Carrying large Plastic injection mould stocks of a large variety of different materials in a wide range of grades and colours can be extremely expensive and is to be avoided if possible. If it is necessary to obtain supplies of a special grade or colour for a particular job, careful estimating of the quantity required will show savings in several ways.First, in having the minimum surplus material to dispose of at the end of the contract.Second, an indirect saving of interest and storage costs, which，on a medium-priced material could be as much as 14 percent each year on the manufacturer’s price of course，in a highly inflationary situation this can show a profit.Third, because stores do not expand and contract as material comes in or goes out，the carrying of unnecessary stocks causes congestion，and it is only wastage of the storekeeper’s time.Any obsolete material should either be used or disposed of as soon as possible. There would seem to be little point in keeping sizeable quantities of say，nylon at per tonne in the hope that one day a use will be found for it.
The disposal price can generally be discounted to allow for the interest and storage costs which would be incurred if it were kept in stock.Similarly with finished components: if a contract ends with unsold components，steps should be taken to dispose of them. Keeping unpaid-for components in the goods outward department of the store awaiting the customer’s call-off is extremely foolish.Not only are there interest charges on the raw material to consider，but there are also charges on the work that has gone into the moulding of the parts. Moulded articles generally take up a vastly greater space than raw materials.And it is all too easy for the moulder who has adequate storage space to allow customers to leave their finished components in the store ‘until they can be collected’，and then to find that all the raw material storage space has been used up as well.The same principle applies to scrap mouldings to be re-granulated. On occasion these have been allowed to accumulate, possibly because of a breakdown of the granulator or a shortage of labour to operate it，to such an extent that it becomes difficult to find space for new material or for good quality finished articles.It must always be remembered that storage space costs money which must be used to the best advantage,and that any material standing in the stores is using up capital resources. It is a very sound principle to work so that all scrap material fit for re-work should be granulated, dried, packaged，and labelled ready for use as soon as possible.
Storage of such scrap，often in inadequate packages，allows it to accumulate dirt and moisture which is then difficult to remove.If a standard material is to be used in a number of colours，the master batch techniques should be employed Many materials are now available from the manufacturers in the form of natural coloured powders，with little except stabilizers by way of additives.They demand a slightly lower price than compounded materials than materials with colour or other additives incorporated by a process involving fusion and granulation. Because they have not undergone the compounding heat treatment, they can be considered as slightly better-quality materials，in that colour especially of acrylic plastics may be slightly better and flow properties，such as melt flow index may be more uniform.These materials are coloured by adding a colour concentrate, known as a masterbatch，which can either be bought from the manufacturers or from one of the many specialist companies now in this sort of business, or they can be made up by the user.
Pigments suitable for making master batches are now readily available from dye and pigment manufacturers.Many material manufacturers publish or supply formulae so that their customers can make up and match a standard range of colours, and some provide a colour matching service. The quantity of colour concentrate added will be between 1 and 5 per cent，depending on the type of colouring matter and the density of the colour required.The coloured master batch is intimately mixed in the correct proportions with the natural powder in a tumble blender, which consists of a drum or container rotated end-over-end- Its volume should be about times that of the material, so that a good free space is left for the tumbling to take place effectively. Baffles to assist in the mixing are attached to the walls of the container. After about 15-20 min mixing, the blended powder is fed to the injection moulding machine，where the rotation of the screw during fusion helps to produce a homogeneously coloured melt.The method adopted when using pigments to match the colour is to make up a quantity of ‘super concentrate’ by mixing the pigments with the powdered plastic. This is adjusted to give the correct colour match on dilution and from the concentrate supplies of masterbatch can be produced It is found easier to weigh out or to measure the exact amount of colour when it is mixed with plastic than when the pigment is in its concentrated form. If a considerable quantity of the super-concentrate is made up, it is possible to make supplies of masterbatch with little batch-to-batch variation.