From my visit to the Imperial War Museum I had the idea of creating a replica of a grenade. I was unsure of what would be the best way to create this and was originally thinking along the lines of creating a silicone mould of the grenade from a clay version and then creating a metallic looking grenade from this. This way I would be able to pick up all of the fine detail of the grenade precisely.
The method that you go through I creating the silicone rubber is as follows:
The silicone rubber is a 2 part chemical and is very good at taking exact copies of very intricate detail.
To take a molding you will need:
Some MDF
Silicone rubber (parts A and B)
Mixing pot
Mixing stick
Hot glue gun
The object to be molded
The first thing to do is to create a box to encase the object to be moulded. MDF is a cheap easy alternative to make this but it can be made from any rigid material. You just need to make sure that the box has 5 sides and that there are absolutely no gaps. There should be a 5mm area around the object to ensure that the mould is sturdy. There is no need for there to be any more of a gap, otherwise it will just be a waste of silicone rubber. For an object that has a flat surface, this can be laid down onto the bottom sheet of MDF unsupported. For this it is important to place double sided tape onto the underside of the object so that it does not move out of place. However, only place this tape around the edges of the object because the more tape that you put on the harder it will be to remove from the MDF. Make sure to cut away the edges of the tape as close to the object as you can so this does not distort the mould.
For an object that needs to be suspended, as in it has detail that needs to be moulded on all sides, a small piece of dowel should be attached with the hot glue gun so that the silicone rubber reaches round to all sides. This dowel, should too, be at least 5mm in length.
To make the silicone rubber you first need to place the mixing pot onto the scales and then press the on button. This means that when you are measuring out the rubber it does not take into account the weight of the pot and so is more accurate. Then measure out the amount of the rubber that you will need. You then need to add the catalyst to the mixture. The amount that you put into the mixture depends entirely on what the manufacture suggests. You then need to stir the mixture very well.
The next stage is to ‘de-gas’ the mixture. This is done to get rid of all the air bubbles in the mixture. This would distort the fine detail that the rubber should pick out. You need to make sure that the mixture does not exceed a third of the volume of the pot as, in the ‘de-gassing’ process the mixture does expand. It is important that the lid of the container is tight on top of the ‘de-gasser’ so that it creates a tight vacuum. To make the vacuum work you need to have the lever of the valve pointing in the opposite direction to the pipe. One of the valves (the shut-off valve) needs to be open at all times otherwise a vacuum will not be created. Put the lid on and then turn the pump on. You will find that when the air is being drawn out of the mixture small bubbles will appear on the surface. As the air pressure then decreases the bubbles get bigger and this causes the mixture to expand and may overflow if the pot is filled too high. Once finished you need to be careful when you let the air back into the cylinder as to not let it in too fast. This will cause particles to fly around in the cylinder meaning they will land in the pot. There may be surface bubbles present after you have taken it out of the cylinder but these will disappear easily if you roll the mixture around in the pot carefully, making sure that you don’t add more to the mixture.
You can now pour the mixture into the mould that you’ve created, making sure that you pour it from quite a height and into one corner of the mould and let it spread around the rest of the mould. Although you must be careful not to add in any more bubbles.
Once this is dry the next step is to make the resin to make the object. There are many different types of resin that you can make for this but the best one to use for this type of moulding is a bench-castable room-temperature curing polyurethane. This particular one is very good as it is fairly quick at setting.
To do this you need:
Fast cast polyurethane resin (part A and B)
Mixing pot
Mixing stick
Filler powder (to add a metallic finish to the resin)
To mix the resin together you need to make sure that you mix the exact same amount of both parts of the resin otherwise it will not set properly. Also add the filler powder at this moment. For this the minimum amount to add is a mixture of he amount of the 2 pars of resin together. For example, if you add 50g of part a resin and 50g of part b resin then you will need to add 100g of the filler powder. But this is only the minimum amount that you need to add.
Coat the inside of the mould with a fine coat of the filler powder and then you are ready to start filling the mould with the resin.
This was the original way that I was going to make my grenade but through discussions with my tutors I decided that the better way to do it would be to carve the grenade shape out of polyurethane foam.
The processes that I went through making the grenade are as follow.
To start with I studied an image that I took at the Imperial War Museum and used this to gage the size of the grenade. I then drew this onto the polyurethane foam and started shaping the block of foam into the right shape. Once it is in the right shape I then sand it to a smooth finish and start to engrave the slits into the surface to create a paneling effect.
After this I decided to cover it in a layer of paper maché to give it a smooth and solid finish. The next step is to paint it. For this I made sure that in the grooves is darker paint so that it looks even more 3dimentional. To give it a metallic feel I coated it in PVA glue after I have painted so it is slightly reflective.
The next stage is to attach the pin, which I have created using a keyring that I have painted to make it look old. There are also additional shapes that are part of the grenade that I have created out of the same foam and then simply attached them on.
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