SE426562B - SET AND DEVICE FOR REDUCING SURFACES FOR CASTED PLASTIC FORMS - Google Patents

Description

7806259-'5 2. In the case of body parts, it is quite sufficient that one side, hereinafter referred to as the front, has a smooth surface, while surface unevenness on the back can be intolerantly tolerated. According to the invention, it has now been found that if the surface of the mold which corresponds to the back of the workpiece is designed so that the plastic material has more difficulty in flowing along it than along the surface of the mold abutting the front of the workpiece, a rolling effect will occur. within the viscous plastic material. This rolling facilitates the release of trapped air bubbles, so that they can be transferred to the back.

It is assumed that the above-mentioned theory is correct, but it is very possible that the scientific explanation is different. However, it has been found that the invention actually works and constitutes a solution to the problem of avoiding surface porosity. In addition, the wave formation and the fiber pattern in the surface are reduced, so that a better finish is obtained after painting. Surface roughness in the varnish is eliminated by the formation of a rough surface with valleys and ridges, which retain the varnish when the solvent evaporates. The flow of the material at the mold surfaces is counteracted, as is the interlaminar flow, so that the flow lines become less noticeable and the parts stronger at the flow line.

How the surface treatment is to take place depends on the geometry of the mold and on how the charge is placed in the mold as well as on whether the strips and lugs are so located that the movement is more difficult on one side than on the other. The processing consists in grinding the surfaces of the mold with an abrasive, so that the desired pattern is obtained on each half of the mold or on each part thereof. Suitable abrasives are those that contain alumina, glass beads, sand and metal. The particle size of the abrasives can correspond to a mesh size of between 0.40 and 0.06 mm.

Each mold surface is treated so that the material flows along it faster on the front than on the back.

This difference in flow rate causes the glass fibers to roll along the rough surface, so that the flow is slowed down and air bubbles can leave the material during the flow and are trapped at the back. 7806259-3 3 If the surface of the mold is highly polished, the plastic material tends to slide in the form of small balls along the hot mold surface, so that air inclusions occur at the top and flow lines become visible by forming a front of molten plastic. On the other hand, when the surfaces of the mold are patterned according to the invention, the depressions will be filled with plastic mass, which prevents the fibers from appearing on the surface, whereby a homogeneous material mixture is obtained at the flow lines and the molded objects become stronger.

If the front is convex, the pattern should be rough on the concave side and fine on the convex - see Fig. 1.

If the concave side is the front side, both sides should preferably have the same pattern - fig. 2. In general, the pattern is used to, by creating resistance on one side, reverse the flow so that trapped air will always be on the back side. - see Fig. 3.

If a single lug or strip is located on a flat surface with a straight edge, the passage from the ingot to this strip or equivalent should be easier than in the surrounding area, so that the flow of material into this thicker area is accelerated, whereby the entire front of the material flow , so that air inclusions are avoided; see Fig. 4.

Fig. 1 shows a body part with a rough pattern on the inside and a fine one on the outside.

Fig. 2 shows a body part, where the length of the front (area) is greater than the length of the inside (area). Both surfaces are finely patterned.

Fig. 3 shows a cross section through the front of a bonnet to a Chrysler eU: B body. Here, too, the outside has a fine pattern, so that the plastic material will flow faster along the fine-patterned surface and, by rolling action, release trapped air, which is transferred to the back, which is coarse-patterned.

Fig. 4 shows the bottom of a mold for manufacturing roofs for a Corvette.

Fig. 5 shows a ready-made Corvette roof, viewed from above, towards the visible surface and shows an even material distribution.

Neither the press pulps used nor the pressing methods form part of the present invention without any known or suitable technique being possible.

In this context, reference is made to the SPI Handbook of Technical and Engineering of Reinforced Plastics / Composites by J. Gilbert Mohr, et al, Second Edition, Copyright 1973 Van Nostrand, especially p. 175-242.

SMC essentially consists of a styrene-dissolved unsaturated polyester, peroxide, a lubricant, fiberglass, a hardener and a filler. The fiberglass is included with between 15 and 60% by weight and preferably with 22-35% by weight. The fiber length is between 12 and 50 mm and the fiber diameter is very small.

The polyester and styrene materials together usually make up approx. 15% by weight of the mixture. The lubricant, for example zinc stearate, and the peroxide are present in small amounts. Furthermore, there may be a thickener, for example magnesium oxide. Finally, the mixture may contain a modifier, for example a copolymer of polymethyl methacrylate and methacrylic acid or acrylic acid. According to a preferred embodiment of the invention, the mold surface can be treated in any suitable manner, so that it becomes rough. The processing can, according to the above, be mechanical and consist of grinding, but you can also use chemical or electrochemical etching as well as processing with an electric arc or plasma. It is per se possible to pattern the mold surface already when it has been cast, but this approach is not recommended.

The invention can be applied for casting workpieces of viscous plastic by, for example, compression molding or injection molding. The viscosity of the plastic material can be between 103 viscosities between 15 x 10 and 108 centipoise. Preferably, SMC is used with a 6 and 50 x 106 centipoise at room temperature. The mold can be of a known type for its general construction and the same is usually made in two halves, a bottom and an upper part. The distance between the two mold surfaces usually varies between 3 and 12 mm.U 7806259-3 The following procedure has been used for the manufacture of the roof section for a Corvette. Fig. 4 shows the lower half of the mold and Fig. 5 its upper. A sufficient amount of press pulp is introduced into the loading space 1 in Fig. 4. The surface areas 3, 5, 7, 9, 11 and 13 have a relatively smooth structure, so that the plastic material can flow freely up to the recesses 15, 17, 19, 21, 23, 25, 27, 29, 31 and 33, where heels are to formed. The parts of the mold surface that have been patterned by throwing glass beads against the mold wall have been marked with 35.

The area 37 outside the heels is treated with alumina having a grain size of 60 to 90 to develop an even more coarse patterned surface than within the area 35, so that the flow rate is reduced after the heels. This deceleration along the more coarse-patterned surface makes it possible for the press pulp to increase its speed at the upper half of the mold, so that air bubbles, which generate pores, are transferred to the underside.

Fig. 5 shows the upper mold half, which has a finer pattern 39 than the lower one. This too has been achieved with glass beads. To harden the molding compound, both mold halves are heated.

Pressing is done by hydraulically pressing the upper half of the mold down towards the lower one. In this case, the press mass flows relatively unhindered along the unpatterned portions of the lower mold half, so that the cavities for the lugs are filled. When this has happened, the material continues to the corner behind the lugs, where an additional rolling action occurs, so that the pores are closed on the underside of the workpiece.

The pressing times and temperatures and the other parameters to be used are known in the art and corresponding data can be found in the literature resp. can be obtained from, for example, Dow Chemical Company or Röhm & Haas Company, both of which manufacture press pulps.

The invention has also been applied to the manufacture of various other body parts.

Claims (2)

1 7806259-3 Patent claims. l. In the case of compression molding of SMC type composite, containing a polyester resin, a filler, a catalyst, glass fibers, a release agent and a low-profile additive, which expands during curing, to produce parts, in particular car body parts, in the assembled state has a more visible side, front side, and a normally invisible side, back side, characterized by using a molding compound, the viscosity at room temperature is in the range 103-108 centipoise, and a mold, the walls of which are unpolished, the wall abutting the future back of the part having a coarser surface structure than the wall abutting the future front, so that it is imparted to a smooth surface by the mass flowing with less resistance along the latter than along the first - said mold wall. A mold for carrying out the method according to claim 1 in the compression molding of SMC type composite, containing a polyester resin, a filler, a catalyst, glass fibers, a release agent and a low-profile additive, which expands during curing and for the production of parts, especially body parts for cars, which parts in assembled condition have a more visible side, front, and a normally invisible side, rear, the mold being intended to work with a press compound, the viscosity at room temperature is in the range 103-108 centipoise, characterized in that the wall of the mold which is arranged to abut against the future back of the part has a coarser surface structure than the wall which is arranged to abut against the future front of the part. REQUIRED PUBLICATIONS: