Frequently Asked Questions

Frequent Asked Questions about manufacture and use of reusable vacuum membranes…
The answers we provide are based upon our and our client’s experience using reusable vacuum membranes made with ACC Silicones Ltd VBS26-35 grade addition cure silicone. Alan Harper Composites Ltd © 2015,2016.
No part of this document may be copied and reprinted without indicating named source and date i.e Information and data supplied by Alan Harper Composites Ltd , UK

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 ?

a/ For standard polyester and vinyl ester clients have achieved over 600 cycles
however if deep draw and narrow shapes in a membrane are characteristic
where they may be a solid silicone shorter service life is expected . This is due to styrene attack coming from both sides of the “V” and meeting in the middle. The styrene needs far longer to evaporate from the silicone membrane in such sections and they become whitened through constant use. They will then become weaker, less elastic and brittle and will then begin to fracture. In these circumstances the Silcoset 153 can be smeared over the surface, allowed to cure and thus locally coat and strengthen the worn out whitening sections. This can be repeated as often as necessary to help the membrane survive longer production use under these circumstances. This type of whitening is accelerated by higher exothermic temperatures in resin rich corners common to these deep V moulding designs.
b/ DCPD based unsaturated polyester resin has proven to reduce the life of a membrane quite considerably . The chemistry of DCPD affects the membrane shortening its usable cycle life to as little as 100 cycles or less. This is evidenced by discolourization through whitening of the translucent membrane blue colour and loosing its self release characteristics.
c/ ACERS or Amine Cured Epoxy Resins are notoriously aggressive to high quality silicone as the amines are absorbed and attack the membrane surface within only a few cycles. Experience to how many cycles (pulls) clients can achieve with ACERS depends on the individual epoxy and nature of the curing agents . between 27 and 35 cycles can be expected . After this time the cured laminate is very difficult to release from the membrane surface and will eventually stick so hard that the membrane becomes torn through or pitted. Higher temperature cures also play a part in longevity issue and the time from wet ACER contact to cure.
SOLUTION:
Our SILFLON coating solution is the only method we can recommend for
rejuvenating these membranes. SILFLON is available in 125 and 510 g packaging and is simply painted or sprayed overt the worn out surface and allowed to cure for at least 10 hours at ambient conditions. It is moisture curing and needs no heat to accelerate. A 125 g bottle is sufficient for 0.7 m2 of membrane cavity moulding area. See SILFLON link…
http://www.alanharpercomposites.com/product/silflon-510g-102172/

What can prevent Addition Cure VBS26 Silicone from curing ?

 

The most common error in using addition cure (platinum cure) 2 part silicone is spraying or painting the mixture onto contaminated surfaces . Most common
contaminates are:
  • 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
Before use of mixed VBS26-35 silicone upon any surface ALWAYS carry out small area test by pouring a small amount in spots of the work part and allow to cure. Heating with hot air will quicken this test . On cure and removal if any surface of the cured silicone appears in any form wet, oily or under cured or is sticky then you can assume the surface from which it came is contaminated and sealing and other tests must be carried out before proceeding with membrane manufacture by spray or brush .

How much Silicone do I need ?

After 5 years experience we recommend to all clients to budget for 7 kg /m2 of total reusable membrane surface . Although the main membrane is between 3 and 4 mm thickness the additional edge seal thickness and reinforced cloth areas which can be the entire surface have been averaged out and the 7kg /m2 is an accurate minimum thickness to work with . The 7 Kg figure is the mixed silicone so 3.5 kg of A and 3.5 kg of B / m2. If on the other hand you may wish to make a bladder then assume 4 kg /m2.

Basic VPI system and flow enhancing...

Please also understand that there are TWO ways which we apply VPI.
Method 1 is place membrane over and pull vacuum to check all is sealing . Mix resin correct volume and pour under membrane at central area . Place vacuum membrane back and apply full vacuum to allow pour to spread and infuse whole product to edges.
Method 2 is as above but pour through membrane at central infusion point. and allow pre determined volume to rapidly fill under membrane in mass which will do exactly same as method 1 but not have to lift membrane to enter resin.
New technology available to above methods 1 and 2: Using our Morphing runner flow enhancing technology the reusable vacuum membrane is manufactured with “SMART” pre designed resin flow channels within membrane to provide rapid spread of resin shot without worries of fibre flow permeability so for example woven rovings can be infused without need for flow enhancing systems like flow mesh or additional flow combination mats !!!
before gel and cure

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.

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