When a part is sent out for quoting to die casters there are some very important issues that need to be discussed and agreed upon between the purchaser and the die caster. One of these is the finish requirement that is needed for the end product that may or may not be included on the part print or separate finish specifications for the part being quoted.
The as-cast external surface finish classifications shown here illustrate variations in production practice. Surface finish requirements should be specified for production at the most economic level.
Selection of the lowest classification number, commensurate with the die cast part application, will yield the lowest cost.
|Cast Surface Finish Classifications and Final Finish or End Use|
|Class||As-Cast Finish||Final Finish or End Use|
|1||UtilityGrade||No cosmetic requirements. Surface imperfections (cold shut, rubs, surface porosity, lubricant build-up, etc.) are acceptable||Used as-cast or with protective coatings;Anodize (non-decorative)Chromate (yellow, clear)|
|2||FunctionalGrade||Surface imperfections (cold shut, rubs, surface porosity, etc.), that can be removed by spot polishing or can be covered by heavy paint, are acceptable.||Decorative Coatings:LacquersEnamelsPlating (Al)Chemical Finish Polished Finish|
|4||Slight surface imperfections that can be removed by agreed upon means are acceptable.||Structural Parts(high stress areas)Plating (Zn)Electrostatic PaintingTransparent Paints|
|5||No objectionable surface imperfections. Where surface waviness (flatness), noted by light reflection, is a reason for rejection special agreement should be reached with the die caster.||Special Decorative Parts|
|Surface finish, applicable to limited areas of the casting and dependent on alloy selected, to have a maximum value in micro inches as specified on print.||O-Ring Seats or Gasket Areas|
Generally, extra steps in die design, die construction and casting production are required for the more exacting finishes, and additional cost may be involved. General Guidelines for As Cast Surface Finish on Die Cast Parts The specification of external surface finish requirements is desirable for selected die casting applications and, in the case of some decorative parts, essential.
The purpose of the guidelines presented here is to classify the as-cast surface finish for die castings into a series of grades so that the type of as-cast finish required may be addressed and defined in advance of die design.
These guidelines should be used for general type classification only, with final surface finish quality requirements specifically agreed upon between the die caster and the customer.
|Typical As-Cast Surface Roughness Guide|
|Typical Surface Roughness (m-inches)|
|Alloy Family / Alloy||Expected in a New Die||Over the Life of a Die|
|Aluminum, ZA-12, ZA-27||63 or better||100-125|
|Magnesium||63 or better||63 should be maintainable|
|Zinc, ZA-8||32 or better||63 should be maintainable|
|Notes:1. Part design, gate location, draft, flow lines, die surface treatments and other factors can impact surface roughness.2. Roughness values for Over the Life of a Die do not include heat checking in the die.3. Die lubricants utilized for special applications may impact surface roughness and the values in the table may not be achievable.|
As-cast surface finish classification does not apply to machined surfaces. Finished machined surface requirements shall be as agreed upon between the die caster and customer and separately identified on the engineering part drawing.
As mentioned before it is imperative that surface quality criteria be defined and agreed upon during the quotation process so that the die caster will consider such requirements for costs, die design, and processing steps during production.
By Leonard Cordaro, President of Premier Engineered Products