Container Liner and Dummy Block: Material, Clearance, and When to Replace
The die gets the attention, but the parts that touch hot aluminium on every stroke are the liner and the dummy block, and they consume more steel than the dies do. A liner is a tube three hundred millimetres across that has to hold its bore while aluminium oxide grinds along it. A dummy block is an elastic tool that has to seal against that bore under pressure and still slide back out. Both are specified by clearance as much as by grade, and both have a replacement point that can be read from the machine rather than guessed.
What each part does, and what ends its life
| Part | Job | What ends its life |
|---|---|---|
| Container liner | The bore the billet slides through. It carries the wear from the billet skin and holds the seal at the die face | Scoring, heat checking and a bore that no longer seals. A washed-out liner shows as flash at the die face |
| Dummy block | Transmits the ram force, seals against the liner, separates from the discard and returns through the container without stripping the film | Creep: the ring grows, the clearance closes, the taper seat caulks. Sticking force can reach about 1 MN on a 200 mm block |
| Clean-out block | Scrapes the built-up oxide skin out of the liner between billets | Its own clearance grows as the bore wears, so it stops removing the skin |
| Stem | Carries the dummy block and the ram load | Buckling and fatigue at the thread and the nose |
| Die backer and bolster | Supports the die so it does not deflect under load | Compressive yield and permanent dishing |
The wear parts around an aluminium extrusion die. Working hardness for each is given on the extrusion die steel selection page; the liner and dummy block are the two that are ordered in the largest sections.
The liner accumulates oxide, and the clean-out block is the only thing removing it
In direct extrusion without lubrication the billet skin, which is oxide, is left on the container bore because the aluminium adheres to steel and the friction stops the skin sliding. The layer builds in front of the dummy pad as the stroke continues, and after roughly seventy percent of the billet has passed it starts to flow inwards across the pad face and into the extrusion. That is where back-end defects and oxide stringers come from.
The control is mechanical. A clean-out block is run through the container to scrape the skin out, and the published practice is to do it every 15 to 20 billets and at every alloy change. How much skin comes out depends on the clearance between the block and the bore, so the block has to be sized to the bore as it currently is, not as it was when the liner was new. Alongside that, three things reduce the rate the skin forms: a scalped billet, a smaller temperature difference between billet and container, and a container temperature that is not far below the billet.
The dummy block is a spring, not a plug
A fixed dummy block has four jobs, and the design only works if it does all four at once: transmit the press force at temperature, keep a secure seal with the container wall, let go of the discard at the end of the stroke, and come back through the container without stripping the film off the bore. It has to be soft enough in behaviour to seal and elastic enough to release, which is why most fixed blocks use a taper that converts axial force into radial expansion.
The published geometry for a 200 mm container makes the point. The block has to leave an aluminium film about 0.4 mm thick in the bore, and it has to return through that bore with about 1.5 mm of clearance so it does not drag the film back out. That means the block’s diameter has to change by roughly 1.1 mm between the loaded and unloaded states, and it has to do it at whatever press force the job is running, including a low one. The three-part design splits the job: a rigid body in compression, a taper, and a separate flexible outer ring. The ring is the part that wears and the part that is replaced.
The clearance numbers, and which state they describe
| Number | Value | State it applies to |
|---|---|---|
| Dummy block hot clearance | 0.254 to 0.305 mm per side; 0.508 to 0.609 mm on diameter | Running, hot, sealing against the bore |
| Clean-out block clearance | Half the dummy pad clearance | Running, scraping the oxide skin |
| Film left in the bore | About 0.4 mm | At the end of the stroke, before withdrawal |
| Withdrawal clearance | About 1.5 mm on a 200 mm container | Unloaded, returning through the liner |
Two sources describe different states, not contradictory numbers. The running clearances come from Saha; the film and withdrawal figures from the fixed dummy block design Sheppard describes for a 200 mm container.
Read the table as a set of states rather than a single tolerance. A block that is correct when hot can be tight when cold, and a block that seals well can still fail to release if the taper seat has caulked. That is why the inspection after a campaign looks for three permanent changes: the outside diameter of the ring has grown, the clearance has closed, and the seat between the taper and the block has deformed.
When to replace, and what maintenance buys
Steel loses mechanical properties progressively under this cycle, so the ring is a consumable on a cycle count, not on a failure. The published figure is a ring change after 30,000 to 40,000 cycles; beyond that, permanent deformation becomes increasingly likely. Between changes the routine is machining the block back to its original dimensions and, in some cases, nitriding it, with high-temperature grease between the taper and the block at every reassembly. Lubrication is not housekeeping: with a sticking force around 1 MN on a 200 mm block, the grease is what lets the block separate from the discard without damaging either.
Which steel, and why not harder
The liner and the dummy block are both made from chromium hot work steel, AISI H13 and its European equivalent 1.2344, and they run softer than a die. The liner is normally 42 to 47 HRC and the dummy block 42 to 48 HRC, against 46 to 50 HRC at the die bearing. The reason is the load: a liner is a thick ring under internal pressure and a dummy block is a part that has to deform elastically on every stroke, so both are toughness-limited rather than wear-limited. Pushing the hardness up to buy wear resistance takes away the ductility that keeps the liner from bursting and the ring from cracking.
Sections are the other reason these parts are a steel order rather than a die order. A liner for a 7 inch container is a bore of about 180 mm, and the wall around it is thick enough that the block is several hundred millimetres across; a dummy block for the same press is a disc of similar size. That is bar and forged block territory, not die-blank territory, and it is where the tonnage sits.
Where this leaves a die steel order
Aobo supplies 1.2344 ESR from 20 to 600 mm, annealed and turned, which covers liners, dummy blocks and backers as well as die blanks. The programme is on the 1.2344 ESR H13 extrusion supply page, the grade on the H13 tool steel page, and the hardness by component on the extrusion die steel selection page. What happens at the die face when the liner stops sealing is on the pick-up and die lines page, and the correction that follows a bad trial on the extrusion die correction page.
Before you machine a block to these numbers
Clearances are press-specific and are set by the block design, the container bore as it currently measures, and the alloy. The figures quoted here are published values for a 200 mm container and for one dummy pad design; they are not a drawing. Measure the bore, follow the block maker’s dimensions, and confirm the clearance hot, with the press at the force the job actually uses.
Sources, Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000, pages 192 to 193; Extrusion of Aluminium Alloys, T. Sheppard, Kluwer Academic Publishers, 1999, section 7.2.1.