Resin parts; why are they made and how do you work with them

Hey Brian,

Resin parts are usually highly detailed add-ons.  Sometimes the manufacturer of the kits makes the resin parts, sometimes they are outsourced from a more specialized company and added into the kit.  They're usually made to replace some of the less detailed mold-injected plastic pieces, or to enhance the detail that is already there.

For instance, I'm working on a Hasegawa Hellcat.  The cockpit was very basic and plain with minimal detail.  So I ordered a Verlinden resin cockpit upgrade kit.  Now this kit is very detailed.  All the plumbing is visible as are all the dials, switches, knobs and guages.  I also plan to enhance this with a PE kit from Eduard.  The result is a model that looks like a miniature plane instead of a toy.

As for cutting the parts out, you can use an x-acto, but it's going to be a long, drawn out, process.  I suggest looking into a dremel/rotory tool.  It'll speed up the cutting process and has a lot of handy attachments.  It'll become an essential tool in your arsenal.

-Fred

Thanks for that help, Fred.  I take it then that resin somehow better lends itself to more detailed molds.

Wish you well on the Hellcat.

Brian

Brian

The dust from resin parts is nasty stuff that you do not want to inhale.  When cutting or sanding the resin parts, use a dust mask or respirator.  Always wet sand to keep the dust down.  When I use a razor saw to cut off casting blocks I will wet that as well to limit the dust.  I rarely use a Dremel for this since it will put a large amount of dust in the air and can contaminate a wide area.

Marc,

I don't believe I would have thought of that.  I do have a respirator for airbrushing.  Will use that, thanks.

I've wondered this too - isn't a mold a mold?  How can it be any easier to detail the mold for resin that it is to detail the mold for styrene?  I would think the only advantage would be for smaller (thinner) parts - but then I see people saying that resin parts are brittle and sometimes break while still in the package.

What is the advantage ofusing resin as a material, rather than using styrene for everything?

Or is it purely and simply a question of being able to charge more money for something you can call a "multimedia" kit?

While I do not profess to know the manufacturing processes for both of them perfectly, there are some very basic differences.  An injected plated model is just that... hot plastic injected into a mold that has 2 sides together.  Because it has 2 side, a part can't have ½ more than 180 degrees around. As in dimension around landing gear strut having the mold line running down the length of both sides.  If it were more on one side than the other it would not come out of that half of the mold. 

This is where I have to assume some things since as I said... I am not sure what material is used for the mold for resin.  So I am going to assume they are similar to those that can be made with some of the AM products used for homemade casting.  Once the detailed master is made, a mold is made that will allow a cast part to be removed with 3D detail.  Again I assume, it is soft enough for parts to be pulled out and then go back to shape for more castings.  And since it is a resin that cures from a catalytic action, and while it may generate some heat from that process,  it is not hot like melted plastic so the mold can be soft enough to allow parts that have a relief surface greater than 180 degrees.

The advantage to all of this is to make better detail not necessarily more detail.  Look at a pipe running thought a wheel well on an injection molded kit.  Round on top but has straight sides down to the bulkhead or whatever the "pipe" is running along.  The resin cast of the same thing will have a better 3D pipe as it will be more tubular.

Anyone that can give more of a "technical" description, please do as I would like to hear it as well.

Cured resin dust is no nastier than any fine dust. It's an irritant but not toxic.  A dust mask would be more than asequate. A respirator is overkill. "Dust from partially cured epoxy can cause respiratory problems if inhaled, and dermatitis if allowed to settle on skin. Dust from fully cured epoxy is inert and considered a nuisance dust."    Source

A simple razor saw or a fine saw blade inyour #1 or #2 handle will do the trick.

Plastic is formed in large injection molding machines under heat and pressure. The top end machines, such as the ones touted by Dragon Next Generation kits have mulitple parts and allow more detailing but are still limited in how fine a detail can be achived.

Resin is poured into a one or two piece mold and requires not heat. It cures more slowly. The nature of the mold allows for finer detailing and even detailing on undercut surfaces. I have worked with a couple Michael Roberts bases that have texture on the bottom of overhanging rocks.

I just started working with my first resin parts (Classic Airframes 1/48 Bf109D).  It's actually pretty easy to work with.  Get yourself a nice X-acto razor saw set.  I picked mine up at my LHS (High Sierra Models) for about $15.  It came with a nice sturdy handle and 3 razor saw blades. 

I use the razor saw to cut off the big blocks.  What works for me is taking a small wood cutting board, hold the part down firmly and then carefully begin sawing the block off.  Don't cut too close to the part so you don't risk accidently cutting the part.  It's almost impossible not to make a straight cut with that razor saw set.

I got a really nice set of sanding sticks which I then use to sand the rest of the resin block, the coarser ones to wear it down quickly, then the finer ones to get a nice smooth finish/edge right down to the part. 

A set of small/fine files would also be helpful, the really tiny ones.

I do all this work either by the kitchen sink or the bathroom sink downstairs (I soooo need to put a utility sink/counter in the garage ) as sawing and sanding will produce a lot of resin dust. 

Some people go a little overkill and insist you use a respirator.  It's no more hazardous than sawdust, paint dust etc. which you encounter doing "do it yourself" handiman projects around the house.  And just like the dust from those kind of things, you don't want to breath it in.  A mask that covers your mouth and nose will be sufficient.  I rinse the parts frequently as I cut and sand to help keep the dust down and clear it away so I can see what I'm doing as the dust will built up on your hands, saws, sanding sticks and on the parts. 

With a little care and patience while you learn the ins and outs, you'll be a pro with resin in no time.  I can't believe I let it intimidate me for so long before I tried it.  Piece of cake.

Most resin parts are relatively expensive because they are hand-made in silicone molds.  Silicone molds allow for greater detail than a stamped mold (ala injected plastic) will.  You can cast entire assemblies as one piece instead of in halves.

While most are hand made (sculpted, molded and cast by a person), there are rapid protoype printers that will take a 3D computer file and print out a resin part in full 3D - allowing for full undercuts and the works.  The biggest drawbacks for RPPs is the initial cost of the printer.

-Fred

alumni72 wrote:

So in essence the difference is in the material and not the mold?  Thanks for the excellent information, everyone!

Actually, the difference is in both.  Injected plastic parts (99% of model kits)are molten plastic being injected into steel (or aluminium), stamped molds.  They melt down large quantities of plastic, inject it into the metal mold and then cool it.

Resin, on the other hand, is a 2 part chemical, the resin and a kicker (the kicker is what causes the resin to harden).  Since both parts come in liquid form from the start, there is no need for a heat source (though the chemical reaction between the resin and the kicker does generate some heat).  Once the 2 are mixed, it is poured into a silicon mold, where they usually cure overnight.

Here's the general gist of how resin casting works (2 part mold method).  First, you build/sculpt your master.  This is basically a fancy buck.  There are limits to what can be cast (shape and/or size) which is why most resin casting is done in multiple parts.  Now you have to build a mold box.  This is a box that is larger than the piece you are casting.  Some people build boxes out of Lego lined with clay.  With the piece your molding suspended (so it's not touching the bottom of the box), fill the box with liquid silicone so that it comes up to the halfway point of the piece.  Now, you're going to want to create plugs, or keys.  The plugs/keys allow you to mate the 2 halves of the silicone mold without them sliding around.  You're also going to want to lay in a pencil or a straw (somethinthing cylindrical that will create a pour channel) from the piece to the top of the mold box.  You also need to add smaller channels from the piece out to the sides so that when you pour in the resin, the trapped air has someplace to go instead of becoming a big air bubble in your solid resin piece.  Once the silicone has fully cured, remove your buck and start over now creating a negative mold of the other side.  Make sure your plugs and keys line up as well as your channels.  When the second side has fully cured, you are set to go.  Put some mold release agent on the inside of the negative relief (so your resin piece comes out of the mold with out any problems), rubberband the mold halves together and pour in your resin (slowly, to cut down on air bubbles).  If you've done everything right, you should have minimal spillage and overflow.

For anyone interested there are several books dedicated solely to resin casting techniques.

-Fred

dariencharlie wrote:

I just opened a kit which has some resin parts. 1] Why are there resin parts

Resin parts provided with a kit are there to offer more detail than conventional molding processes can.

Resin parts sold seperately are purely designed to offer yet another avenue for the modeler to empty their wallet. [;0]

Woo-hoo, hehehe, hahaha..... I crack me up. LOL 

Hey - I don't have heaps of experience with resin parts, but I've always separated them from the block by scoring right at the junction of the block and part with a sharp hobby knife. I make a few strokes with light pressure on one side, then turn it over and score lightly on the other side.  In about two minutes, the part and block are separated, there is no dust generated, and I have a very clean parting that needs only a nominal amount of light sanding to clean up. 

Hope this helps.