Frequently Asked Questions
How do you prepare a mould to spray silicone onto?
1/ The face mould should be calibrated accurately using thickness wax to the final thickness design of the part. This sometimes is too expensive to do and can be carried out more economically but less accurately using a grp moulded part. In this case, the part rear face needs to be smoothed over to flatten any sharp points and edges and free from any loose fibres.
Note – The problem using a moulded part is that internal corners may be bridged and therefore be extra thickness and inaccurate. As the sprayed reusable vacuum membrane does not shrink it will naturally copy any inaccuracies in the calibration into itself.
2/ The surface of the calibration does not need any release agents applied simply needs to be clean and as smooth as possible.
3/ If the calibration is done using a moulded part then it must be well cured (made and left to cure for at least 4 days) and trimmed to final dimension . If, when replacing into the face mould, there are small gaps at the trimmed edge then these will be filled with non-sulphur modelling clay (our part number 102152). Other clays which usually contain some sulphur are NOT suitable . If the calibrated part is made by hand lay then smooth over its cured B face with 280 sandpaper and if necessary gaps and large surface roughness can be flattened using body filler and rubbed smooth.
4/ The flanges of the mould should at least 100 mm wide (130mm ideal) and be left clean ready for the normal reusable vacuum membrane calibration for the seal and mini vacuum channel.
5/ All flanges should not have sharp corners if they are designed with changes in height or direction. Any changes in a direction up or down should be made through at least a radius of 25mm or more . The greater the radius the better.
How many cycles will a reusable vacuum membrane last ?
What can prevent Addition Cure VBS26 Silicone from curing ?
- Any natural or synthetic rubber containing any form of sulphur
- Condensation cure silicone – RTV
- Modeling clay like Plasticine which often have sulphur base, and any Tin/
- Amines. . Use instead our Silicone safe modeling clay part number 102152
- Under cured polyester mouldings which are breathing styrene gas .
- Carbon Monoxide gas even in minute ppm levels
How much Silicone do I need ?
Basic VPI system and flow enhancing...
How do you get the glass into hard to reach/ tight corners?
We will take one of our clients case as an example, with Dartford Composites Ltd, who have been very successful in moulding their rail carriage seat (a monolithic laminate of 3 mm in fire retardant resin, is moulded in minutes by our VPI process under a reusable vacuum membrane. Fill time under 8 minutes with negligible waste after pour of 2.2 kg mix).
There are three features which allow the fibre to be contoured to stay in these multiple corners before VPI.
1/ The fibre is combination mat like the original Choramat Rovicore and now copied would wide you as far as China at lower cost. The main features of this glass is that it is full weight in one sheet /ply so in this seat it is one layer of 1200 g/m2 It is stitch bonded so can be massaged very easily into three-dimensional shapes and in the case of this seat virtually no tailoring but cutting is necessary so is quick to load .
2/ The gel coat tack is used to help the glass laying stick into the negative corners. If the gel coat is left too long and becomes “dry” then it is very difficult to push and keep the glass in position before the membrane is vacuumed down. In these cases the gel coat is re-tackyfied using mist spray of acetone.
3/ The membrane is made with extra thickness in the internal corner shapes of the part so is semi-rigid and act like intensifiers in these regions which are ideal to assist perfect and correct fibre pressure in these otherwise tight detail. In normal infusion consumable film use these areas would be very prone to bridging which is far less a problem with reusable moulded membranes which fit perfectly. Mind you the operator still has to quickly push down into these corners as the vacuum builds to assist the male membrane’s detail is hard down in these areas. This might take the operator about 30 seconds. Remember at this stage the resin is already flowing as it has been deposited on bloc in the bum area of the seat mould.