Poor Cavity Fill/Continued Porosity

Poor cavity fill can result from a variety of process and engineering factors. In the case of vacuum-assist die casting, poor cavity fill and/or continued porosity can result when there exists a blockage or failure to achieve open flow path between the cavity and the vacuum tank. The below possible causes and solutions are representative of issues that can cause a reduction or elimination of the efficacy of the vacuum-assist method in the casting process.

It is recommended to first ascertain the level of vacuum being achieved in the die cavity for reference. This can be done by placing a rubber mat over the pour hole while the die is in the closed condition. Slit the rubber mat and place a vacuum gauge within the slit. Turn the vacuum pump on, ensure the tank is at vacuum pressure, and activate the system by opening the solenoid. Check the vacuum gauge at the pour hole to determine if an effective vacuum level has been obtained through the cavity.

Possible Cause

Solution

Plugged Vacuum Block

Metal solder and lubricants can build up in the serrated tooth pattern over time according to process variables and alloy being cast. This can reduce the effective flow air for air to evacuate through the block. The best solution is to setup a regular maintenance schedule where the blocks are cleaned with a brass brush once or more per shift.

Plugged Filter Filters on the vacuum system will require regular cleaning. The frequency depends upon the specific casting process and alloys being cast. Filters can be monitored for indication of blockage by placing a vacuum gauge before and after the filters on the system. Any differential between the gauge readings on the pump side and on the die press side may indicate a need to check the filters. Midland FL-0016 Media Filters have quick-connect ends for easy change out when one become clogged. Midland-FL-0803 Knockdown Filters have removable baffles for easy cleaning, and a ball valve to quickly drain any trapped lliquid.
Pinched or Punctured Vacuum Hose It is critical to system integrity that the vacuum hoses and connections be checked periodically for blockage, restriction, and/or punctures. These can all reduce the free flow of air from the cavity to the vacuum tank.
Excessive Die Leakage It is possible, that there exists so much air leakage into the die cavity, that the vacuum system is unable to sufficiently evacuate the air in the cavity, or the Valve-Less Blocks do not provide sufficient area to allow full air evacuation in the allotted time. Vacuum systems and Valve-Less Blocks are sized based on calculations which take into account the volume of air that must be evacuated, and the time in which that air must be evacuated to allow for cavity fill. While safety factors are built in to the calculation, changes to process and/or die press may result in air leakage that over comes the vacuum system and/or Valve-Less Blocks. In this case the process engineer or operator must assess variables such as die alignment, fill times, and whether the die is properly held closed during injection.
Misalignment of Vacuum Block(s)

photo-of-pinched-off-vacuum-serration

If blocks are not aligned properly it will restrict the effective flow area through which air from the cavity and system passes. Check the alignment of Valve-Less Vacuum Blocks by placing a piece of lead solder on the serrated part of the block and then closing the die. Next, measure the cross sections of the vertical angles of each serration using a caliper. The critical factor is to ensure that the thickness of each vertical angle corresponds to its opposite vertical angle for each serration.

If the measurements of the thickness of two opposite angles differ greatly, it indicates misalignment of the stationary and ejector side halves. Misalignment can reduce or completely choke off the air flow through the blocks. It will be critical to ensure that blocks are properly installed using the pucks on the back sides as reference edges. Instructions for correctly installing blocks can be downloaded in the .pdf links on the Valve-Less Vacuum Block Installation page.