Room Temperature Polyurethane Mold Making Procedure

Room Temperature Polyurethane Basic Mold Making Procedure

This procedure is one example of mold making procedure for room temperature cure tooling and part molding systems. High Temperature applications (above 130 ºF) require high temperature tool building systems and are not covered under this procedure.

This document consists of:

Section #1: Definitions, in other words what you need and what they are

Section #2: Mold Making Procedure

Section #3: Recommended Materials:

Note:

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Section #1: Definitions

1. Master/ Pattern:

Master or pattern is an exact duplicate of the parts to be produced. The mold is cast as a duplication of the master. If the mold material or the part material have shrinkage, the master must be made oversize to compensate the shrinkage.

2. Frame:

You need a structure holding the mold material. Clearance around master in all direction should be around 1/2". Wood frames should be fabricated out of 1" to 1-1/2" plywood. Wood frames are recommended to keep weight to a minimum.

3. Mold Material:

Typically, the mold material will be a slow cure, low shrink material. Polyurethane mold making material should be:

Slow cure

Low shrink

Good wear characteristics

Appropriate hardness

The recommended material is shown below. The part material (such as plastics or concrete) will be cast into the mold cavity, which will be created when the master is removed from the mold.

4. Pot Life

Pot life refers to the time that a poured material takes to start solidifying. You need to finish pouring the material within this time.

5. De-gas (De-air)

This is the process of removing air that is introduced into the material during the mixing of the pre-polymer and curing agent (or the A & B components.) Typically, a vacuum of 28 to 29 in hg is required to completely de-gas the material. Often de-gassing is needed to eliminate entrained air that may cause an uneven mold surface and premature tool wear.

6. Dry Air

When you are storing pre-polymer and/or a curing agent, the product in the container that is exposed to ambient air must be blanketed with air (-40 ºF dewpoint) or dry nitrogen. You need to close the lid tight after filling the space above material with dry air or dry nitrogen. Urethane reacts with moisture and expands or "foams". Dry air or nitrogen will prevent the penetration of moisture into the urethane, whenever you are storing the prepolymer and curing agent.

7. Green Strength

Sometimes the point where the part or mold material exhibits sufficient strength not to tear or deform when handled or de-molded is called "Green Strength". See part 8 of the following segment.

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Mold Making Procedure

Build a wooden frame out of 1" or greater thickness plywood, leaving 1/2" spacing around all sides of the master. Formica on the mold parting line will help in cleaning excess material from the mold when pouring parts.

Cut five 2" diameter holes in

- the four corners

- the center of the mold

Cut 7/8" diameter holes

- In between the 2" diameter holes

These holes are to pour through and to allow the air in the cavity to escape when the mold material is poured. Exact location is not critical.

Place the master (or pattern) on 3/16" or 1/4" thick sheet wax that has adhesive on one side. The " feature" side is to be facing up on the master. The adhesive side of the wax to the master will keep the master from floating while pouring the mold. Trim the wax flush with the sides of the master. The wax will make the master 3/16" to 1/4" thicker and creates a pour line that the mold will be filled to when pouring parts.

Spray the complete surface area of master and table with mold release. Be careful to avoid puddling of the release. Too little release will cause the master to stick in the mold and too much will wash out the features of the master.

Place the prepared frame over the master, be sure to center the master in the frame opening. Weight or clamp the frame over the master to avoid leakage of the material. Use a level to ensure the mold is flat.

Calculate the proper weight of material to fill the cavity. Multiply the cubic open area by .044 to arrive at pounds of material required to fill the mold. When hand pouring, add 3 - 5 pounds of material to this weight to cover the residue left in the pour container.

Mix the polyurethane mold making material at the right ratio. Stir the material with mechanical agitation for a minimum of thirty seconds per twenty pounds of material. De-gas the material for two minutes or until the material is air free.

Pour the mixed mold making urethane material into the mold through any outside 2" diameter hole. Pour the mold through this opening until it is 1/4" to 3/8" below the top edge of the frame. Do not overfill the mold, or it will not sit flat when pouring parts. Failure to follow this procedure may result in trapping air pockets in the mold. Pouring can only be done in other openings if the material is coming up the frame wall of that opening.

The mold must cure for at least sixteen hours at a minimum of 70 ºF before de-molding the master. Check the mold for green strength* before attempting to de-mold the master. If the material tears or easily chips off, cure the mold longer before de-molding.

If the part resists de-molding, use air pressure to release the master. Inject air around the mold parting line until the master releases. Avoid prying the part loose; other wise you could damage the mold.

10.

Once the part is de-molded, trim the flash from the parting line of the mold and inspect the mold for dimensional and visual accuracy.

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Products

Molding Material:

Mold making polyurethane materials are available in various hardness. The recommended materials includes: MPP-A50A, MPP-A65A, MPP-A80E, MPP-A85A, MPP-A95B

Mold Release:

Available at Northstar Polymers @$75 plus shipping and handling for 40 LBS in 5 gallon pail. Quantity discount may be available.

Wax:

#165 Regular Sheet Wax, Adhesive Back

12" x 24" sheets & specify thickness required

Freeman Manufacturing & Supply

Cleveland, OH

800-321-8511

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Please Note: These instructions only apply to certain types of molds, which do not require strict tolerance, are using only room temperature cure material, and are for low volume production. It is also meant to provide only the basic idea of simple molding using polyurethane resins. Please consult mold-making specialists for further information.

Notice: All of the statements, recommendations, suggestions, and data concerning the subject material are based on our laboratory results, and although we believe the same to be reliable, we expressly do not represent, warrant, or guarantee the accuracy, completeness, or reliability of same, or the material or the results to be obtained from the use thereof, neither do we warrant that any such use, either alone or in combination with other materials, shall be free of the rightful claim of any third party by way of INFRINGEMENT or the like, and NORTHSTAR POLYMERS DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, OF MERCHANTABILITY and FITNESS FOR A PARTICULAR PURPOSE.

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