Thanks Thanks:  0
Needs Pictures Needs Pictures:  0
Picture(s) thanks Picture(s) thanks:  0
Results 1 to 9 of 9

Thread: Resin shrinkage

  1. #1
    Join Date
    Aug 2009
    Location
    Brisbane
    Posts
    158

    Default Resin shrinkage

    I have been clear casting for some time using a pressure pot to reduce the risk of air bubbles. I am using a clear casting resin (30ml resin/10 drops of hardener) as recommended by the manufacturer and pressurise to 58 PSI. I am using tube in silicone moulds. The shrinkage exposes the ends of the tube when set. I fill the moulds to the maximum level. Just wondering if the pressure is too high which may be causing the shrinkage. I would appreciate any advice which will assist in overcoming this problem.

  2. # ADS
    Google Adsense Advertisement
    Join Date
    Always
    Location
    Advertising world
    Posts
    Many





     
  3. #2
    Join Date
    Apr 2020
    Location
    Seaford, Vic
    Posts
    397

    Default

    Quote Originally Posted by daryl1 View Post
    I have been clear casting for some time using a pressure pot to reduce the risk of air bubbles. I am using a clear casting resin (30ml resin/10 drops of hardener) as recommended by the manufacturer and pressurise to 58 PSI. I am using tube in silicone moulds. The shrinkage exposes the ends of the tube when set. I fill the moulds to the maximum level. Just wondering if the pressure is too high which may be causing the shrinkage. I would appreciate any advice which will assist in overcoming this problem.
    Any reason why you are using pressure instead of vacuum to get rid of the air bubbles?

  4. #3
    Join Date
    Aug 2007
    Location
    Saskatoon, SK, Canada.
    Posts
    1,436

    Default

    If there is minimal room between the end of the tube and the moulds when you pour, the high pressure collapses the silicone to some extent, exposing the tube. Just for grins try casting a single blank at just 10 or 15 PSI and see if you still have the issue. If there are no bubbles as well then you don't need to use such a high pressure. If a few increase the pressure a little. Another "trick" to try to reduce the bubbles is to warm the resin, add the MEKP, and gently stir the resin to mix. The less air you mix in the fewer bubbles you have to deal with.

    RSD when you use vacuum on the resin it can boil off some of the volatile compounds changing the properties of the resin. It also makes the bubbles expand and if there isn't sufficient time for the bubbles to float away before the resin starts to set and cure you'll have more and larger bubbles. Pressurizing keeps the bubbles in solution so the don't form. Much like a diver having nitrogen bubbles in the blood. Great as long as they are deep but take the pressure off with a rapid assent and the bubbles expand and they have the bends.

    Pete

  5. #4
    Join Date
    Apr 2005
    Location
    Nerang Queensland
    Age
    66
    Posts
    10,766

    Default

    You are using Polyester resin that shrinks. Not much you can do with that resin, but you could use an Epoxy or Polyurethane resin instead, that have far less shrinkage and not as brittle too
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

  6. #5
    Join Date
    Apr 2005
    Location
    Nerang Queensland
    Age
    66
    Posts
    10,766

    Default

    Quote Originally Posted by RSD View Post
    Any reason why you are using pressure instead of vacuum to get rid of the air bubbles?
    Because casting uses pressure not vacuum. Vacuum is used for stabilising. Yes vacuum can be used to remove bubbles, but timber (for hybrid blanks) is full of air and takes ages to get removed, so in vacuum the resin goes off long before the bubbles are removed. In pure resin, it does work, but the effective temperature is a lot higher so can cause temperature issues (ie cracking and heat can actually cause bubbles). Lastly the OP is casting with a tube in, so vacuum can draw the air from inside the tube, into the resin.
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

  7. #6
    Join Date
    Apr 2020
    Location
    Seaford, Vic
    Posts
    397

    Default

    Quote Originally Posted by dai sensei View Post
    Because casting uses pressure not vacuum. Vacuum is used for stabilising. Yes vacuum can be used to remove bubbles, but timber (for hybrid blanks) is full of air and takes ages to get removed, so in vacuum the resin goes off long before the bubbles are removed. In pure resin, it does work, but the effective temperature is a lot higher so can cause temperature issues (ie cracking and heat can actually cause bubbles). Lastly the OP is casting with a tube in, so vacuum can draw the air from inside the tube, into the resin.
    I'm horribly confused now! On youtube I often see them forming bowl blanks of wood and epoxy - but after they pour the epoxy into the mould with wood in it they put it into a vacuum pot for curing - but are you saying that they should be pressurising it instead?

    If you had access to a large vacuum chamber and a large pressure chamber would you put a river table in the vacuum chamber or the pressure chamber?

  8. #7
    Join Date
    Feb 2016
    Location
    Perth WA Australia
    Posts
    828

    Default

    Most guys that make large resin tables will remove bubbles prior to the pour, so either vac or pressure will work. Once poured they'll just hit any surface bubbles with a torch.

  9. #8
    Join Date
    Apr 2005
    Location
    Nerang Queensland
    Age
    66
    Posts
    10,766

    Default

    Quote Originally Posted by tonzeyd View Post
    Most guys that make large resin tables will remove bubbles prior to the pour, so either vac or pressure will work. Once poured they'll just hit any surface bubbles with a torch.
    Removing bubbles from resin for a river table before casting is a waste of time. The majority of bubbles in a river table came from the wood, too much heat, or whilst pouring/mixing the resin. As river tables don't fit in a pressure pot, best way is to ensure timber is sealed, resin is warm but a low exotherm so it doesn't get too hot, hit the poured resin with a heat blower and/or misting spray of metho that will pop bubbles.

    Quote Originally Posted by RSD View Post
    I'm horribly confused now! On youtube I often see them forming bowl blanks of wood and epoxy - but after they pour the epoxy into the mould with wood in it they put it into a vacuum pot for curing - but are you saying that they should be pressurising it instead?

    If you had access to a large vacuum chamber and a large pressure chamber would you put a river table in the vacuum chamber or the pressure chamber?
    Hybrid blanks are best cast under pressure NOT vacuum. As I previously said, wood contains a lot of air, so under vacuum you would end up with honeycomb resin under vacuum. When you say you've seen cast blanks put into pot, they are usually referring to a pressure pot, not a vacuum chamber. Pressure pots are solid and run off a compressor, whereas vacuum chambers are usually clear run with vacuum (for stabilising using a stabilising resin like Cactus Juice).

    River tables are not cast under either vacuum nor pressure as they are just too large to fit. The pressures involved with casting and stabilisation (pressure and vacuum) need to be very strong and are not cheap, if they fail, serious injury can occur even death (pressure vessels go off like a bomb)
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

  10. #9
    Join Date
    Aug 2017
    Location
    Canberra
    Posts
    585

    Default

    Quote Originally Posted by RSD View Post
    I'm horribly confused now! On youtube I often see them forming bowl blanks of wood and epoxy - but after they pour the epoxy into the mould with wood in it they put it into a vacuum pot for curing - but are you saying that they should be pressurising it instead?

    If you had access to a large vacuum chamber and a large pressure chamber would you put a river table in the vacuum chamber or the pressure chamber?

    You might be mixing up vacuum and pressure. The pressure pots are the big metal ones with screw down lids. Thats what I see most people use. The converted paint spray cans are pressure pots.

Similar Threads

  1. Resin shrinkage!
    By wm460 in forum WOODTURNING - GENERAL
    Replies: 7
    Last Post: 29th July 2016, 12:54 AM
  2. Shrinkage In The Cold
    By Gags_17 in forum WOODWORK - GENERAL
    Replies: 14
    Last Post: 26th February 2009, 06:52 PM
  3. Gap Filler Shrinkage
    By Wassy in forum DOORS, WINDOWS, ARCHITRAVES & SKIRTS ETC
    Replies: 14
    Last Post: 4th November 2007, 10:00 AM
  4. Shrinkage
    By Jon in forum WOODWORK - GENERAL
    Replies: 7
    Last Post: 28th May 2003, 02:54 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •