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 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

So, I see I've exceeded the character limit. Here's the continuation from post #1.


Well, that's it for now. Next I'll walk through applying the mold release, sealing the containment field and securing the side walls of the box. Hopefully the material will be in and we can pour the lower half of the mold.

2-12-

Finally received the MOLD STAR 30 material to pour the mold halves for the sorter scallop. This material suits our needs in that it cures in 6 hours, and being silicone, will not require mold release. I'll still use some release, but normally silicone does not require release agent.

Here's the part in question just to refresh:



Today I started with turning a couple of sprues to pour the urethane material into and one that serves as a vent. Venting is required on most parts to allow air to escape and fill the mold cavity.

I turned Delrin rod stock to a 10 degree taper so it would form a funnel in the mold.



Next, a 10 degree angle is milled in the end of the sprue form that matches the radius of the part. Scheme here is to have both sprues vertical when the mold is sitting upright in the pouring position.



The sprues are hot glued to the part, the excess glue is trimmed away. This step is not necessary, but if left it will be reflected in the mold cavity and need to be removed later.





I didn't take a picture of the next step, but I made a block with a 1" diameter radius to support the glue on sprue and cut the excess flush with the bottom mold form on the vertical bandsaw.

With all the parts of the form tended to, it's time to apply mold release. I'm using paste wax and Silicone spray. Apply a couple of coats of paste wax, let dry and buff with a soft cloth and assemble the mold with the core in place.



The leveling plate is set on a stable surface and leveled.



The containment field walls are hot glued to the leveling plate, working all around to get the best seal and stabilize the form. I used DOW High Vacuum Grease to seal some of the joints, but Vaseline works well, too. Before the walls are put in place I wiped down the pieces with a paste wax rag and buffed. Here's the form with the lower half poured with the MOLD STAR 30.

Pouring the material in this manner will form a parting line in the same location as the original part.







Here's one of the sprues before the upper half is poured.



The upper half of the mold being poured. Didn't have enough hands to pour and take the picture, but the trick is to pour slowly in the lowest part of the mold and allow the material to flow over the form.



The pins serve two purposes. First, they form the hole for the mounting bolts, second, they are used as registration keys to help align the mold halves when the mold is assembled.





Next time, we'll take it all apart, trim up the flashing, reassemble and pour the part in urethane.


dla Excellent work DLA

About 25-30 years ago I worked in Composites R&D at an aircraft engine company and did a huge amount of just what you are doing. We would mix silicone or urethane in 5 gal buckets much like mixing drywall mud. For our use, we would always degas our mix as well. A small batch of 12 oz would blow up to the size of a sponge cake as the entrapped air would escape then collapse down to a small size again. We did a lot of oven or autoclave cures so air would be bad.

At the same company, I also worked in a Photoelastic Stress Analysis Lab (a s precursor to computer stress analysis) where epoxy models of metal parts are made using techniques just as you are using. We would make a plexiglass containment box in which our mold was made. The molding material was clear electrical grade silicone, degassed and oven cured. We would use little cubes of silicone cut from scrape cured material to hold the object off of the bottom of the plex box and encapsulate the object in one pour. After the mold hardened we would remove it from the box and carefully, very carefully, cut the mold with a scalpel to within 1/16" or less of the (usually metal) object almost forming the two mold parts (common Isopropyl Alcohol is a great lubricant for this procedure). Then we would tear the two mold halves completely apart which would leave a clean break along the object resulting in NO discernible parting line. After removing the object and carving one or more sprues in the mold halves we would "glue" the two mold halves back together with a very thin film of the mold material and then post cure the in an oven.

For our Photoelastic models we would use a special epoxy that we would heat to a temperature that would yield a consistency near that of water. After the epoxy cured we would cut the mold open to remove the finished part. The molds were generally for one use only, that is all we needed.

Continuing a bit off of the molding topic, the epoxy models where at a "no stress" state at room temperature. A test rig was made to bend, spin or compress the model depending on type of stress was being studied. Often the rig was put into a vacuum chamber to eliminate any atmospheric effects on the models. If, for instance, you were studying the effects of centrifugal forces on a compressor blade you would not want to have aerodynamic factors influencing the results. In this case we would slowly spin the blade on a rig that would represent a compressor disc while the temperature was increased to the point where the epoxy started to soften. We would then evacuate the air, increase the rotation to the test speed to where the model would stretch from centrifugal force. Once the deformation was achieved we would slowly lower the temperature so that the "stretch" was locked in resulting in a model with internal stress. When removed from the test rig the model was studied by passing light through (it was translucent) and viewing it with polarized lenses. The light would have a rainbow effect with each color indicating a level of strain which through calculations could be scaled to a real world engine part. The shape of the rainbow showed how the strain flowed through the part. Less complex parts such as beams or sheet metal could be tested at room temperature by merely loading them while passing light through. The link below has a short video showing how the stress builds inside a part as it is loaded/unloaded. We also made parts using rapid prototyping methods (absolute leading edge in the 's) that have become what is today know as 3D printing.

It has been a long time since I did this work, I hope I remembered it correctly.

http://stresstechlabs.com/photoelastic.html

Excellent work DLA

About 25-30 years ago I worked in Composites R&D at an aircraft engine company and did a huge amount of just what you are doing. We would mix silicone or urethane in 5 gal buckets much like mixing drywall mud. For our use, we would always degas our mix as well. A small batch of 12 oz would blow up to the size of a sponge cake as the entrapped air would escape then collapse down to a small size again. We did a lot of oven or autoclave cures so air would be bad.

At the same company, I also worked in a Photoelastic Stress Analysis Lab (a s precursor to computer stress analysis) where epoxy models of metal parts are made using techniques just as you are using. We would make a plexiglass containment box in which our mold was made. The molding material was clear electrical grade silicone, degassed and oven cured. We would use little cubes of silicone cut from scrape cured material to hold the object off of the bottom of the plex box and encapsulate the object in one pour. After the mold hardened we would remove it from the box and carefully, very carefully, cut the mold with a scalpel to within 1/16" or less of the (usually metal) object almost forming the two mold parts (common Isopropyl Alcohol is a great lubricant for this procedure). Then we would tear the two mold halves completely apart which would leave a clean break along the object resulting in NO discernible parting line. After removing the object and carving one or more sprues in the mold halves we would "glue" the two mold halves back together with a very thin film of the mold material and then post cure the in an oven.

For our Photoelastic models we would use a special epoxy that we would heat to a temperature that would yield a consistency near that of water. After the epoxy cured we would cut the mold open to remove the finished part. The molds were generally for one use only, that is all we needed.

mikegt4,

Very interesting and informative post. I do degas certain material types, but mostly try to use material that doesn't require degassing. An example is the ECON-80, it has a low viscosity so, it doesn't trap and hold air bubbles, plus it cascades well in the funky mold configuration we're working with. I do degas the material I've used for the shifter boots. Although degassing is not normally required, I had an issue with small bubble entrapment.

I've never attempted to pressure cure any materials, simply because I don't have a pressure vessel. We should probably explore the options regarding air bubble entrapment.

I really like that idea of using silicone blocks for part support. That one will be added to any future projects for sure.

dla

Here's the construction process for a basic mold box. Copied from the Smooth-on site, with a link as well. http://www.smooth-on.com/pages.php?pID=53&cID=11

Thought it might be easier to reference.




Constructing a Simple Mold Box


The purpose of a mold box is to contain the liquid rubber (after it is poured over and around a model) until the liquid turns to a solid. A mold box does not have to be a complex structure &#;depending on the size and configuration of your model, often a coffee can, cake pan or plastic bucket will suffice. If you make molds of flat &#; two dimensional models on a regular basis and require a mold box there are a number of advantages in constructing your own mold box.

Advantages of Constructing A Mold Box


&#;Easy To Construct
&#;
Minimal Assembly Required

&#;
Reusable

&#;
Adjustable (to adapt to different size models)



Materials Needed:

Original Model Used In This Presentation: Terra Cotta Cameo Decorative Plate.

Dimensions: 15" long x 10.5" wide x 1" tall (38 cm x 27 cm x 2.5 cm)

Flat Baseboard -- (Plywood or Acrylic Sheeting)
Dimensions: 20" long x 16" wide x ½ " thick (51 cm x 41 cm 1.3 cm)

Retaining Pieces&#; (4) 2" x 3" (5 cm x 7.6 cm pieces of wood or acrylic)

(4) 2" x 22" (5 cm x 56 cm pieces of wood or acrylic)

Screws: 1" (2.5 cm) Clamps

Modeling Clay or hot melt glue gun,
Smooth-On Super Instant Epoxy.

Assembly :

Step 1. Cut and Assemble Retaining Walls
To accommodate our model, we have constructed retaining walls out of ½" (1.3 cm) thick acrylic strips. We selected acrylic because most mold rubbers release easily from acrylic. Wood can also be used. Four pieces measuring 2" x 3" (5 cm x 7.6 cm) were cut for the shorter side of the retaining wall and four 2" x 22" (5 cm x 56 cm) pieces were cut for the longer side of the retaining
wall. These pieces were then assembled together in an "L" shape with 1" (2.5 cm) screws. (See Figure One Below).

Step 2. Secure Model To Baseboard
The baseboard should be at least twice the size of the original model to allow enough "working space". Secure the model to the backboard by applying a bead of hot melt glue around the perimeter of the reverse side of the model. Press model firmly onto baseboard and create a tight seal where the model meets the baseboard. This will prevent liquid rubber from leaking underneath the model.

Step 3. Assemble Retaining Walls Around Model
Place retaining pieces around the model, making certain there is at least a ½" (1.3 cm) clearance (gap) between the cameo and retaining wall. This ½" (1.3 cm) gap will equal the wall thickness of the cured rubber mold. (Figure Two)

Step 4. Clamp Retaining Walls Together
Fasten the retaining walls together with C-clamps and apply hot melt glue to any seams where the liquid rubber may leak out. This includes seams where the retaining walls meet the baseboard and also where retaining walls meet one another. Important: Mold box seams not properly sealed will result in rubber leakage &#; which equals lost time, dollars and material. (Figure Three)

Step 5. Apply Sealer To Model: Smooth-On SUPERSEAL.
Being made of terra cotta, the cameo and any other porous model must be sealed. Models made of water/sulfur based clays must also be sealed as well. Apply 2 coats of SuperSeal to entire model and surrounding forms (let first coat dry 7 minutes before applying next coat, letting final coat dry for at least 1 hour).

Step 6. Apply Release Agent &#; Smooth-On Universal Mold Release.
For easiest release, apply Universal Mold Release after SuperSeal is dry. Spray a light mist coating over surface of model and surrounding forms. Brush over surface and into areas of detail. Follow with another light mist coating and let dry for 15 minutes before applying rubber.

Step 7. Pour Mold Rubber
Mix and pour mold rubber onto model and let cure. Be certain that the liquid rubber levels off at least ½" (1.3 cm) above the highest point on the model. Let rubber cure overnight.

Step 8. Removal of Retaining Walls.
Finally, after rubber has cured, remove the retaining walls away from the cured mold and flex rubber mold to remove original model. (Figure Four)

Step 9. Demold.
Remove cameo from the cured rubber.

Urethane Casting vs. Injection Molding

How do I know which is right for my project?

Overview

Cast Urethane and Injection molding have both benefits and drawbacks. When creating a plastic part, it can be complicated to differentiate which process is best for your application, volume, size and complexity. It is important to understand the advantages and weaknesses of each before deciding on a production process that is right for your project.

thingyfy contains other products and information you need, so please check it out.

Processes

Cast urethane is the use of a silicone mold to produce a plastic part using the force of gravity to fill the part. The process begins by creating a master pattern, typically by 3D printing or in some instances, machining. This pattern is then placed into a constructed wooden frame which silicone is poured into, making the mold. This mold is then used to re-produce high quality prototype or production level parts with a two-part polyurethane mixture poured into the mold. Once the part has set, it is then removed from the mold, where it often gets finished and painted, correcting any voids or flash as a result of the low-pressure process. These silicone molds can be re-used up to twenty times and can easily be regenerated as needed.

Injection molding is the creation of a plastic part using a steel or metal mold or &#;tool&#; and injecting a molten plastic into the tool. A tool is constructed based on the part dimensions, material selection, and part quantity among other part specifics and is inserted into an injection molding press for molding. In this press, the thermoplastic material, which is selected based on part requirements, is dried, melted, and injected into the mold via a reciprocating screw. The material cools in the mold and is ejected from the machine where it is ready to be handled. Tooling and finishing are then carried out if necessary.

Pros and Cons

Understanding where one process excels beyond the other can help drive the part designer to properly plan for the correct manufacturing process. Below are a few topics to consider when creating a part, and how these processes lend themselves to each.

Prototyping

Receiving prototype parts in a quick and inexpensive way is critical when creating, iterating and launching a product. Cast urethane is a great solution for this since the molds are made of silicone, keeping cost and manufacturing time to a minimum while making it easier to iterate the design. At this stage, getting quick part feedback can help uncover and offer the chance to correct part issues, reducing risk for costly tool modifications or retrofitting if left until the injection molding phase.

For more information, please visit Urethane Casting Services.

Upfront Tooling Costs

While injection molds can be amortized over parts in high volume to flip tool-to-part cost ratio to it&#;s advantage, the high pressure and temperature environment tends to result in higher tooling costs in comparison to cast urethane. This is due to the need for a complex steel or metal mold with internal systems built into the tool to produce quality parts for a long, repeatable and reliable lifetime. Since cast urethane tools are much more basic and designed for low volumes, the upfront costs tend to be a fraction of their injection molding counterparts. Because of these differences, it is important

to consider estimated annual part volumes and stability of part design when choosing between the two processes.

Part Volume

When it comes to part production, annual and lifetime volumes can quickly help designate what process is best for your application. While the quick turn and low cost tooling benefits of cast urethane outweigh injection molding in those categories, injection molding takes the win when it comes to production volume cost savings. Not only will tool price amortization yield long term savings across a higher part volume, but the cost per part is significantly reduced in injection molding. As a result of the high level of complexity in an injection molding tool and press, the cycle time to create a part can be as short as 30 seconds to minutes. In urethane, the process typically yields one to two parts per day. Injection molding automates the manufacturing process, and you are able to get hundreds to thousands of parts in the matter of days to weeks, all while having the long term benefit of a high quality tool that will last you for years to come.

Why Mack Prototype?

Our company is unique because we have the means to do both urethane casting and injection molding &#; among other processes &#; in the same facility. Typically, we recommend starting with the cast urethane to prototype parts, make or approve part design and confirm material selection. Due to the iterative nature of prototyping, our team works closely with customers until they are satisfied with their urethane prototype part.

Once the customer has given approval and frozen the design, we then move forward into injection molding to scale up production volumes. Injection molding has the advantage of manufacturing a medium to high volume of parts in a thermoplastic material consistently and accurately. Our 104 years of expertise in injection molding makes us an excellent extension of your manufacturing arm for plastic parts.

Want more information on Custom Sheet Metal Fabrication Services? Feel free to contact us.