Definition: A piece of documentation that lists the following: place of manufacture, project number, material to be molded, melt and mold temperature, electrical requirements.
On most hot runner tools there should be a tag, metal stamped or printed name plate located on the side of the mold. This tag is showing the project number, of the unit, the material it was designed for and the temperature maximum it was developed for both in mold temperature and material temperature.
In the older days there also may have been data relating to pressure on this tag or additional tags. The important fact here is that the tag is usually developed by the hot runner supplier, and it may or may not be attached to the side of the mold by the mold shop that built the tool, dependent on the way the tool was built. That is not a critical issue, but one should be aware of where to go if in fact the data is not listed on the tool.
What does the data mean? It is a guide to where to run the drop temperatures, and the mold temperature. The hot runner system is in fact an extension of the nozzle on the machine, in that it is not melting or softening the material at this point but only transferring the melt from the machine to the cavity. The system is made up of heated sprue bushing, manifold and drops with various gate configurations.
The sprue bushing tends to center the manifold and system, and an additional pin helps to locate the horizontal plain of the manifold. Dependent on the system of manifold to drop, screwed together or floating, the tips of the gate are located into the mold base, cavity area. The heat applied to the manifold and drops while done in a sequence that allows for the manifold to heat, thermally expand and then for the drops to heat, and thermally expand applying pressure to the manifold if floating to seal the manifold to the drop. If screwed together than it still expands but the sealing part is non-critical since it is already sealed.
Since there is a thermal expansion occurring, both in the manifold and mold itself, than we must know the range of temperature that must be used for the manifold and drops. With the manifold this is important so as to position the opening to the proper seal off point of the drops. One can calculate the thermal expansion of the manifold mathematically (done in the design phase) and then one can also measure it by bench testing the manifold system on the bench.
An example is a material such as PP running at a temperature of 440° F on a drop distance of say 24" equals a growth to centerline of 0.062" when at temperature. Not much one might say, but in a compression manifold to drop system and the drop diameter is 0.250" we have effectively decrease the area of flow through the junction by 25%. This than can equate to a higher pressure drop, a possible shear point due to mismatch and turnover rate, and even the creation of a dead spot to collect and degrade material.
In the drop area, the temperature is very critical as the opening of the gate and clearance have all been calculated and by using a temperature outside of the range we can than cause thermal expansion of the pin /drop so as to seal off the gate opening. Yes this is dependent on the type of gate, design of the system etc.
An example that the author encounter was with a multiple edge gate drop, in that having arrived in late afternoon, and inspecting the operation it was noted that the machine was using maximum pressure, and extremely high heats to fill some small parts. Being that they only ran 2 shifts per day, the author asked and arranged that the start-up be at a lower temperature. Upon arriving at start-up the heats were set to 100°F lower and upon start-up and running it was noted that full parts were produced and pressure used was much lower. The issue being that he thermal expansion was closing off the gates, resulting in the use of extreme pressures to fill the parts, due to pressure drop.
The final issue is mold temperature. Since mold temperature affects the entire mold base with thermal expansion, it thus is tied together with the thermal expansion of the hot runner. Critical are areas of touch or seal between the hot runners drops (typically) and the mold we have a heat lose, (not the topic here) but this can be a great concern and is related more to thermal couple placement. Thus if the material temperature is increased above recommended (hot runner system) one may be able to compensate for some of the thermal expansion by increasing the mold temperature but than one also increase cycle time. Also if one were to run the mold temperature lower than recommend and the manifold at proper temperature, alignment problems may also exist, due to steel not growing to calculated size.
All the above should be checked out with the hot runner supplier, and at the design stage, all parties should be in attendance so as to discuss where the parts shall be run, how much pressure is available, and what has been calculated as necessary to fill and pack the parts.
SL Silvey
23042012.01
silveysplastics@hotmail.com
