This article was originally published in Practical Engineering 1940 Vol No1. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the subject and its place in industry.
The Production of Synthetic Resin Moldings Explained
There are several forms of plastic moulding material manufactured at present, the most familiar being named "bakelite" It is a synthetic resin produced by a chemical reaction in which carbolic acid and formaldehyde (distilled from wood) play the most important part, but the exact details of the process are omitted as not being considered essential to the present article.
For the manufacture of utility articles obtainable in vast quantities, this resin is mixed with a base termed in the trade a "filler," and this filler varies according to the class of article required to be moulded. As for instance, some articles have to be hard and necessarily fragile, others perhaps have to be so light and weak that the filler used is to gain strength, whilst in other cases the filler decided upon is one which will stand the constant contact of boiling liquids. In some plastic moulding powders very finely ground wood is used as the base; other materials include French chalk, china clay, asbestos and wool fibre, also in the majority of moulding shops all the scrap and waste from previous mouldings are reground and used as filler mixed with a further proportion of resin.
Bakelite moulding compound is used for fabricating a great variety of articles purchasable at very competitive prices. It is very durable, not greatly affected by boiling liquids, and is a non-conductor of electrical current. The natural colour of bakelite resin is amber, but any other colour is produced by the admixture in the filler of a suitable dye.
This form of moulding material differs considerably from bakelite in structure, also in manufacture, although the modern method of producing small bakelite articles is by injection (explained later), acetate is always injected into a closed mould, heated separately and brought to a fluid condition in a container fitted to the moulding press for that special purpose.
In Fig. 1 will be seen a small cup and an ashtray. Both of these have been .produced in plunger-type moulds from bakelite powder loaded directly into the mould.
In Fig. 2 are shown three coil formers, the largest of -which is moulded in bakelite by the injection method, the remainder being fabricated from cellulose acetate.
The Process Employed
The process used in the manufacture of plastic mouldings is fairly simple, and does not call for a great deal of skill on the part of the operator. Quickness in ejecting the moulded article, combined with close application to the time allowed for “cooking" (which is the period estimated by the controller as sufficient to produce in the compound a state as near to perfection as possible) are the only points the person operating a moulding press has to consider.
Bakelite moulding work cannot in any circumstances be termed a "pleasant occupation," owing to the heat which is necessary and the annoying smell and rapidity under which the operators have to perform their duties.
With all forms of pressure moulding there are many snags and drawbacks. The mouldings shrink upon cooling — which has to be guarded against and allowed for; also, in time they "grow" by the normal action of the pressure confined in the moulded object continually struggling for release. Moulded metal articles are affected by this expansion more than synthetic resin, although in all mouldings it is a serious drawback and considerably restricts the fabrication of moving parts.
Experiment in this direction is continuous, and one enterprising firm is now moulding shaft bearings from bakelite with the admixture of graphite to the resin compound. The advantage of this will at once be understood when it is explained that the shafting is run dry, without oil or any form o£ lubrication, owing to the graphite reducing friction to a minimum.
The shrinkage in plastic moulding is to a great extent overcome by allowances in mould sizes when these are being made, of a percentage averaging 0.0006 per inch. The figures representing allowances are usually entered on the mould drawing by the designer, as much experience is necessary in making calculations, also it will be realised that bulk and the filler being used in the moulding compound will considerably vary the amount of contraction. In some instances, the bakelite powder is made into lozenges by the use of special machinery before being loaded into the moulds, which facilitates the accurate measuring of the material being used, but this method has its limitations as where moulds have fragile or structurally weak internal parts, the danger reduces the practicability.
There are several practical methods of obtaining and applying these two very essential elements. In the case of heat, steam, gas and electricity can be used efficiently, whilst in the case AI pressure, hydraulic and electrical AR-universal, also hand pressure obtained with a fly press is often used with success where the more elaborate mechanical processes are not available.