Jump to content


OK, what am I doing wrong?


  • You cannot reply to this topic
7 replies to this topic

#1 plowboy

plowboy

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 3,426 posts
  • Location:in the middle
  • Full Name:Roger Hayes

Posted 02 April 2012 - 02:48 AM

I made a couple of molds this weekend to cast parts for myself. Both are two piece molds. One is a mold for disc brakes and the other is a wheel that I made by modifying a kit wheel. So far,I've only had one failure on the disc brake mold. But I've only had one success with the wheel mold. On the first few tries,the rim would come out perfect,but the center would always have an air bubble! Too big to try to repair. Plan B: inject the resin with a syringe. I made a hole for the syringe with another hole beside of it to let the air escape. I injected the resin into the mold until I would see solid resin with no air coming out of the hole. I thought this has gotta work,but NO! Now, I have a perfect center with two big blank spots in the rim! Once again, I made holes in the mold to let the air escape in the two areas that were blank. Same result. :angry: I have resigned to make another mold for this wheel,but this time,I'm just going to make a simple one piece mold for it.

I have a few other parts and wheels that I have made that I also want to make copies of that will require two piece molds. How do I keep the air out? Should I inject the resin on one side of the mold and let the air escape on the other? Should I slush cast and have an area to let the air escape? I really don't want to go to the expense of buying a pressure pot just for what few parts I'm going to cast. Plus, I don't see how the pressure pot would help since the resin sets up so fast. Seems to me that once it starts setting up,the pressure pot isn't going to help anyway. Does the resin set up slower inside the mold? I will appreciate any info anyone can give me.

#2 RyanSilva

RyanSilva

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 745 posts
  • Location:Massachusetts
  • Full Name:Ryan

Posted 02 April 2012 - 03:04 AM

Resin speed fluctuates with not only its mixing time of a specific brand/model resin you are using but also the temperature and mass of the object you are casting. thin walled items will set up slower than a thicker object but the speeds arent very noticeable with a fast kicking resin. The biggest problem I see with folks casting items as a hobby is they use a fast set resin..and it really ticks people off when they get bad parts which is understandable, Most resin casters have been there.

While mold design and air key ways play a big part in how easily the air travels in and out on a mold, the big culprit is the fast set resin, how fast is yours? I recommend a medium speed resin without pressure pots, heck smoothon makes a very slow resin also that will give you 20 minutes of MIXING time..which if I was a patient person who wanted to get the most out of my resin with more than enough time to knock out most of the air bubbles. id use it. It has a demold time of 2-4 hours but that can be a bit longer depending on size of item and air temp.

Checkout smooth-on smoothcast 305 and 310 if you wanted to try a new resin.


Now the molds themselves, have any pictures? how are they vented? ive found when casting tires using 1 vent hole that is both a fill port and vent. tap of the mold a few times on a hard surface when almost filled, will knockout the air pocket..however that's using smoothcast 305 medium set resin so i got enough work time, you may not have that.

You can try putting your resins in the fridge before you mix to buy you a minute or two, some folks do that..i never tried it though.

#3 Erik Smith

Erik Smith

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 2,799 posts
  • Location:Spokane, WA
  • Full Name:Erik Smith

Posted 02 April 2012 - 03:55 AM

Ryan has some great tips. I am a novice caster but have learned quite a bit in the last few months. I have also wasted plenty of RTV and resin!
As Ryan stated, set time depends on a number of factors. What resin are you using? I started with alumilite's "hobby" kit - with the tan resin. It sets wry fast and is pretty thick, making it difficult for me to get everything in place before I could get the bubbles out. I switched to Alumilite's white resin and that helps some - it is closer to the consistency of water and gives you a little more working time. So check the viscosity and set time.
As for the mold - post some pics and the experienced casters here could throw some tips and ideas your way. I have made a few two part molds and usually have to do them twice to work out errors in layout, etc.
Bubbles are a pain.

Edited by Erik Smith, 02 April 2012 - 03:55 AM.


#4 mr moto

mr moto

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 858 posts
  • Location:Baton Rouge, LA
  • Full Name:Manuel J. Martinez

Posted 02 April 2012 - 04:26 AM

Without actually seeing your mold "up close and personal", it's hard to know what the problem is but usually if a bubble keeps forming in the same place that tells you where to put a vent hole.
The other posters are giving good advice and with experience you'll find what works for you. I never had a lot of luck with Alumilite although now that my skills are better I could probably deal with it. Anyway, I prefer SmoothCast 305.
As far as mold design, I no longer have any "fill ports". The molds are designed so that you just pour a puddle of resin into the lower mold half and it squeezes out through the vents as you close the mold. That way you have pressure forcing the resin through the mold. Look at the squeeze out on these molds:

Posted Image

That's what you want to see when you close the mold. You can see in the picture below how each part has a "well" in the lower mold half to hold the resin.

Posted Image

Good luck! Just keep trying and you'll get it.



#5 plowboy

plowboy

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 3,426 posts
  • Location:in the middle
  • Full Name:Roger Hayes

Posted 02 April 2012 - 06:23 AM

Thanx for all of the advise everyone! I definitely need a slower setting resin. I have the Alumilite Mini Casting Kit and the resin in it sets up really fast. I guess it would be good if you're in a hurry,but that's part of the problem. You have to go full speed to mix it up,pour it and get your molds together before it sets up and becomes useless. I probably only have ninety seconds to two minutes working time once I pour in the hardener. I made three other molds for wheels that are just the simple one piece type. I'll see how they work later tonight.

#6 Mike Kucaba

Mike Kucaba

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 2,071 posts
  • Location:Mesa,AZ
  • Full Name:Michael J Kucaba

Posted 02 April 2012 - 08:12 AM

If you want to use up the quick resin on your present mold,just mix some up and only fill one side of your mold an let it cure.You'll be able to see if your getting a bubble,and your mold will be open to atmosphere to let air escape. I can't really tell which half to fill first without seeing the mold halves.

Ryan's suggestion about chilling the two parts does help,and you can also chill the mold. Some wheels are hard to cast,especially the ones with small detail such as the lug nuts.

#7 Greg Wann

Greg Wann

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 541 posts
  • Location:Glendale, Arizona
  • Full Name:Greg Wann

Posted 22 April 2012 - 10:01 AM

One thing about making a mold is you have the option of creating a new parting line for your reproduced part. I find many times it is just a good idea to observe and follow the original parting line on small parts. This means that a portion of the part will be in the top of your mold. When I do this I put a little resin in this area before closing the mold. So I am putting resin in both sides. Keep a toothpick handy too. Many times I will stick the end of it in a cavity to help chase out and remove bubbles. I use SMOOTH ON TASK 8 resin and it will cure very fast so I have to work quickly too. There are times I mix small amounts and then only do certain parts in the mold. It is important that you stay focused and stay relaxed while you are making parts. It is sometimes difficult to stay tuned in when all kinds of distractions are going on around you. I find it difficult to keep myself on track. Even as I am now typing my wife is asking me questions and one of my dogs is barking at something and I can't focus.
My lunch is ready now, I will have to come back and finish this entry. YES Dear! I will be there shortly. Lunch is over and the wife left. ate SOS. Good stuff!
I pretty much have to get psyched up to make parts if I can't stay on track. Here is a pic of three molds that make a Judson Dual Supercharged engine from a 64 Olds Cutlass kit. You can see all the vents I cut in the mold top. The mold pictured above is a very nice mold to me. I first started making parts by setting them all on one flat surface and no venting. The flash around the parts was quite thick. Then I dreamed up the pyramid shapes and it was just what I was hoping for, flash that was as thin or thinner than some you will occasionally see still hanging on new kit sprues. I am probably not the first guy to dream this up but it was a big help to me. I use Task 8 because it will withstand higher temps than most resin. A club member bought a resin 57 Chevy convertible kit from someone. He had it in his car trunk for only a half hour under the Arizona summer sun and it was warped badly. I thought about my parts going out in the mail from here and thought the worst so I switched from the Smooth On 300 I was using.

Attached Files


Edited by Greg Wann, 22 April 2012 - 10:45 AM.


#8 Greg Wann

Greg Wann

    MCM Ohana

  • Members
  • PipPipPipPipPipPip
  • 541 posts
  • Location:Glendale, Arizona
  • Full Name:Greg Wann

Posted 22 April 2012 - 11:12 AM

When you can clearly see the color of your mold through the flash around your part you will know you did good!. Here is a pic. Once your parts have fully cured you will need to be cautious of opening your mold. If you are not carefull you may break the very part you are making. When I first open a mold, I place it top up in my left hand and use my left thumb to pull up a corner of the top while my right thumb is pulling down on the same corner of the bottom part then I turn it in my hand while slowly working it open. Before I open the mold I remove all the little spots that have raised to the top through the vent. This will allow me to have it set perfectly flat to remove the parts from the mold. I found it is a good idea to let it lay flat and take a flat sharp tool to push down on the flash next to the part to help break it free. Sometimes your fingernail will work well. I usually use one of my free Starbucks wooden coffee stirrers sharpened with a sanding stick to be helpful here. There is just so much to learn and know when casting parts. Once the flash is free, it might still be tricky to get it freed from the silicones tight grasp. And then when it finally breaks free it flips out of my fingers and hits the floor and bounces away *&^%$))#$@ &^%@!!!&**()()()(^^^^%%%%%$#@$#!!! Yes! It happens often. I have tile floors and keep a flashlight handy.

Attached Files


Edited by Greg Wann, 22 April 2012 - 11:35 AM.