plastic injection molding using insulated runner molds, an excellent option when a large number of plastic parts need to be manufactured quickly.
Advantages Over Hot Runner Systems
Properly designed insulated runners, with thermally controlled gates, offer several advantages over hot runner mold systems, including:
Dependable reproduction – no “dead spots” in mold
Uniform temperature distribution – thermal insulation forms through melt deposited at the wall
Economical process for continuous operation of uniform cycles without extended interruptions
Easy to maintain – quick to clean
Fast production color changes
Molds are less expensive to build and to maintain than hot runners
Minimal heat loss – thermal equilibrium reached quickly
Low energy input on startup – reduces energy cost
More consistent volume of polymer per part
Faster molding cycles
Elimination of runner scrap — less regrind
Improved part finish
Decreased tool wear
Insulated runner systems are inexpensive alternatives to traditional hot runners, where the mold is not heated, but the runner channels are extremely thick and stay molten during continuous cycling.
This feature provides the flexible gating advantages of hot runners without the added cost of the manifold and drops of a heated hot runner system.
Cold Runner Molding Has Advantages
However, as with hot runners, the insulated runner system also has disadvantages when compared to cold runner molding in certain situations. The increased level of technology required to manufacture and operate the insulated mold results in:
More complicated mold design
Higher mold costs
More difficult start-up procedures
Possible thermal degradation of the polymer melt
More difficult color changes than with cold runners
Insulated Runners had an oversized internal runner cut into both the top clamp plate and the “A” plate. This runner was very thick and relied on the thickness of this runner-cull to keep the plastic in a molten state as long as the molding machine was cycling. The walls of the runner were solid with only a molten center core providing melt delivery. These led to cylindrical drops (also very thick) and generally to top-center-gated parts.
This system needed fast, uninterrupted cycles to keep the gates open and even momentary interruption caused one or more gates to freeze off.
Startup was also tricky with these molds. Methods included hand injection of multiple shots into the mold before going to auto, making one big shot and going to auto, or boosting the back pressure way up and extrusion filling the runner cull.
Later the gate drops were heated with a probe which made startup easier and also made keeping the gates open easier, even allowing a brief disruption the the cycle. With very fast cycles (3 to 6 second range) the heated probe insulated runner can have a fairly small thickness and in some cases, be reground and re-used in the product.
Insulated Runners had an oversized internal runner cut into both the top clamp plate and the “A” plate. This runner was very thick and relied on the thickness of this runner-cull to keep the plastic in a molten state as long as the molding machine was cycling. The walls of the runner were solid with only a molten center core providing melt delivery. These led to cylindrical drops (also very thick) and generally to top-center-gated parts.
This system needed fast, uninterrupted cycles to keep the gates open and even momentary interruption caused one or more gates to freeze off.
Startup was also tricky with these molds. Methods included hand injection of multiple shots into the mold before going to auto, making one big shot and going to auto, or boosting the back pressure way up and extrusion filling the runner cull.
Later the gate drops were heated with a probe which made startup easier and also made keeping the gates open easier, even allowing a brief disruption the the cycle. With very fast cycles (3 to 6 second range) the heated probe insulated runner can have a fairly small thickness and in some cases, be reground and re-used in the product.
Though sometimes a bit tricky to startup and keep running, these systems could offer advantages over not only cold runners, but hot runners as well. These include:
Quick cycles
Less regrind and scrap, though the thick cull wasn’t generally used back in molding
The tool was less expensive to build and maintain
Less chance for melt leakage.
Color changes were very fast compared to hot runners, as the whole colored cull was pulled after the molding machine’s barrel was cleaned. Often color changes can be preformed in 5 minutes with less than 5 pounds of scrap
Even if heated gate drops were employed, fewer and less sophisticated controllers were needed.
Yes, the insulated runner is an old technology, but if you have a multi-cavity, fast-cycling job using a commodity resin like PP or PE with frequent color changes, and want a more economical tool that is easier to maintain, then consider insulated runner tools.
Insulated runner molds have oversized passages formed in the mold plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path.
The advantages of an insulated hot runner system include simple design and low mold cost. The disadvantages of this type of hot runner system include the following:
Unwanted freeze at the gate
Fast cycle time needed to maintain melt state
Long start-up periods needed to stabilize melt temperature
Problems in uniform mold filling
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Properly designed insulated runners, with thermally controlled gate, offer several advantages over hot runner mold systems, including:
Dependable reproduction – no “dead spots” in mold
Uniform temperature distribution – thermal insulation forms through melt deposited at the wall
Economical process for continuous operation of uniform cycles without extended interruptions
Easy to maintain – quick to clean
Fast production color changes
Molds are less expensive to build and to maintain than hot runners
Minimal heat loss – thermal equilibrium reached quickly
Low energy input on startup – reduces energy cost
More consistent volume of polymer per part
Faster molding cycles
Improved part finish
Decreased tool wear
However, as with hot runners, the insulated runner system also has disadvantages when compared to cold runner systems. The increased level of technology required to manufacture and operate the mold results in:
More complicated mold design
Higher mold costs
More difficult start-up procedures
Possible thermal degradation of the polymer melt
More difficult color changes than with cold runners
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