an unsaturated polyester dissolved in styrene, peroxide, lubricant, fiberglass low profile additive, curing agent and filler. The fiberglass constitutes 15 to 60% of the mix by weight and preferably 22 to 35%. The fiberglass is 12 to 50 millimeters long and has a very small diameter. The polyester plus styrene usually constitutes about 15% by weight of the mix. The lubricant, e.g., zinc stearate and the peroxide are present in small amounts. A small amount of a thickener such as magnesium oxide may also be present. An impact modifier such as polymethyl methacrylate copolymer with 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.

Treatment of the mold surface includes any process which will roughen to mold surface by removing portions of the surface to form an irregular, rough surface. Such treatment includes the mechanical abrasion treatments discussed above, chemical and electrochemical etching as is known in the art, and electrical treatment such as electric arc or plasma treatment whereby material is removed from portions of the mold surface to form pits as is also known in the art. The desired texture can also be imparted to the mold surface by the initial casting of the mold, but this procedure is not preferred.

The process of the present invention is applicable to the molding of articles from viscous plastics. Such molding techniques include SMC techniques, bulk molding compound techniques and injection molding techniques including reaction injection molding. It is particularly suitable for the use of SMC in the manufacture of automotive body panels.

The viscous plastics can have a viscosity of from 1000 to 10x107 centipoises. The preferred SMC has a viscosity of from 15x 106 to SOx 106 centipoises at room temperature, the viscosity being measured, for example using a Brookfield Viscometer.

A part having a convex curved visible surface may have a coarse texture on the concave side and a fine texture on the convex side (see Fig. 1). In effect, the texture is used to reverse, through resistance on one side of the part, the flow pattern of the material so that whatever air is enclosed in the part will be on the backside (se Fig. 2).

If 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 immediately between the boss OI rib and the material supply (charge), to accelerate flow into this thicker area so that the entire front of the material flow reaches the shear edge simultaneously to avoid enclosing air (see Fig. 3).

The mold that can be used in the practice of the present invention includes any of the well-known molds of the prior aiL. These are usually two-piece molds having a bottom section and a top mating section. The space between the two faces ot the mold usually ranges from 3 millimetres to 12 millimetres disregarding reinforcing members. The area of mold surface (in square centimetres) ranges from 5000 to I x 10'2cm2.

The following procedure has been used in manufacturing the removal roof section of the Corvette automobile. The bottom half of the mold used is shown in Figure 3 and the top half of the mold used is shown in Figure 4. A quantity of sheet molding compounds sufficient completely to fill the mold is placed in charge placement area 1 of Figure 3. Areas 3, 5, 7, 9, 11 and 13 are relatively smooth areas which allow the sheet molding compound to flow freely to the mold recesses which form bosses.

Bosses are shown at 15, 17, 19, 21, 23, 25, 27, 29, 31 and 33. The dotted areas 35 represent areas of the mold which have been textured by impingement, of glass beads against the mold. After the charge is placed, the mold is closed.

The area behind the bosses indicated by 37 are textured by 60 to 90 grit aluminium oxide to produce even a rougher surface in the area than 35 so as to slow down the flow of the sheet molding compound after it has entered the bosses. This slowing down of the travel of the SMC at the rougher surface allows the sheet molding compound at the upper mold surface to increase its speed relative to the lower surface, roll over and carry the bubbles which causes porosity to the lower surface.

Figure 4 shows the upper mold surface which has a smoother texture 39 compared with the texture of the lower mold surface.

The upper surface is textured by impingement with glass beads. Both the upper mold surface and the lower mold surface are heated in operation so as to cure the sheet molding compound. The surface of the mould shown in Figure 4 has a uniform texture 39 which is less rough than the average texture of the mold surface of Figure 3. Both the upper and lower mold surfaces are heated so as to cure the sheet molding compound to form the cured roof panel.

In the operation of the mold the upper mold half is brought down by hydraulic pressure against the sheet molding charge in the lower mold half. The pressure causes the charge to roll in the textured areas trapping porosity in the lower surface of the panel. The charge flows relatively free across the areas of the lower mold surface which is not textured to fill the cavity which form the bosses. After the filling of the bosses the remaining sheet molding material flows to the corner areas behind the bosses where, again, it is subjected 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 conditions utilized in the molding techniques are known in the art and also probably available from the Dow Chemical Company and the Rohm & Haas Company, two of the manufacturers of the resins used to make sheet molding compounds.

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

WHAT WE CLAIM IS:- 1. A method of molding a thermosetting liquid having an initial viscosity between 1000 and 7 x 10t centipoises at room temperature and containing chopped glass fiber comprising a first step of placing the thermosetting liquid in a mold having two heated dies, one of the dies having at least one region with a rougher surface than the opposed region of the other die, and a second step of bringing the two dies together thereby to form a cavity, so that the thermosetting liquid flows to fill the cavity prior to setting, the flow of the thermosetting liquid being retarded over the die with the rougher surface as compared with the other die, such that a rolling motion of the glass fibers is produced, thereby to form a sheet with a rough surface and a smoother surface, the smoother surface being intended to be the viewed surface of the sheet 2. A method according to claim 1 wherein said rougher surface of one die has been formed by the initial casting of the die.

3. A method according to claim 1 wherein said rougher surface of one die has been formed by removal of material from the surface by abrasion, etching, electric arc or plasma treatment.

4. A method according to any one of the preceding claims, wherein the initial viscosity of the thermosetting liquid is between 15x 106 and 50x 106 centipoises.

5. A method substantially as any one herein described with reference to the accompanying drawings.

6. A sheet produced by a method according to any one of the preceding

Claims (1)

1. claims.