The reason hot runner manifold system heaters prematurely fail has a great deal to do with the original installation procedures, the type and quality of heaters used and the temperature controller used.
In order to understand the causes and related problems associated with heater failure we should first understand the construction of a heater (see Figure 1).
(1) The sheath of a heater can be made from many types of metal tubing. The better heaters are constructed using a stainless steel sheath, which resists rust and corrosion.
(2) The core of the heater can be made from ceramic or highly-densified mag-nesium oxide. The better heaters are made from magnesium oxide because ceramic cannot be swaged.
(3) The internal thermocouple should be made from braided thermocouple wire. The most common thermocouple used in the hot runner manifold system is type J, (Fe-CuNi) Iron and Constantine. The white wire is positive (+) and the red is negative (-). If type J thermocouple wire colors are different than red and white, or if the color has been changed because of heat, the positive (+) wire is magnetic and can be detected by the use of a magnet. The magnet will have no effect on the negative (-) wire. If thermocouple splicing is required, the splice will have to be made with type (J) wire. If another type of wire is used, the temperature reading will not be correct or the controller may not work properly. The thermocouple may or may not be grounded.
(4) The heater wire used in most heaters is a Nickel-Chrome resistance wire - not much thicker than the hair on your head. This wire is tightly wound on the core material and spaced for proper resistance. Some heaters are progressively wound. This changes the watt density in selective areas of the heater, so it can produce more heat in an area where it may be required. These heater wires are connected to the heater leads by nickel terminals.
(5) The heater core is then placed inside a precut length of tubing, which has a cap welded or crimped to one end. The core is centered in the sheath using a fiber or cardboard insert. The assembly is then filled, using vibration, with Magnesium Oxide Powder. The end cap or seal is then attached.
(6) The inexpensive heaters are now ready for sale.
(7) The more expensive heaters are assembled in an oversized sheath and will go to the next step where they are swaged or reduced in diameter to the specified size. This process produces a superior heater because of the density increase of the insulation. Some are then centerless ground, and others have special coatings added all of which help to make a higher quality heater. These processes also affect the price of the heater, but the additional cost is justified with extended heater life.
(8) Magnesium Oxide is very hygroscopic. This means that the humidity, mold sweating or leaking, hydrocarbons and other forms of contamination in the molding environment will affect the heater's life.
Temperature Control Types
Temperature control has a great deal to do with the life of your heater. There are many different types of temperature control on the market today. It will be impossible to cover all of them, so this article will cover the benefits, as they would affect heater life.
(1) Soft start is used to dissipate the moisture that has gathered in the heater because of humidity and other contamination in the environment. If this contamination is not removed before the heater reaches 212xF, the moisture may turn to steam and dissipate some of the Magnesium Oxide Powder (M.G.O.) as the steam exits the heater. After all the M.G.O. is dissipated from any area of the heater, the heater coil can touch the sheath and short out. This will cause the heater to go to ground and the sheath to arc-weld itself to the inside of the heater hole.
(2) A voltage-proportioning, phase angle-fired type of controller eliminates the thermo shock created by an on-off type controller. The voltage proportioning type of controller provides only the amount of voltage required to maintain set point. An on-and-off type of controller provides full-rated voltage to the heater until it reaches set point and then no voltage until the heater calls for heat. During this on/off cycle, temperature swings can be very great - causing product quality problems, which also could cause premature heater failure because of expansion and contraction of the thread-like heater element.
(3) The ground fault detecting system detects excessive moisture or other resistance to ground and prevents the control from applying high voltage until the heater is dry or the resistance to ground has been eliminated.
It is very important that moisture be dissipated from the heater before full-rated voltage is applied. If a soft start-type of control system is not available, you can dry your heaters by using manual mode at 10 to 15 percent for about 20 minutes. This should add considerable life to your heaters. When it comes time to replace your existing control system, compare and purchase the best you can afford. It will save you many hours of downtime due to heater failure.
Heater removal can be very difficult, if the heater has arc welded itself to the inside of the heater hole. You will continue to experience heater failure if the heater is not removed with care. When a heater is removed by drilling with a size-for-size drill, the drill will cause scallops in the heater hole, which will cause hot spots in the heater that will allow the heater to overheat and fail. The only good way to repair this damage is to redo the heater hole by making it oversized or opening it up to the next size heater. Sometimes this is impossible. Make sure heaters are available before you open up any heater holes. Also remember that if the heater manufacturer supplies you with oversized heaters, buy some spares. This special heater will not be available for quick delivery if you have a failure.
(1) Set up the manifold so the heater hole is parallel, square and centered with a mill or drill press. Drill the core out of the heater using a cement drill that is smaller than the I.D. of the sheath. This should prevent damage to the heater hole when the core is completely removed. Use a drill that is 1/64" smaller than the heater. Dull sides of the drill, so it will only cut on the end. Making sure proper alignment is held, drill out inside of the sheath. Use water to make sure that the drill is kept cool or it may catch and break or cause other damage.
If the inside of the sheath is removed and the heater body is still stuck in the heater hole, make a tool from an old ejector pin that is shaped like the end of an ink pen. Grind away all but one side of an ejector pin until it is sharp but substantial. Place the sharp, chisel-like end between the heater sheath and the heater hole. Tap on the end of the ejector pin forcing it between the sheath and the heater hole. This will break the weld and allow you to remove the heater without doing any serious damage to the heater hole. Before replacing the heater, ream the hole with the proper size reamer to make sure it is clean. Apply a heat transfer media to the heater and slide it into the heater hole. Do not force. This heater will be wet from being on the shelf and should be properly dried before it is subjected to full-rated voltage.
(2) Many manifolds manufactured today use a tubular-type heater. Some of these heaters are removable and others are cast in and are not removable. The non-removable tubular heaters will have to be serviced by the manifold manufacturer. The removable type of tubular heaters are usually placed in a machined channel and held in by cover plates or some type of potting/heat transfer media. After the cover plates are removed, the heater should be easily removed. Heaters that have a heat transfer/potting media are removed by forming a piece of brass in the shape of a round chisel and driving it between the heater channel and the heater. This will lift the heater from the heater channel.
(3) When reinstalling the heater that is held in by cover plates, place the heater in the heater channel and replace the heater retainer plates. The problem with this type of heater installation is that there is a limited amount of contact with the heater channel unless some type of heat transfer media is used.
(4) When reinstalling heaters that have a heat transfer/potting media, remember these heaters are available in a pre-bent or straight form. If you would like a pre-bent heater, order it when the system is ordered or delivery may be a problem. If you choose to use a straight-length heater you can save some time and money. The straight-length heaters are annealed and reasonably easy to bend. Start in the middle of the heater channel and work to either end. This way the ends of the heaters will be the same length when you are done. You will need a piece of hard wood, a piece of brass, a hammer, a soft-faced hammer, gloves and a helper. Gently tap the heater into the heater channel, making sure that the heater sheath is not cut or broken. When the heater is in the heater channel, the heater should be compressed making it a (D) shape. This can be done using a hydraulic press or with a hammer and a one-inch wide piece of brass. This will give the heater full contact with the sidewalls of the heater channel. By re-potting the heater with the manufacturer-recommended potting compound, you should have a good heat sink for your heater - providing even heating and long heater life.
(5) Before removing the tubular heater, make sure it needs to be replaced. If the terminal is broken off because of a leak or other damage, the heater can usually be repaired. The termination pin on most tubular heaters is a solid pin, one to one-and-a-half inches long. The sheath can be cut away using a hacksaw to expose a new lead to which you can connect your power lead.
(6) CAUTION: Do not remove more than 0.750 of an inch from the heater sheath and remove only what is required to make your connection. Also use caution in making the termination, so as not to create a short between the sheath and the heater element.
Preventive maintenance is the key to any successful hot runner application. When a hot runner system is part of your plans, contact your hot runner manufacturer. They may be able to show you designs that will aid in the maintenance of the hot runner system. They also may be able to assist you in training plant personnel, which could save a lot of headaches and related downtime.