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In another post was asked by brino to start a post on resin casting so here goes.
This will be a very basic overview for now as I am just getting back into this after a break of many years. As a result I don't have a lot of photos to use, and no current but in later posts after I have made some stuff I can get into more detail if there is interest.
Resin can be a useful material for many projects. It is a relatively easy and cheap method for individuals to duplicate a part or for creating a piece of material that can then be formed using other tools.
Resin is very popular with model makers as it can make parts with greater detail than injection molded plastic. It also requires far fewer resources / less investment so is better suited to niche subjects. A resin kit can be cost effective with only hundreds or even dozens of kits sold, where an injection molded kit has to sell thousands of kits to turn a profit. There is a large resin aftermarket ranging from parts to enhance detail, and conversion parts to change a model kit into a different variant to full kits of subjects ignored by the large kit makers. There is even some use by the large model companies for special edition kits with enhanced detail or unusual variants that don't justify the costs of tooling up new plastic parts.
Resin is also popular with prop makers and costumers, puppetry etc.
Many resins are machinable and resin blanks are are used by pen makers.
Casting resin is generally a 2 part material similar to epoxy or the binder used in fiberglass. There are two common types of casting resin, polyester based and polyurethane based. Polyester based resins tend to have long cure times, have a much stronger smell and frequently have significant shrinkage. Polyurethane resins have very little shrinkage and a much less pungent aroma so have become more popular for casting projects.
As with any chemicals precautions should be taken to avoid skin contact or breathing the fumes from the resin components. Once cured resin is inert and fairly harmless, although when cut or sanded (and I assume machined) it creates a very fine dust which is an irritant and a dust mask is recommended. Wet sanding is also effective at reducing the dust produced.
Resin comes in a variety of colors, the most common resins are white to light tan but there are translucent and transparent resins including "water clear" resins which as the name suggests are very clear. There are dies and other additives that can be used to change the color, consistency and other characteristics. Metal powders are available to give the resin a convincing metallic appearance. The quantity used will change the depth of color, and by varying how well it is mixed can result in a uniform color throughout, or swirls and spots. Since these are mixed into the resin, the colors go all the way through the part, it is not just on the surface. Mixing resins of different colors can also be done to provide multi color effects.
Materials can be added into the resin while it is being poured, or the resin just poured over it. Inserting a metal rod or wire to give a thin piece added strength is common and a dense material could be used to give more weight to the base of a cast item for a more solid heft or greater stability in a tall part. For parts that are going to be machined, adding materials like scrap pieces of exotic wood inside the resin can create an interesting appearance to the finished part when they are exposed.
It is common to place items inside a clear resin part which can then be seen embedded inside the part. Of course items placed inside resin need to be compatible with the resin. Incompatible materials can have a variety of results but the most common reaction is the inhibition of the resin from properly curing.
Using the resin
After measuring the two parts out by weight or volume (varies by the specific resin) they are mixed together which creates a chemical reaction. Most resins are a 1-1 ratio. Curing time varies from just a couple minutes to an hour or more depending on the resin used. The end product resembles plastic, but there are a lot of different types of resin available, some hard and brittle, some soft semi-flexible, there are even some with a rubber like consistency.
Fast curing resins have the advantage of being able to make many parts in a short period of time. A fast curing time means an even shorter working time, leaving little time to mix the resin, pour into the molds and to do any final actions preparing the mold.
Long curing times can provide a great deal of working time, but of course limit how many parts can be cast in a session. Curing times of 5-20 minutes are common.
Molds
The molds for resin casting are generally made from latex or silicone. Latex is cheaper and is better for some types of casting. Silicone is generally the preferred mold material. Like resin the mold making material is generally a 2 part material that may be mixed by weight or volume depending on the material. Some popular silicone mold material is mixed 1-1 which makes measuring easy, but a 10-1 mix is more common. The catalyst side is often dyed to help see that it has been well mixed into the base material. Mold materials usually have a much longer set up time than resin, often with a working time of 20-30 minutes and curing time of 4-24 hours.
Silicone molds can capture extreme detail with some able to replicate detail at a microscopic level. Like resins there are different grades of silicone with different strengths, stretchiness (allowing more undercut detail in a part without tearing the mold), curing time, and form. There are additives to thin or thicken a liquid mold making material. There are also mold making putties that can be used to make a mold of a part that can not easily be cast in a conventional mold, such as a raised emblem attached to a large item.
Silicone molds will degrade through use, small bits will occasionally be torn lose while removing parts, and the mold may tear. Undercut details are particularly hard on a mold as the repeated stretching of parts coming out will result in damage. The life span of a mold varies greatly based on the specifics of the part being cast and the level of detail required on the part. A very complex mold requiring a high level of detail on the cast part may only be good for a half dozen casts and could be do badly damaged after the first use to continue using. A well designed mold will generally be good for 20-50 castings, and a simple part with simple detail could be good for hundreds of casts.
Single sided molds are the easiest to make, but limit the type of detail that can be captured. Two sided molds are more difficult and time consuming to make / use but can allow for very detailed parts to be cast make. It is possible to make even more complex molds using 3 or more sections, but generally it is easier to split the part into multiple pieces and cast them individually.
Plastic sheets can be a cheap and easy way to make mold forms. Legos or Megablocks blocks make a great reusable form. A plastic cutting board is a good working surface since silicone and resin do not stick well to most.
The curing process for resin, latex or silicone is a heat reaction, so the ambient temperature as well as temperature of the materials when used can speed up or retard the curing time significantly. Keeping the material cold prior to use can slow down the curing time, but it is possible to slow the curing rate so much that it fails to fully cure. Similarly a cold mold can have a negative impact on the curing rate / quality. Some resins require that the mold be heated to 100 f (38c) or more prior to use for the best result, while others work just fine at room temperature. Silicone is very heat tolerant, and also used for making molds for casting with "low temperature" metal alloys with melting temps up to 300 f (150c). Using a fast curing resin that has been stored in a hot location can result in an extremely short working time.
The shelf life of resin casting materials and silicone mold making materials can be quite long when unopened, but once opened will begin to degrade, so it is best to buy in quantities that can be used up within a couple of months, although there are air displacement products available that will extend the storage life. Once they have been used to make a mold or cast into a part, the materials are very stable.
Issues and some advanced techniques
The major issues when making a mold is to avoid large undercuts in the mold. An undercut is where part of the mold will go over the part, essentially retaining the part inside the mold. Silicone is very flexible so it will tolerate some undercut, but this creates a weak point where over time the mold will likely tear. Small tears are usually not a problem, and it fact it is common to slit the silicone mold when casting complex parts. These do create weak point though and eventually piece of the silicone will be torn lose either removing a detail from the part, or creating an undesired flaw which will have to be removed from the part before use. The two major ways to eliminate a large undercut is to redesign the part, or to make a multipart mold spliting at a point that eliminates or at least reduces the undercut.
The second major issue is air bubbles. This is a bigger issue for the resin, but one of the common ways to reduce bubbles in the resin, will highlight bubbles in the mold. To some extent air bubbles can be reduced through technique and design of the mold. Thinking like an air bubble and eliminating places where bubbles can become trapped will eliminate a large portion of them.
The mixing process results in adding a lot of air to both the resin and silicone. Some of this can be reduced with careful mixing, but the nature of mixing is going to add air no matter how careful and trying to hard not to get air bubbles can have the result of not getting the 2 parts fully mixed which is a bigger problem. Badly mixed the product will not fully cure. Even thin resins are syrupy, so air bubbles do not easily escape. Silicone is even thicker.
The pouring technique is another way to reduce trapped air. By holding the container several inches above what it is being poured into / onto and pouring slowly so the stream is thin, many air bubbles will be allowed to escape.
When pouring the silicone mold material over a part, care should be used to allow the material to pool and rise slowly, and avoid trapping air around the part. A toothpick can be used to prod around the part while the silicone is still liquid if trapped air is suspected.
When pouring resin into a mold it is also important not to block the entrance to the mold with resin, so the air in the mold can escape.
With slower curing resins, air bubbles can often be massaged out of the mold and the the resin carefully pushed back in or additional resin added to fill the void.
For parts where some repair of the part with a filler putty is acceptable, these methods are often adequate.
Clear resins are much more difficult as the techniques above primarily eliminate surface bubbles, that effect the detail. Internal air bubbles are less of an issue with an opaque resin unless they create a weak spot. With a clear resin internal air bubbles will be visible and even small air bubbles can have a negative effect on the appearance of the part.
There are three common methods for eliminating air bubbles, vacuum and pressure.
By putting the resin or silicone in a vacuum chamber the liquid material will foam as the air bubbles expand and pop. The use of vacuum is also known as degassing. As the product will expand to 2x or more in size it is critical that the volume of the material be much less than the capacity of the container or it will bubble over. As this is done after mixing the product it is only suitable for products with long working times. This is also usually done before pouring the material to make a mold or to cast a part so a downside is there is a possibility that air can still get trapped while pouring. Vacuum is impractical for use on resin after being poured into a mold, as it will cause some of the resin to be forced out of the mold along with the air. Vacuum can be used on an uncured mold to help eliminate any fresh air that was trapped during the pour. This is a fairly slow process not practical for use with many resins.
The other method is using a pressure pot. With this method instead of using vacuum to draw the air out, pressure is used to crush the bubbles as well as to force some of the air back into solution (think of the reverse of taking the lid off a bottle of soda, when you open the lid, pressure is released and bubbles form). This does not actually remove the air, but it makes the bubbles very small. Typical pressures used are 40-80psi or about 3-6x normal atmospheric pressure. This pressure is maintained until the material solidifies so the air can not re-expand when the pressure is released. Because this is a faster process than vacuum it can be used with relatively quick setting resins.
The third common method is a hybrid method using vacuum to degass the silicone, and pressure to reduce the bubbles in the resin. Because silicone is flexible, silicone used to pressure cast resin has to be treated with vacuum or pressure, or the trapped air can become an issue even in a cured mold. Air bubbles in a mold made under normal pressure may not be visible, but under pressure these bubbles will constrict resulting in small bumps and irregularities on the surface of the cast part.
Adding a pressure pot and / or vacuum chamber along with the related hardware (air compressor, vacuum pump etc) to a resin casting operation adds significantly to the initial cost but results in much higher quality castings. There are of course DIY solutions out there and most resin casting pressure pots are just converted paint pots.
Finally a few examples (pictures
)
Setting up a multi piece single sided mold. This is actually not ideal as there is a significant undercut on many of these pieces. However by slitting the silicone along the length of the part I was able to make 5-10 copies of these parts before the mold started to fail. As this was for personal use, that gave me years worth of parts even with the high cull rate for malformed parts. Yes I sue Legos for the forms. You can buy individual blocks from Lego, so ordered several useful sizes and color coded them to make it easy to find the size I need. Each piece is sitting on a blob of special model clay compatible with the silicone (many model clays are not compatible). When done there will be a small pour stub left that can be cut off the part.
After adding the mixed silicone
For these wheels and rescue litters I made two part molds. As with the first mold shown the wheels are sitting on a blob of clay holding them at the right height and creating a pour stub. The litters were a little tricker and were just floated on the silicone. To make the cast of the litters, some resin is poured into the mold and the two halves smashed together, any excess resin being squeezed out of the mold at the seam line.
The second photo here shows the first stage of the mold. After this cured I removed the small bits of silicone stuck to the parts, then brushed some warmed (liquified) Vaseline over the silicone to prevent the second half from sticking to the first
Example of parts, these parts were made without vacuum or pressure so you can see both the quality that is possible just by using some care and "thinking like an air bubble, as well as some of the small air bubbles that are inevitable without vacuum or pressure. These of course were selected from the best examples, and there is a high cull rate, although even malformed parts can often be used for other things, like a spare tire where the bad side won't show.
Center console for an emergency vehicle
Tires / wheels
Rescue litters
This will be a very basic overview for now as I am just getting back into this after a break of many years. As a result I don't have a lot of photos to use, and no current but in later posts after I have made some stuff I can get into more detail if there is interest.
Resin can be a useful material for many projects. It is a relatively easy and cheap method for individuals to duplicate a part or for creating a piece of material that can then be formed using other tools.
Resin is very popular with model makers as it can make parts with greater detail than injection molded plastic. It also requires far fewer resources / less investment so is better suited to niche subjects. A resin kit can be cost effective with only hundreds or even dozens of kits sold, where an injection molded kit has to sell thousands of kits to turn a profit. There is a large resin aftermarket ranging from parts to enhance detail, and conversion parts to change a model kit into a different variant to full kits of subjects ignored by the large kit makers. There is even some use by the large model companies for special edition kits with enhanced detail or unusual variants that don't justify the costs of tooling up new plastic parts.
Resin is also popular with prop makers and costumers, puppetry etc.
Many resins are machinable and resin blanks are are used by pen makers.
Casting resin is generally a 2 part material similar to epoxy or the binder used in fiberglass. There are two common types of casting resin, polyester based and polyurethane based. Polyester based resins tend to have long cure times, have a much stronger smell and frequently have significant shrinkage. Polyurethane resins have very little shrinkage and a much less pungent aroma so have become more popular for casting projects.
As with any chemicals precautions should be taken to avoid skin contact or breathing the fumes from the resin components. Once cured resin is inert and fairly harmless, although when cut or sanded (and I assume machined) it creates a very fine dust which is an irritant and a dust mask is recommended. Wet sanding is also effective at reducing the dust produced.
Resin comes in a variety of colors, the most common resins are white to light tan but there are translucent and transparent resins including "water clear" resins which as the name suggests are very clear. There are dies and other additives that can be used to change the color, consistency and other characteristics. Metal powders are available to give the resin a convincing metallic appearance. The quantity used will change the depth of color, and by varying how well it is mixed can result in a uniform color throughout, or swirls and spots. Since these are mixed into the resin, the colors go all the way through the part, it is not just on the surface. Mixing resins of different colors can also be done to provide multi color effects.
Materials can be added into the resin while it is being poured, or the resin just poured over it. Inserting a metal rod or wire to give a thin piece added strength is common and a dense material could be used to give more weight to the base of a cast item for a more solid heft or greater stability in a tall part. For parts that are going to be machined, adding materials like scrap pieces of exotic wood inside the resin can create an interesting appearance to the finished part when they are exposed.
It is common to place items inside a clear resin part which can then be seen embedded inside the part. Of course items placed inside resin need to be compatible with the resin. Incompatible materials can have a variety of results but the most common reaction is the inhibition of the resin from properly curing.
Using the resin
After measuring the two parts out by weight or volume (varies by the specific resin) they are mixed together which creates a chemical reaction. Most resins are a 1-1 ratio. Curing time varies from just a couple minutes to an hour or more depending on the resin used. The end product resembles plastic, but there are a lot of different types of resin available, some hard and brittle, some soft semi-flexible, there are even some with a rubber like consistency.
Fast curing resins have the advantage of being able to make many parts in a short period of time. A fast curing time means an even shorter working time, leaving little time to mix the resin, pour into the molds and to do any final actions preparing the mold.
Long curing times can provide a great deal of working time, but of course limit how many parts can be cast in a session. Curing times of 5-20 minutes are common.
Molds
The molds for resin casting are generally made from latex or silicone. Latex is cheaper and is better for some types of casting. Silicone is generally the preferred mold material. Like resin the mold making material is generally a 2 part material that may be mixed by weight or volume depending on the material. Some popular silicone mold material is mixed 1-1 which makes measuring easy, but a 10-1 mix is more common. The catalyst side is often dyed to help see that it has been well mixed into the base material. Mold materials usually have a much longer set up time than resin, often with a working time of 20-30 minutes and curing time of 4-24 hours.
Silicone molds can capture extreme detail with some able to replicate detail at a microscopic level. Like resins there are different grades of silicone with different strengths, stretchiness (allowing more undercut detail in a part without tearing the mold), curing time, and form. There are additives to thin or thicken a liquid mold making material. There are also mold making putties that can be used to make a mold of a part that can not easily be cast in a conventional mold, such as a raised emblem attached to a large item.
Silicone molds will degrade through use, small bits will occasionally be torn lose while removing parts, and the mold may tear. Undercut details are particularly hard on a mold as the repeated stretching of parts coming out will result in damage. The life span of a mold varies greatly based on the specifics of the part being cast and the level of detail required on the part. A very complex mold requiring a high level of detail on the cast part may only be good for a half dozen casts and could be do badly damaged after the first use to continue using. A well designed mold will generally be good for 20-50 castings, and a simple part with simple detail could be good for hundreds of casts.
Single sided molds are the easiest to make, but limit the type of detail that can be captured. Two sided molds are more difficult and time consuming to make / use but can allow for very detailed parts to be cast make. It is possible to make even more complex molds using 3 or more sections, but generally it is easier to split the part into multiple pieces and cast them individually.
Plastic sheets can be a cheap and easy way to make mold forms. Legos or Megablocks blocks make a great reusable form. A plastic cutting board is a good working surface since silicone and resin do not stick well to most.
The curing process for resin, latex or silicone is a heat reaction, so the ambient temperature as well as temperature of the materials when used can speed up or retard the curing time significantly. Keeping the material cold prior to use can slow down the curing time, but it is possible to slow the curing rate so much that it fails to fully cure. Similarly a cold mold can have a negative impact on the curing rate / quality. Some resins require that the mold be heated to 100 f (38c) or more prior to use for the best result, while others work just fine at room temperature. Silicone is very heat tolerant, and also used for making molds for casting with "low temperature" metal alloys with melting temps up to 300 f (150c). Using a fast curing resin that has been stored in a hot location can result in an extremely short working time.
The shelf life of resin casting materials and silicone mold making materials can be quite long when unopened, but once opened will begin to degrade, so it is best to buy in quantities that can be used up within a couple of months, although there are air displacement products available that will extend the storage life. Once they have been used to make a mold or cast into a part, the materials are very stable.
Issues and some advanced techniques
The major issues when making a mold is to avoid large undercuts in the mold. An undercut is where part of the mold will go over the part, essentially retaining the part inside the mold. Silicone is very flexible so it will tolerate some undercut, but this creates a weak point where over time the mold will likely tear. Small tears are usually not a problem, and it fact it is common to slit the silicone mold when casting complex parts. These do create weak point though and eventually piece of the silicone will be torn lose either removing a detail from the part, or creating an undesired flaw which will have to be removed from the part before use. The two major ways to eliminate a large undercut is to redesign the part, or to make a multipart mold spliting at a point that eliminates or at least reduces the undercut.
The second major issue is air bubbles. This is a bigger issue for the resin, but one of the common ways to reduce bubbles in the resin, will highlight bubbles in the mold. To some extent air bubbles can be reduced through technique and design of the mold. Thinking like an air bubble and eliminating places where bubbles can become trapped will eliminate a large portion of them.
The mixing process results in adding a lot of air to both the resin and silicone. Some of this can be reduced with careful mixing, but the nature of mixing is going to add air no matter how careful and trying to hard not to get air bubbles can have the result of not getting the 2 parts fully mixed which is a bigger problem. Badly mixed the product will not fully cure. Even thin resins are syrupy, so air bubbles do not easily escape. Silicone is even thicker.
The pouring technique is another way to reduce trapped air. By holding the container several inches above what it is being poured into / onto and pouring slowly so the stream is thin, many air bubbles will be allowed to escape.
When pouring the silicone mold material over a part, care should be used to allow the material to pool and rise slowly, and avoid trapping air around the part. A toothpick can be used to prod around the part while the silicone is still liquid if trapped air is suspected.
When pouring resin into a mold it is also important not to block the entrance to the mold with resin, so the air in the mold can escape.
With slower curing resins, air bubbles can often be massaged out of the mold and the the resin carefully pushed back in or additional resin added to fill the void.
For parts where some repair of the part with a filler putty is acceptable, these methods are often adequate.
Clear resins are much more difficult as the techniques above primarily eliminate surface bubbles, that effect the detail. Internal air bubbles are less of an issue with an opaque resin unless they create a weak spot. With a clear resin internal air bubbles will be visible and even small air bubbles can have a negative effect on the appearance of the part.
There are three common methods for eliminating air bubbles, vacuum and pressure.
By putting the resin or silicone in a vacuum chamber the liquid material will foam as the air bubbles expand and pop. The use of vacuum is also known as degassing. As the product will expand to 2x or more in size it is critical that the volume of the material be much less than the capacity of the container or it will bubble over. As this is done after mixing the product it is only suitable for products with long working times. This is also usually done before pouring the material to make a mold or to cast a part so a downside is there is a possibility that air can still get trapped while pouring. Vacuum is impractical for use on resin after being poured into a mold, as it will cause some of the resin to be forced out of the mold along with the air. Vacuum can be used on an uncured mold to help eliminate any fresh air that was trapped during the pour. This is a fairly slow process not practical for use with many resins.
The other method is using a pressure pot. With this method instead of using vacuum to draw the air out, pressure is used to crush the bubbles as well as to force some of the air back into solution (think of the reverse of taking the lid off a bottle of soda, when you open the lid, pressure is released and bubbles form). This does not actually remove the air, but it makes the bubbles very small. Typical pressures used are 40-80psi or about 3-6x normal atmospheric pressure. This pressure is maintained until the material solidifies so the air can not re-expand when the pressure is released. Because this is a faster process than vacuum it can be used with relatively quick setting resins.
The third common method is a hybrid method using vacuum to degass the silicone, and pressure to reduce the bubbles in the resin. Because silicone is flexible, silicone used to pressure cast resin has to be treated with vacuum or pressure, or the trapped air can become an issue even in a cured mold. Air bubbles in a mold made under normal pressure may not be visible, but under pressure these bubbles will constrict resulting in small bumps and irregularities on the surface of the cast part.
Adding a pressure pot and / or vacuum chamber along with the related hardware (air compressor, vacuum pump etc) to a resin casting operation adds significantly to the initial cost but results in much higher quality castings. There are of course DIY solutions out there and most resin casting pressure pots are just converted paint pots.
Finally a few examples (pictures
)Setting up a multi piece single sided mold. This is actually not ideal as there is a significant undercut on many of these pieces. However by slitting the silicone along the length of the part I was able to make 5-10 copies of these parts before the mold started to fail. As this was for personal use, that gave me years worth of parts even with the high cull rate for malformed parts. Yes I sue Legos for the forms. You can buy individual blocks from Lego, so ordered several useful sizes and color coded them to make it easy to find the size I need. Each piece is sitting on a blob of special model clay compatible with the silicone (many model clays are not compatible). When done there will be a small pour stub left that can be cut off the part.
After adding the mixed silicone
For these wheels and rescue litters I made two part molds. As with the first mold shown the wheels are sitting on a blob of clay holding them at the right height and creating a pour stub. The litters were a little tricker and were just floated on the silicone. To make the cast of the litters, some resin is poured into the mold and the two halves smashed together, any excess resin being squeezed out of the mold at the seam line.
The second photo here shows the first stage of the mold. After this cured I removed the small bits of silicone stuck to the parts, then brushed some warmed (liquified) Vaseline over the silicone to prevent the second half from sticking to the first
Example of parts, these parts were made without vacuum or pressure so you can see both the quality that is possible just by using some care and "thinking like an air bubble, as well as some of the small air bubbles that are inevitable without vacuum or pressure. These of course were selected from the best examples, and there is a high cull rate, although even malformed parts can often be used for other things, like a spare tire where the bad side won't show.
Center console for an emergency vehicle
Tires / wheels
Rescue litters
