UPKEEP & MAINTENANCE OF MOULD
1. Introduction:
Injection moulds play a vital role in production of qualified components. Consistency in the quality would also depend on the quality of mould. Unreliable mould would need frequent adjustment, repairs etc. It is therefore necessary that mould should be reliable
in its function and moulding of qualified components.
One of the prime objectives of any moulder is that the moulds in use should function well to the maximum possible production quantities. A mould properly designed and made may not remain necessarily proper, unless kept and maintained in a proper manner. Therefore, upkeep and maintenance of moulds are important practical aspects of a moulding shop.
The following points are to be taken care for an effective maintenance of the mould
- Upkeep
- Maintenance
- Specification sheets for all moulds.
- History sheets.
- Instruction manual on upkeep and maintenance
2. Upkeep
Upkeep means that the mould should be kept well cleaned. Before closing the two halves of the mould, an antirust spray may be used to prevent the rusting of the mould parts. The mould should be stored preferably at a place where the humidity is low and ambient temperature is not excessive. Moulds which are stored near a chemical factory or storage, the cavity/core must be chromium plated otherwise due to the chemical vapour, the core and cavity will get corroded or eroded. Rusting can be very much minimized if the moulds are kept covered by PVC sheet covers.
3. Maintenance
It means that w should keep the moulds and bring them to production in a worthy condition. We can do a preventive maintenance of the moulds and it would cover
examining mould for small damages. Check whether any problem with clearance and the wedge blocks. If it is there, increase in the clearance of the moving parts,
the adjusting of the locking wedges. The cleaning of the water channels is a must otherwise it will get rusted inside the hole and finally the flow of water will not be sufficient. Then check for guide pins. Sometimes, any guide pin or cam pin may found shaky in its holding hole. In that case, as a maintenance action, either a new pin may be put or the existing one may be copper plated at the holding surface.
4. Specification sheet:
Specification may vary from mould to mould. It gives complete information about the mould, which can be stored in a computer. Each sheet may be stored in a computer, allotting a suitable file number. This also helps to the persons, who are not familiar with the moulds and might have an occasion to handle maintenance and upkeep of moulds.
5. History Sheets:
This pinpoints the defects, which are occasional / which are repetitive / which are chronic in nature. This helps to take appropriate steps to find the solutions to the defects and take corrective measures. It also reveals the management a lot of information about the performance and capabilities of the shop floor personnel.
6. Instruction Manual:
Upkeep and maintenance actions are almost similar for all the moulds, but some special precautions may be needed for some moulds. It is therefore a good practice to prepare a manual on “moulds upkeep and maintenance”. Technical personnel involved in upkeep and maintenance can follow it.
7. Points which affect Physical Mould Life:
The following points to be seriously considered which will affect the physical mould life.
- Inadequate physical strength of mould parts to withstand the loads encountered and caused by clamping force , injection pressure, ejection force etc.
- Improper mould making practice
- Improper allowance for fatigue in mould studies in a frequent
- cause for breakage
- Improper heat treatment specification
- Lack of lubrication of sliding faces
- Incorrect machine set ups.
- Poor maintenance practice and rough handling of the moulds
- Moulder not adhering to the suggested start up and
- maintenance procedure
- The use of dirty and contaminated plastics
- Abrasive, erosion or corrosion fillers such as glass and
- some other additives.
- Highly humidity
- Dirty and corrosive cooling water
8. Types of Maintenance (frequency)
There are four levels of maintenance activities and one breakdown maintenance. All these are explained in detail below. The types are
- Routine Maintenance LEVEL I
- Inspection Maintenance LEVEL II
- General Maintenance LEVEL III
- Major Maintenance LVEL IV
- Breakdown Maintenance
8.1 Routine Maintenance – LEVEL I
This is a very simple preventive maintenance. Before the mould is removed from the machine, it is to be heated (if it is chilled) so that all surface condensation is evaporated. With the mould still warm, internal mould surface should be cleaned with shop safety solvent to remove any residual dirt and stain. The ejector system moved fully forward then sprayed with approved rust preventive before the ejector system is retracted and closed. Lenses and highly polished surfaces should be protected by
following specific instructions. All water lines should be drained and blown free of all residual water to avoid build up of rust due to standing water. Check and assure all bolts , plates, clamps, etc are in place and tight. Bag the last shot as an example of the typical quality of this run. Any components missing or cavity blocked off should be noted and attended to. Store these parts with the mould.
8.2 Inspection Maintenance – LEVEL II
It is done for every 20,000 cycles and also at the end of production run. Using the check list and visual inspection technique, the mould is looked over. Any minor repairs that are necessary are noted. If not, needed repairs or future required touch up notations are kept with the mould history log for future evaluation. Any components missing or cavity blocked off should be noted and attended to. A sample from blocked cavity should be retained for the mould maker to make repairs. The mould should be washed with safety solvent to remove the varnish and build up from the moulding process. Notations for work to be done during the general maintenance procedure
should be noted on the form for future work. Bent, worn or broken ejector elements should be noted. The mould should be removed from the production and elements replaced
8.3 General Maintenance – LEVEL III
This is after 1, 00,000 cycles of moulding. Only competently trained tool room technicians should perform this maintenance. All plates are separated and their faces cleaned. Highly polished surface should not be cleaned with brushes and rags. Clean with facial tissues or cotton balls. All components are checked for wear. Any excessive wear is noted and a determination is made to repair, replace or continue to use. All rough areas outside the cavity detail area are to be worked out. All moving parts are to be lubricated if required. Vents should be checked for depth, width and land as
compared to the tool drawing specifications. “O” rings, seals and gaskets should be checked for integrity. All water lines are to be pressure tested for any leaks
and re-certified for flow capacity. Water lines that have built up scales are restricted should be pressure cleaned with descaling agent. The ejector system is to be examined for proper alignment. Any damaged hole or pin should be reworked Replacing broken return pins, re-plating or re-texturing to be done as a result of the material eroding the mould surface. Replace all springs after 50,000 cycles. Nature of work done should be recorded in the general maintenance file.
8.4 Major Maintenance – LEVEL IV
This is after 250,000 cycles. This maintenance should be performed by skilled tool makers. Before maintenance starts, there should be four complete shots delivered with the tool for study. Two shots should be from initial mould qualification. This gives a visual record of what was acceptable when the mould was new and fully functional.
The second two shots should be the most recent parts produced before the tool was pulled for maintenance. Comparison of the both shots will find an excellent indication
of wear and abuse the tool was suffered. All components determined and authorized to be replaced should be removed and new components constructed and installed in accordance to the original design if previously certified parts are not there. Worn leader pins, bushings and all bearing moving surfaces should be checked for wear and replaced or repaired. Plates and mould cavity surfaces should be checked for parallelism and ground flat required. Mould cavity surfaces should be cleaned and polished as required to the original surface requirements. All moving components should be checked for easy movements, adjustment to be made as required.
Springs are to be replaced with new springs to avoid fatigue.
8.5 Break down Maintenance:
It is done unexpectedly when an accident occurs to the mould is in production. The problem may be , water leak, core/cavity damage, cooling line blocked, ejector system failure and heater failure in case of hot runner mould, etc. We have to study the problems along with production engineers and to bag a last shot from the production.
The maintenance of the mould is done with reference to the decision taken in the outcome of the meeting.
9. Mould removing, cleaning, and storage
Whenever a mould production is over and it is to be stored in a store room we should do maintenance for the mould. This mould has to be taken for production after sometime. We should heat the mould to room temperature before removing from the moulding machine to evaporate any surface condensation. Using a mild, clean shop safety solvent and soft, clean towels gently clean internal mould surfaces to remove residual dirt and grime. Move the ejector system fully forward and spray both mould halves with shop approved protective rust preventive before retracting the ejector and closing the mould. Drain and blow free all residual water to avoid rust buildup.
No water should be trapped inside the mould and it is critical. Make sure all bolts, plates and clamps are properly placed and the safety straps are secured and tightened.
Remove mould, check and clean it again, if required. Bag the last two or four shots to store with the mould including feed system as examples of the expected run quality.
Store the moulds on storage racks notably out of shop floor. Don’t let water lines stick out when storing moulds on racks. Wrap the mould with plastics. Ensure that air is dry when wrapping, or put a dissident air absorption bag inside so that temperature
changes don’t cause condensation inside the wrapping that could form rusting.
10. Maintenance of idle mould
Basically a repeat of activities already done during preventive maintenance during production. We have to check the flash faces. Then remove stains and special attention to the given to air vents. You must check the cooling channels. Replace “O” rings if necessary. Check slating ejector pins, normal ejector pins, guide pins, and pins for slide movements on wear and tear and bending. If necessary, replace. Check springs and replace if necessary. Replace them in sets. Check on leakage after assembly.
Check sprue bush on dented marks. Check slide movements. Blow cooling channels before bring the mould down.
11. Preventive Maintenance
The preventive maintenance of the mould mainly depends on the role of designer and the mould maker. If they do their job very well, then this maintenance will be very easy.
11.1 The Role of Designer:
The designer must study the part drawing first. He must identify the area of concerns and to be discussed with customer to avoid on doing mould and moulding problems.
He must discuss to customer about the part design which include sharp corners, inadequate draft, and limited areas of ejection etc. which call fragile steel sections in the mould. He should discuss with them to determine and modifications can be made in the part design. Mould designer should incorporate into the mould design provision for mould maintenance. Critical area should be inserted for ease of repairs. Materials should be used which are resistant to galling when slides are required and wear strips and locks should be accessible and adjustable. Steel should be selected for strength and hardness properties that will ensure reliable performance. Mould bases for high production moulds should be fabricated from steel that will not collapse and more under stresses induced by the moulding operation.
11.2 The Role of the Mould Maker:
Mould maker must follow the mould design, any changes in connection, should be with concurrence of the mould designer. If a mistake in design has been noticed, the mould maker should inform the designer so that the correction can be made & recorded.
Identifying steel for parts is important. After hard machining he should send the steel for stress relieve or send when appropriate. Heat treatment of steel should be documented to ensure that proper hardness have been achieved..
12 Maintenance Control
An effective maintenance system will reduce the risk of needless downtime and poor quality parts in both short and long run moulding condition. Maintenance report is important that individual records are kept for each mould and injection moulding work. These records will document their history and should contain the following
- Machine/mould specifications including accessories.
- Record all modifications
- Maintenance logs
- Downtime logs
- Manufacture services call reports
- All alterations to the machine since installation.
13 Frequency of Maintenance
Frequency of maintenance is determined by four factors. They are
- Material
- Complexity
- Moulding material
- Abuse
Material: Aluminium or soft tools will suffer wear and tear in a shorter period of time than tools made of conventional tool steel. Mould made of hardened steel will last longer than those made of conventional mould steel.
Complexity: Moulds with intricate mechanism or parts requiring unreasonably high precision will require more maintenance than a simple two plate mould of low precision parts. Slides, lifters, internal cores, hydraulic and mechanical systems, hot runners, complex ejector system, all add to the maintenance schedule.
Moulding material: Materials with high melt temperatures wear out moulds quicker than those with low temperatures. Higher melt temperature will expose the metal to more heat and enhance material wear. Filler materials are abrasive to metal, they will tend to wash the mould steel away after thousands of cycles.
Abuse: Excessive clamp pressures, high injection pressures, jerking the mould, opening and closing the mould, not lubricating the appropriate components, multiple ejection, crashing the mould closed or closing up on the partially ejected parts are a sure road to increased maintenance.
Abuse can also be defined at the work place. A shop that is dirty, open to the outside dust and dirt, machines with grease and absorbent on the floor all creates an atmosphere for mould to wear out quicker. Area exposed to acid vapour, area of high uncontrolled humidity or salt water, dry area with constant wind and dust cause premature wear on the mould. When the mould is clogged or restricted cooling circuit, this also is an abuse which is called Environmental Abuse.
So the upkeep and maintenance of the mould is very essential to keep the mould to run beyond its life.
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