The cycle is as follows: the piston is removed from the cylinder and the cylinder filled with plastic and tamped down inside with an odd bit of metal, as if filling a pipe. The unit is switched on and, after a while, the unmistakable but not unpleasant smell of hot plastic will be noticed; a little longer, and molten plastic will start to exude from the nozzle. The nozzle is then wiped clear of plastic and placed in the in- gate of the mould and, immediately it is in place, the piston is pressed home.

The unit is removed from the mould, the piston removed and the cylinder refilled with plastic. The mould is opened and the component removed whilst the fresh plastic is brought up to moulding temperature in the cylinder. The mould is reassembled and the cycle repeated.

Having said all that I must admit that I do not do things in an upturned vice myself, since I have a small arbour press which is rather more rapid in action. The principle is the same however in both cases and the vice is perfectly adequate.

Temperature

One other requirement is for some means of regulating the temperature.
The simplest solution that I know to this problem is a Simmerstat from an old electric cooker. This device is in effect a means of switching an element, or anything else for that matter, on and off at short intervals. As the knob is turned toward the hotter end of the scale the element, or what have you, is switched on for a longer period and off for a shorter period. In the with it jargon of today, the mark-space ratio is varied.

This is taking us rather a long way away from our injection moulding, so suffice it to say that such a control fitted to the bench so as to regulate the output of the mains socket is very useful for other things besides controlling moulding units; soldering irons for instance.

However, back to the problems of moulding. I have indicated the need to control the moulding temperature without really stating why, so I must repair the omission. When a plastic is heated, it melts, and the more it is heated the more runny it becomes until a point is reached at which it starts to decompose and gives off a most unpleasant smell.

When a plastic has been heated to this degree it is quite useless to try to mould it, as the properties for which it has been selected just do not materialise. If the plastic is not heated enough, it will not run into the mould properly and will fail to fill it. Failure to fill the mould could be the fault of the mould design as well, but I will deal with that later.

What should happen is this: when the piston descends, it should do so easily and smoothly and come to rest with a small jerk when the mould is full. If the temperature setting and the rhythm of the worker are well matched, then this will happen time. after time, and you will wonder why I made it sound so difficult. Sometimes, however, the piston comes to a soggy halt, and the ‘mould will fail to fill; assuming all is well with the mould the heat should; be turned up and a slower and more deliberate rhythm`. adopted.

Of course, the piston sometimes fails to stop at all and molten plastic squirts, from every joint of the mould. This is fortunately rare and the cure simple: turn down the heat and,or adopt a faster rhythm. This same cure works wonders with components exhibiting heavy flash. The object of the exercise is to get the plastic just hot enough to fill the mould completely, and no more.

I have said nothing yet about the plastics them selves, but I will give some notes on them and their characteristics after I have dealt with the moulds.

The sides of the mould must taper toward the parting line as shown in the diagrams, and this feature, termed draft is the most awkward design limitation on moulded components, though a little forethought in choosing the parting line can minimise the effect.

The other feature common to all is an ingateorhole-for-squirting- plastic-in-by.

In Fig. 4(a) the ingate is fairly large and injects directly into the component through the top plate. This means that there is a large lump of unwanted plastic sticking out of the side of the finished component. It will be appreciated that the component must also be more difficult to extract from the mould, though a small punch placed in the ingate and tapped gently will do wonders.
In Fig. 4(b) the ingate is on the parting line and may be reduced to a small diameter for a small distance at the lower end to reduce the size of the moulding marl: on the component. It is important that the length of this reduced bore is kept small since injection is usually into a cold mould, and considerable cooling of the plastic may take place before the material ever reaches the cavity. Opening the mould results in immediate release of the component, but the force required to keep the mould closed whilst moulding is higher than in 4(a).
Fig. 4(c) is an indirectly injected version of Fig. 4(a) which also incorporates the small final diameter of 4(b). It does have, however, one other merit in that when the first wave of incoming plastic is cooled by the walls of the ingate, the resultant slug of cooler plastic is injected into the cold Well and the mould filled from the hotter plastic in its wake.

There are many other types of mould but all are derivatives or cross breeds of these three, so I will waste no further time on generalities but get down to the description of an actual mould.

All the above operations can be done without removing the job from the chuck, so ensuring concentricity even with the sloppiest lathe. With the work still mounted in the chuck, a tool shaped as shown in Fig. 6 is mounted on its side in the toolpost, so that when the cross-slide is operated it cuts a groove across the face of the stationary workpiece. If the tool is fed in to cut only 0,005 in. or so at a time, no difficulty will be experienced in cutting the cavities for the spokes. After cutting the cavity for one spoke the work is indexed round to the next position, and the next spoke cavity cut.

It will be appreciated that the cavities for the spokes must be made radial and that, to ensure this, the tool must be accurately set to centre height.

This is best done by unscrewing the chuck with the job still in the jaws and putting a lathe centre in the headstock; the tool may then be set at centre height with the aid of a magnifying glass and sheet of white paper held behind the lathe, to provide an evenly illuminated uncluttered background.

If the lathe used has a suitable indexing mechanism, then all is plain sailing, but should one not be fitted it is usually a fairly simple job to arrange a mounting for a change wheel or other suitable gear at the other end of the headstock spindle. (See Fig. 7,)

A piece of strip metal with the end bent at right angles can then be used as a `pecker` in the teeth of the gear wheel, and will be found quite rigid enough for the modest loading experienced in this type of work.

Some experiment is necessary to determine the actual clearance here but is not difficult nor protracted. In use, the mould is merely assembled and stood on a flat steel plate under the moulding rig, After moulding is complete the mould is pulled apart and the wheel knocked out.

Now, if the area of the wheel as seen from the back is less than the end area of the piston, then the pressure of the end of the moulding rig upon it will keep the mould closed without the need for clamp screws. In practice, spoked wheels inSgauge and below can be made in this Way, but forO gauge or larger gauges and for disc wheels in smaller gauges it is necessary to screw the mould together A simple way is to pass two screws down through the top cap and outer ring and into the underlying metal plate.

It will be appreciated that by replacing the appropriate section other related types of wheel may be produced. Outer rings producing fine or coarse scale flanges have been mentioned, but the spoke section may be readily replaced to produce disc, three-hole or split-spoke wheels, and the core insert may be changed to vary the axle hole diameter. This should suffice for this month; next month We shall discuss driving wheels and plate moulds.

Fig 11 shows the mould, which is laminated from flat plates, hence the term `plate mould. When a mould of this type is made, the first thing to do is to drill the dowel holes and fit the roughly shaped plates together. The outer faces of the mould can then be finished. The cavity is marked out on the centre portion of the mould, and the axle centres or other reference points

So far, we have the moulding rig itself and the production of some simple moulds. All of these have been in metal and fabricated in one way or another. It is possible, however, to cast moulds in either metal or epoxy resin.

The bare bones of the process. are as follows: an original pattern is made and mounted on a back plate, if flat, or embedded in modelling clay up to the line, if not. The whole thing is then coated with a parting agent (a quick squirt with the wife`s, aerosol polish) and a casting taken from it. This is trimmed up and fitted with a metal top plate in the case of the flat pattern and the necessary in-gate, etc., drilled. In the case of the awkwardly shaped pattern a second casting of the other side must be taken to form a complete mould.

I have been deliberately sketchy in the details of mould production in this manner since I have failed to achieve the same quality and precision in moulding from the cast mould as from the metal mould. Whilst in other hands the process is demonstrably feasible there is a lack of crispness about the mouldings which I have made so far. Hence, since this is supposed to be an explanation of the things I can do, and not, as with some writers, an admirable and lucid explanation of something that I can’t, I will abandon this facet of plastic moulding to an abler pen.

Back on firmer ground again, I think the only further information required before every modeller becomes his own plastic moulder, is some information on plastic materials.

The centre plate into which is cut the common flange of the frame is -018 in. thick and the outer two are -O64 in., i.e., the width of the flange less the thickness of the centre plate. Into these are cut the projecting flanges of the frame. Fig. 3 should make this clear.

Examining the plates in order we have: Plate 1 which is the cover plate with the in-gate or ‘hole for putting in the plastic‘ in the top. Plate 2 is split into two and has filed into the joint edge the cavity for the flange of the crossbar with the ‘chairs’ for the rail at the ends. Two pieces of rail are secured half in—half out of the plate to provide the slot for the rail in the ‘chairs’.

There is also a long slot which provides distribution between the in-gate in plate 1 and the four inlets in plate 3. Plate 3 provides the main flange of the frame and has a loose centre section (plate 3 1/2). Plate 4 provides the other flange of the side frames and has both a loose section (plate 4 1/2) and an inset piece of rail for the top chair.

Plate 5 is the other cover plate. No screws or dowels have been shown in the drawing but in fact there are four 1/4 inch diameter pins through the corners of the plates and two 3/32 inch diameter pins passing through plates 1, 2, 3 1/2, 4 1/2, and 5. All these loose parts, pins, etc. tend to make moulding and extraction slow but Don, using my press and equipment made about four hundred frames in a week. The mould itself accounted for about nine hours work and Don put in about forty hours! So, all told, we managed to produce ‘A’ frames for less than one minutes work apiece.