Definition:    1) A hardened steel insert in an injection mold that contains a heated passage for material to flow from the machine nozzle seat to the parting line of the mold (2-plate), also be used in hot runner systems, 3 plate molds. 2) A system for transferring material in a mold without waste so as to eliminate a cold runner sprue.

To continue the discussion of the heated sprue bushing one of the main concerns has to be location and how it fits into our mold. Note that we are discussing a heated sprue bushing that is all inclusive, and includes its own end cap / discharge.  When talking with the manufactures of and reviewing CAD data of the design it should be understood that we have a thermal exchange going on. This thermal exchange is that the sprue bushing is hot, and the mold is cooler. When there is contact we have a heat sink between the two, meaning that the sprue bushing shall give up heat at that contact area.

What this means is that we need clearance on the sprue bushing in most cases around the body, and that that only the area closest to the tip is what locates the discharge end of the bushing. It is mentioned here that on many heated sprue bushing that we may have choices of replaceable tips, and different design for the discharge area of material  into the part and or runner.  What is important here is that manufactures recommendation for contact area and water line placement within the mold be followed. The contact area at the tip between the sprue bushing and mold has to be calculated to take into effect the thermal expansion of the heated sprue bushing.  This being both the expansion in length and diameter though diameter is minor in this aspect.  The sealing area in most cases would list a diameter that is plus but never minus in diameter dimension. This allows for a snug fit and prevents  a possible hang up spot when filling the part and or runner. This contact between the mold and heated sprue bushing in some cases is only 1mm to possibly 3mm depending on manufactures, thus minimizing the contact and thermal transmission of heat lost to the tip itself.  

In some cases the water temperature in this area has been specified to be higher than what may be used in the normal processing temperature for a material and must be checked prior to construction.  One manufacture has this issue on certain tips and it has resulted in many an issue in cycle times. This should be researched for what is being manufactured.  Also the distance from the heated sprue bushing to  the water line placement should be checked and followed. The concern here is that the plastic material has to be cooled to a temperature so that it breaks from the heated sprue bushing and the runner/part cleanly and not string. One of the major causes of stringing (and subsequence damage to tool) is that the material has not frozen enough to break cleanly. This is due to lack of thermal exchange, and can be due to mold temperature, material temperature, sprue bushing temperature, and or too large of opening in the sprue. It is fairly basic that if the material is not cool enough and we pull on the joint area, it may break on the outside but with the inside still molten or soft, it could string.

The cooling media to these lines should be on a separate circuit from the manifold or mold heater so as to be able to control it to a measured steel temperature and prevent string and or other issue that can occur.

At the head end of the heated sprue bushing the location and alignment is made on the lower end of the head to the mold and again is a minimal contact area but more so than at the discharge end. Further the top and bottom of the head should be sandwich with insulator washers so that the locating ring is isolated from the heated sprue bushing and the base of the head compresses to an insulator washer against the mold base.  This in effect isolates the heat further and gives a better control of the temperature of heated sprue bushing. Should these insulator washers not be used than the heated sprue bushing now has a greater heat sink and it could be found that a higher temperature setting need be used and that maintaining the heat, and flow of the plastics into the feed system is an issue.

A good illustration of this can be found at: page 2 of 8 ;  http://www.dme.net/wwwdme/PDF%20brochures/Integral_Heated%20lr.pdf

SL Silvey

silveysplastics@hotmail.com