Maximizing Productivity of Nobake Molding Operations.

With proper planning and consideration of the common variables associated with nobake molding, foundries can optimize their unique setups to increase productivity.

Many foundries use nobake sand, but, because each approaches molding differently, each sees significantly different productivity levels. While there is no "standard" operation, there are definite operating standards to be followed.

Typically, problems with unproductive lines are related to bottlenecks, space constraints, flaws in layout design or inadequate staffing. This article outlines the variables to consider when designing a nobake loop for maximum productivity as well as things to reconsider in current operations.

Design Planning

The first step in optimizing a nobake molding line is to determine:

* mold size--What is the maximum and minimum size pattern the loop will handle? Mold size will affect the roller size, the rollover parameters, overhead hoist needs and aisle width;

* flaskless or flasked molds--Flasked molds will require a provision for handling the flasks in the mold prep area;

* production rate--Be realistic and ask other foundries about average production on similar lines;

* available floor space--Will a building expansion be needed? Without adequate room around the line, you'll end up paying for inefficiencies associated with cramped space;

* binder system (Table 1),

* environmental considerations--Local, state and federal regulations can affect a foundry's ability to expand or install a new production line;

* materials needed for the molds (sleeves, chills, vent rope, cores, sand, sand additives, coatings, adhesives)--Where will the material storage and supply routes be located? In general, consider materials that will be included in the mold with regard to storage and restocking paths;

* desired staffing level--List the activities to be completed on the mold before pouring and study them to set the personnel level;

* mechanization--To a point, the more mechanized the loop, the less manpower needed and more consistent mold quality. However, mechanization requires higher capital investment;

* projected pattern runs--Do most orders require five or 500 molds? This will tie into the mechanization of the line and overall design. The greater variety in the number and size of patterns run per day, the more flexible the line must be in handling setup.

Once these fundamentals are answered, consider production activities. Equipment breaks down, and maintenance will be required. With proper design, a foundry can continue to operate while minor elements are repaired. This may require space around key areas, such as space for forklift access to the rollover and roller conveyors.

Upgrading Loop Design

Major processes in most nobake lines are pattern setup; mold filling and facing sand; mold stripping, coating and drying; coresetting; mold closure; and transporting molds to the pouring area.

Time each area of the loop over an extended period to determine where bottlenecks occur. The standard time or each molding stage will depend on production requirements. For example, 30 molds/hr is the desired production rate, then one mold half must be made every minute. Typical bottlenecks are:

* rollovers--This equipment may turn over and strip the pattern slower than needed. A faster rollover or dual rollovers may be required;

* coating application and drying--This operation also may exceed the production time. Dual drying lines can help correct this problem;

* closure area--In many instances, it takes longer to close, weight and push the molds to the specific pouring line than it does to complete other process steps. Auto-closure devices, automated mold conveyor systems and additional staffing may be needed;

* coreset area--The coresetter may have to search for cores or retrieve them from storage which may back up the production line. Have adequate space around the coreset area and have service personnel supply the storage for the coresetters;

* molding/mixer area--The most common bottleneck in most lines is at the molding area. Molds are being made only if the mixer is putting sand in a pattern box. Problems typically are not an issue of sand-flow volume (most mixers only run 1-2 hr/shift) but a matter of staffing and the number of activities to be completed at the molding station. The molding station will determine the potential speed and productivity of the line. Time lost at the beginning of the molding process cannot be made up later.

Pattern Setup

Most foundries design their nobake line without a pattern setup person to prepare the molds for the molding area.

A setup lead person can make the difference between a fair production rate and maximum production rate. This person sets up the pattern, places chills, makes minor repairs, cleans the pattern and applies a release agent. The setup person also adds risers or sleeves, vents, sprues, lettering for heat numbers, rebar, wires and supports, and ram-up cores or firecrackers. Other responsibilities include pattern movement (new patterns in/finished patterns out) and flask placement and handling.

If a setup person is handling these activities, the machine operator can concentrate on filling molds and compacting for maximum quality. To figure out if a setup person is needed, add a worker to the process and determine if:

* more molds are realized per hour or per day from the molding loop;

* other process steps are able to keep up with increased mold production; If more molds were produced and the process line was able to keep up with the additional molds, use the following formula to see if the setup person's cost can be justified:

(Increased Molds per Day x Average Revenue or Profit per Mold) - Cost for Setup Person per Day = + or - Cost Savings.

Mold Filling

In addition to filling the mold with sand, the mixer operator is responsible for compacting and striking off molds; venting the patterns/molds; adding sleeves and sprue covers; and adding ram-up cores. If the mixer operator has extra time, put a worktable behind or beside him, and have him make simple cores with the strikeoff sand.

After the mold is filled, the binder must develop sufficient strength to be stripped from the mold. The following three concerns are important at this stage of the loop design.

Number of Patterns--How many patterns do you run at a time? Experience says 5-6 patterns, but that depends on scheduling needs. If too few patterns are run, a bottleneck will occur at stripping waiting for binder to set up. If too many patterns are run, it can take too long to finish orders.

Binder Set Time--How fast should the binder set? The slower the setup speed, the more space is needed between the molding station and rollover.

Space--Is there room behind the molding area to store molds waiting to be stripped? A mixer operator should never have to wait for the binder to set at mold stripping. The "18F rule" states that the chemical reaction either doubles or is cut in half for every 18F (10C) change in sand temperature. Since a typical mix is 97-99% sand, sand temperature influences binder setup speed. The other key variable is the type and amount of catalyst, acid or co-reactant.

Mold Stripping

At mold stripping, consider:

* maximum and minimum mold sizes--Variations outside the rollover's capability will require hoists or cranes to strip the mold;

* obtaining the straightest draw possible/stripping method (hand, hoist and trunion, rotary table or rollover)--The stripping of the mold must be straight, or deep pockets and small areas will break. Financial needs, flexibility in work mix, pattern equipment setup and space constraints affect the stripping method;

* bottom boards--Bottom boards minimize warpage as the binder finishes curing. To lower labor handling costs consider automation for returning the bottom boards to the rollover area;

* pattern equipment flow--The patterns must flow back to the setup with minimal handling;

* molds stuck in the flask at stripping--Take the mold off the line for stripping so the line can continue;

* cleaning the mold halves (airblowoffs, cleaning tools and disposal)--After stripping and before coating and coreset, loose sand must be removed from the mold or the castings will have sand defects;

* scrap mold removal and disposal-- When molds are scrapped, there must be an easy method and enough space for removing them from the process line to save downtime;

* binder system--The type of nobake binder will affect: the amount of space needed for the binder to develop strength for stripping; the odor at the molding station [sand temperatures above 100F (37.8C) will affect the intensity of odors at mixing]; and the handling strength of the molds.

Binder Type

Binder characteristics are noted in Table 1. Consider the following basic rundown when designing or updating your molding operation.

Phenolic Urethane-- This system consists of a two-part binder system and a liquid catalyst. The Part I binder is a phenol-formaldehyde polymer, and Part II is a polymeric MDI with solvents and additives. Part III is a liquid catalyst.

Furan--These two-part systems cure by a condensation reaction (water is produced as a byproduct). Acid catalysts promote the cure. The type of binder, strength and amount of the acid control the cure rate.

Ester-cured Phenolic--T his two-part binder uses an alkaline phenolic resin (it contains sodium or potassium ions that have replaced hydrogen on the phenolic hydroxyl group and has a 7.0 pH or higher). An ester co-reactant controls the cure rate.

Mold Coating

After the mold is stripped from the pattern equipment, it must be finished for closure and pouring. A refractory mold coating can improve surface finish and reduce casting defects. If a refractoy mold coating is used, the following must be considered:

* method of application (brush, swab, spray or flowcoating)--The coating method will affect the time needed for coating and ventilation. Hand methods, such as brushing and swabbing, take longer than spraying or flowcoating and require a longer process line. Spraying applications in which there can be large amounts of fugitive coating and carrier can lead to ventilation concerns;

* coating carrier type (water or alcohol) and safety considerations-- Water-based coatings require longer process lines and dedicated drying ovens, while alcohol-based coatings can be burned off on the line. Alcohol-based coatings are highly flammable, and ventilation is needed at the mixing and application station;

* application time--If coating is time-consuming, it may be better to coat the mold off-line, bringing it back when it is ready, so the line can continue operating;

* drying equipment-- Good air flow in an oven and a cooling zone at the end of the oven are two improvements that help guarantee the mold coating is dry and the surface is strong;

* mixer and storage vessel--This variable concerns the size (55-gal drum, mini-tank or bulk) of the storage and mixing containers. The mixing container must be close to the coating station for refilling and quality checks. Room to remove and replace containers is needed to forego production delays.

Coresetting

After the mold halves are cleaned and coated, the cores must be set. The space needed for storage depends on the number of patterns being run, the cores per mold and the type of core storage container (racks, pallets, etc.). For example, if five patterns are run and each pattern requires two different cores per mold, then space for 10 different cores is needed. If the cores are stored on pallets, then space for 10 pallets is needed. For smooth flow, standardize container size and type to avoid space and movement problems. Other considerations include:

* type of coresetting (hand, hoist or automatic)--Most coresetting is done by hand or hoist. These methods are more flexible to pattern changes. Automatic coresetting technology is available for high-production patterns to improve productivity and quality;

* storage material movement (removing unused cores, replenishing cores run and cores for new jobs)--A large enough area is required to store both the cores for current and upcoming jobs so the line is not delayed as cores arrive from storage;

* method of moving cores (hand, forklift or hoist)--The size, weight and quantity of cores to be run and the storage medium affect this decision;

* packing material removal--If cores arrive in packing material to minimize breakage, removing this material is a space constraint;

* scrap core disposal and removal-- Controlling this variable can be as easy as figuring in a location to store a scrap core tub near the process line.

Mold Closure

If the cope half is not closed straight onto the drag half, many casting defects can occur. The type of closure device and lineup system is critical to keep from knocking sand into the mold, crushing cores, preventing runouts and minimizing mold shift.

Several types of lineup systems are available: shift buttons, offset parting line, pins and bushings, or lineup pins. Lineup pins and bushings can be a very effective option for mold lineup.

Other considerations at this area are:

* gluing (bottle or adhesive gun)--The time it takes to apply a glue and allow it to dry will affect space needs;

* handling of molds (hand, manipulator, crane, jib hoist or mechanical closure)-- Each of the mold handling methods has advantages and disadvantages, depending on the mold size. The better the device can be controlled and held straight, the better the quality of the molds;

* weights--Put the mold weights on most molds early, after the molds are closed. The weights can level warpage in the molds and prevent runouts;

* bottom boards--Bottom boards must be stored and returned to mold stripping;

* clamping, banding or pinning of molds--The effectiveness of a particular method is dependent on the size, weight and venting of the specific molds being poured.

Pouring Area

Once a mold is ready to be poured, the following must be considered:

* method of moving molds to pouring (manual or power rollers, crane, hoist or conveyors);

* metal spillage issues (roller type, repair or removal of damaged rollers, metal removal and disposal);

* mold breakdown (let sand fall on floor, limit spillage to bottom board or catch sand on conveyors);

* mold removal for shakeout;

* removal of weights (if used);

* metal pouring area--Are molds poured on the rollers, on the floor, or are the molds moved through a pouring area to cooling lines?

Loop Staffing

A typical loop setup has 4-9 workers, depending on the variety and complexity of the patterns, production rate desired and the materials used.

The basic staffing for each area of a nobake loop includes:

* one pattern setup/changeout person;

* one mold filling person--Two are sometimes needed if many materials are added (chills, ram-up cores, etc.) or no setup person is used;

* one mold stripping (manual or mechanical operation) person--This depends on consistency of strip time and quality of the stripping device;

* one mold coating person--This could require additional staff, depending on coating method;

* one to two coreset workers--This depends on the cores being set per mold and core storage flow;

* one to two mold closers--This depends on closure method and type of lineup device. Shift buttons and offset pattern lineup methods typically require two people, due to straight-closure difficulty;

* one person to move molds to pouring in a manual operation/unmanned with conveyors.

All production areas must be balanced to keep the line flowing consistently. If all factors are considered, a productive and environmentally acceptable nobake line will result.