How to Pour Resin without Air Bubbles - Tips for Perfect Castings

How can I avoid Bubbles in my Resin Baby Castings?

If you are experiencing bubbles, whilst Resin Casting 3D Baby Casts or creating Cold Cast Metal Baby Casts, there may be a number of reasons for this. In fact you may be asking yourself the following questions, which I will attempt to answer throught this article:

• How to pour resin without bubbles?
• How to eliminate micro bubbles in resin?
• Why is my resin foaming?
• How to cast urethane without bubbles?
• How to degas resin without a vaccum Chamber?

The most common sources of moisture, which might react with liquid Urethane Resin, are:

• Humidity -  Urethane Resin will react with moisture in the air, particularly if the resin has a long pot life (working time).
• Opening & Closing of Containers – It is important not to repeatedly open and close Parts A & B, more than is necessary, as this can introduce moisture from the air, to the unused portions in the containers.  This will especially be troublesome in damp or humid weather conditions.  After dispersing the required amount of resin for each pour, immediately place the lids back on the containers and store in a dry cool place.  You can also try using XTEND-IT, from Smooth-on, which is designed to extend the shelf life of moisture sensitive polyurethane products by displacing air in the containers.

• Contaminated Mixing Sticks & Containers – some stirring sticks and containers, depending upon what they are made of, can absorb moisture and transfer it to the polyurethane resin. Wooden stirring sticks and paper containers can absorb moisture from damp or humid working environments. Therefore, to avoid this, ensure you are working in a room temperature environment, with good ventilation.  Also, try using only plastic or metal mixing tools and containers, which will not absorb moisture, which could then be introduced to your resin mixture. 

• Applying Excess Release Agent – If you apply too much release agent to your silicone mould, this can generate what looks like, micro bubbles in the cured resin. Only apply a thin spray of the release agent, from a good distance, to ensure a very light coverage.

• Adding Fillers – when adding mixers or fillers, like metal powders, when cold casting, many fillers absorb moisture, which when mixed in with the urethane resin, can cause the resin to bubble or foam.  To prevent this, try spreading your filler out on an ovenproof sheet and bake in a cooking device, which you do not use for domestic purposes (such as a Mini Oven, which you use purely for studio purposes).  Heat at 150 degrees F for a couple of hours, then leave to cool before using. This will help moisture to evaporate from the fillers before casting into the resin.

• Slow Layer Casting – when using a slow setting resin, this can give time for the resin to absorb moisture. To prevent this, ensure that you cast slow setting resins in a room temperature environment. Avoid casting in a damp or humid weather conditions or environments.
• Correct Mixing – not mixing the correct volume of part A and B can also affect the outcome of your resin casting project, as can mixing to enthusiastically, which can introduce air-bubbles.  Ensure that you mix slowly and gently, to avoid introducing air-bubbles, and disperse any visible air bubbles in the mixture before pouring, by pressing them onto the side of the mixing container.  
• Correct Pouring – when pouring the resin into your silicone mould, pour slowly, from a good height, rather than simply dumping the resin into the mould, to avoid introducing air bubbles. Pour high, in one direction, to generate a long, narrow flow. Let the resin find it’s own level within the mould. By pouring from a height, you will create a long, narrow flow of resin, which will slowly fill the cast, rather than filling it with a flood of resin, which is more likely to introduce air bubbles. 

As you can see, there are a number of factors that can affect how resin performs and cures, which in turn will impact upon the outcome and results of your Resin Casting projects.  This includes; the correct mixing of Parts A & B, using the right amount of materials and release agent, surface and environmental contaminants and humidity.

If you are experiencing air bubbles in your Resin Baby Castings, ask yourself these questions: 

• Where have you stored your Resin?  It must be kept as per manufacturers instructions, in a dry, non-humid environment, which will not introduce moisture. 

• Where have you Cast your Resin?  Make sure that you are casting in a non-humid environment, at room temperature. Do not cast in cold, damp envioronments.

• What have you used to mix the Resin? 
  Some mixing sticks and containers can absorb moisture. Therefore, try using only metal or plastic mixing tools.

• Is your mixing apparatus clean and dry? 
  Even the tiniest amount of moisture will contaminate resin, interfering with curing and causing air-bubbles. Make sure that all of your mixing apparatus and work surfaces are completely clean and dry. 

• Have you washed the mould with water? 
  It is fine to wash your silicone mould, in between resin casting, with a little soap and water, but it is important, to ensure that no moisture remains inside, or on the surface of the mould, before you start resin casting.  Dry your moulds thoroughly before casting.  You can also try warming up your moulds, before casting, to ensure there is no surface moisture, which may contaminate the resin.

• Have you done a small cup test?  
  Do a small cup test, as instructed below. This will help to determine if the resin is at fault, or if the problem only occurs when you cast into the mould?

Tips for avoidng and minimising Air Bubbles in your Resin Baby Casts

1.  Perform a Cup Test  - 
   If you are experiencing issues with your resin not curing, or bubbles in your resin casts, you will need to identify and eliminate which variables may be causing this, by performing a cup test.  This involves mixing a small amount of Part A and Part B, and pouring into a disposable polyethylene cup to see what happens. 
   If the resin cures as expected, you can conclude that there is something else affecting the material when poured into your mould. For example, if you discover that your resin is curing as expected in the pot, but is not curing properly when mixed and poured into your silicone mould, there must be a surface contaminant or something in the mould that is interfering with the cure of the resin. 

2. Cast your Resin in a Room Temperature Environment - Resin performs best , when cast in room temperature environment. Resin does not perform well in cold or damp environments. If you are working in an environment, which is below room temperature, this will result in moisture contaminating the resin.  Make sure that you heat up your room, before opening any of the containers or mixing utensils and before you begin mixing or pouring.  Pre-heat your work room and ensure that you generate enough heat, so that the room stays warm enough, throughout the entire mixing, casting and curing process. If you don’t want to switch on your central heating and heat the whole house, use a small electric heater to heat the room you are working in and keep the doors closed.

3. Dry your Silicone Mould before Casting  - 
   It is important that your silicone mould is clean and thoroughly dry. Even the tiniest amount of surface moisture on your mould can contaminate the resin, preventing it from curing and causing air-bubbles.
4. Warm your Silicone Mould before Casting - another thing that can cause issues with air bubbles and resin not curing, is if you are casting into a cold mould.  You should therefore, bring your mould to room temperature, espeically in cooler weather conditions.  Pourng into a cold mould, will cause 'creeping', where the resin resists flowing to the coldest part of the mould (usually the toes in 3d baby castings) and creeps up and away from the cooler parts of the mould, to the warmest part, resulting in large air-bubbles or even missing toes. 
   To avoid this, you can try heating up your silicone mould before resin casting. You can try heating on a radiator or hair-dryer, or ideally,  place the silicone mould within an industrial oven, or Mini Oven, at 150 degrees for about 10-15 minutes. Note: 
   Do not place the silicone moulds in your own domestic oven, used for cooking purposes, as the silicone may exude non-food safe fumes when heated up.
5. Warm your Resin before Casting - 
   This is particularly important if your resin has been kept in a cold environment, below room temperature, or if it has been allowed to freeze.  
   In this case, warm a bowl of water in a microwave, so that it is warm, not hot or boiling.  Place your resin containers in a grip seal plastic bag and allow the bag to sit inside the warm water bath, for about 10 - 15 minutes. (Do this whilst your are warming up your silicone mould).  
   Note:  Do not allow water to leak into your resin containers, as this will contaminate the resin and prevent it from curing.  It is a good idea to warm your resin and silicone moulds, regardless of the time of year, because, depending upon where you have stored them; they may be below room temperature, even in the summer. 
   Be careful not to overheat your resin. If it is too warm when you mix it, you will shorten the pot time (mix time) and you risk it setting before you have completed casting.

6. Take care when mixing - 
   Mix slowly and carefully, whilst scraping the sides and bottom of the cup, to ensure Parts A and B are thoroughly mixed and to avoid introducing air bubbles. If you notice air-bubbles, whilst mixing, before casting, disperse them on the side of the pot with your mixing stick. Note:  Some mixing tools and apparatus can absorb moisture from your work environment.  Therefore, for best results, try using only plastic mixing containers and metal mixing sticks to minimise the chances of moisture contamination.  
   Avoid using, Paper, PVC or Styrofoam Cups, which can absorb moisture.

7. Cast with Care - 
   When casting into your silicone mould, pour your resin, in one direction, from a height, which creates a high, narrow steam. This will minimise air-bubbles entering the resin as you cast it.  
   Whilst pouring the resin, direct the flow of resin towards the highest part of the mould and let it find its own way. It will naturally find it’s way to all recesses of the mould. Casting in this way will minimise air bubbles.  
   Note: Casting resin and plaster is very different; so do not be tempted to cast in the same way that you would when casting plaster baby feet.

8. Apply heat - 
   Once you have cast your moulds, if you have been experiencing issues with air bubbles, you may wish to try applying a heat source, as a means to bring air-bubbles to the surface and disperse them. You can try using a Heat Gun, to draw out and disperse surface bubbles.  Move the Heat Gun quickly over the resin cast, at a height, no closer than 1 ½ inches away, to avoid damaging the mould. Repeat as necessary to remove air bubbles from the surface of the resin.   Warming the mould and resin will help cure the resin quicker. Note:  I recommend that  only a general dry heat to be used, rather than blasting with heat, using a butane torch.  Some people use a psychical flame to blast surface tension, which would not be my own preferred method.  Be careful when using heat, as sustained, high temperature heat, including localised, excess heat, will affect the resin, which may result in fisheyes in the casting and warping.
9. Vibrating Table - if these simple steps don't work to minimise air bubbles in your casts, you may want to consider casting on a Vibration Table, to help disperse air bubbles.  However, I have personally got good results, following all of the steps above.
10. Use a Vacuum Chamber or Pressure Pot - 
    If you are using a Polyurethane Resin, it may be extremely difficult to remove all of the air-bubbles. However, in the case of Resin 3D baby Casting, the aim is to bring the air-bubbles to the upside of the cast, which will not be visible when the casts are framed.  
    However, If you continue to struggle removing bubbles from your casts, it may be necessary for you to consider using a Pressure Pot or Vacum Degasser, which should then enable you to produce bubble free casts. 
    You will need a Chamber & Pump/Compressor, which can reach at least 50psi within a miniute. There would be no need to go higher with psi. Do not exceed 60psi.  To remove air bubbles, you would either need to put your Resin in a degassing chamber, to remove the air from the resin mixture, prior to casting.  Alternatively, you can cast them in a pressure pot. Important Note If you cast your Resin Casts in a pressure chamber, your silicone mould would also need to have been cast under pressure, to avoid distortion in your mould.

Important:  This article has been written to offfer guidance and potential solutions to frequently asked questions and problems, that people may experience, during Silicone Mould Making,  Resin Baby Casting and Metal Cold Casting.   Results are variable depending upon the skill of the Life Caster and their working environment. Always refer to the Technical & Safety Data Sheets provided with your Silicone and Resin products, before using any material.