Mould Repair:
1. The best time to repair a mould is when it requires the least amount of effort and expense. All available means of repair depend upon the kind of damage and the design of the mould. Various methods can be adopted for mould repair.
2. Minor damage in the parting-line area can be repaired by grinding, subject to limitations imposed by the required dimensions.
3 Some times a damaged spot can be drilled out and closed with pin or raised from the back.
4. Exchanging single component or group of parts such as ejector, guide pillar, bushes etc.
5. Plating / Re-plating the areas subjected to high wear.
Mould Maintenance:
1. Emergency basis at the time of breakdown.
2. Preventive maintenance of the moulds between production runs.
Preventive Maintenance:
1. The concept of preventive maintenance can well be understood from the well-known axiom.
2. “Prevention is better than Cure” and “ A Stitch in time saves nine”.
3. A mould is subjected to wear & tear, require periodic cleaning & lubrication, readjustments and necessitates frequent checking to retain their basic accuracies and efficiency. The principle therefore lies is discovering the symptoms and conditions that may lead to major breakdown later, well in advance, in the initial stages itself by period inspection, thus avoiding the probable major breakdown in the near future.
4. An analysis of the causes of manufacturing, damage and breakdown can serve as a reliable basis for chalking out a useful system for preventive maintenance.
Advantages of Preventive Maintenance:
1. Reduction of downtime
2. Decrease in rejects
3. Compliance with terms of delivery
4. Reduced cost per repair
5. Longer useful life of the mould
6. Increased overall productivity
Preventive Maintenance – The Role of the Mould Maker
1. Mould maker should follow the mould designs strictly.
2. Any changes should be made in concurrence with mould designer.
3. Steel identification should be done methodically.
4. Mould parts should be stress relieved and heat treatment of steels should be documented.
5. Mould parts which could easily be replaced (less costly) should be made less hard than one, which is difficult - Justification while deciding.
6. Provision for the locking of the round inserts, not to rotate / axial movement.
7. Ejector guide incorporation for long run of mould.
8. Oil groove machining over the guide pillar
Proper air venting
9. Proper tightening of cooling nipples and pipe over it, no way the water should leak or spill over the moulding area.
10. Recognize which parts of the mould may be more readily susceptible to damage because of their design and / or function. In such cases manufacture spares so that if such a part fails, the loss in production will be considerably less.
Preventive Maintenance Procedure:
1. Blow out coolant from cooling network with compressed air.
2. Remove scale by circulating HCL solution in the cooling lines and neutralize.
3. Remove plastics remnants from all mould parts.
4. Make the air vents free.
5. Check the surface finish and re-polish if necessary.
6. Check guiding elements for wear and damage. Replace if needed.
7. Check the ejectors for wear and damage. Replace if necessary.
8. Check moving parts (slides, revolving cores, collapsible cores) for wear and repair/replace.
9. Check the fasteners for proper tightness.
10. Check ejector assembly for easy movement.
11. Check springs for fatigue and permanent distortion. Replace if necessary.
12. Check hot runner connections.
13. Grease guiding elements.
14. Grease moving, sliding and revolving components.
Maintenance Schedule:
TO RUN THE MOULD CONTINUOUSLY FOR LONG PRODUCTION RUN
1. Daily Care: Proper oiling and greasing over the sliding and rotating parts.
2. Weekly care: Check Mould clamping bolts, bolts holding sliding parts, scoring if any on guide pillar, coupling bolts and it’s damaged portions.
3. Monthly Care: Major Dimensional variation in the product (inspection of product), flashes in the ejector pins (to rectify ejector guide area).
4. Yearly Care: Replacement of certain parts, which are under constant wear and tear like, slides or springs etc.,
Some useful precaution: Adequate greasing over the moulding and non-moulding areas of the mould for rust prevention. We should use rod of soft material like Copper, Aluminum etc., while working on moulding machine to remove component, runner etc..
Causes of mould wear & damage and its remedies:
1. Neglect: Most often damage results from continuing to run the mould after flashing occurs. Major damage results from closing the mould on the material itself, bits of flash, chips from sheared undercuts, cracked parts of runner or torn gates or any other foreign objects.
Remedy: Flashing is a self-aggravating situation. Hence at the first appearance of flash in any mould immediate attention is essential. There probably is a build-up of some sort on the parting line, back of the striper plate or between slides. Correction at this stage can be made by toolmaker at small cost. Often this is simply a matter of skimming off the parting line or the refitting of inserts& adjustments of wedges.
2. Accidents:
1. The accident happens due to various reasons:
2. Mistake in mould installation
3. Continuing to produce in a malfunctioning machine.
4. Continuing to operate a mould, which has started to squeak & squeal.
5. Improper assembly of moulds, failure to tighten screws, leaving metal chips or grinding dust in the mould by Tool Room Technician.
6. Remedy
7. It is necessary to check all mould-clamping daily. In ultra-high speed operation, a check after each shift is recommended. This check should be combined with a cleaning and lubrication of all sliding components using a minimum of lubricants.
8. Hand Tool Damage
9. Due to use of screwdrivers, knives or cutters etc for removal of sticking parts, flash, short shots etc. from non-automated moulds.
Remedy: The best prevention for this is a fully automatic mould running in continuous production. Otherwise sharpened soft brass or copper bars with facilities for re-sharpening are mandatory.
3. Water Hazard: Corrosion due to contact of water on unplanted surfaces. Water forms in the moulds from condensation, seepage through porous metals, leaky pipefitting and O – rings. Rust & salt deposits from inside of the cooling channels.
Remedy:
1. Chromium plating on the component area and nickel-plating of inner surfaces of cooling channels.
2. A cold mould should never be closed; let it warm up above room temperature before closing, as condensation will occur in the mould even if it is closed.
3. While disconnecting cooling lines, first disconnect supply lines and then air blow the remaining water. Finally apply little oil for protection from rusting.
Acids: Mould surfaces may get damaged due to attack of acids after exposure to corrosive materials when overheating decomposes some thermoplastics. Over heating can occur in the politicizing cylinder, in the mould cavities as the result of too small gates, and inadequate venting & cooling systems.
Remedy: To reduce/eliminate the attack, mould surfaces should be chrome / nickel-plated.
Mould temperature control & prevention of excess heating in the plasticizing cylinder. Adequate venting and slowing the rate of mould filling by increasing the runner & gate dimensions.
Fatigue: Fatigue is a major cause of mould damage, which leads to a breakdown of mould components due to continuous cyclic loading of more than 1 lakh cycles. Fatigue failure of ejector pins is one of the most common causes of mould breakage.
Remedy: Use of good quality tool steel & periodic stress relieving of the mould. Avoiding sharp corners and high length to cross-section mould components.
Cold deposition: This process permits the repair of a damage of less than 1 cm2 with a thickness between 1 micron and 1 mm. The operation is similar to electric welding. The cathode is attached to the work piece and anode is dipped into the electrolyte and brushed across the spot to be treated. Metals deposited are nickel, copper, cadmium etc. As the metal deposition is done in the cold condition no distortion of the mould takes place.
Welding:
1. Welding is frequently used to remedy damage such as local dents, breaks or little cracks or if small corrections are needed .
2. When it is necessary to weld, a few simple but important steps must be taken:
3. Know the type of steel, which is to be welded.
4. Prepare the surface, which is to be welded. Completely remove all defects by grinding or machining. The surfaces to be welded must be completely clean and dry.
5. Select the proper electrode. The electrode wire material should be of the composition as the mould material or at least similar.
6. Preheat the mould to about 1000F below the tempering temperature of the steel. Heating above tempering temperature will draw the temper and soften the mould. The maximum preheating temperature is 9000F
7. Weld, Preheat the mould if the temperature drops below 5000F, before the welding is complete.
8. After welding allow the mould to cool to hand warm or 2000F.
9. Reheat the mould to the preheating temperature and hold for one hour per inch of mould thickness. Two hours should be the minimum time held.
10. Allow the mould to cool to room temperature in still air.
11. TIG process and welding with coated electrode wires is the most common for repair purposes. Every welding process, however, is a hazard to the mould because it generates high thermal stresses. Improper workmanship may cause distortion and cracking which jeopardize function and performance of the mould. Therefore one should always contemplate whether or not other means of repair are available. Check the ejectors for wear and damage. Replace if necessary.
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