plastic injection mould maintenance

Every mould user has to undertake some kind of maintenance activity.Since faults and damage to the complex mechanism of these tools inevitably occurs even when the greatest care is taken. However, as already hinted, experience reveals such activities to consist of most spasmodic and irregular inspections of the moulds. Moreover, attention is bestowed upon them only when some specific error, fault or visible damage is encountered of sufficient magnitude as to cause severe interference with normal production,standards of accuracy, quality of finish and so on, in the finished moulded article.

A little consideration, of course, will show this method, or rather lack of method if one regards the matter from the right angle, to be one which necessarily must incur a considerable loss of time and money, and not a little dislocation of the normal routine work of a tool-room.

Nothing is so disruptive of successful production runs than the prevalence of numerous breakdowns involving removal of the mould from the machine while undergoing repairs or adjustments.Also, nothing is more likely to cause irritating interference with production programmes of new mould manufacture passing through the tool-room than a constant stream of repair work coming in, from moulds badly needed to be retained in production service.

Since some kind of maintenance service, therefore, must be afforded any usage of moulds, it follows that such services should be of the most compelling character possible to devise, so that every conceivable aspect of moulding practice likely to involve a mould failure is covered by a pre-determined scheme of activity.

Indeed, it is perfectly true to state that the maintenance facilities available in any moulding shop should form one of the most crucial cornerstones in the organisation, for only too often is valuable time and money lost in this direction.

Disadvantages Arising from Unplanned Maintenance

In an endeavor to spread greater understanding of the need for planning all maintenance activities on an orderly and intelligent basis, some of the chief disadvantages which it has been noted in practice often accrue when this phase of moulding operation is thus neglected, are listed here. Such a list will no doubt prove a very informative and helpful guide to those who hitherto have tended to neglect the subject of maintenance service in the moulding works.

Speed of Production Affected

Where moulds are afforded no skilled and continuous maintenance oversight the speed of production will very rapidly tend to decrease after the moulds have been in operation for a short time. This will arise from the increasing prevalence of numerous minor misadjustments,faults, misalignment, etc” needing correction, for which purposes the mould will have to be removed from the machine and withdrawn from production use for a particular time.

Such a feature will be especially noticeable in the case of moulds from which very lengthy production runs from each set-up are required.Since all moulds, however carefully they are designed, constructed, and handled during use, will begin to show normal signs of wear upon component members, and a consequent departure from the initial degree of reliability, smooth_running,and accuracy particularly when the tool has been engaged in producing say a hundred thousand or more articles.

The more intricate the mould mechanism has to be, the still greater will be this tendency to wear and error. Similarly, with compression moulds which have to be manually operated, considerable wear may soon occur due to rough handling such tools often receive at the hands of operators. Unless such errors are discovered and corrected while in their more or less harmless stages, very noticeable diminution in the quantity and quality of output will take place.

Accuracy and Quality of Moulded Article Suffers

injection mould maintenance

Regarding the finished moulded product, here again wear in the mould mechanism, will tend to produce a harmful effect upon the accuracy and finish of the article. This will begin to exhibit departures from original accuracy, surface finish and general high quality, much more quickly than should normally be expected, because of the accumulated effects of such wear,maladjustment, distortion between mould members, undue slackness between working parts and so on.

Each of these features if taken individually may be regarded as relatively unimportant and seemingly of a minor character not requiring the attentions of a skilled maintenance tool maker. Yet in conjunction with each other, coupled with prolonged omission to correct, may over a long period cause thereto build up into mould trouble of very extensive and serious proportions. Such as complete or partial failure of mould mechanism to function; or, on the other hand, in very drastic dimensional errors and deviations in the finished products

In this connection, it is well worth remembering that numerous apparently puzzling features occurring during production of a moulded article may, in reality, be due to uncorrected wear, or excessive slackness of some mechanical member of the mould. And as sometimes is believed, it is due to variations in the powder or material being used. Indeed, the someone could give a number of revealing examples where the sudden emergence of inexplicably unusual variations in the sizes or finish of a moulding have been found, after prolonged examination to arise from a simple mechanical fault in the mould itself .Moreover, this fault incidentally was easily corrected.

One such example must suffice to indicate what the author has in mind. The example chosen well illustrates the above point. It consisted of a box-like component, roughly rectangular in outline with certain portions of two opposed side walls thickened up to provide encircling bosses for relatively large cross-located holes required through each of these walls- The component, by the way, was injection-moulded from cellulose acetate.

The mould was fairly intricate,having two sliding cores cylindrically shaped and moving within round holes in the mould blocks. These cores were in turn actuated by means of inclined pins attached to the stationary block which were caused to pass through suitable inclined. holes machined in the core slides themselves. Thus by this well- known and widely used method the cores were retracted at the same time as the main blocks were parted.

The cored holes had to be maintained to fairly close limits, i.e., plus or minus .002 in. After the mould had produced about 10,000 or so of perfectly satisfactory mouldings, the inspection department, encountered difficulties in holding the hole tolerances. At first the variations were so slight and ranged close to the upper limit that nothing was deemed necessary. This went on for some time until a point was reached when the tolerances began to be exceeded very substantially on a large proportion of the mouldings.

Examination of the mould was made whilst the tool was still affixed to the machine, but no apparent cause could be found. Several members of the tool-room personnel, after a long contact with this trouble averred the real cause to be in the grade of material and its rate of shrink varying from batch to batch.

Persistence and unaccounted explanation of the trouble eventually led to the removal of the mould from the machine and a dismantling of the affected cores. It was then found that some wear had occurred on the shanks of these cores and within the bored holes in which they operated.

This undue slackness was allowing the cores to rock slightly at the instant of ejecting, such action causing the incompletely solidified moulded article to be distorted and the hole made slightly larger than the projecting end of the core! Simple cause,but how difficult to locate.

In efforts to discover the trouble, considerable time and energy were expended by a number of individuals, and moreover, a large number of exceedingly doubtful components had been run off,some of which later were found to be unsuitable for the client’s purpose.

Working Life of Mould Impaired and Shortened

The working life of the mould itself may also become shortened very materially, or its working efficiency impaired permanently because of the persistence of some undetected fault, which as time goes on radically worsens until a major breakdown occurs and irreparable damage is inflicted on some vital mould member.

Thus neglect of simple elementary maintenance attention may lead to serious consequences and result in a considerable loss of investment which will be shown in the moulder’s final profit account.

Cases have been known where a neglected simple fault in a mould mechanism has eventually led to a complete failure and the total ruin of the tool as a production unit.

Instances have occurred where an order has been undertaken for the manufacture of a very large quantity of mouldings, and before the total number has been produced the mould has been allowed to wear out, or become ruined as the outcome of some neglected fault or damage.this generally could and should have been observed at an early stage and its effects foreseen by one skilled and specialising in mould maintenance service.

In such circumstances , another mould has had to be made up in order to complete the ordered quantity, and of course, the moulding manufacturer was unable to pass this added tool cost on to the client. Thereby mould buyers have to suffer not only worry and loss of production time, but additional financial outlay on the new tool which may be irrecoverable.