M14 Cheek Pad: Anyone with Experience in Injection Molding?

[Richard Turner/Turner Saddlery]

I am planning on reviving a cheek pad that I designed over 20 years ago. The original design required laminated leather spacers to give rise off the comb of the stock. The original design worked perfect; however, within a year, the one and only glue that would work with the set-up was discontinued and the formulation changed. It was a Non-Flam Aircraft contact cement that was very thin and was applied through pressure and would vacate water/moisture when under pressure then the glue would fill the void, but remain flexible. Kind of like laminated plywood. The folks who regulate solvents saw fit to ban the key ingredient in the glue that made the whole process work with the pressure molds I had designed. The original molds I had made was for an exact fit for the issue M14 stock.

What I am looking to do is still use the same outer shell, but have a closed cell foam that is the correct height and thickness. There should be a company that can either make the interior riser by injection molding the foam or maybe using a CNC or 3D Printed part. With the old design, one could vary the height needed by adding sheet foam or foam tape to give extra rise off the comb of the stock. With a proper molded foam interior, the piece will conform to many other stock profiles and has been used on the M40A1 and similar stocks. Up until the glue fiasco happened, about 1997, the USMC at Quantico, VA, was considering and evaluating its use on their then DMR which used the M14 platform. The lack of a suitable glue shot the whole project.

Any help, knowledge, or advice in the realm of injection molded/3D Printed products would be greatly appreciated.

Here is a photo of the cheek pad from Chandler's Book "Death From Afar I" showing how the cheek pad mounts to the stock. The whole point was to be functional, but still retain a military appearance similar to the M1C/D cheek pad. I also provided two photos so you can see interior of the original piece showing the laminated leather.

Semper Fi,

Richard Turner

http://i1273.photobucket.com/albums/y417/tursad/M14-M1A%20Cheek%20Pad/M14M1ACheekPad_zpsa3fc95c9.jpg (http://s1273.photobucket.com/user/tursad/media/M14-M1A%20Cheek%20Pad/M14M1ACheekPad_zpsa3fc95c9.jpg.html)

http://i1273.photobucket.com/albums/y417/tursad/M14-M1A%20Cheek%20Pad/IMG_20150912_155203_zpsboruqxfb.jpg (http://s1273.photobucket.com/user/tursad/media/M14-M1A%20Cheek%20Pad/IMG_20150912_155203_zpsboruqxfb.jpg.html)

http://i1273.photobucket.com/albums/y417/tursad/M14-M1A%20Cheek%20Pad/IMG_20150912_155229_zps24htxyf7.jpg (http://s1273.photobucket.com/user/tursad/media/M14-M1A%20Cheek%20Pad/IMG_20150912_155229_zps24htxyf7.jpg.html)

http://i1273.photobucket.com/albums/y417/tursad/M14-M1A%20Cheek%20Pad/IMG_20150912_155325_zpsdnv0qd5r.jpg (http://s1273.photobucket.com/user/tursad/media/M14-M1A%20Cheek%20Pad/IMG_20150912_155325_zpsdnv0qd5r.jpg.html)

[george r]

That looks like an interesting project. I have had a brush or 2 with injection molding and unfortunately you said "Any help, knowledge, or advice in the realm of injection molded/3D Printed products would be greatly appreciated." giving me an opening for little more than gossip. Actually its because I like the sling I got from you so much I'll tell you what little I know.
If you want to try a 3D printed form inside your cheekpiece, most of those processes are not very strong. Look on the web for Shapeways. They are kind of inexpensive but they are valuable because they can print in nylon. You will need to supply a CAD model. You could get a part for around $15-$50 depending on complexity or weight and print time. A CAD model is nice because you design in the tapers and swoops and stock fit interface to exactly what you envision, though I can see your leather covering can only morph so far.
I'm betting you were considering rubber foam because a hard cheek weld may not be necessary with a scope. But if you considered solid rubber maybe with lightening holes you might be able to mold that in a pourable mold. A 2 part urethane mixed and poured into a silicone mold. You can buy those materials at McMaster-Carr. You could carve a wood pattern to make the female mold in silicone rubber, which could peel off the pattern and then peel off the molded part. It might last for 50 parts before it tore. Some castable urethanes are not long lived and will revert to their monomers after time so you would need to learn which is appropriate.
You could get a 3D service house to make a nicer pattern from your CAD model and they could make an ABS form using the same silicon mold process. That could be tough even if it were hollow. That process might cost less than $1000 but piece parts might still be $40. They can work with you to get to you target cost.
If you wanted foam rubber consider RIM reaction injection molding. it is a low pressure process, maybe 200 psi that generates it's pressure in a lightweight mold when the 2 part elastomer reacts and foams and fills the mold. The part has a skin on it and you can vary the density by experiment. It might even look good enough to be visible on the ends. Kind of like a nerf toy football but you can adjust the density.
I found real injection molding to be very expensive because of the cost to fabricate the molds. If you are making only 1000 pieces each piece would have $30 mold cost even for cheap aluminum molds. It is very common to have them made in China at a quarter or a third of domestic manufacturing costs and then join the offshoring world. Hope this helps and good luck

[Richard Turner/Turner Saddlery]

george r

Sir:

Thank you for your expertise on this subject. I had envisioned using something like closed cell foam such as used in the 1970's era issue OD sleeping bag mats or a sponge type rubber such as used in flip-flops (thongs) but not quite as stiff as that used as the foot bed section of Birkenstock sandals. I spoke with a tech at a foam products company yesterday and after reviewing photos of the cheek pad, he stated that the thin taper on the edges of the insert would rule out being cut by the water-jet system they use. I may be able to have a the width of material I need with the material cut in the flat, in strips and they could skive (taper) the edge on each side and leave the molding up to me. I have tinkered with some of the closed cell foam aforementioned and it seems it will mold and compress slightly under moderate heat. If I go this route, I guess I'll have to come up with an aluminum two piece mold that can be heated. I do have an older pneumatic large format hot foil stamp machine with adjustable heat controls that might be converted to use a two piece mold. I used to use it to brand leather coasters before I acquired a more advanced model.

I am also waiting on some samples of extruded rubber to come in, its called dual-durometer, and is close to the shape I need. It is not a hard rubber, but is a little more dense than foam rubber and will compress slightly, which should help conform to the comb of the stock.

Thank you again for your advice.

Regards,

Richard


That looks like an interesting project. I have had a brush or 2 with injection molding and unfortunately you said "Any help, knowledge, or advice in the realm of injection molded/3D Printed products would be greatly appreciated." giving me an opening for little more than gossip. Actually its because I like the sling I got from you so much I'll tell you what little I know.
If you want to try a 3D printed form inside your cheekpiece, most of those processes are not very strong. Look on the web for Shapeways. They are kind of inexpensive but they are valuable because they can print in nylon. You will need to supply a CAD model. You could get a part for around $15-$50 depending on complexity or weight and print time. A CAD model is nice because you design in the tapers and swoops and stock fit interface to exactly what you envision, though I can see your leather covering can only morph so far.
I'm betting you were considering rubber foam because a hard cheek weld may not be necessary with a scope. But if you considered solid rubber maybe with lightening holes you might be able to mold that in a pourable mold. A 2 part urethane mixed and poured into a silicone mold. You can buy those materials at McMaster-Carr. You could carve a wood pattern to make the female mold in silicone rubber, which could peel off the pattern and then peel off the molded part. It might last for 50 parts before it tore. Some castable urethanes are not long lived and will revert to their monomers after time so you would need to learn which is appropriate.
You could get a 3D service house to make a nicer pattern from your CAD model and they could make an ABS form using the same silicon mold process. That could be tough even if it were hollow. That process might cost less than $1000 but piece parts might still be $40. They can work with you to get to you target cost.
If you wanted foam rubber consider RIM reaction injection molding. it is a low pressure process, maybe 200 psi that generates it's pressure in a lightweight mold when the 2 part elastomer reacts and foams and fills the mold. The part has a skin on it and you can vary the density by experiment. It might even look good enough to be visible on the ends. Kind of like a nerf toy football but you can adjust the density.
I found real injection molding to be very expensive because of the cost to fabricate the molds. If you are making only 1000 pieces each piece would have $30 mold cost even for cheap aluminum molds. It is very common to have them made in China at a quarter or a third of domestic manufacturing costs and then join the offshoring world. Hope this helps and good luck

[StockDoc]

Mr Turner, you might want to talk to these people, they make a waterbase Polyurethane glue that is used on leather. Might help in the right direction.

http://www.bondpolymers.com/polyurethane-leather.html

[george r]

Sounds like you are rolling, If you are considering an extrusion which by its nature has a constant cross section you might consider paying for a custom die. In aluminum extrusions something that would fit in a 4 inch circle would (when I was buying them years ago) cost only about $600. A die for extruded foam or plastic could be a lot cheaper. Also you can cut some foams with a hot wire and kind of end up with an extrusion. But maybe not too long at a time.

[NMC_EXP]

Richard

I was a non-metallic materials engineer for a heavy equipment manufacturer. Mostly worked with rubber but had experience with plastics, both injection molded and reaction-injection molded.

There was a big shake up 20 years ago due to the greens getting so called "ozone depleting substances" banned. In addition to Freon that eliminated a lot of common solvents used in adhesives.

If I understand you, you are looking for a foam insert to be used with a leather outer shell?

The only foam product we had was a reaction-injection molded engine oil pan cover for sound reduction. The material was a polyurethane. The molding process was two stage: (1) Inject the hard outer shell, (2) remove the outer shell from its mold, insert it into mold #2 then inject a layer of foam onto the interior of the shell (at the same time another outer shell was being produced in the other mold).

Final product had an smooth, solid outer skin with a layer of urethane foam bonded to the inside of the shell.

If all you need is a foam profile you might consider "CNC knife cutting" or "water jet cutting" of bulk foam block or sheet. This is an easy way to do prototypes if not actual production.

I've been away from this for a decade and no longer have access to resources at work. Best bet would be to do a search for "foam prototypes".

There were a lot of smaller rubber/plastics companies which were willing if not specialized to do limited prototype runs.

http://www.westernrubber.com/capabilities

Western rubber has foam capabilities with CNC knife and water jet cutting.