Maintenance can help you get the most of permanent molds.
"An ounce of prevention is worth a pound of cure." This sage advice is certainly true with permanent molds, as attention to the regular maintenance required for proper care of tooling and equipment optimizes the usable life of the molds, saving your foundry both valuable time and money.
Every tooling and equipment component has a usable life that varies with the application, the design robustness and the attention paid to maintenance. Some components are designed to be sacrificial to save costs in the long run and to minimize the effects of other more significant problems. But, failing to properly maintain inexpensive components, sacrificial or not, may lead to costly tooling damage and lost production time, not to mention scrap castings, all of which limits a foundry's ability to meet customer expectations and to make a profit. Indirectly, a lack of proper maintenance also can force schedule changes or result in capacity limitations and missed customer deliveries.
Manufacturing companies may experience downtime resulting from many varied sources including time allotted to preventive maintenance or due to unplanned events, often the result of inadequate regular maintenance activity. However, production facilities are particularly at risk when basic maintenance is ignored. Tooling and equipment components are designed with anticipated maintenance and replacement requirements, and most of the basic requirements are known and understood by experienced foundrymen. However, because employee training often is not comprehensive and cause-and-effects are not clearly understood, the result is maintenance procedures may not be followed consistently in daily foundry practices.
Lubrication is probably the most important requirement for properly maintaining permanent mold tooling and equipment. Many foundries use graphite as the lubricant with an oil carrier. The oil will burn off, leaving the graphite.
Proper lubrication starts with die assembly. High quality anti-seize grease should be used on bolt threads to prevent seizing from rust and high temperature use. It's not uncommon for iron and steel molds to rust during periods of non-use. Die coating protects some surfaces when left on the mold during storage, but what about the noncoated surfaces? Figures 1 and 2 show the results of inadequate lubrication to mold components.
[Figure 1 and 2 ILLUSTRATION OMITTED]
The general practice is to lubricate appropriate locations during the shutdown of a die to ensure protection from freeze-up. This includes return and ejector pins from the front and backside of the die and the ejector plate carrier bolts, as well as leader pins, angle pins and bushings. Bolt threads used for die assembly, in addition to these components, should be lubricated during die preparation, prior to preheating the die for coating. The ejection system requires additional lubrication at least once during each shift of production.
The ejection system is the most at risk for failure if not lubricated properly. Effective lubrication practice can protect these components, even in severe conditions for extended periods. Even without severe conditions, lack of lubrication leads to galling of the pins, perhaps not only destroying the pin, but also damaging the die.
Going though the motions of lubrication does not ensure it will be effective. It's not uncommon to find heavy lubricant buildup on the ejection components on the backside of the die with rust where it counts at the die and pin interface. It's also more difficult to reach the pins at the bottom of the mold than at the top due to some machine frame access restrictions. Don't overlook the ejection, system carrier bolts when lubricating critical locations.
It's important for workers to confirm the effectiveness of their maintenance techniques, making sure that the proper lubrication is applied where it is needed, all the way to the end of a pin. One way to confirm that pins are getting the proper lubrication is to look for rust or galling following the application of lubricant. Another indicator is to look for a flame at the parting line when the mold opens, which shows that the oil covering the pin, particularly the end in the mold, has ignited.
Even the best lubrication practices will not eliminate all ejection system problems and failures. For example, die cleanup and blast media can become lodged in the counter bores of headed return pins. This can cause mushrooming of the return pin head, creating casting flash and dimensional problems since the die will be held open. This media can also become lodged in ejector pin holes in the die, as well as in mounting plate holes, leading to binding and galling.
An example of this is shown in Fig. 3. A sleeve ejector pin from a new die was damaged by blast media trapped in the relief hole from the back of the die.
[Figure 3 ILLUSTRATION OMITTED]
Media trapped in the counter bores of a double plate ejection system, where the head of the flanged pin is captured, also can limit the free movement of pins needed for differences in temperature between the die and ejector plate as a result of linear expansion. The use of [CO.sup.2] blasting, rather than more abrasive techniques, minimizes these problems. Even a combination of glass beads and aluminum chips is less abrasive than sand or steel media, thus promoting longer die life. Care should be taken during die cleanup to protect these areas, and each pin should be checked for free movement prior to each production run.
Both binding and galling as a result o the problems previously discussed often produce bent ejector pins, and binding can easily lead to bent castings. A loose ejector pin can create a dimensional issue on castings produced, depending upon the pin setting tolerance requirements. In the double nut pin and plate ejection system, which utilizes a hex and jam nut on each side of the ejector plate, nuts tend to become loose, requiring maintenance. Some of this can be eliminated using toothed lock washers between the nuts or a serrated face on the inner and outer nut.
Though proper lubrication and adjustment will not make an ejector pin last forever, downtime, scrap castings and rework will be minimized. Pins will eventually lose their sharp corners and round over, leaving raised flash on the casting to be removed with additional processing labor. The ejector pin and return pin holes also will occasionally need to be reamed to remove buildup of graphite, blast media and die coating that may keep the oil from carrying the clean graphite to the desired location while leaving clearance for venting.
Some casting shapes have venting requirements beyond what can be handled using tilt pouring and ejector pin placement because some features of the casting may be too thin for adequate ejection surface area or they may be located in the non-ejector half of the die. Screw vents are often used in these cases. Most designs include flats placed on the perimeter of a pin, similar to a hexagon shape. The pin is placed in a round hole, leaving gaps for air to escape. They can be manufactured in any desired diameter and placed strategically, even on angled surfaces, to allow an escape path for air. The key to effective use of a screw vent is to turn the screw from the backside of the die each time the die coating is sprayed over the pin. This will break the seal covering the air gaps on the flats.
Crosshatching also is used on larger surfaces to address venting requirements. It works like the peaks and valleys created by die coating in that air is allowed to move and disperse into the network of interconnected shallow, sharp grooves. Due to surface tension, the aluminum will not fill into the sharp corner, providing the necessary gap for venting.
A spline is a variation of crosshatching that is used for round casting features. These sharp grooves are positioned in a mold along the angle of pull and perpendicular to the parting line, and they may become filled with die coating causing a void.
Inserts are often designed into molds specifically for the venting advantages they provide, as well as for tooling feature-manufacturing requirements. A thin deep rib running the width of a casting can exhibit multiple air traps. These may be eliminated with a series of ejector pins and/or screw vents, which may not be practical for other reasons. Air traps can be effectively controlled using insert line venting if the same care is taken to keep the blast media and die coating free of the clearance gap. A feeler gauge can be used to break the die coating seal after spraying during production runs. Insert line venting can lead to flash, and is often designed and sized with this expectation. Some additional processing time can be justified, particularly on large complex castings.
Permanent mold alignment systems are similarly affected by lubrication, die coating and die cleanup procedures. Early component failure and casting dimensional errors are two of the more prominent problems. Leader pins and bushings are designed with a minimum clearance of 0.001 in. and a maximum clearance of 0.005 in. in the new condition. Depending upon casting dimensional requirements, shift tolerance and the condition of the other bushings, an entire casting production run could possibly be scrapped. Even minor wear leads to misalignment immediately as the die opens, creating drags and bent or cracked castings. When the die coating is damaged, you have soldering and die wear.
Know your shift limits and know the related risks of misalignment. These limits should be a part of maintenance control plans, and bushings should be changed before they exhibit signs of excessive wear (Fig. 4). Compare the cost of a $15 bushing, or even a full set, to the cost of a large number of unusable castings. Having to scrap parts due to shift could be an expensive lesson.
[Figure 4 ILLUSTRATION OMITTED]
Depending on your shift limits, leader pins and bushings should be discarded while still in relatively good condition, when they are starting to exhibit signs of wear. Reasons for failure include galling from blast media that can easily be trapped in bushings installed in blind holes. Open holes are often made blind by stuffing insulation on the back of the die. Also, lack of proper protection of pins and bushings during die coating starts the galling and wearing immediately. Use a pipe for proper protection of pins during die coating (Fig. 5) and use pins to protect their mating bushings. In some tooling applications, bushings must be threaded into the mold using a slot in the flange of the bushing. A liberal application of high quality anti-seize grease during installation can make it easier to change these regularly.
[Figure 5 ILLUSTRATION OMITTED]
When this wear is ignored long enough, the die alignment will be shifted, possibly resulting in unusable castings. Shift on the casting and flash are visual signs of component failure and the need for maintenance.
Another visual sign of component wear is shown in Fig. 6. The angled pin grease grooves are disappearing at the point where the block would stop in the closed position. The mating bushing also may show an out-of-round condition. This type of component wear can allow flash.
[Figure 6 ILLUSTRATION OMITTED]
Iron bushings are intended to be sacrificial to steel pins. However, a cast iron die does not require a bushing to prevent galling to a steel pin. A reamed hole can be maintained effectively with proper care during production setups if machine bearing and alignment systems are kept in good condition, but the use of a bushing provides for easy repair regardless of how long it lasts.
Machine alignment problems can sometimes be recognized on deep cavity dies, when the cavity feature is much deeper than the length of the leader pin and the die coating shows signs of sliding against the opposite half. This could be the result of a loose mounting system, a machine gib, a corner block out of adjustment or a more serious bearing system problem.
The small AutoCast machines utilize a recirculating ball bearing system. The sacrificial component is the steel bearing track bolted to the machine frame sides, which requires specific adjustment and torque ranges to maintain bearing pre-loading and alignment of the moving ram. This allows the ball bearings to have running clearance between the steel ball track connected to the ram. Improper maintenance may lead to failure of the steel ball track at the point of incorrect adjustment to the track.
Failure can also be the result of abrasive contaminants in the bearing system from the use of sand cores and die cleanup in the machine. Again, [CO.sup.2] blasting techniques would minimize this problem, and minimizing abrasive die coating overspray also will reduce premature bearing wear.
Preventive maintenance checklists like mold inspection sheets can significantly reduce problems, particularly when they include accountability (Fig. 7). Keeping these on file provides the opportunity to identify training needs when problems arise.
Fig. 7. A mold inspection checklist like this one can serve as verification that lubrication and maintenance procedures are followed.
Mold Inspection Checklist
Customer Product# Mold# Cavity# Date/Initials
O.K. Items to Check
[check] Leader Pins: bent, damaged, loose bolts.
[check] Leader Pin Bushings: damaged, missing, loose.
[check] Ejector Pins: bent, damaged, loose nuts, correct setting per process sheet, excessive wear/roundness, aluminum buildup around pins.
[check] Ejector Sleeves: damaged, worn, loose, correct setting, loose nuts.
[check] Metal Cores: bent, damaged, buildup, undersized/undercuts.
[check] Parting Line Surface: remove buildup, flash, nicks.
[check] Casting Cavity Surface: check for damage, cracks, chips, buildup, undercuts.
[check] Mounts Plate Bolts: checking tightness, missing bolts.
[check] Pour Cups/Angle Plates: flush with parting line, loose bolts, undercuts.
[check] Ejector Plate: free operation, loose carrier bolts, head of return pins flush with/below surface.
[check] Mount Rail: loose bolts, missing bolts, damage.
[check] Vents: open, flush/depressed per process sheet, damage.
[check] Core Prints/Vents: remove trash, buildup, check damage.
[check] Visually Checked Last Shot for Other Defects.
Repair Defective Items Found or Tag Mold with Card Detailing Items Needing Repair.
Lubrication efforts must be effective to avoid problems. Look for signs of rust and galling to be sure. Train new employees on the focal points for wear, like on bushings, casting flash, and on castings showing parting line shift, to detect maintenance needs early. Consider using go, no-go style tools to make it fast and easy to determine if components are worn beyond the desired limits. Detail maintenance needs to be completed prior to the next run upon die shutdown, making sure to keep the last casting shot with the die to review for visual signs of component wear.
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|Date:||Feb 1, 1998|
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