Wednesday, April 29, 2015

Vacuum Forming Part 2

I am currently working on a new vacuum forming rig.  This rig is going to be different from conventional vacuum forming because it will use female tools instead of male tools and pull the heated plastic into the mold rather than stretching it over the mold.  This is similar to how spar baths are made.  

Because most of the "bucks" I have made so far are male tools, I am making the rig adaptable so I don't have to remake those tools and all new tools will be female form the design phase.    

There are a more than a couple of reasons I am building my rig this way and they are -

1. Time where you have just seconds to evacuate all the air with a vacuum form using positive tools.  Provided there is a good seal, vacuum forming into a female tool means that the need to have the part finished quickly does not apply anymore.   

2. Heat where you have just seconds to get the plastic down and air out before the plastic begins to cool. Pulls made into a female tool can be heated during the process or re-heated if required. 

3. Suction where you must align the plastic to the platen to get a good seal to make vacuum forming work.  In my system, part of the set up will be alignment and a seal tests before any heat is applied.  Once in position, the plastic is not removed for heating.  

4. Part size where the temperature and the plastic types/thickness affects the final part size normally resulting in parts that are slightly over-sized or have paper thin walls.  When making a male tool for vacuum forming, you must take this size increase into consideration. If you want to use thicker plastic to make a thicker part, you lose detail.  With female tooling for vacuum forming, the part is the exact size of the mold very time and no detail is lost, even when using thicker plastic.  It is therefore possible to have sharper lines or use thicker plastic on deeper pulls.  In some cases, parts made using a female mold can look as good as injection molded parts.   

5. Material wastage is reduced.  Generally you need a the sheet of plastic to be at least 40% larger than the footprint of the part you are making a pull from. So you also need a massive platen and heater plus suction to make this work. I believe it is possible to make parts like this from way smaller equipment using female tools on the vacuum forming rig. 
Proof Of Concept Test #1
The plastic to make the dome in my photo was just 450mm x 450mm x 3mm. If this could be done on a male vacuum form tool, I would suggest that the sheet of plastic would need to be at least 1200mm x 1200mm.  The technical term for what I have done here is Vacuum Assisted Slump Forming as there was not physical tool used apart from the tube of the vacuum chamber.  The dome is 250mm in diameter.       

6. Ease of use. At that size (1200mm x 1200mm), it is now a 2 man job just to set it up, not to mention the energy needed to push the plastic down in the first place (unless you have access to a motorized pro version of that size). With female tooling, once you get a seal, you can start the heating slowly and evenly and get tight edges right to the base. 

7. Fails due to plastic burning are common with DIY vacuum forming because you are on a time line. You need to get as much heat into the plastic as you can to allow the plastic to form properly.  As the plastic heats up, the plastic usually riased toward the heating element before slumping once the critial temperature is achieved.   If the plastic touches the heating elements, the job has failed.  In my system, the plastic can't bow up because it is already under suction.  As soon as it softens, it is pulled down into the tool.   

Sometimes with vacuum forming on a male tool, you won't get enough heat at the edges and as a result, get sloping sides. Look how tight my vacuum form is. The only thing I did wrong with that dome in the first test was I did not use a frame and as the plastic heated and stretched and was pulled down.  It reached a point where it warped and eventually broke the seal. If I had used a good frame and clamping system, I would have been able to have a nice flat surround and I did that in test #2.

Proof Of Concept Test #2

8. Generally, vacuum forming is noisy because most DIY systems use a vacuum cleaner and suction must be on the whole time. I am not using a vacuum cleaner, rather the vacuum pump and chamber I made for degassing resin and silicone.  With my system, once the vacuum is started (and assuming a good seal is made), the pump can be shut off.  It need only be turned back on to increase vacuum as required. 

 The Vacuum System

Suction is measured in units of mercury where the weight is based on a cylinder or tube filled with mercury.  In the image above, the amount of suction is shown on the vacuum gauge in both inches and centimeters of mercury.  To put this in perspective, the very best vacuum cleaners will pull about 6"HG.  My system can go to 30"HG which is just over one negative atmosphere.
The Heating Unit

The heating unit used here is a fan forced halogen cooking appliance.  What I like about this unit is it has a quick stop mechanism, thermostat and timer.  once I learn the temperature rating required for each type of plastic, I can set the temperature to that and not have to worry about burning plastic.      

 I will update this post once the former is made.

More to come soon...


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