Brillion Iron Works' recipe for success with plastic tooling.

Having converted many conventional patterns to plastics, BIW's plastic pattern shop leadman details process and cost considerations.

In the 1970s, many U.S. foundrymen had a low opinion of plastic tooling. Primarily mass-cast epoxies, the plastics' quality was at the mercy of room temperature and humidity, while wear characteristics and life-expectancy were not much greater than those of wooden tooling. This perception has persisted, and it has taken a long time to convince foundrymen to give subsequent generations of plastic tooling another try.

Personnel at Brillion Iron Works (BIW) were also suspicious of plastic tooling, until a management team from the foundry made a 1982 tour of European foundries and found many of them extensively using a new generation of plastic tooling. Another look at the material was in order.

That year, a specially ventilated, temperature and humidity-controlled room was built at BIW and the firm purchased its first plastic injection machine for use with polyurethanes. Fourteen years and two more injection machines later, BIW uses more than 2100 plastic patterns, and many more have been modified with plastic in some way or another.

Plastic patterns offer quickened lead times and cost savings, but plastic is also useful with pre-existing metal or wooden patterns. While you can build metal tooling that is dimensionally perfect to the blueprint, the castings will invariably have some discrepancies. If you move the tooling from one molding unit to another, the castings will again change. The quickest, easiest way to fine-tune those dimensions is with plastic.

This article details both construction specifications and quoting considerations for modern plastic tooling.

Negative Construction

There are many different materials that can be used to construct pattern negatives, including laminates, mass casts and quick-setting materials. BIW has used all of these and more, but tooling, production and product variables will determine what each foundry uses. Currently, BIW uses a 3-part, low density backfill applied over two layers of an epoxy surface coat. This process provides a dimensionally stable, lightweight negative that the foundry keeps for repairs, modifications, retooling and checking core fit.

The negative construction procedure is as follows:

1. Mount a single-shrink master on a 1/2 or 5/8 in. aluminum plate, leaving about 1.5 in. around the master. Spray on a graphite-filled automotive primer to the wood master to seal it.

2. Melt 1/8 in. string wax around the base of the master to seal it from any plastic seeping underneath, and prevent tear-up on the parting of the negative when removing the master.

3. Insert two 1/4 in. metal dowels into the plate to locate it to the negative.

4. Build a plywood frame to fit on the edge of the plate, making it 1.5 in. taller than the highest point of the master. Set frame aside.

5. Apply several coats of wax release to the master. Let dry and buff between coats.

6. Apply two layers of surface coat, allowing it to become tacky between coats (45-60 min).

7. Squash the frame on.

8. BIW's 3-part backfill consists of 350 g of lightweight sand filler, 100 g of resin and 42 g of hardener. It is weighed and mixed in a 5-gal pail and then mixed in a jiffy mixer. Once packed into the negative, it is cured for 12 hr, demolded, sanded and polished. Adjusting the amount of sand used in the backfill lends flexibility to its strength and density.

Pattern Construction

1. Apply several coats of wax release to the negative.

2. Construct an undersized pattern (cut or machined from blocks of wood, plastic or metal). A 0.090 - 0.120 in. thick liner of a flexible, Shore A 30-40 durometer material is made by brushing 4-5 thin layers onto the negative.

3. Once the-liner is cured, pull it from the negative, apply release agent to the negative and liner, and place the liner back in the negative. Figure 1 shows a typical negative and liner.

4. Prepare the plate from which the negative was made with anything you want cast into your undersized pattern - threaded T-nuts, aluminum blocks for doweling, date tags, reinforcements for ribs, etc.

5. Mix the fast curing material and pour it into the negative that contains the liner. Pour it up to within 1/8 in. of the top of the negative and set the plate on. Demold it after about 20 min. Repeat process for additional cavities.

6. Place all undersized patterns in an oven and bake 12 hr at 150F (65C). This ensures that the only curing and shrinking of the final pattern is in the urethane injected later.

7. Remove patterns from the oven and cool them to room temperature. Sandblast and mount them to the plate.

8. Drill a 1/2 in. hole through the plate and about 1/4 in. into the undersized pattern. Then drill a 1/4 in. hole all the way through the pattern. The location of this hole is usually over the deepest part of the cavity.

9. Insert 1/2 in. tubing through the 1/2 in hole in the plate until it is seated into the undersized pattern. Drill holes in the plate for inserting straws for vents. Apply release agent to the negative.

10. The undersized pattern, being attached to the plate, is suspended to the negative. Fill the mold with urethane.

If the pattern design has feather edges at the parting line or has offset partings, BIW uses the final mounting plates for cover plates for the process, machining out the plates where needed and injecting the patterns permanently into them. The plate is always undercut to mechanically lock the pattern to it.

The injected materials BIW uses depend on the application. For prototype and low-production tooling, a machine-applied polyurethane (Shore D 90) that sets up in 40 sec is used. The speed of set-up provides the ability to inject multiple cavities in one day, and the ability to quickly repair broken or worn tooling.

For high-production tooling, BIW uses a urethane (Shore D 60) that can be applied by machine, syringe, caulking tube, or fed by gravity. This material offers control of offset partings, facilitates rapid engineering changes and cuts the cost of tooling and repairs.

BIW also has 128 plastic coreboxes for phenolic urethane amine coldbox and oil sand cores. They are made in much the same way as the patterns, with a variety of materials used to accommodate production needs. Plastic for corebox construction offers even greater advantages than for patterns because one half is made directly off the other, creating a better seal around the cavity to prevent gas from escaping.

Quoting Considerations

BIW has a shell molding unit, along with nine green sand units and a loose floor for specialty castings. When quoting a part, the tooling material must be considered. Because of the abrasiveness of several of the foundry's green sand units, and especially its shell sand line, only brass, iron or steel tooling are considered.

Other quoting considerations for tooling include: molds per year; tooling life expectancy; number of cavities; cost of tooling; design complexity (simple designs can just as easily and quickly be machined from aluminum); ribs/thin sections; deep pockets; offset partings; engineering changes; time; and rapid prototyping. Because of all these variables, tooling materials include wood, epoxy, polyurethane, urethane, aluminum, brass, gray and ductile iron, and stainless steel.

When considering molds per year and life expectancy, BIW must decide if a job is better off with high-wear metal tooling, or if multiple sets of plastic tooling are more cost-effective.

The foundry checks all its tooling for wear standard increments based on the number of molds made from it. A check consists of dimensionally laying out a casting and visually inspecting the tooling. Wear plugs have also been inserted into all plastic tooling to aid inspection. Table 1 shows standard increments for a variety of tooling materials.

For the number of cavities and the cost, BIW has found that plastic tooling is better vs. metal based on each additional cavity. This is a 10-40% savings, depending on all the other factors mentioned above. For ribs/thin sections and deep pockets, plastic tooling requires extra time to incorporate metal where strength and support are needed. Metal is also sometimes used where deep pockets are known to be a problem for molding.

Offset partings, mounting or setting tooling into plates all need to be accounted for. An advantage of plastics is the ability to control clearances on offset partings to reduce flash around the cavity.

Time is increasingly important and shows up in all the quoting factors already mentioned. The lead time for building metal tooling, along with mounting, gating, sampling, fine tuning and receiving approval can run 14-16 weeks. Plastic tooling can cut 3-4 weeks from that time - even more when coupled with rapid prototyping methods.

Table 1. Tooling Wear Inspection Schedule

Material            Life Expectancy         Wear Check (Molds Made)

Wood                    500-1500                      1000
Epoxy                  3000-5000                      1500
HMP                   25000-40,000                    7500
Urethane             60,000-100,000                  10,000
Aluminum             30,000-50,000                   25,000
Brass               100,000-200,000                  25,000
Gray Iron           200,000-300,000                  25,000
Ductile Iron        300,000-500,000                  25,000
Stainless           500,000-750,000                  25,000

Rapid prototyping is becoming more common, and speeds the process considerably if the customer has the part on a CAD file. Historically, BIW's prototype tooling has become its short-term tooling out of the necessity to satisfy customer time frames. Since wearability is not a strength of most of the rapid prototyping processes, an avenue to consider is to make your negatives from the CAD file, then construct the tooling from higher wear plastics.

General quoting guidelines for plastic tooling are as follows:

1. Hours (including negative construction)

* Single cavity surface mounted - 24-36 hr

* Single cavity offset partings and/or injecting into plate - 40-120 hr

* Multiple cavities surface mounted - 3-6 hr per additional cavity

* Multiple cavities offset partings and/or injecting into plates - 60-120 hr

2. Total cost of plastic materials (negatives and patterns)

* Single Cavity - average $0.14 per cu in.; $1.60/lb

* Multiple Cavities - average $0.16 per cu in.; $1.87/lb

BIW has had great success using plastic tooling on a large percentage of its patterns. The material offers a production and design flexibility unavailable with other pattern materials.