Plastic Mouldings

Kitmondo 06 May 2014

This article first appeared in Practical Engineering 1940 Vol1 No2. Therefore, the information contained within pertains to 1940. The article discusses new types of equipment launched into the market at the time.

Some few years ago, the only method of charging a mould was by taking the upper and lower forces apart and inserting the material, but this has been superseded by the modern method of injection. For this process, special presses or moulding machines are needed, and in the former they are of the double action variety, viz., the lower platen rises and closes the mould, then a plunger descends entering a small cylinder, which is screwed into the top of the mould, into which the bakelite powder or lozenge has been previously placed.

Undoubtedly, this is a great improvement on charging the mould direct, as it reduces the "flash," which is a certain amount of resin spewed out between the two mould forces by the internal pressure, and is a great trouble should it become thick, having to be trimmed off after moulding as it is very unsightly.

Cellulose Acetate Moulds

Moulds for this material are always charged by the injection method. It differs only from the bakelite injection mould by having a cylinder attached to the press containing a fairly large quantity of the material in a hot fluid state. A great many large and elaborate moulds are used in the process of forming and casting this class of resin; the mouldings, such as cheap fountain pens, pencils, etc., have to be produced in very large quantities, to support a competitive market, and maintain a profit.

The rapid and effective stripping of these small articles after moulding has produced many wonderful and elaborate mechanically-operated moulds, especially for the production of such articles already mentioned, and having internal threads. Rapidity of operation is the most essential point, and the design is always directed with this end in view.

One of the most effective mechanically-operated moulds at present is made by having a complete circle of stems made of steel which forms the internal bore of a pen or pencil barrel. These stems pass through a plate and on the back end of each is fitted a small gear wheel. In the centre of this circle of gear wheels, and in mesh with them, is a larger wheel to which a handle is attached. When the moulding is completed this handle is given a sharp turn, which causes all the stems to revolve together, unscrewing them from the moulded thread inside the barrels. An even quicker method than this has been recently invented, and has proved quite practical against considerable adverse criticism, i.e., after moulding, and when the material is still in a hot plastic state, the steel stems with the moulding thread on, descend through the plate, stripping off the barrels. It had been found after considerable experimenting that the moulded thread was not unduly injured and regains almost its normal shape and size upon cooling.

Location Pins

A cellulose acetate mould produces the small coil formers with the top force made from good quality mild steel and left soft; the bottom force is made from the same class of material, and into this, the core pins made from cast steel, hardened and tempered, are a press fit just burred over at the bottom end to retain them. A slight taper will be noted on the core pins, which is very essential to facilitate stripping.

The location pins made from cast steel hardened and tempered, are of different sizes to locate assembly the same way round each time. In the making of this mould, very little bench work is necessary, and the only real fitting is the three rectangular core pin holes, which have to be carefully located. The splits, which are made from cast steel, are probably the most tricky job; there are really two pairs of each, which must be exactly the same to centralise. They are shaped up to a predetermined size, the core pin slots run through on the horizontal milling machine, then the shallow recess is end milled in the top and bottom sides of each to form the flange on the coil former. The outside taper is then machined on the horizontal milling machine with the aid of the swivel vice to set up for the angle.

In some tool rooms, about .010in. would be left all round to be ground off after hardening, but in many, just a few thousandths would be left on for the fitter to work off with a file. In any case, all mould surfaces have to be polished before and after hardening. This is very essential with all moulding dies to avoid sticking, and also to produce a finish on the moulded article.


Emery powder mixed with paraffin oil used on sticks of wood or pieces of fibre are very handy tools, and afterwards a final surface highly polished can be obtained with rouge. Polishing is not so essential with cellulose acetate moulds as with bakelite, the moulding pressure being considerably less. In quite a lot of modern tool rooms good quality mild steel would be used for the splits of this mould, and afterwards skin hardened in the cyanide bath, but it must be distinctly noted that it would have to be "black" mild steel which is hot rolled, as "bright mild steel" or "cold rolled steel" would distort in the hardening process, and render the part useless.

Bakelite Mould Making

A small cup is made in a very simple three-impression mould. The bottom bolster is made from mild steel with cast steel moulding forces inserted into it, after being hardened and ground. The top force is a mild steel plate with plungers to form the inside wall of the cup, positioned by a spigot and held on by screws. This type of mould is fairly simple to make. A small protrusion will be noted on the top of the plungers entering the top plate, without this the mould would take twice as long to make, and would mean a great deal more trouble.

Best quality mild steel is turned to size in the lathe representing the bottom bolster and the top plate. The “top" plate overlaps the body, and a step is turned on this also. The two flanges projecting from the mould body are used for opening the mould. The top plate is now marked out in pitch diameter circles (PDC) to the required sizes. These sizes are not important with the exception of noting that the thin part of the wall is sufficient to withstand the great internal pressure which is necessary for moulding. No set rule is laid down, but it is generally safe when this wall is of a thickness which is not less than one half the outer diameter of the moulded object. Each of the circles is  now  divided up into three equal spaces at staggered intervals of 60°, the outer circle defines the position for the locating pegs, and the inner circles the centres for the moulding inserts.

The plate and bolster are now clamped together, and the three holes drilled to accept the location pegs, a tight fit in  the plate and a nice slide fit in the bottom bolster. These pegs are usually lathe turned from cast steel, and approximately .015in. is left on the diameter which is ground off, after hardening. One peg is made larger than the other  two to ensure  that the mould is always assembled in the same position. When the pegs have been fitted satisfactorily, so that the plate can be put on and taken off without undue effort, it is clamped together again, and three holes, which in this case would be -1/2in. diameter, are drilled through both top and bottom members, this then represents a datum line for each mould bore. The bottom bolster is now mounted on the lathe face plate, and each hole opened out to a size which will accept the cast steel insert, and this will be ground to suit.

Two fairly small holes will be noticed in the bolster—these are to knock the cast steel insert out if need be, and should on no account be omitted.

Polishing the Faces

The inside moulding surfaces of the inserts are then highly polished by rotating at high speed, and a surface produced first with emery cloth dipped in paraffin, and finally rouge powder on a stick. The plungers are turned with a spigot on the top end, which will be 1/2in. diameter, and a sliding fit in the hole already in the top plate. This centralises them, and also maintains direct relationship with the bottom force on assembly. Before being hardened, holes are drilled and tapped to accept three cheese-headed screws, which are also counter-bored in the top plate, so that the heads are just below the surface. The ejector, which is shown in the sketch, forms the bottom of the cup, and must be a good fit in the cast steel insert, otherwise the moulding material will be forced down between. To minimise the possibility of this occurring a taper of 1 is made on the head part, the moulding pressure then having the effect of closing the surface against leakage.

An Ashtray Mould

A cigarette ashtray mould is a single impression mould of fairly simple design, which would be loaded with bakelite power when in operation equal spaces at staggered intervals to ensure an easy and uninterrupted 60°, the outer circle defines the position flow immediately fluxing occurs. Although it appears simpler than the cup mould, the cost would be greater, as it is mostly milling machine work. The procedure would follow very much the same lines as the cup mould.

The ejector studs form the three half-round sunken slots as well as pushing the job out, and to prevent them twisting, it will be noted on the sketch that the shanks are rectangular. Both forces are fairly difficult to produce, and require a lot of highly skilled milling to reproduce the equally spaced flats which will be noticed on the external circumference.

The polishing will be a tedious process, having to be mostly accomplished by hand. A very simple handy tool for this work can be made from a piece of mild steel with a saw cut in the end, into which a strip of good quality emery cloth can be inserted, and then drawn back along the remainder. This instrument will be found extremely handy in getting into awkward corners.

Impression Knob Mould

This is a simple form of plunger type mould for producing wireless cabinet control knobs, and although there are many designers averse to this method, the lower force or cast steel inserts were made up of two parts to facilitate machining. The flutes were produced on a slotting machine, although failing this, they could have been filed by an experienced toolmaker. The mould did not give as much trouble as was anticipated, except for occasionally having to be “stripped down" and the flash cleaned away from the butt faces of the inserts.

The top insert acted as the plunger, and protruded above the mould when loaded and before being placed between the press platens. To strip after moulding, the tool was placed on metal parallels under a fly press, and the complete top and bottom inserts were pushed right through and then pulled apart by hand to withdraw the component. Pins were fitted by drilling holes slightly over the centre line to facilitate assembly.