Die-Casting and Plastic Moulding in Motor Car Manufacturing

This article first appeared in Practical Engineering 1940 Vol1 No4. The information within the article is accurate as of 1940. The article informs the reader of developments in Manufacturing at the time.

Sand-Casting

Sand-casting can be applied to the majority of metals, such as cast-iron, special alloy steels, aluminium and other light alloys, and zinc-base alloys. In the case of aluminium, elektron and zinc-base alloys, however, it is now more usual to make the casting in metal moulds, using metal cores, the process being termed die-casting. Among the parts which can be produced by this method are pistons, universal joint, housings, shock absorber bodies, fan belt pulleys, fans and even dip-sticks; door handles, hinges and window-winding mechanism lend themselves to die-casting, as do carburettor parts, radiator grilles, and a number of smaller components and body fittings.

Two Types of Die-Casting

There are two types of die-casting— those produced by allowing the metal to flow into the mould under gravity, and castings obtained by forcing in the molten metal under pressure. In the first instance the casting has a satin-smooth finish resembling dull silver or pewter; a pressure die-casting, however, such as a door handle or filler cap, has an even more perfect finish. In either case the arrangement of the mould, and the core, if a hollow object is to be cast, is very much the same.

Casting Aluminium Pistons

For the sake of simplicity let us take an aluminium piston produced by gravity die-casting. In the first place, the external shape of the piston will be accurately cut in the metal mould or die, making the necessary allowance for the machining required in this instance.

The core of a piston is a fairly elaborate item; allowance must be made for the gudgeon-pin bosses, probably a lip around the lower inner edge of the skirt, and possibly stiffening ribs on the underside of the crown. When a baked sand core is used, this can be broken up in order to remove it after the casting has cooled. With a metal core, of course, this is impossible, as it will be obvious that a solid core would be trapped inside the piston.

As a result, the core is made up in a number of sections. There is a square or possibly circular central piece. Around this are arranged four or more sections, as shown in an accompanying illustration; when the sections are fitted accurately together, they produce a smooth and practically unbroken contour of the interior surface of the piston, although in practice it is usually possible to detect the lines caused by the junction of the core pieces on the inside surface of a die-cast piston. Finally, a round bar is passed through the mould and the core in order to provide the necessary gudgeon-pin holes.

With the mould and core sections assembled together, all is ready for pouring. The molten metal is poured in through a passage which takes the form of an "S," the object of the “hump" being to eliminate gas bubbles and porosity. Air escapes from the mould through a similar passage on the opposite side.

Removing the Core

When the metal has hardened, the core is extracted by removing the sections one by one. First, the gudgeon-pin bar is withdrawn. Then the centre section is removed. This enables the two smaller pieces, at right angles to the gudgeon-pin bosses, to be moved inward sufficiently to clear the lip on the skirt and to be lifted out. The larger sections surrounding the gudgeon-pin bosses can then be moved in toward the centre of the piston and withdrawn.

This procedure in the die-casting of an alloy piston—which is, incidentally, used by the makers of the Lo-Ex pistons on Austin, Morris, Standard and a number of other well-known cars—has been explained in some detail, as the principle adopted is very much the same as in the case of other die-cast articles.

Pressure Die-Casting

For the more elaborate castings, or small parts on which a high finish is desired with the minimum of machining and polishing, pressure die-casting is widely used. One advantage is that the walls of the casting can be of thinner section, while a dense casting is obtained. In America, pressure die-castings are produced under pressures of from 1,000-2,0001bs. per sq. in., which enables sections as thin as .030in. or less to be used in very small castings.

For general die-casting, zinc-base alloys are widely used, since they offer a fairly high tensile strength in the neighbourhood of 35,000-40,0001bs. per sq. in., and are economical in use, although where weight is a deciding factor an aluminium-copper-nickel alloy combines lightness with a tensile strength of 21,000-25,0001bs. per sq. in. Elektron provides greater lightness and strength, but requires specialised handling.

Carburettor parts, which have to withstand considerable vibration, and which must remain proof against petrol leakage, are frequently cast in zinc alloys. An additional advantage of pressure die-casting in this case is that the intricate and deep coring necessary to produce a modern float-chamber bowl, complete with petrol wells and jet seatings, can be obtained with the minimum of trouble, while subsequent machining is usually eliminated.

Similarly, small and intricate parts, such as the brackets carrying speedometer mechanism and similar items in which a high degree of accuracy combined with rapid repetition is required, can be produced very economically. It is even possible to incorporate accurate screw threads in the original casting, although this is not usually done in the case of aluminium alloys. When door handles, window winders and similar parts are produced as pressure die-castings, steel shanks can be incorporated as inserts in the casting.

Radiator Grilles

In America, very extensive use is made of die-casting in the production of radiator grilles. All the General Motors and Chrysler group of cars now make use of die-cast grilles, being influenced by public demand which has resulted from the more substantial and impressive appearance of a die-cast grille, as compared with a sheet-metal pressing. It is worth noting, incidentally, that a die for a one-piece or two-piece die-cast radiator grille for a medium-price American car can be produced for between £2,000 to £3,000, whereas equivalent dies to produce a grille from stampings would probably cost £15,000 to £20,000.

Use of Plastics

A feature of modern production which is similar in many respects to die-casting is the production of fittings and components from plastic materials. The best known of these is the phenolic resin, of which bakelite is a well-known example. The process begins with the manufacture of the moulding powder. It is possible to trace the origin of a moulded part on the car back to three primary substances, coal, lime and water, since the raw materials for the manufacture of phenolic resins are phenol and formaldehyde. Phenol is produced from coal tar; formaldehyde, which is produced by the oxidation of methyl alcohol, is now a synthetic product obtained from carbon dioxide and ammonia. The resin is produced from the phenol and formaldehyde in a steam-heated still with the aid of an acid or an alkaline catalyst, the latter generally being used for the production of moulding powders.

Preparing the Powder

The powder is prepared for moulding by grinding the resin, and mixing it with a filler wood-flour is generally used for the majority of applications. Pigments or dyes are added to give the required colour, or to imitate the surfaces of various types of wood. The raw materials are bulked together and mixed under heat, emerging in sheet form to be cooled, broken up and ground to the desired fineness. The stage to which the materials are taken during the mixing and rolling is important, as different types of mould call for various types of powder.

The powder is then placed between dies, and under the influence of heat and pressure becomes a paste which flows into the shape of the mould, in much the same manner as does the molten metal of a die-casting. Small metal fittings, such , as sockets, threaded shanks, terminals and so forth, can be placed in the mould if necessary to be incorporated in the finished moulding.

Among the many parts of a car which are manufactured in this manner are ignition equipment components, such as distributor cap, rotor, etc., in addition to gear-lever- knobs, ashtrays, switches and similar items. In some cases the complete facia board and instrument panel are produced from one plastic moulding, having a better finish and more attractive appearance than a pressed-steel panel. Minor bearings which do not carry heavy loads, such as those used for carburettor controls, can be moulded from special powders which contain graphite and consequently need no lubrication.

Moulded Timing Gears

Where considerable strength is needed, laminations of paper, cotton or linen are incorporated in the moulding, as in the case of the moulded timing gears which are extensively used on modern cars. The material is manufactured by impregnating rolls of paper or asbestos sheet with a solution of resin in spirit. The sheets are passed through an oven to evaporate the solvent, and to allow the resin to penetrate the fibres. The impregnated material is then cut up into sheets of suitable size, which are stacked together and squeezed in a hydraulic press, the pressure exerted being in the order of 1 to 2 tons per sq.in. From the resulting compressed fabric, timing gears, fibre bearings and other items are cut.

Direction Indicators

From a production engineer's point of view both die-castings and normal plastic mouldings produced in dies have considerable advantages. Often a plastic moulding will supersede a die-casting. In the body-work, for instance, moulded fillets or frames are already being used by several manufacturers around the openings in the doors, rear panels and quarters, which receive the window-glass.

On the other hand, die-castings are frequently used in conjunction with plastics. In the case of a trafficator, for instance, the translucent plastic material forming the indicator arm is moulded around the zinc base die-castings before the arm is assembled into the complete trafficator. A similar process is followed in the case of door handles produced from die-castings, which are subsequently sheathed with plastic material. A number of other examples of this principle can be found on the average car.