the draft angle will need to increase. To illustrate this idea, a feature that is 50 mm high may require 1 degree of draft to properly remove it from the mold, but a feature that is 10 mm high probably will require 4 to 5 degrees. Insuffi cient draft angles are one of the causes for a casting defect referred to as a sticker. A sticker is defi ned as excess metal on the surface of a casting caused by a portion of the mold face remaining on the pattern. Having a casting that is prone to forming stickers will increase cost due to addi- tional scrap, grinding and/or rework. Finish stock is added to those
surfaces of a casting that will require subsequent machining. T e task here is to design the casting with the amount of fi nish stock that will allow the casting to be machined successfully without adding too much material for removal. Finish stock allowances will vary with the type of material being poured, the size of the casting, and the molding process used to produce the casting. Sometimes design- ers will attempt to minimize fi nish stock allowances on the casting to allow the machine shop to maximize speeds and feeds and thereby speed up the machin- ing process. While this thought process is valid, the designer needs to work closely with the metalcaster’s engineer- ing team so the stock amount does not get too low; the minimum amount of fi nish stock must be within the limits of normal process variation to avoid additional problems such as surface inclusions in the machined surface and areas of non-cleanup.
2
Parting lines Did you realize a simple
change to the parting plane
could reduce the cost of your casting? T e determination of where the parting line will be on a casting design seems innocuous, but the parting line loca- tion can eff ect the cost of the pattern equipment, the number of cores used in the design, the ability to easily remove the pattern from the mold, grinding and fi nishing costs, and potential scrap rates. Making an eff ort in the design stage to keep the parting plane fl at is the best practice to adopt. While off set part- ing lines are common in metalcasting, they tend to drive cost up because the pattern equipment will be more diffi cult to produce, thus, more expensive. Some
off set parting lines can only be achieved through the use of an extra core. T is situation should be designed out if at all possible to reduce overall costs. Off set parting lines are much more
likely to produce fi ns that will need to be ground off during the fi nishing process. If the off set is deep, there could potentially be problems drawing the pattern out of the mold and perhaps even sand compaction issues because of the diffi culty in getting even squeeze pressures during molding. Reduced scrap can be achieved if the
design allows the casting to be made in one half of the mold. T is is possible be- cause this will eliminate a defect referred to as shift. Shift is defi ned as inadvertent movement of one half of the mold in re- lation to the other half. Shift appears as a step in the casting at the parting line and will cause additional grinding to blend the surfaces in the best case, and scraping the casting in the worst case.
3 Tolerances Does the design contain dimensional tolerances that are
out of the standard tolerance range for the metalcasting operation? Cast toler- ances are highly dependant on the type of molding process used to produce the part, but other factors such as casting size and pattern construction also play signifi cant roles. T e design engineer- ing team should review readily available industry standards for dimensional tolerances with the metalcasting engi- neer to determine what can be achieved and then apply the correct tolerances as required to help the facility produce a conforming casting. In regards to pattern construction,
there is a wide spectrum of materials available that can be used to make a pattern, including soft and hard wood, a large variety of cast or machinable plastics, and metals such as aluminum, iron and steel. However, the design team needs take the pattern material into consideration early in the development cycle. For example, a wooden pattern will not be as capable of maintaining tight tolerances like a machined metal pattern can. However, metal pattern equipment will be more costly to produce than wood or plastic patterns. T e decision must be made to determine the best balance of tool cost vs. dimensional capability.
4
Surface Finish Requirements
Are there cosmetic require-
ments for your casting? Have the surface fi nish requirements of as-cast surfaces been clearly communicated and un- derstood between the end user and the metalcaster? T is often is an area that lacks proper defi nition and can translate into higher casting costs if incorrect assumptions are made. Surface fi nishes related to texture are directly related to the molding and fi nishing processes used, and they should be discussed dur- ing collaboration with the supplier at the quoting stage. T ere are a number of surface fi nish comparator plates available that can be used to specify the desired surface fi nish of the casting. Cosmetic surface fi nish requirements,
such as the amount and type of visual defects allowable on the casting should also be well understood. If the metal casting engineering team is made aware of tighter defect requirements, they can engineer the process to improve the surface fi nish. Some steps that the met- alcaster can take are placing ingates and risers in the correct location in relation to the critical casting surfaces, or the use of alternate metal fi ltering methods. Requiring a casting to have high
levels of visually defect free surfaces will increase the scrap rate and can easily add 15% or more to the cost of a casting.
5
Section Thickness Castings tend to be less prone
to a variety of problems when
casting wall sections are held to a consis- tent thickness. If the design has a wide variation in section sizes, this could po- tentially cause a higher scrap rate, which could translate to increased casting cost. T in casting wall sections can cause
a problem with incomplete metal fl ow through the section, resulting in a casting defect referred to as misrun. T in sections also have the possibility to crack or break during solidifi cation, shakeout and handling. Transitions between thick and thin
sections must be correctly designed to avoid problems with the casting. One basic engineering concept is that stress will concentrate in areas where there is a drastic change in section size. To limit the stress in a casting, these transition areas should be designed to incorporate a
Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 27
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