The longevity of mold life is determined by mold care, cleaning and press set up.
Tool life is key to a companies bottom line and profitability. With the design and building stages complete, it is time to set the mold in the press and develop a process. This article will not only outline key settings that are paramount to mold life longevity. It will also outline what preventative maintenance steps must be taken to extend the life of the mold.
Machine Set Up
TONNAGE: It is important to verify that adequate tonnage is being used for each mold. Using too little tonnage can lead to the mold being blown open due to injection pressure exceeding tonnage requirements. Excessive tonnage can cause tool damage by excessive compression on parting lines, vent areas and mold components. Use the following formula for determining proper tonnage for each mold:
Cavity Area = Length x Width of mold face (in/2) Parts Area (Pa) = Cavity Area × # of Cavities Tonnage (T) = Parts Area × *(T) 3 tons/in2 (Note: thin-walled or high viscosity applications may require up to (T)5 tons/in2)
LOW PRESSURE CLOSE: Setting up low pressure close on the press is a crucial part of mold protection. High pressure lock up position should be set no higher than .05 above actual mold touch position. In addition, Mold should be cycled, reducing LPC pressure until mold is unable to lock up. Pressure is then raised slowly, allowing just enough pressure for the mold to transition from low pressure to high pressure lock up. In addition, the mold close timer is then set .5 secs above the actual mold close time requirement. For example, if the actual mold close time is .76 secs, then the mold close timer is set at 1.26.
CORE SET UP: Proper core set up is a crucial part of extending mold longevity. Core pressures and speeds should be limited to the requirements of their application. It is important to note that pressure can limit speed, but speed does not affect pressure set points. It is imperative that cores fire based on position, not time to prevent crash conditions. Core operation will affect the speed of your cycle, but how they operate should be monitored closely for signs components are setting or pulling erratically by both sounds and sight.
MOLD OPEN & CLOSE: Clamp speeds are definite drivers of cycle time. But it is important to note that faster speeds are not better, if they endanger or affect tool wear or damage. Close fast speeds should be monitored for signs of lunging, which risks mold drop and potential misalignment of pins and components. The transition from close fast to close slow should be smooth, and the close slow condition should be in affect just prior to component/ pin match ups. The transition between Mold breakaway and mold open fast should also be a smooth transition, the open fast segment occurring after all components have cleared pins.
EJECTION SET UP: Part ejection is a key event that requires careful set up. Improper set points can endanger mold life through over-stroking, or if parts fail to eject properly and are closed up on between the mold halves. Forward positions should be based on how much separation is needed to properly remove parts from the mold. Over-stroking causes excessive strain on ejector pins and lifters. Part extraction should never require bottoming out the ejector plate. Pressure set points should utilize only the required amount to reduce stress on ejection components.
HOT RUNNER & VALVE GATES: Start up and shut down methods are key functions that reduce or extend mold life. Poor start ups lead to over-packed cavities requiring mold tear down and service. Before running parts, valve gates should be fired manually to verify they set up properly, and in good working condition. Plastic should be purged through all drops of the hot runner to verify the drops are warm and ready for start up. Hot runner shut downs should include running the barrel dry, and an immediate reduction in hot runner temperatures to reduce the chances of degradation.
MOLD WATERING: Watering is an important part of extending mold life. Increased mold temperatures adversely affect mold life. Mold temperatures should be limited to minimum requirements to achieve acceptable part aesthetics. In addition, mold half temperatures should never require more than a 20F variance between the stationary and movable halfs. Temperatures above this scope risks the potential of tool damage due to improper heat soak conditions and poor tool mating conditions. In addition, it is important to clearly mark curcuits, as well as supply and return requirements to prevent incorrect installation of the mold.
MOLD CLEANING & CARE: Last but certainly not the least of considerations is mold cleaning and care procedures. Molds in production environments should be inspected, cleaned and greased a minimum of once per shift. Watch for signs of wear, such as galling, parting line wear, burrs and metal shavings.
Schedule a regular preventative maintenance procedure for the mold to be serviced by tool room personnel. Keep service records for the mold, and review repetitive service/ repair events to establish preventative maintenance frequencies to reduce the chance of unplanned service events.
Verify slide slots are greased, and slides function properly. Watch for signs of detent failure, or loose gibs. Follow the rule that any time you are between the mold halves for cleaning & inspection, slide positions are verified as correct as you exit the mold
Rust preventative should be used in cases where a mold will not be used for more than 6 hours. Textured and polished areas should be coated thoroughly to prevent rust damage.
By following the suggestions above, a mold’s life can be extended above and beyond the expectations of your company. Proper preparation in the initial set up stage will not only increase the longevity of mold life. It serves to protect the mold from unplanned damage events and reduce tool wear. This will reduce the frequencies needed for preventative maintenance events. Proper inspection and care will also extend the life of your mold. The difference between success or failure for a company’s tooling approach hinges upon its set up, implementation and maintenance procedures.
Garrett MacKenzie is the Owner/ Editor of Plastic411. He has been in the plastic injection industry for over 30 years, with over 15 years experience in Process Engineering and Management. He now offers training programs for engineers, processors and set up personnel.
See http://www.moldertraining.com for more information.