Urethane: Materials and Methods

I'll add, to do this correctly, it's going to take more than one post. I'll do this post, then add-on until we've made the journey just to keep it all together.



I've been using castable Urethane materials for a bit over 10 years. It started as a want, later becoming a need. I had been researching methods and materials for a couple of years to fill a void in the supply of NOS and aftermarket rubber seals and the like for the Classic Car hobby.


Some of the applications I've encountered, are low production, with no hope of being reproduced by any of the major suppliers. Traditional method tooling costs alone for some of the parts I've reproduced can exceed $20,000. With that kind of overhead, the 3 major players in the U.S. restoration rubber and weather strip business just can't afford to invest that kind of money in tooling for low volume repop parts.


Now, NOS parts for Classics, are well........ you know what they are. Hard to find, very expensive and even if stored in the best of conditions, probably won't last once put into service. I mean, if you should replace a fan belt or radiator hose every 4 years, what can be expected of a 43 year old part?


The upper and lower shifter boots for /69 Impalas (GM refers to them as "Seals") are very hard to come by NOS or used. I gave 500 bucks for this set. As a rule they sold around .00 a set and up...... when you can find them. I might add that since these have become available the price of a NOS boot has dropped dramatically.



Here's one I made:



And here is a cowl extension rubber for -69 Impala, Caprice, Belair and Biscayne in the same foam type rubber as original.




So, that's the why.


The how is, done correctly, a longer story. The actual process is really not a difficult one. The materials are hazardous and just like anything else, deserve the same respect as painting or handling any chemicals. READ THE MSDS!!!


All the research I've done has been online. Just about anything you want to know regarding mold making and casting urethane is online...... then again, the stuff you can't find online you learn the hard way. I can help with that up to a point. I learn something new every part I cast. Each part is different in it's own way, it may have heavy under cuts, thick/thin sections, or cast-in inserts. Maybe they have any combination of all the above.


Some can be simple one-part molds and there are multi-part molds. I'll try to show what I've done with those as we go along.

The process also has another positive.... It's cost effective. I do have a tidy sum invested in equipment, although it's not prohibitive, I would have not purchased some of this equipment if this was a one-shot venture for me. I have something of a part-time mold shop going on here.


Some of the equipment you would need to do a one time project would include:

Electronic Gram Scale

Mold Forms

Glue Gun

Mold Release


A simple mold will consist of a containment field made from sulfur free clay, Masonite, Lexan or just about anything that will contain the liquid material until cured. I normally use Delrin and/or Lexan due to the fact that's what is easiest for me to access.


The mold form serves to hold the model in place and stable while the mold is being formed. I try to make the molds in two parts, simply because that's the easiest way to go. Some parts require multiple part molds and the complexity increases with every section that's added.

Here's the mold used for the Chevrolet clutch rod boot posted elsewhere:



And the reproduction boot:



So, this is the first installment. I'll leave a link to a preferred supplier who has a ton of information and videos showing their products in use.

http://www.smooth-on.com/

What ever the part may be I study the part and determine basically three things. Weight, Durometer and how I'll approach the mold configuration.

Weight is straight forward, simply weighing the part tells me how much material is required to cast the item. Most of the material I'm using is 1:1 and can be mixed by volume. However using an accurate gram scale helps maintain closer ratios and at the bottom of the bucket, there is no unused A or B component.

Durometer of the original part is critical if you're interested in reproducing an accurate representation of your part. I have Durometer Testers for both A and D Shore materials. "A" Shore material will be your softer, rubber type material and "D" Shore is for the harder, plastics. While not totally necessary, a tester will provide the best measurement of the hardness/softness of a given material.

As a rule, a rubber band is 25-30 Duro, a car tire is 55-60 Duro. Here's a chart the will give some insight regarding Durometer:

http://www.smooth-on.com/Durometer-Shore-Ha/c/index.html

Now, mold configuration is sort of a learned process and can take a fair amount of time to accomplish. It's two fold with a lot of preparation involved, but it's like painting..... You get what you put into it.

Look at almost any cast, injected molded part. Try to envision what the MOLD looks like, not the part. Try to see what's not there. That is what the mold looks like. The mold is a reverse of the part. At first, I had to look at parts a VERY long time to see the mold, now I can see it almost immediately unless it is a complex part.

The simple part is the containment field or mold box. Here's a better description than I can give:
http://www.smooth-on.com/pages.php?pID=53&cID=11

Here is a tutorial that got my attention early on. The intricate detail in reproduction of a given part is unbelievable. This is also an example of a very simple project using the most basic techniques. Check it out.

http://www.smooth-on.com/gallery.php?galleryid=157

2-4-

Still Waiting for materials. In the meantime, here is the project so far. Doing a few things different than I have in the past, so if this turns to do-do, we'll all see it at the same time.

I made a leveling plate to rest the mold form on during the casting of the form. It's 12"x24"x3/4" Delrin with 4 3/8-24 socket head Allen bolts for leveling legs. All I'm shooting for here is a level surface that's easy to move around the shop. A couple of machinists levels and a couple of minutes, it's done.






This is the part. Obsolete and very expensive when available from the original vendor. We could CNC this part out of Delrin, but there are some contours that are challenging. The bigger issue is that in Delrin, the fingers tend to snap off when put into a bind. 80-85 Durometer urethane is hard enough to maintain the shape and has enough flex to prevent breakage.







This is the inner structure of the mold form. It's purpose is to support the original part inside the containment field during the casting of the mold material. BTW, this time I'll be using a 30 Duro Silicone material. The mold material should be softer than the part or harder than the part to facilitate removal. If both sides (mold or part) are the same Durometer, it is difficult, if not impossible to remove the mold halves or part. You should be able to "peel" either the mold from the part or the part from the mold.





Here's the part in place on the inner form.





The part, inner form with the containment field walls.





Here the white Delrin pins will serve to form the holes for the mounting bolts. We could just drill these after the casting, but it will save that step later. Since these parts will be ongoing for some time to come, it's just smart to "mold in" as many machining steps as possible.





dla

I had posted in the "What did you do in your garage" thread a couple of weeks ago. Post # http://www.garagejournal.com/forum/showthread.php?p=#post There was some interest in the process of using Castable Urethane, as promised, here's a walk-thru of the process and materials.I'll add, to do this correctly, it's going to take more than one post. I'll do this post, then add-on until we've made the journey just to keep it all together.I've been using castable Urethane materials for a bit over 10 years. It started as a want, later becoming a need. I had been researching methods and materials for a couple of years to fill a void in the supply of NOS and aftermarket rubber seals and the like for the Classic Car hobby.Some of the applications I've encountered, are low production, with no hope of being reproduced by any of the major suppliers. Traditional method tooling costs alone for some of the parts I've reproduced can exceed $20,000. With that kind of overhead, the 3 major players in the U.S. restoration rubber and weather strip business just can't afford to invest that kind of money in tooling for low volume repop parts.Now, NOS parts for Classics, are well........ you know what they are. Hard to find, very expensive and even if stored in the best of conditions, probably won't last once put into service. I mean, if you should replace a fan belt or radiator hose every 4 years, what can be expected of a 43 year old part?The upper and lower shifter boots for /69 Impalas (GM refers to them as "Seals") are very hard to come by NOS or used. I gave 500 bucks for this set. As a rule they sold around .00 a set and up...... when you can find them. I might add that since these have become available the price of a NOS boot has dropped dramatically.Here's one I made:And here is a cowl extension rubber for -69 Impala, Caprice, Belair and Biscayne in the same foam type rubber as original.So, that's the why.The how is, done correctly, a longer story. The actual process is really not a difficult one. The materials are hazardous and just like anything else, deserve the same respect as painting or handling any chemicals. READ THE MSDS!!!All the research I've done has been online. Just about anything you want to know regarding mold making and casting urethane is online...... then again, the stuff you can't find online you learn the hard way. I can help with that up to a point. I learn something new every part I cast. Each part is different in it's own way, it may have heavy under cuts, thick/thin sections, or cast-in inserts. Maybe they have any combination of all the above.Some can be simple one-part molds and there are multi-part molds. I'll try to show what I've done with those as we go along.The process also has another positive.... It's cost effective. I do have a tidy sum invested in equipment, although it's not prohibitive, I would have not purchased some of this equipment if this was a one-shot venture for me. I have something of a part-time mold shop going on here.Some of the equipment you would need to do a one time project would include:Electronic Gram ScaleMold FormsGlue GunMold ReleaseA simple mold will consist of a containment field made from sulfur free clay, Masonite, Lexan or just about anything that will contain the liquid material until cured. I normally use Delrin and/or Lexan due to the fact that's what is easiest for me to access.The mold form serves to hold the model in place and stable while the mold is being formed. I try to make the molds in two parts, simply because that's the easiest way to go. Some parts require multiple part molds and the complexity increases with every section that's added.Here's the mold used for the Chevrolet clutch rod boot posted elsewhere:And the reproduction boot:So, this is the first installment. I'll leave a link to a preferred supplier who has a ton of information and videos showing their products in use.What ever the part may be I study the part and determine basically three things. Weight, Durometer and how I'll approach the mold configuration.Weight is straight forward, simply weighing the part tells me how much material is required to cast the item. Most of the material I'm using is 1:1 and can be mixed by volume. However using an accurate gram scale helps maintain closer ratios and at the bottom of the bucket, there is no unused A or B component.Durometer of the original part is critical if you're interested in reproducing an accurate representation of your part. I have Durometer Testers for both A and D Shore materials. "A" Shore material will be your softer, rubber type material and "D" Shore is for the harder, plastics. While not totally necessary, a tester will provide the best measurement of the hardness/softness of a given material.As a rule, a rubber band is 25-30 Duro, a car tire is 55-60 Duro. Here's a chart the will give some insight regarding Durometer:Now, mold configuration is sort of a learned process and can take a fair amount of time to accomplish. It's two fold with a lot of preparation involved, but it's like painting..... You get what you put into it.Look at almost any cast, injected molded part. Try to envision what thelooks like, not the part. Try to see what's not there. That is what the mold looks like. The mold is a reverse of the part. At first, I had to look at parts a VERY long time to see the mold, now I can see it almost immediately unless it is a complex part.The simple part is the containment field or mold box. Here's a better description than I can give:Here is a tutorial that got my attention early on. The intricate detail in reproduction of a given part is unbelievable. This is also an example of a very simple project using the most basic techniques. Check it out.Still Waiting for materials. In the meantime, here is the project so far. Doing a few things different than I have in the past, so if this turns to do-do, we'll all see it at the same time.I made a leveling plate to rest the mold form on during the casting of the form. It's 12"x24"x3/4" Delrin with 4 3/8-24 socket head Allen bolts for leveling legs. All I'm shooting for here is a level surface that's easy to move around the shop. A couple of machinists levels and a couple of minutes, it's done.This is the part. Obsolete and very expensive when available from the original vendor. We could CNC this part out of Delrin, but there are some contours that are challenging. The bigger issue is that in Delrin, the fingers tend to snap off when put into a bind. 80-85 Durometer urethane is hard enough to maintain the shape and has enough flex to prevent breakage.This is the inner structure of the mold form. It's purpose is to support the original part inside the containment field during the casting of the mold material. BTW, this time I'll be using a 30 Duro Silicone material. The mold material should be softer than the part or harder than the part to facilitate removal. If both sides (mold or part) are the same Durometer, it is difficult, if not impossible to remove the mold halves or part. You should be able to "peel" either the mold from the part or the part from the mold.Here's the part in place on the inner form.The part, inner form with the containment field walls.Here the white Delrin pins will serve to form the holes for the mounting bolts. We could just drill these after the casting, but it will save that step later. Since these parts will be ongoing for some time to come, it's just smart to "mold in" as many machining steps as possible.dla

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Urethanes are versatile compounds comprised of nitrogen, hydrogen, and oxygen; it is a crystalline compound, an ester of carbonic acid (heated with alcohol and an acid catalyst). Urethane is a common sealant because of its natural resistance to water, oil, and oxidation. The material is abrasion and extreme-temperature resistant. Urethane is also an excellent choice for casting low volume, durable parts while maintaining production-level quality. Read on to learn more about urethane casting utilizations and design strategies.

In basic terms, urethane casting involves designing the master part pattern, making a mold from the pattern then pouring or casting the resin to produce urethane casting parts or urethane casting prototypes. Each step involves multiple processes and can be rather complex.

Making the Master Part Pattern

The first portion of making the master part pattern is to perform the design of the component utilizing mechanical engineering principles and a 3D modeling platform, such as SOLIDWORKS. The CAD model of your part will be utilized to make the Master Part Pattern for the part to be cast. For the 3D model design, one should consider how the part will be molded and how the urethane casting material will be removed from the mold. The following tips are helpful when making the Master Part Patter CAD model:

1. Since the mold for urethane parts is likely comprised of silicone, you do not need to design mold relief or draft angles.
2. Consider incorporating more complex features, such as deep/narrow holes, internal cavities, and channels, with secondary, post-cast processes.
3. Gates should be designed for resin and curatives to be injected. The size and location will be dependent upon the volume of the cast part and the profile of the casting pattern.
4. Vents should be designed to allow the escape of trapped air inside the mold.
5. When splitting your part into segments, you need to have locators on the mating surfaces to prevent the shifting of the mold halves. Shifting can change the parting line of the molds and create issues with dimensioning or tolerancing of the final parts.

Ultimately you should simply design the master part pattern with the end goal of production of urethane cast parts.

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Manufacturing the Master Part Pattern

Once the CAD model is finalized, it is sent to a manufacturing provider which typically chooses to print the Master Part Pattern. Most often the printing is accomplished by the PolyJet 3D (PJ3D) Fused Deposition Modelling (FDM), or Stereolithography (SLA) methods. CNC machining may be utilized to create the master part pattern but 3D printing is typically more cost-effective. The main goal of the master part pattern printing is to create an accurate silicone mold for the urethane molded parts.

Making the Urethane Part Mold

Most urethane part molds are made from pouring liquid silicone around the parts to create a mold. First, a box is constructed which will be utilized to hold the master part for the pouring of silicone around it to create the mold. Next, the master part is securely placed in the box. Thirdly, the liquid silicone materials are poured into the box, around the master part pattern. The silicone is cured in typically several steps. Curing is often accomplished utilizing one of two methods: condensation curing or addition curing. Typically, the mold is split in two for the removal of the master part, or the molding process is designed to create a two-part, mating mold. Another option for a silicone mold is called a skin mold and it is created by pouring liquid silicone on the master part pattern, layer by layer until the desired thickness is achieved. Sometimes, features to be molded that may be susceptible to fatigue are replaced with stronger materials.

Casting the Urethane Part

Urethane casting of parts involves the utilization of a type of polyurethane or urethane casting resin to fill a silicone mold for part production. The urethane casting material is typically two components, resin, and curative agent, which chemically react upon mixing to form the urethane material. The two components are mixed in a container and degassed, to remove any air bubbles. The molds may be filled with urethane utilizing pouring or a pressure-fill method, such as screw injection. The urethane cast part is then cured and the mold is split into two parts so that the cast part may be removed.

Post-casting operations such as grinding, cutting, CNC machining, drilling, or milling may be performed on the urethane cast parts as needed. Gates or vents may be smoothed or cut. Holes or channels can be added. But the urethane cast parts do have a low melting point so coolants must be utilized during machining as a preventive measure.

Urethane Casting Applications

Urethane parts may be utilized for a wide variety of components from automobile parts to medical devices to consumer products and even prototypes. Here are a few examples of common urethane casting parts:

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• Shore D urethanes can be utilized to make tough and durable parts such as enclosures, cases, consumer electronics, medical devices, packaging, and even toys.
• Softer urethanes, Shore A urethanes, with higher elasticity are useful for flexible, yet impact-resistant parts. Examples include rubber gloves, overmolds, buttons, keypads, rubber boots, and buttons.