Most casting processes, such as die, permanent mold or sand casting, require a draft angle or taper to the part (die or permanent mold) or pattern (sand). The purpose of a draft angle is to allow clearance for the pattern and/or part to eject smoothly from the mold. If a mold wall is perpendicular to the part or pattern ejection, there is ample friction or interference between the mold and the pattern/part which will not allow one to release from the other. This can result in damage to either the mold or the pattern/part.
So, what happens if the completed design or current billet part has vertical walls? The designer creating the casting drawings and models will need to identify any vulnerable walls and modify them with added stock. If the stock cannot be present on the final design because of weight or size restrictions, notes need to be included to machined it off in post processing. In some cases, the addition of a core is needed create a vertical wall. All options drive up the cost.
V-Process is different from most castings.
V-Process has a unique advantage that draft angles are not required to eject the mold. In fact, V-Process castings can have zero degree draft.
This process is an advancement of traditional sand casting. A pattern is CNC machined from polyurethane and built with vacuum vents thru the pattern that allow air to travel thru it. A vacuum pump is applied thru these vents and a thin coating of Surlyn film is drawn tight to the pattern. A flask is placed on the filmed pattern and filmed with dry, unbound sand. A second coating of film is applied to the top of the flask and the vacuum pump is transferred to the sand. The sand now holds its shape like a brick of vacuum sealed coffee allowing the pattern to be removed. The cope and drag remain under vacuum throughout pouring and cooling. The is vacuum released after the casting has cooled sufficiently the sand falls away easily.
Draft is not needed because the initial step of “filming” the pattern forms a barrier between the pattern and the sand. This film allows friction loss and prevents tool wear. Since the sand is loose and not bound, the sand and part separate and there no friction.
Utilizing a vacuum instead of a binder to hold the sand shape offers unique advantages.
- True net shape with high dimensional accuracy
- Elimination of extra post processing machining
- Pattern and tooling construction are less expensive and easier to modify
- Patterns have unlimited life because the file protects it from the sand
- Remove the extra design and CAD time to include draft
- The fine sand used offers a highly cosmetic finish