How to make a one off replica part

We had a Tweet from @coledude in sunny Australia asking if it was easy to make a mould out of an existing part, so we decided to write up a quick tutorial for him and anyone else who would like to know. For this example we are using a motorbike part.

If there is a part, say of a motorcycle that is broken or a vintage part that is hard to obtain then making a fibreglass replica part would be the cheapest and maybe you’re only option, please find below the basic techniques for this process;

You will need the original part to use as the plug to make a copy mould, then you use this copy mould to make a part to fit the motorcycle. What you must remember though is you cannot get an absolute perfect copy as each time you use fibreglass there is a small amount of shrinkage which will affect the overall dimensions.

• The first thing is to prepare the surface so that the fibreglass will not stick to it, and will release easily
• Wax the surface with a good amount of wax, then polish it back so it is shines like it would on the bonnet of a car, you will need to repeat this process 5-6 times giving time for the wax to dry before polishing off. This process will take at least half an hour but you will have built up 5 layers of wax on your part.
• Get some gel coat and catalyse to 2% and then spread over the waxed surface. Make sure to cover all of the part, do it to a depth of at least 800 microns, which is like a thick painted surface and leave to cure for at least 3 hours.
• When cured get the fibreglass mat (450 gm is best), wet the surface with catalysed resin and apply the chopped strand mat over the top. Wet the mat and consolidate with a roller, getting all the air out. Apply a second coat of mat and repeat the process, wait and leave it to cure before applying a third and forth layer of mat but do this the next day. The reason is to try and lesson the shrinkage of the mould you are making.
• Now you can if you are really clever obtain a zero shrink resin system to make your mould, It is a filled polyester and specially designed to make moulds, but only moulds .In which case do all the 4 layers at once. This resin is special and is not designed for finish parts though so you will need normal resin for this.
• Ok you have a mess of resin all over your original part, gently release the battery cover away from your fibreglass. without damaging the original you could use small wooden wedges to prise them apart.

Now you have made a mould

• You now have to repeat the process on your mould, so wax the surface again at least 5 times
• Apply a gel coat of the colour you want the finished product to be. If you want a carbon Kevlar part then apply clear gel coat
• Then apply 2 layers of 450 gm mat if making your part out of fibreglass or one layer of carbon Kevlar, Carbon Kevlar is very difficult to work with and the exact pattern of the mat is all important on the finished product, if you are using this then lay and consolidate carefully. I would personally leave this type of mat to the professionals unless you know what you are doing.
• Wait until cured and release from you mould you should then have your replica part.

No responsibility can be held for damaging original parts. If you want to make a number of replicas then flanges need to be added to the mould which is a specialised process and the mould should be made bigger than the original allowing the part to be trimmed to a depth. Also if there are sharp returns on the original, then a split mould would have to be made. This is what fibreglass moulders do for a living and is a whole different ball game.

PLEASE NOTE: This advice is given in good faith for guidance only, and is given without guarantee, or warranty, and users should use their own judgement on the suitability of there specific project.