The molten plastic is injected into the cavities of the mold that form the shape of the product. First variations of hot runner systems started appearing in manufacturing in the mid-20th century but did not become standard in the industry until the 1980s, driven by the need for consistent quality and productivity improvements.
A modern-day hot runner system uniformly heats injected plastic using a temperature controller. The precise over-temperature monitoring and controls help to avoid resin burning. The role of thermocouples in hot runners is to ensure a steady resin flow temperature.
In the event of nozzle failure, the plastic can back up and flood the channels, causing a complete shutdown for system purge and repair. Although hot runners can be expensive to purchase for manufacturing, they can help pay for themselves in reducing scrap and improving throughput.
Hot runner mold designs and their performance have advanced over the past decade, in many parts thanks to 3D printing and thermal-flow research. However, thermocouples continue to be a critical component of the overall process. Depending on the mold and controller design (grounded/ungrounded, explained in more detail later), thermocouples also have their predefined position to ensure an accurate reading.
Replacement thermocouples are cheap, but even the cheap parts need careful consideration to achieve maximum use. When selecting a thermocouple for your new mold or as a replacement, considering the right insulation, thermocouple type and calibration tolerance can be the difference between success and failure or at least how often you replace them. Also, remember that the hot end-design can vary based on the requirements of the specific applications and the controllers utilizing thermocouples.
Polyimide Thermocouple Wire
Hot runners most often use a polyimide taped thermocouple wire as the lead wire. Polyimide tape is a high-performance insulating tape that gets fused to the thermoelements (conductors) as insulation, and also forms the jackets of the thermocouple cable. FEP is used as the adhesive agent during the fusing process.
Polyimide provides substantial moisture and abrasion resistance and high dielectric strength. Polyimide does not burn, is very lightweight and is chemically inert. It also has excellent flexibility.
Polyimide taped thermocouples operate up to 500°F (260°C) and are unaffected by rapid temperature variations. In repeated cycling over 392°F (200°C), however, the FEP adhesive agent will tend to fail. Most commonly, the failure mode is seen as unraveling of the polyimide tape.
Polyimide has a single-use operation temperature of 600°F (315°C) primarily because of the aforementioned weak point of the FEP adhesive. Nevertheless, the temperature surrounding the leads in hot runner systems cannot exceed 482°F (250°C). Because of this, polyimide is an ideal choice for hot runner temperature-controlling lead wire and thermocouples.
Polyimide taped thermocouple cable is also used in the aerospace industry, power generation, laboratories, petrochemical plants, cryogenic applications, pharmaceutical industry and autoclaves. Depending on the application, the wire may need additional modifications to ensure suitability for the use. Available options include metal overbraid, galvanized half-oval armor, twisted/shielded pair, small-diameter HF/D overall jacket (one insulated, one bare conductor) and special color codes. Calibration test reports are available on all thermocouple cables.
PEEKSenseTM Thermocouple Wire
For application where mold temperatures are exceeding 572°F (300°C), TE Wire & Cable R&D team has been working on a new non-fiberglass design that can provide all the benefits of polyimide without the FEP adhesive failure. We refer to this product as our new PEEKSenseTM thermocouple wire. It is the next generation of thermocouple wire for superior temperature sensing. There’s no polyimide tape at all, so there’s no worrying about the insulation unwrapping while being in application.
Since there is no fiberglass used at all, operators have a better time handling the wire. One of the main complaint about fiberglass wire is its tendency to be abrasive and cause itching on the surface of your skin. There are no complaints of this sort with PEEKSense. We have seen this type of thermocouple outperform all insulated thermocouple wire in this temperature rating in terms of number of uses the user is able to get out of it.
In terms of product specifications, PEEKSense is currently offered in Types J and K and is featured in 24awg, with the likelihood of more gauge sizes to come online soon. It is known to have a high tensile strength, superior chemical resistance and strong abrasion resistance. The insulation also does not melt, does not leak under high pressures and can withstand 752°F (400°C).This product brings together the best of TE Wire & Cable’s temperature-sensing capabilities and our quality guarantee to reach greater temperatures for even more time.
MGO Mineral-Insulation Thermocouple
The replacement thermocouple is connected to a mineral-insulated (MI) thermocouple or probe through a connecting transition. These kinds of thermocouples are insulated with a single- or dual-wall sheath made out of materials varying from Inconel® to stainless steel. Inconel is a registered trademark of Huntington Alloys Corporation. Inside, the thermoelements are encased with magnesium oxide (MgO), which helps to prevent contaminates from the sheath to come into contact with the thermoelements as well as insulate the thermoelements. In more complex MgO assemblies focused on temperature accuracy, there are dual-wall MI thermocouples as well as the new patented dual-wall low-drift MI thermocouple. They can also come in two-, four- and six-conductor styles.
Grounded vs. Ungrounded
- Ungrounded thermocouples are used to isolate the thermocouple from other sources of voltage (also known as noise) that could affect your temperature reading. In this case, the junction of the thermocouple remains fully encapsulated with a filler material nearest the junction and the outer encapsulating. The downside to using a thermocouple like this is that there will be a slower response time due to the material covering the junction.
- Grounded thermocouples are used when noise is not as much of a concern or when it is known that noise will not be present while taking the temperature from the thermocouple. In this case, the thermocouple junction is placed immediately against the encapsulating sheath, and so it is closer to the exterior environment allowing for a faster response time in getting the temperature.
Some of the most common causes of thermocouple failure include:
- Thermocouple not reading correctly - thermocouples will eventually fail, that’s the reality of working with temperature. However, if thermocouples are exhibiting erratic or incorrect readings, the first area to check will be the cold end to ensure leads were terminated correctly. Second, will be the actual design of the mold controller (grounded/ungrounded) to ensure proper thermocouple design was provided. Lastly, ensure the location and installation of the thermocouple.
- Lead short out - this failure mode is more common with fiberglass leads. Due to the general nature of the injection molding and mold temperatures, the impregnation on the fiberglass over time will carbonize, allowing it to short between the leads. If the temperature surrounding the lead wires is over 392°F (200°C), we recommend a polyimide or fluoropolymer insulated leads.