Foams, the basic's
Foams are nothing more than a solid product with air pockets inside it. You can take almost any material and add a frothing agent, or simply blow air into the product as it passes through the inlet port. Exceptions to this rule are items that oxidise with the introduction of air, steel being one of these but it can also occur in the manmade range of plastics. Polyurethane has been the mainstay of foams however recently Epoxies have been adapted to produce foams.
LPUr Foam under pressure
To change the mechanical property of foam, firstly we set the chemical filler and frothing ratios. We then use a system of low pressure to control the overall structure. In its most simple form we make an open topped mould allowing easy pouring of the foam mix before it expands. To this we then cap the mould with a simple top and add weights to the cover, these weights allow expansion and compression with the excess foam venting through the join.
From a wide range of polyurethane or epoxy base chemicals a range of filler and frothing agents can be used. The fillers give additional mechanical properties to the foam. It's within this chemical and filler compilation that you can make a foam, which is soft enough to push your finger into, suddenly under impact, such as in a car bumper backing in an accident, behave differently and suddenly have a high resistance, it absorbs and dissipates without deforming.
CNC trimming or finishing of Foam's
Sometimes casting the part is only one part of the job. When we made centre consoles and door arm rests both had to be machined for accuracy of the outer shape or pocketed allowing for room within its shape. With the armrest whilst the outer shape was accurate enough from the mould it was necessary to machine the inner to allow for parts of the door mechanism to pass through. With the Centre console we had to machine over the top as after the leather trim was applied items such as Ashtray's had to be fitted with a specific edge to hold it tight in place. Why not incorporate this into the mould? well because if we did we then lost the flow of the foam as it entered the mould in a liquid state and started to cure. Many options are available to overcome this but at the quantity and price point simply making a machining jig and CNC cutting out the back were quickest and therefore most cost effective. We will always evaluate the complete job and put forward the options, sometimes a close argument between differing approaches and other times a clear cut option can be put into action either way we know of many options from the simple to the complex.
Foam by its very nature has a different cell structure every time its made.
Our years foam "blowing" has taught us how best to control these ever changing factors to produce parts that are very similar time after time.
The first problem to overcome is the mould technology. Foam's will react to the contact patch of the mould to produce its surface composition. Certain products, for example silicone which gives a very good finish in a plastic part causes the foam cell to breakdown and "powder". However we use silicone moulds and simply add a wax polish "barrier" and thus the foam parts cell structure remains intact.
Then it is necessary to make sure the chemical mix is identical, for this we measure the compounds out separately with a 3 sigma set of scales.
Next it is important to fully mix the compounds, and within a very fine time tolerance, typically you have less than 15 seconds to mix the compounds.
Finally the temperature is very important. We store all our chemicals in a temperature controlled room, and we carry out the manufacturing processes in a thermally insulated room, we have a table of air temperatures to cure times and observe all the above factors carefully.