Causes Of Wear and Damage Of Plastic Mold
Most often ,the damage results from continuing to run the mold after flashing occurs- Major damage results from closing the mold on the material itself, lts flash，chips from sheared undercuts, cracked parts of runner or torn gates, stringers from nozzles or from sprues that are too hot or improperly controlled, and the inexcusable crime of allowing the mold to clamp on granules of raw material.
The sources of damage include:
- The result of closing the mold on foreign objects such as inserts, tools, screws, nuts, broken ejector pins, etc., caught between the parting line surfaces. In many cases the smallest bits of foreign objects cause the greatest damage, especially if they are caught at the cavity edge.’ These small bits are not detected by properly set low-pressure closing protective devices. Clamping on a tiny object, even if it is a plastics granule, concentrates the entire press clamp tonnage on this very small area—exceeding the elastic limit of any mold material regardless of quality and hardness.
- The result of the use of screw drivers, knives or cutters, etc., used to assist in the removal of sticking parts，flash, short shots, etc.，from non-automated molds.
- The result of contact with water on unplated surfaces. Water forms in the molds from condensation, seepage through porous metals, leaky pipe fittings and “0”-rings. Careless handling of hoses and feed lines during hook-up leaves water on the mold surface. This is not harmful if detected immediately and carefully removed. Corrosion is progressive and even if the molds are stored after being sprayed with an antioxidant, a few drops of water or condensation can cause tremendous and costly damage.
- Attack from acids after exposure to corrosive materials which may form when some thermoplastics are decomposed by over-heating. Overheating can occur in the plasticizing cylinder, the hot runner system or in the-mold cavities, as the result of too small gates, inadequate venting or cooling systems.
- The result of accidents caused by a number of things. Mistakes in mold installation, continuing to produce in a malfunctioning machine, continuing to operate a mold which has started to squeak or squeal will result in damage. Damage will be done to molds that refuse to close or open normally or will not eject properly if their operation is allowed to continue without ending the problem. An alert and well-trained foreman can hear trouble starting as he passes through the molding room. The aforementioned types of mold damage are not limited to the pressroom. Tool-room technicians contribute their share by the improper assembly of molds, failure to tighten screws, leaving metal chips, grinding dust or polishing abrasive on or near sliding surfaces.
- Fatigue is a major cause of mold damage which leads to a breakdown of those mold components that are subjected to the maximum stresses while cycling correctly over long periods of time. This usually occurs, if it is going to occur at all, after 100,000 and before 300,000 molding cycles. In a multiple-cavity mold, components identical to those that break first may last in finitely longer. Fatigue can manifest itself in components subject to compressive loads as well as those in torque, tension, or bending.
Thermal shock has a questionable contribution to the deterioration of molds. So far as our analysis goes, unless rapid heating and cooling of the mold is part of the cycle, there is ordinarily only a small effect in comparison with hydraulically and mechanically induced stresses. Molds run in the higher temperature range—500 to 800° F deteriorate more rapidly; this is an historical problem with the die casting dies.
Molds that are still on the drawing board offer innumerable opportunities. You can build a cheap mold that will be in trouble and not last very long or you can build in the well-known factors that provide long life and minimum mold maintenance will be experienced. A mold that has become unusable because of wear，abuse, and improper construction has passed the opportunity for preventive maintenance. The options here after this happens are few unless the economics permit major replacement with corrected design.
It is important to specify chrome plating for thermoset molds before any wear occurs. New molds for this work should be plated immediately after the samples and mold operation are approved. Never run production before plating in molds for thermosetting materials. Periodic checks after the chrome-plated mold is in full production are desirable to find evidence of mold wear, which will show up first in comers and high flow areas. A simple check may be made by swabbing a copper sulfate solution in the mold areas;
The copper plates out on the surface, the chrome has gone and must be replaced. Instruments are available that will measure the thickness of chrome plating and predict potential failure. Mold components should be replated as soon as the chrome has gone. Mold parts must then be stripped, repolished, and replated. The superficial hardness of chrome Plating is the equivalent of 68 Rockwell C whereas the average compression mold steel is about 56 Rockwell C. The relative resistance to wear has been determined to be approximately equal to the ratio of the squares of the hardness numbers—thus emphasizing the importance of plating to minimize Wear.
High volume transfer molds for thermosets often use replaceable inserts made from high Rockwell steel or hardenable carbides. It is equally good practice to provide for the subsequent installation of an insert when it is expected that substantial wear may occur. Without this advance planning, it may be difficult or impossible to install an insert when and if it becomes necessary.
Control of Flash
The best quality blow molds are made with replaceable pinch-off inserts.At the first appearance of flash in any mold, cleaning is essential. There probably is a build-up of some sort on the parting line, back of the stripper plate or between slides. A check will determine whether or not the clamps or other fastenings have loosened. The initial appearance of flash suggests a complete checkup of the press. Stress rods have been known to break within one of the nuts hiding the failure.
The most successful program of preventive maintenance is based on scheduling each mold out of production regularly into the tool room. The last shot molded should be attached to the mold with its record tag. In the tool room, the mold is disassembled as needed for complete cleaning and careful inspection. After completion of essential repairs, replating, etc., moving parts are lubricated; mold is sprayed with an antioxidant and returned to storage or production. This is the best and least costly maintenance; nothing is more costly than unanticipated shutdowns at peak production periods. You pay for this service as a protection or you pay later as a loss; loss expense is greater than the prevention thereof.
When a clean and otherwise properly operating mold is flashing, corrections can be made by the toolmaker at small cost. Often this is simply a matter of skimming off the parting line or the refitting of inserts, adjustment of wedges or the installation of oversize knockouts. Such corrections are facilitated by making advance provision in the mold design for such maintenance. Flashing is a self-aggravating situation. The longer a mold is run and flashing, the faster the flash area increases since it gradually adds to the projected area that the machine is capable of clamping adequately.
Hand Tool Damage
Very strict enforcement is recommended of well-defined and publicized rules of acceptable shop practice concerning the presence of ferrous tools in the molding room. Sharpened soft brass or copper bars with facilities for re-sharpening are mandatory. When hard brass-or bronze tools are provided it must be recognized that they will work-harden and must be annealed and reshaped daily by a responsible person. The best prevention of tool damage is a fully automatic mold running in continuous production with minimal startups.
A number of preventive measures can be taken to avoid corrosion by water. The most popular is plating, usually chromium on thermoset molds and nickel or chrome on thermoplastics molds. It must be recognized that plating can be porous, especially if the steel substrate is pitted or has been subjected to corrosion previously. It is not safe to depend on plating for protection from water damage. Where chillers are used for mold temperature control, condensation of moisture on the mold surfaces can sometimes occur even while they are in full operation. The best solution for this problem is an air conditioned pressroom,or at least a humidity controlled atmosphere. When a chiller or very cold water is used for cooling, it is important to anticipate the shutdown time and allow the mold to warm up to above room temperature. Only then is it practical to clean and spray the mold before closing. A cold mold should never be closed; let it warm up above room temperature before closing. Condensation will occur in the mold if it is closed while cold. When cooling lines are being disconnected, the best practice is to disconnect the supply line hose first and then use the air nozzle to blow the remaining water from the mold into the drain line. This procedure prevents subsequent movement of water from the channels into the parting line or other openings.
Not all damage to molds is confined to molding surfaces. Rust and salt deposits form inside of the heating or cooling channels in spite of the use of well-designed water treatment systems. Delicate molds with correspondingly thin steel wall sections separating inner and outer surfaces have been found to rust through from the inside. The best practice results from nickel plating these inner areas. Electroless nickel plating solutions can be pumped through the water lines and, since this process does not depend on the passage of electrical current, the plating develops evenly if they have been thoroughly cleaned before the plating. This must be done before any rust has formed.
To reduce or eliminate the attack of the mold surfaces by corrosive plastics the products of their decomposition, plating can be most helpful. The plater must know exactly what the attacking medium is to suggest the proper a e. Gold has been used for some very corrosive substances. Chrome is attacked by some decomposition products, and stainless steel is not immune to corrosion. Nickel plate appears to be second only to gold plate in corro-l0n resistance. It must be recognized that the plating may not have completely covered the surface to be protected. This fact necessitates extreme care in the molding room to avoid the overheating of the compounds which trigger the attack. Besides mold temperature control and prevention of excess heating in the plasticizing cylinder, corrosion may also be minimized by a reduction in the speed of mold filling and the provision for adequate venting. Slowing the rate of mold filling may be accomplished by machine control and by increasing the runner and gate dimensions. Frictional heat from an excessively small gate is easily cured. Venting at the extremities of the cavity, plus added vents along the way, reduce the back pressure and permit a temperature reduction along the way. In some cases it helps to vent the runner system.
Mold fastenings loosen from many causes: clamping force causes mold compression, the impact and bursting pressures caused by rapid filling of the mold, tensile stresses as the mold is pulled open—all these forces are augmented by inertia and acceleration during fast cycle operation. This loosening of fastenings can happen inside the mold as well as at the point of fastening in the press. In far too many cases, a mold is set into the machine with only four clamps to hold each half to its platen. These clamps may be adequate in a static position but become entirely inadequate if the mold on the moving platen is quite heavy or if there are slides or split cavities of large size. Other factors are:
- When deep part, or parts, in the case of multiple cavities, is combined with low draft angles;
- When sticky materials are being molded;
- Molding in fast cycles; or a combination of all of the foregoing.
The strength of mold clamps must be calculated from the pull-back strength of the press and this force compensated by an adequate number and size of screws that can be torqued to a total equal to the pull-back force multiplied by a safety factor of two. It is obviously useless to use stronger fastening devices to hold the mold in the press than are used to hold the mold together. Conversely, since the internal fastening devices are usually difficult to reach, it is extremely important to make sure that their combined force is well in excess of the pull-back force. This is another burden that the mold designer must bear. Poorly designed and cheap molds fall apart under the production forces. In the prevention of clamp loosening, it is necessary for someone to check all mold fastenings 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 lubricant. A wipe or spray-on and wipe-off is the best procedure.
Whenever a mold resists opening, closing, ejecting, or other operation after it has been lubricated, it must be shutdown, taken out of the machine, and disassembled without any additional exercise of force to overcome the inoperative condition.
This is a subject that must be understood and appreciated by molders and mold makers for tools operated in continuous long-term production. Molds that have delicately proportioned core or cavity members are particularly subject to fatigue. All molds, even those with the best possible design will fatigue after long periods of production. The designer and mold maker must consider this factor and study the steel data carefully before any material selection is made. Some steels are more fatigue resistant than others. The mechanisms of steel failure are being studied by certain groups of metallurgists, and it is hoped that all of the basic data may soon be available. A rule of thumb that is reported to be reasonably reliable is to obtain data on the percent elongation at the elastic limit of the steel under consideration, at the hardness that is anticipated for the desired service. When this figure is below 3%, steel is recommended. The alternative is to use a steel of lower hardness to gain increased elongation. There are obstacles and unknowns in this course of action. Very few steel companies are willing to release these specific data and prefer to evaluate their steels in relative terms only. Experience has shown that some steels having an elongation above the 3% value have poor performance records in fatigue failure.
One preventive measure is recognized; the metallurgists recommend periodic stress relieving. This can greatly extend the life of questionable mold sections that have not yet exhibited cracks. It consists of heating the parts to the same temperature or just below the temperature at which the mold sections were tempered originally. The plating must be stripped before this annealing operation since the stress relief temperature is usually above the plating stability point. Experience is the only available teacher at this time to suggest a desired time interval for this operation. Expensive mold sections merit this preventive care. Mold components with sharp or nearly sharp fillets and cores with a high ratio of length-to-cross-section will require the most frequent stress relief.
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