The choice is cores: when it's time to pick a coremaking method, the weight, dimensions and production rate of the casting will help steer you in the right direction.

Metalcasters often preach to their customers that they must look at the total cost of a part, from casting through machining and assembly, in order to see the full economic benefit of a casting. The same sermon could be delivered to metalcasters when they're choosing a core system.

Wayne Rossbacher, owner of Foundry Consulting Inc., Sugar Grove, Ill., and an expert in core manufacturing, tells a story of a metalcasting facility that was trying out a new type of core. After a few months, the coreroom manager spoke out during a preliminary meeting about a new job at the facility. He didn't want to use the new core sand any longer because it cost him more in the coreroom for materials and operation. But the cleaning room manager argued that the new cores were saving him money in finishing--more money than was being eaten up in the coreroom. Both managers were concerned with their own department budgets rather than the total resulting cost of the casting to the facility.

"When deciding what type of core to use, metalcasters must look at the entire cost picture and not just at material cost and speed of production," Rossbacher said. "Internal budgets should be flexible enough to allow those changes for the better overall price."

Choosing the most economical coremaking process for a given part is a simple way to cut costs at a metalcasting facility, although the choices of core types are complex. The first step in considering a core is to gain full knowledge of what your current core processes are costing you.

"The biggest fault of metalcasting facilities is that they don't know the cost of things like maintenance, cleaning and scrap rates," Rossbacher said. "For instance, the sand for shell cores is more expensive than other sand cores right now, but most shell cores are hollow, so it weighs a third of what other sand cores weigh."

Employ a Variety

Generally speaking, it's a good rule of thumb to use two to three coremaking processes, particularly if you are a jobbing shop. "Not one process is going to solve all your problems," Rossbacher said. "When you get a job, the size, shape, etc., of the part will warrant what type of core is best and most economical. No single process fits all the criteria."

Choosing a coremaking system depends on several factors, but the three main considerations are the weight, dimensions and production rate. As with any operation in your metalcasting system, the ideal coremaking process will save money and improve the quality of the casting. In the case of coremaking, a dimensionally accurate core will translate into a dimensionally accurate casting. This has sparked a focus on self-hardening sand systems.

The two most-used core types are shell and coldbox. Shipments of castings made with shell cores accounted for 19.5% of total shipments, while castings with coldbox cores accounted for 48.9% of shipments in 2005 (Table 1). Both are suitable for fast production and produce dimensionally accurate cores. Both core types could be used in many applications with little difference in quality or cost, but certain applications may favor one process over the other. Hydraulic valve systems, where higher hot strength, critical dimension and better surface finish are required, predominantly use shell cores. But, when the job is larger than a 2-3-in. gate valve, coldbox cores step in.

Shell cores have a superior casting finish, low weight and, with hollow cores, superior breakdown after casting, but their size is limited, curing time is longer and corebox cost is high. Most coldbox cores need a corewash to alleviate problems of erosion, but they can be made in a variety of corebox materials and a larger size range.

"Choosing a coremaking process lies in the kind of work you are doing or want to do," Rossbacher said.

Most cores used today are made with chemical binders. The main coremaking processes, shell, hotbox and coldbox, can be used with a variety of binder systems, although not all binders and coremaking methods make a good match.

In the shell core process, sands are mixed with a phenolic resin and heat-cured to produce the cores. In hotbox coremaking, furfuryl resin binders are used. Coldbox techniques are based on chemicals or air-drying methods. Each manufacturing method has its pros and cons (see "Don't shy away from cores."). When you're researching your core types and core manufacturing methods, keep in mind these key questions:

* What is the dimensional precision required?

* What's the surface finish required?

* What is the cost of the patterns and material?

* What is the volume of the job?

* What is the overall cost for floorspace, equipment and energy?

Aiming for the highest core properties is not always the goal. If you're hollowing out a hole in a place where no one will see the surface, core quality becomes secondary, and you can use a less expensive core with lesser qualities. Customers, however, may still ask for the better surface finish in cases where it's not necessarily needed. This is where understanding the application of the part and what the customer is really looking for may allow you to produce a better product for less.

Consider All Options

Although the most widely-used, shell and coldbox cores are not the only options. Cores can be made using a number of different binder systems and coremaking methods. A third core type that fills in where shell and coldbox can't is nobake. Nobake cores have no size limitations, but typically have slower speeds of cure. So, for large jobs with short production runs, nobake cores stand out.

For many metalcasting facilities, oil sand cores are still an option, as well. "Oil sand cores get a bad rap for the amount of gasses that come out of the cores when the casting is poured," Rossbacher said. "But recent studies have shown that the gasses of the oil cores come off slower than the other cores, and most of it comes out after the casting has solidified."

However, oil sand cores don't produce cores as dimensionally accurate as other core types. According to Rossbacher, a general rule of thumb is, if a part has been made with oil sand cores for years and is produced in low volumes, it may be best to stick with the oil. But if it's a high production run, newer job or a job that's proven difficult, you may want to start thinking about updating.

Stock Your Shop

If you've determined that it's time to add or update a coremaking process at your facility, one last element to consider is the equipment needed. The equipment can be selected based on three of the main designations for core binder systems--heat-activated, coldbox and nobake (Table 1).

In the hotbox and warmbox systems, sand can be mixed in a muller or mixer and then blown into the corebox using a core blower, or in the case of oil sand, a core shooter. Shell cores can require either a machine for the dump method of filling a corebox or a core blower. The shell sand can be purchased ready to use, so there may be no need to purchase mixing equipment.

Coldbox sand can be mixed with either a continuous or batch mixer and put in me corebox by hand or using core blowing equipment. Nobake systems can use batch mixers or high-speed intensive mixers to prepare the core sand. Compaction tables may also be required to compact the sand in the corebox.

If you're simply trying new core sand, you may be able to use existing coremaking equipment with minor changes to expand your coremaking repertoire.

However, if you decide an entirely new coremaking method should be added, remember to track the price of equipment, facility space used and energy spent when determining its true cost to the facility.

For More Information

"Examining Key Variables of a Coldbox Coremaking Operation," AFS Cured Sand Committee (4-1), J. Cavanaugh, D. Gilson, MODERN CASTING, June 2003, p. 32.

"Does Your Core Sand Measure Up?" J. Werling, MODERN CASTING, Feb. 2003, p. 37.

"Benton Achieves Quality, Quantity and Consistency in Coremaking," R. Foti, MODERN CASTING, June 2000, p. 35.

Shell

Advantages

* Superior casting finish

* Fast production

* Low-weight cores

* Self-venting cores

* Good dimensional accuracy for intricate designs

* Storage excellence

* Superior breakdown of core after casting

Disadvantages

* Raw materials are expensive; capital outlay is high

* Cores removed at a high temperature can result in core distortion

* Corebox cost is high

* Long curing time of 1.5 to 2 minutes leaves operator standing

* Limited in size

* Heat retention properties can result in soft metal areas in casting

Coldbox

Advantages

* Speedy production (in seconds)

* Good tensile strength 

* Excellent dimensional accuracy

* Good abrasion resistance

* High density

* Exceptional collapsibility

* Low gas content

* A variety of corebox materials can be used

* Low tooling cost

* Absence of irritating odors and extreme heat

Disadvantages

* High cost of resins and catalysts

* Requires clean and dry sands

* Special equipment is required

* System is sensitive to sand temperature

Hotbox

Advantages

* High production rates

* Fast curing times

* Core driers not required

* High collapsibility

* High core strength

* High core stability

Disadvantages

* Sand section limitation

* Some blowing heads may require water cooling 

* Venting is more controlled

* Sand temperature is critical

* Ventilation is necessary in the machine area

Don't Shy Away from Cores

"The tradition has been for metalcasting facilities to not want to have cores in designs," says Mike Gwyn, vice president of technology at Advanced Technology Institute Corp., Mt. Pleasant, S.C. "But metalcasters should look at cores as an advantage."

Your chosen coremaking method will only take you so far. Good core design will allow you to achieve the complex shapes your customers are looking for. Following are some tips to designing cores offered by Gwyn:

* Try to design tooling so that the parting plane of the mold enables cores to be set directly on the parting plane.

* Plan for venting and provide passages in the core for the gas to get out.

* A coresetting gauge at the point where the core is placed in the mold can help the operator be sure the core is in the right place.

* For cores that have a lot of complexity, use a fixture to assemble the cores and place the core assembly in the mold.

Growth of Coremaking

Cored casting shipments are forecast to grow 0.5% per year through
2012, despite the loss of casting shipments. Meanwhile, the use of
oil bonded and hotbox cores are projected to decline.

Table 1. Growth of Coremaking Process for 2005 to 2012

                          2005

Process      Casting Shipments   % of Total
                (000 tons)       Shipments

Oil bonded          60               1
C[O.sub.2]          100             1.6
Hotbox              180             2.9
Shell              1,200            19.5
Coldbox            3,000            48.9
S[O.sub.2]          250             4.1
Warmbox             650             10.6
Nobake              700             11.4

                               2012

Process      Casting Shipments   % of Total   % '05/'12
                (000 tons)       Shipments      AGR *

Oil bonded          20               0.3        -14.5
C[O.sub.2]          40               0.7        -12.3
Hotbox              120              2.0         -5.6
Shell              1,160            19.7         -0.5
Coldbox            3,050            51.7         +0.2
S[O.sub.2]          100              1.7        -12.3
Warmbox             810             13.7         +3.2
Nobake              600             10.2         -2.2

* Annual Growth Rate