Over the years, heating systems for hot runner systems have changed as much as hot runners themselves have. The word "hot runner" itself explains the process and keeping the runner hot is a simple idea.
Consider the hot runner as a body - the heating elements are the heart, the controller is the brain, and the thermocouples are the nerves that connect the entire system together. And, like a body, if one of these elements fails - no matter how much a company has spent - then the system will no longer work.
When selecting replacement parts for your heating system, cost should not be as critical as most companies make it. The cost of heating elements between a good manufacturer and a bad one is negotiable compared to the total investment. The production time and quality of the parts gained by selecting a respectable manufacturer will more than make up the difference. Keeping in mind the following tips when selecting a manufacturer will ensure less downtime due to a faulty product.
Manifold Heater, Cartridge Heater
Cartridge heaters are utilized around the flow channel to ensure uniform temperature. It is important to keep the distance between the heaters and the manifold equal or greater than 1x the diameter of the heating.
Thermocouple placement should be located equally distanced between the heating element and the flow channel and should be at least 1.5" deep to ensure an accurate reading.
If an internal thermocouple is utilized, it is important to ensure that it is located towards the center of the heating element (at least 2" away from the lead end) depending on whether the controller is grounded or ungrounded.
Some of the most common causes of failure include:
- Lead short out. This can be corrected by changing the lead type. If fiberglass leads were utilized, this could be the cause. Hot runners by nature create gases, which over time saturate the fiberglass material, allowing it to short between the leads. Depending on the ambient temperature around the lead area, Teflon leads can be utilized to correct this, as it is more resistant to gases. However, the temperature surrounding the leads cannot exceed 250'C.
- Internal thermocouple not reading correctly. This can be caused by two different reasons. One reason is the thermocouple must be located in the center of the heating element. If not, you will never obtain a correct temperature of the flow channel. The other reason is whether or not the unit is grounded or ungrounded. Consult your controller manufacturer to determine this.
- A performance issue. In a standard heater the resistance wire is evenly wound. To enhance performance, a distributed wattage heater is recommended. This is where the resistance wire is stacked at each end to compensate for the loss of heat due to various reasons. This allows for a more even heat curve.
Tubular Heating Elements
Tubular heating elements are inserted into a milled slot into the manifold. This allows for a more accurate location of heat at the areas that require the most (i.e., nozzle exits).
Tubular heating elements are for the most part the heater of choice. They are reliable, relatively inexpensive and there is no extra cost for gun drilling the manifold. But more importantly, they perform the job well.
Tubular heaters do have two drawbacks. One is availability. It can take from six weeks standard delivery to as little as a week (if the manufacturer is running that diameter that week) to get a new part. Unlike cartridge heaters, tubular heaters have longer delivery times because of the machine setup time.
The other drawback is the design. If the manufacturer does not have a template of your system, it is extremely difficult to match some of the more complex layouts. For this reason, more companies are changing to highly flexible tubular heaters. These can be easily inserted into a manifold by anyone, resulting in shorter down time. This type of heater is capable up to 95 watts per square inch and is easily bent on site in minutes. A stainless steel plate or insulation plate is recommended to hold the heaters in place, and a dovetail design can replace this plate if a space is not available.
The thermocouple location should be maintained as explained above. If a problem arises with standard transfer heaters, it may be that the terminal area is not manufactured to bendable environment. Also, the slot may be too large or the diameter tolerance of the heater may be too wide, giving an uneven notch and an uneven temperature.
The torpedo system is one of the first hot runner heated nozzles introduced to the moldmaking industry. The concept is simple - a cartridge heater is inserted into a gun-drilled hole running through the center of several flow channels. When replacing a torpedo-style cartridge heater, several things should be remembered.
- Does the hole have a flat bottom? This is important for the thermocouple to sense correctly, as air is an excellent insulator. With standard construction cartridge heaters, the disc end is concave due to the manufacturing process. To ensure an accurate measurement, a gun-drilled hole with a flat bottom and a flat bottom cartridge heater should be used to achieve optimum contact.
- What is the diameter of the hole of the cartridge heater being inserted? It is important that close tolerances be maintained in this area. With the high watt density required within this type of heater, a centerless ground heater is highly recommended. Standard tolerances by most manufacturers are q 0.002". With a centerless ground heater, a q 0.0008" tolerance is achieved. This greatly increases the life of the unit due to more contact within the body of the nozzle, allowing a better transfer of heat from the cartridge heater to the nozzle body.
- Where is the thermocouple located? The thermocouple must be located at the disc end to ensure proper temperature measurements.
- What are the requirements for the internal thermocouple junction? As today's manufacturers of controllers have different requirements, consult your controller manufacturer for these specs if you do not already have them.
External Heating (Coil Heater)
Coil heaters have been introduced to the hot runner system - greatly increasing the cycle speed and the quality of the product produced. Due to an even heat around the nozzle body, the material is not subject to excessive temperature changes, resulting in less degradation of material. When replacing a coil heater, consider these points:
- The profile of the heating element. A flat or square cross section is far superior to a round profile. This is because of contact - greater contact provides for easier nozzle control and faster recovery time. With a round profile-heating element, the only contact is at the zenith of the arch. But with a flat profile, the contact is across the entire surface of the heating element. A special manufacturing process is required to obtain this contact with the nozzle.
- The correct pitch of the coil heater.> To achieve an even pitch across the nozzle, the coil heater needs to be wound tight at each end and spaced in the middle. This allows the heat to re-disperse over the nozzle, allowing for custom profiling and ensuring even temperatures across the flow channel.
- Internal thermocouple location. The internal thermocouple should be located as close to the tip as possible.
- The thermocouple junction. The unit must be spec'ed out to match the controller being utilized.
- The coil I.D. The coil I.D. should be smaller than the nozzle O.D. in order to achieve a good contact. For front load systems, a pressed-on or pushed-on sheath design is recommended if a clamping strap is too large to install.