In our last column, we discussed the five things profile extruders need to know about profile die design—proper land length, land-length ratio, drawdown, considerations for sensitive materials, and decompression (Plastics Technology, January 2010, p. 16 or online). Once you have taken all those five matters into account, you are ready to have your tool sent out for machining.
But your work doesn’t stop there. Sometimes modifications need to be made. Here are other issues to consider while the die is being built and once it’s ready to start running.
1. Drop the land length in the low-velocity areas.
2. EDM (via carbon or wire) flow channels to increase the flow to the feeder section.
3. Wire EDM the wall-section openings where flow is deficient or “starved.”
Any of the three methods is acceptable; however, I prefer the approach to be “steel safe.” Let me elaborate.
Assume that the land-length ratio was properly executed (you should validate this) and the desired overall width dimension and wall thickness were achieved. Wire EDM the appendages to half the difference between desired and actual wall thickness. Wire cutting less than the actual difference is “steel safe.”
The output (Q) is derived from the equation:
Q= (wh³ΔP) ÷ (12µL) where w is the channel width, h is the channel height, µ is the melt viscosity, and P is the pressure.
You can see that the output flow is a function of the cube of the wall height or, in layman’s terms, the wall thickness. It is better to make two wire cuts than to overshoot the intended wall thickness, which would lead to a heavy appendage.
What if the land-length ratio was not properly executed? Then drop the land length by removing steel to reach the proper ratio (Fig. 2). This can be accomplished by ball end mills, tapered end mills, or carbon electrodes.
Assume that you have achieved all critical dimensions. However, the extremities of your appendages taper in wall thickness and your flow velocities are slightly retarded at the edges. Wire EDM the leg extremities equally on each side of the wall.
In all cases, it is critical that after each die modification the surface be polished to remove the recast (residual rough surface) left from the wire burn, or mill marks from the mill, as the flow will be retarded from the rough surface. It also is assumed that the adapters and spreaders are designed to provide an equal cross-sectional mass-flow balance to the die lip, that the melt is kept under compression, and that a streamlined design approach was taken.
Wall-thickness differences should not be greater than 1.5:1 on standard profiles utilizing the land-length ratio approach. There are special approaches to solving die-design problems when the dimensional specifications exceed the 1.5:1 ratio.
Daniel Cykana has dedicated his 40+-year career to profile extrusion. He worked in the Advanced Technology Group of Bemis Mfg. Co., Sheboygan Falls, Wis., for 32 years, specializing in die and product design and processing. Contact Danielcykana@aol.com or (920) 918-3250 or extrusionprocessing.com.