How to avoid contamination and waste in injection-molding shops?
Keeping material clean
The importance of cleanliness in the plastic mould injection shop is nowhere more apparent than making mouldings free from contamination. Safeguarding the material in the sack is an obvious matter， but is often ignored.
Partially used sacks or containers should be re-sealed or covered and raised off the floor，in case of water or oil leaks. Material stored by the machine for use during the following shift should be raised on a pallet and covered，in case dust or dirt collects on the outside of the sacks.
It is very easy to tip dirt or foreign granules into the feed hopper from a dirty sack. The lid should also be kept on the feed hopper. It often is not!
It is not always realized how difficult it is to eliminate black specks in white mouldings (or white specks in black ones). One black granule in a hopper full of white material can contaminate as many as half the mouldings produced from the full hopper. With strict inspection，, this can mean a great loss of production time and material.
Protecting machines and granulators
For critical jobs, almost clinical cleanliness is desirable, and it is absolutely impossible to make good mouldings in a light color if a granulator working on black material is operating in the vicinity- The high static charge which all dry plastics carry attracts dust，and even the mould if warm and dry， attracts statically charged dust.
When working to critical limits, the only safe course is to isolate the machine through plastic film screens on a light framework and blow in clean air to maintain a slight positive pressure inside the screen. The alternative is to do away with all granulators and other dust sources within the moulding area， or to enclose the granulators in dust-proof (and noise-proof) boxes.
Care of re-work
Scrap mouldings, sprues, and runners to be re-ground and re-worked must be kept free from contamination. They, too, take up a static charge and attract dust and dirt.
Some plastics also attract moisture and, in a few days, even with relative humidity as low as 50 percent, will become too damp to mould. The same care should be exercised with re-work material and virgin plastic in keeping it free from contamination.
Airlines as a hazard
A very common cause of contamination in the moulding shop is the compressed air system. It may，of course, contain moisture which is itself a hazard. However, it is more than likely that the main use of the compressed air line will be for cleaning out feed hoppers and the feed throat of the machine.
Of course, the plastic or dust and dirt must go somewhere, and once airborne, it may land anywhere. The most damaging place is in the feed hopper of an adjacent machine. This use of compressed air is not to be recommended.
It is much better to use a vacuum cleaner，either one of the powerful industrial models or even the small hand machines used in the home and produced by several domestic appliance manufacturers.
Moisture in compressed air can also contaminate mouldings if used for air ejection- The moisture can settle in the mould to produce splash marking, but there are often traces of oil in the air that can make mouldings quite dirty. It is wise to make sure that the compressed air is clean and dry when used in this way by using a sump and filter.
Getting rid of contamination
A great deal of needless waste often occurs in getting rid of contamination. It is pointless to continue moulding if the components produced will not pass inspection. The contamination may be in the feed hopper or in the bag of material used to charge it，and the first action is to discard this material.
The 9 procedures are similar to that recommended for color changing:
- first, empty the feed hopper (retaining the material for later examination);
- continue to mould until short mouldings result;
- screw back as far as the screw will go (this depends on the amount of material left in the screw flights);
- purge the cylinder to air;
- thoroughly clean the feed hopper and the feed throat;
- put a small quantity (a few handfuls) of material in the feed throat;
- screw back and purge;
- repeat once or twice or until satisfied that no more dirt is coming from the cylinder;
- re-fit the feed hopper, put in a small quantity of material and start moulding again. If the mouldings are satisfactory, the feed hopper can be filled and the lid put in place; if not, the process is repeated.
This method of getting rid of contamination from the cylinder uses the minimum of material and avoids waste.
Failure to clean the cylinder in the manner described, for example, by just emptying and re-filling the hopper with clean material and then continuing to mould, can result in the making of dozens or even scores of contaminated mouldings.
Optimizing production efficiency
Dirt and contamination of material is not the only cause of waste in the moulding shop. It can also result because not enough thought has been put into the best method of doing a job. It may be argued that this has nothing to do with housekeeping, but if that concept is considered in its broadest sense as eliminating any factor in operation that is detrimental to profitability.
The kind of project concerned involves moulding very small articles, particularly in heat-sensitive materials, such as acetal and nylon. By normal methods, using a sprue and runner system, the situation often occurs where for every 20 g of mouldings, there may be 40 g or more of sprue and runner.
This is not wasted, but the cost of re-granulating and mixing with virgin material must be taken into account. If the material has become at all oxidized during its passage through the machine, it may not be possible to use it for the job in hand, and it then has to be sold as scrap for between 10 and 30 percent of its original price.
Very often, the size of the machine available determines how the job is to be done. They were required at the rate of 30 million per annum. A 40-cavities mould was made in which the runners weighed 40 g and the mouldings 20 g. The cycle time was 30 s (determined by the need for the runners to be hard enough to eject), but if the caps could have been moulded without runners, the cycle time would have been between 1 and 2 s. Two single¬cavities molds could easily have produced the annual requirement, but 45 tonnes of material were moulded, and 30 tonnes had to be re-ground using the 40-cavities mould.
If they had been moulded without re-work, only 15 tonnes of material would have been processed. In deciding how best to do a job，the side issues involved may be more decisive than the actual job itself. In the above case，the cost of the material used was not just the buying price per tonne, but that cost plus the cost of re-grinding 30 tons of material at，say，£50/tonne.