Control of Plunger Clearance

By Edmund Herman, President
Creative Concepts Co. Inc.
Utica,  Michigan

Introduction
This article looks at how plunger lubrication could be a much simpler situation if the plulnger tip-to-shot sleeve clearance were to be controlled, and the article also explores the mechanisms that cause the plunger tip-to-shot sleeve clearance to change and the existing control technologies that must be implemented to control it.

Clearance Needs
Molten metals under die casting pressures will show a mark at a clearance of 0.003 in. (0.076 mm), start to enter a clearance of 0.005 in (0.127 mm) and flash into larger clearances.  The greater the clearance and the hotter the components, the deeper the flash will penetrate.  At atmospheric pressure, molten metals will not generally enter a 0.010 in. (0.254 mm) clearance when the die members are as hot as 800 F (427 C) and will not enter slightly larger clearances when the die members are colder.
     The clearance between the plunger tip and the shot sleeve should be between 0.002 in. and 0.005 in. (0.051 mm and 0.127 mm).

Temperature Variation - The Detail Devil
The root cause of the clearance variation is temperature variation, and specifically temperature differentials between the plunger tip, shot sleeve and die and temperature differentials throughout the shot sleeve.  Metals expand when they get hotter, so as temperature changes, the metal component changes size.  When a single component, such as the shot sleeve, is at different temperatures in different places, it changes shape as well as size.  Some regions are expanding more than other regions simply because they are at different temperatures.  When adjacent regions of the same component change size in different proportions, the component changes shape.

Example Calculation
Assume that a brand new 2-in. (50 mm) diameter steel plunger tip is at 60 F (15.5 C) (due to the interanl water cooling) when the first shot is made.  Also assume that the brand new shot sleeve is at 80 F (26.7) when the first shot is made. Then, assume that after several hours of steady running, the plunger tip stabilizes at 400 F (204 C) and the shot sleeve stabilizes at 550 F (288 C).  To simplify the problem, it will also be assumed (unrealistically) that the die also stabilizes at 550 F, and it is properly fitted to the shot sleeve so there is no mechanical constraint imposed by the die.  Since the coefficient of themal expansion for steel is 0.00000633 in./in. - F, the change in clearance will be:

Initial temperature difference = 80 - 60 = 20 F
Final temperature difference = 550 - 400 = 150 F
Relative Change = 130 F

The Relative Change in temperature will change the clearance by:

Clearance Change = 130 F x 2 in. x 0.00000633 = 0.00165

In this example, the clearance will get larger than what it was originally since the shot sleeve experienced a greater change in temeprature than did the plunger tip.  However, the change is probably not significant.  If the plunger tip increased 130 F more than the sleeve, the clearance would have decreased.  Still, if the starting clearance is between 0.003 in. and 0.005 in., the change is probalby not significant. 
     The industry was created and grew with shot sleeves in the 1-in. to 3-in. diameter range, so the above situatoin was about the norm.  There was little need for sophisticated control. 
     Today, many aluminum die casting machines are operationg with a 6-in. (152mm).  If the clearance of a properly fitted plunger tip increases 0.005 in., it will seize, and if the clearance decreases 0.005 in., it will flash and then seize on the flash. 
     Generally, the plunger tip and shot sleeve both get hotter as the machine runs from a cold start to a condition of relative temperature stability.  This means that, at least, they both expand.  The situation would be much worse if on occasion one got colder and the other hotter.
     The issue can no longer be ignored.

It Gets Worse
A complication is that the outside diameter of the shot sleeve is constrained at the end near the die parting surface.  If the die gets hotter than the sleeve, a space can open, allowing flash between the cover die and shot sleeve.  More commonly, that end of the shot sleeve gets considerably hotter than the cover die.  Three reasons exist for that.  First, the high velocity flow of molten metal across the end of the shot sleeve dumps a lot of heat into that part of the shot sleeve.  Secondly, that end of the shot sleeve often has a thinner wall than the rest of the shot sleeve, so the heat that enters the sleeve material tends to drive up the temperature more than if there was more mass, and the escape route for the heat is restricted.  Thirdly, the die around the sleeve not only has more mass, but is usually water cooled.
     The result is that the end of the sleeve at the die's parting surface cannot expand freely as it increases in temperature.  It expands, but not freely.  The only way it can expand is to elastically (hopefully elastically because plastic deformation will result in very short shot sleeve life) deform inward, actually reducing the inside diameter.  It behaves as though it is colder when it is actually hotter.  The rest of the shot sleeve, which is most of it, will expand freely.  The result is that the inside diameter of the shot sleeve is not the same throughout the length of the shot sleeve.  The condition is illustrated in Figure 1.  Some shot sleeves are actually made with a taper on the inside diameter at the die parting surface end to help compensate for this phenomenon.

Figure 1 - The pattern of interal diameter change of a typical large - diameter shot sleeve.

     A practical operating strategy is to run enough cooling water through the plunger tip so it remains small enough to not seize in the die parting line end of the shot sleeve, and then let the molten metal set in the shot sleeve for a few seconds so that some of it will solidify at atmospheric pressure to stop the flashing around the plunger tip as it travels through the regions of the shot sleeve where the clearance is large.  Needless to say, such oerating practice destroys any semblance of metal condition control that could possibly help make the casting solidify correctly.  The metal in the sleeve is partially solidified, and portions are at all different temperatures, at unknown temperatures and not always at the same temperatures from shot to shot, as illustrated in Figure 2.

                         

Figure 2 - Metal temperature pattern in shot sleeve before metal is pushed into die.

     As shown in Figure 2, the hot molten metal sets on the bottom of the shot sleeve for some time before being injected into the die.  That results in the heat that is lost from the molten metal to the sleeve entering the bottom part of the sleeve.  The result is a non-uniform heating o fthe sleeve and the resulting non-uniform expansion of the sleeve.  The characteristic distortion of the shape of the sleeve is shown in Figure 3.

Figure 3 - The typical distorted shape of the shot sleeve.

     Typical practice mitigates these distortions to a manageable degree by running the machines slowly, keeping the dies and shot sleeves cold and allowing some solidifcation in the shot sleeve to form a seal.  All these practices are detrimental to the control of casting solidification since they take from the operator the critical solidification control mechanism of temperature.  (These same operating practices minimize similar problems within the dies due to their design and {lack of} control).

What Can Be Done?

Fortunately, the necessary corrective action does not need some new technological breakthrough.  The problem stems from temperature-driven size and shape changes.  Temperature control is "old hat" stuff.

     HEATED SHOT SLEEVE: The first step is to implement heated shot sleeves.  This author suggests electric band heaters controlled through thermocouple feedback, similar to what is routinely done on the extruder barrel of injection molding machines.  Although similar, the system needed on die casting machines is not as complex as used on injection molding machines.  The sleeve should be maintained at 650 F (343 C) and preheated to that temperature before any metal is ladled into it.  This practice will extend shot sleeve life, reduce the temperature variation of the ladled metal, reduce the heat lost from the molten metal to the sleeve. Heated Shot Sleeves.

     When the hot and temperature-controlled shot sleeve is used, the plunger tip lubricant might need to be re-formulated to work with the higher temperature sleeve.

     THERMALLY CONDUCTIVE SLEEVES: Another thing that helps is to use thermally conductive sleeves (Thermal Control Shot Sleeves).  Thick walls help, but copper-clad sleeves are also available (i.e. Pegasus Industries, Inc.).  The copper is highly thermally conductive and helps maintain a uniform shot sleeve temperature.

     NO SHOT DELAY: The shot should be made as soon as the metal is ladled into the sleeve.  The air trapped by the sloshing of the molten metal in the sleeve is a minor issue compared with the damage done to the metal temperature as it sits in the shot sleeve.  However, this step cannot be taken until the clearance variation is eliminated since without clearance variation control, a solidified skin is needed to seal the plunger tip when the clearance becomes large.

    PROPER SIZING TO DIE: The next issue is the proper sizing of the shot sleeve outside diameter to the accommodating hole in the cover die.  It was shown above that this is hopeless for large diameter shot sleeves when the temperature is uncontrolled.  When fitted to avoid flashing between the shot sleeve diameter and the cover die, compression of the inside sleeve diameter is unavoidable.

     The situation changes if the sleeve is preheated to 650 F before the first shot is made and then maintained at that temperature.  For example, suppose the outside diameter of the sleeve where it fits to the cover die is nominally 8 in. when machined at room temperature.  At 650 F, it will be:

8 in. x (650-80) x 0.00000633 in./in.- F = 8.029 in. dia.

     Then, when the die comes up to its operating temperature of 450 F, the hole will expand:

8 in. x (450-80) x 0.00000633 in./in. - F = 8.019 in. dia.

     The accommodating hole in the die can be made 8.024 in. diameter at room temperature.  There will be 0.005 in. of compression fit when the die is still at room temperature and the sleeve has reached its controlled temperature.  If the die is preheated at the same time, the interference fit will not materialize.  But, if the interference fit does happen, it will not create sufficient deformation of the inside diameter to seize plunger tip as there will be some elastic expansion of the die.  Then, as the die reaches its operating temperature, there will be a 0.005-in. clearance to the sleeve which will not accept flashing.

     CONTROLLING PLUNGER TIP CLEARANCE: The final step is to stabilize the plunger tip temperature.  Control systems are now on the market that can be made to do this without installing sensors (i.e. thermocouples) into the plunger tip.  One such system is available from DieTherm Engineering LLC.  That system computes the energy being removed by the cooling water by measuring the water flow rate and the inflow and outflow temperatures, computing the heat removal rate, comparing the heat removal rate to the required heat removal rate and adjusting the water flow rate as required.  Although this author knows of no application to the plunger tip, the system has been highly successful at controling casting solidification through control of die cooling lines.  It is technically feasible to apply the system to the plunger tip.  Application of such a system would require that it be interfaced with the shot monitoring system such that the length  of each biscuit is measured, and the heat energy to be removed calculated for each shot and the control system then made to the adjust the water flow rate accordingly.  Shot system monitroing systems are installed on most die casting machines, and several good systems are on the market.  It is only a matter of interfacing the two systems and some computer programming.  It is not rocket science, and it is not a matter of "discovering" some unknown characteristic of the die casting process.  It is here now.

So What!

Working around the plunger tip problems is a huge cost to operating a die casting facility.  These costs are in down time, replacement parts (e.g. plunger tips and shot sleeves) and poor casting quality.  Casting quality suffers because the die caster is working around the plunger problems instead of being able to adjust the process to optimize the solidification of the casting.  Controlling plunger tip clearance is less complicated and less costly than working with the problems resulting from not doing it.

About The Author

Mr. Edmund Herman is the president of Ceative Concepts Co. Inc..  Edmund has over 38 years of experience in the die casting industry.  Edmund has been involved with NADCA/SDCE for almost as many years and has authored numerous papers.   Edmund was recognized by the North American Die Casting Association with the Herman H. Doehler Award for his work for the advancement of the die casting industry.  Edmund is a certified NADCA instructor and teaches several courses including ones he has developed. 

Contact: edmundaherman@comcast.net