User:Metal5000/sandbox

= Lost PLA Casting =

Lost PLA (Poly Lactic Acid) Casting is process for creating cast metal parts by using an expendable 3d printed pattern to produce a mould that is then burned out in a kiln. Molten metal is then poured into the mould cavity and allowed to solidify. The process is derived from lost wax casting. The main difference from Lost PLA Casting to lost wax casting is that the wax pattern is typically produced from an existing mould, whereas the lost PLA pattern is created on a 3d printer. This eliminates some of the steps needed in the lost wax process and can represent significant time and cost savings in low volume or prototype castings.

Process

1.      A digital 3d object is created by means of CAD or other 3d modelling software, or from 3d scan data.

2.      This digital 3d file is then processed to ensure that the polygonal mesh meets certain criteria necessary for the slicing software in the next step.

3.      Software typically referred to as a ‘slicer’ or ‘slicing engine' is employed to convert the 3d polygonal model into a file format that the 3d printer can use to produce the physical pattern. G-code is a common file format that has many different variations depending on the software and 3d printer hardware used. Typically the pattern is printed hollow or with a sparse infill to enable a clean burnout in step 9.

4.      Once the G-code file has been created, the PLA pattern can be printed and removed from the printer.

5.      Support material that may have been used in the printing process is removed.

6.      Further preparation of the pattern may include sanding or chemical processing to remove visible printing artefacts.

7.      A metal delivery system is attached to the pattern to form channels in the mould to facilitate molten metal to fill the mould cavity. This can be made of suitable wax or can be part of the 3d printed pattern. Metal casting foundries typically utilise a ‘sprue tree’ that allows many castings to be produced from the same metal pour.

8.      The pattern and metal delivery system is then encased in a refractory mould material such as investment plaster or ceramic shell, which becomes to mould.

9.      Once the refractory mould material has had sufficient time to cure and dry, it is placed in a kiln to burn out the PLA and any wax that formed the metal delivery system.

10.  After the majority of the PLA and wax has been burned away, the mould is then fired at a higher temperature in order to vitrify the mould material, drive off chemically bonded water and to vaporise the remainder of the PLA pattern and metal delivery system.

11.  The mould can be left to cool naturally to enable cleaning of debris or ash from the wax and PLA by means of compressed air or rinsing with water.

12.  The mould can be re-heated to the required temperature in the kiln, this can improve the casting by keeping the molten metal fluid for longer as it moves through the mould.

13.  The mould is now ready to be filled with molten metal. Traditional foundries utilise the gravity pouring method where the molten metal is decanted into a pouring basin which is attached to a sprue, the main channel that forces to metal to flow into the mould cavity via runners and in-gates. The casting is then quenched in oil or water, or allowed to cool normally. Internal stresses can be purposely caused by rapid cooling, making further heat treatment possible. In the case of investment plaster, quenching in water can remove most of the mould material