Hallo
 
Well here it is July already and summer is here! Not sure where the time went this past month but it has gone. Hope this finds everyone enjoying their own summer or winter as best they can.
 
This month's topic is on foaming, and no not the stuff on the top of that glass of bier! But what we can do with some baking soda and vinegar in plastic. Yes a cheap foaming agent is that box of Arm and Hammer that is in the grocery store. I have in my past done quiet a bit of this and really had very good luck. In one case we actually dropped 1 pound of plastic out of a part, and they even looked better. In the other it was the prototype of the plastic wine cork.
 
Well my trip to California went great, and while there accomplished a lot (I think) around the old homestead. With the help of a sister and nephew a new coat of paint and many others of those projects that needed to be done. The most interesting one was the use of ceramic beads that were hollow to act as insulation and also abrasion resistance for an acrylic paint.  The surface finish was like fine leather, and it tended to dull the finish of the paint a bit.   It did extend it so it was filler.
 
Anyway this weekend it is time for some two wheeled fun, so till next time enjoy and as George Carlin once stated ( among some great sayings), keep on learning
 
TA-
Steven
 
Silveysplastics@aol.com

360-882-3183

 

 

http://archive.constantcontact.com/fs034/1101663388820/archive/1102138661635.html 
 

 
Definitions: Foaming 1. a collection of minute bubbles
 
Foaming of plastics materials is done in a few ways, by chemical additives, by gas, and by mechanical means. This article will discuss foaming with the chemical type. 
 
Chemical foaming agents are chemical substances that decompose by reacting with the influence of heat, and than create gas, which in effect creates a gas and results in a foamed material.  There are safety concerns with some plastic material and which blowing agents may use with those types of materials.
 
There are two basic types of chemical blowing agents. One is an exothermic reaction in which the chemical components well actually give off heat during the reaction to foam. These as a class can produce larger and or irregular shaped foamed cells. Also with this class of blowing agent allowing time to degas before painting and or coating is advisable. This time frame may be days or weeks dependant on storage conditions.
 
The other types are endothermic blowing agents which adsorb heat thus the results tend to be better surface. They can sometimes be painted inline or immediately, and do not need storage for out gassing. The results for cell structure are a more uniform bubble that is usually smaller in size than that of the exothermic.
 
There are also some combination type products on the market.  These are products that use both types in one product to produce unique characteristics within the product. These are usually related to surface appearance.
 
The issues with foaming agents can be that the end product does not have that glossy uniformly plastic look and may have large areas of splay and or a motley appearance to the product. There are newer grades currently that help in eliminating this issue.
 
In processing with a foaming agent there are many tricks of the trade so to speak. The following mainly apply to those whom use a standard molding machine to foam with.
 
1-     Lower clamp tonnage. Since we are not packing the part we do not need the high clamp tonnage that would be used on a solid part.
2-     No packing of the part. We do not wish to have a cushion on the shot with high pack pressure. Usually only enough pack pressure to keep the screw from bouncing back is necessary.
3-     Screw recovery should be at last possible moment of the cycle. This is because the foam works by heat, and since we do not have a shut off nozzle we have to be concerned with this aspect of the cycle.
4-     Extremely fast fill into our part. This effects a shearing action and maintains pressure on the material so that hopefully the foaming occurs in the mold cavity itself and not the feed section. Sometimes the shear through the gates can act as the needed heat to assist in the foaming reaction.
5-     Short shoot the mold. Remember we are foaming the part. By short shooting we allow a space in the cavity for the foaming to occur. In truth there is a reduction of density.
6-     Tooling issues
a.       Gate into the thin section
b.      Venting has to be excellent, especially the exhaust.
c.       Cooling has to be excellent so as to set up a tough skin, if cycle is important to you.
d.      BrCu components must be plated as there are corrosion issues.
e.       Use good draft in the tool where ever possible.
f.        Texturing can assist in hiding some but not all of the gas issues.
7-     Knowing the actual activation temperature of the blowing agent in use is important.
8-     Keeping the rear section of the screw as cold as possible keeps the gas in the material, and does not allow the gas to escape out the throat of the machine.
 
 
Knowing and having all data on the foaming agent of your choice is probably the best piece of advice that can be given. Also knowing what is trying to be achieved is also important. If the foaming agent is for sink control than the process is typical molding while if it is for weight reduction than all the above and more apply. Using excessive foaming agent is wasteful and usually does not give the result necessary. 
 
 
 

 

  

 

When size, stress and repeatability definitely have to be identical from 1 cavity to 96 cavities in the parts, give me call, or give BTI a call direct and tell them Steve sent ya.