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Mold components being put into a fluidized bed for cleaning. Note: Fluidized bed combustion systems use a heated bed of sand-like material suspended (fluidized) within a rising column of air to burn many types of materials. The scrubbing action of the bed material on the material particles enhances the combustion process by stripping away the carbon dioxide and char layers that normally form around the material. This allows oxygen to reach the combustible material much more readily and increases the rate and efficiency of the combustion process. The heated suspended material cleans away the plastic material within the system, without damaging the metal surfaces of the hot runner. The suspended material flows through the runner channels. It is a combination of the controlled heat, the velocity of the sand material and the controlled atmosphere that makes this a very effective cleaning system.
The question is often asked: When should a tool PM be scheduled? The answer requires examining the many factors that affect the function and maintenance requirements of a tool. Some of these variables are:
- Material (fillers/additives)
- Processing techniques/parameters
- Throughput/total shots
- Steel types used to build the mold base and cavities
- Frequency of start-ups/shutdowns
- Cooling media quality/treatment
Fillers and Additives
Today’s materials contain many different types of additives to enhance the performance of the molded part. These materials include glass, minerals and many other types of fillers that are very aggressive to all components in the mold as well as the machine barrel.
Wear, due to the abrasiveness of the material, adds to the maintenance frequency a tool requires. In general, when
fillers or additives are used it is recommended that preventative maintenance frequency be increased beyond the normal recommendation.
Normal Care during Shutdown and Start-up
Some materials used require very high melt and mold temperatures. In a valve-gated hot runner system, seals in the valve pin actuator are adversely affected by higher temperatures. With all valve-gated systems, it is important that mold cooling be left on after the hot runner has been shut down. This allows removal of heat from the hot runner system and does not allow the mold plates to absorb the residual heat from the heated components.
Removal of the heat from the system protects seals and o-rings from damage. Exposing the seals and o-rings to high temperatures repeatedly will accelerate their degradation. Exposure to heat beyond the specifications for the seals will degrade them to a point that they will fail causing leakage of the air or hydraulic fluid used to actuate the valve pins.
If your facility adheres stringently to procedures and processes then this should not adversely affect the maintenance intervals. However, if there are occasions when improper care is taken when shutting down and starting up the mold then the maintenance frequency should be increased.
Cycle time, injection pressure and velocity of the plastic through the hot runner system can also have an influence on the maintenance frequency required for each mold. Much like a car, the harder and faster you push it, the greater the
frequency of maintenance.
Many tools run 24 hours a day, 7 days a week. The number of cycles on a tool can vary greatly throughout a 7-day period. With a cycle time of 6 seconds, a mold would complete 100,800 cycles throughout a 7-day period; compared to a mold running a 30-second cycle, which would complete 20,160 cycles over the same period of time. Many shops will base their tool PM on total shots on a mold versus time as a consequence.
Mold Material Selection
The choice of steel for cavities, cores, gates and hot runner components will also affect the frequency of repairs. With filled materials, gate steel and nozzle tip material selection is very important. Since it is a wear item it is always preferable to have the ability to easily change out the gates in each individual cavity.
Frequency of Start-Up
The frequency of mold start-ups is a major factor when determining mold maintenance frequency. It is during start-up that many molds experience wear and damage. Hot runner systems are much like the barrels on a molding machine in that they require a soak time once the temperature reaches the set-point. This soak time allows all material within the system to reach a temperature at which it is thoroughly melted.
Many hot runner systems are damaged when injection pressure is allowed to go to the maximum the machine is capable of—trying to push un-melted plastic through the hot runner.
Having a mold start-up procedure in place, and the discipline to follow it, can save costly downtime due to problems that can arise when trying to start-up prematurely. Of course, taking too long to start-up can also be an issue with heat-sensitive materials, so care must be taken in both situations. If your shop does many color or tool changes that require frequent stopping and starting of the machine, the preventative maintenance frequency should be increased.
Cooling Water and Molding Environment
Most tools are cooled using water. Temperature for cooling with water can range from chilled to approaching the boiling point. Treatment of water is important to prevent the buildup of scale in the cooling lines within a mold. If scale does build up in cooling lines, the effectiveness of mold cooling is decreased and this can result in the need to increase cooling time. Maintaining cooling water and checking it regularly, will prevent the need to have cooling lines within a mold cleaned more frequently.
The environment that a mold runs in will also be a factor in the frequency of tool maintenance. Moisture is an enemy when it comes to a mold. Electricity and water are not compatible.
Running a mold with chilled water in a high-humidity environment increases the probability of condensation forming on the mold. This can lead to electrical problems. The goal therefore must be to prevent moisture from coming in contact with your hot runner system and a first step is to understand the environment in which the tool is working.
If your mold is running in a moist and humid environment make sure that electrical inspection is part of the preventative maintenance program. It is ironic that the heat and water, which is so vital to facilitate the optimized performance of a tool, can also be its worst enemy if not checked and monitored.
The issues discussed in this article are some of the most common considerations for determining the maintenance schedule of a mold. Each tool will have specific requirements based upon many factors. It is best to develop a conservative tool-specific maintenance schedule that takes into account the unique needs of the mold involved and then gradually move out the frequency until a reliable but practical limit is found.