The Most Difficult Part of Restoring Old Motorcycle is Rubber Parts Supply. DIY Production Challenge [Part 1]

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There are many things to be concerned about when restoring an old motorcycle. After all, the condition of the rubber parts is a major concern for anyone. We decided to make our own rubber parts, which are not easy to find. Here, we will try to make a part using a plaster mold, which is a relatively practical method. Everyone is surprised by the quality of the parts! Let's start by making a mold from a master part.

"Meter cable support" which is conspicuous without it

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The green Muscat color of the 125B8, Kawasaki's first full-scale mass-produced model for 1963, was a limited-edition "fruit-colored" model sold by Kawasaki Fukushima Sales. The green Muscat color was a limited edition model sold by Kawasaki Fukushima Sales, and was apparently inspired by the Muscat fruits of Fukushima Prefecture. A one-off rubber meter cable protector was added for added protection.

Molding used parts that are in good condition but ticking over.

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The master component is the stock part itself, which has deteriorated to the point of ticking over. The same parts are used for the front brake cable and meter cable. The master parts were placed on a plastic plate, and then the plaster was poured into the plastic after making a bank with aluminum tape.

Making a formwork with aluminum tape

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Aluminum tape has a surprisingly strong waist, making it easy to maintain its shape. Let's stack several sheets and stick them together to firmly reinforce the bank of aluminum tape. There are several types of aluminum tape specifications and thicknesses, but Maxi Itabashi, who is in charge of production, says that the type with adhesive on the aluminum surface (the type without protective paper attached to the adhesive part of the aluminum tape) is easy to use. After the aluminum bank is completed, a release agent for plaster called "Kali soap" is applied to the inside of the mold and the master model with a rag. Even after the plaster hardens, the hardened plaster will crack without the release agent. Let's proceed with the work carefully.

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In FRP modeling, polyresin is applied to glass fibers to harden the resin and reproduce the shape, but since plaster has no fibers, it is surprisingly brittle. That's why plaster is mixed with fiber to make it stronger. This time we're making a small mold, but the plaster alone will crack after it dries, so we'll reinforce the plaster with sisal (hemp fiber). Sisal is sold in the same way as plaster. It has more tensile strength than you might imagine, and it definitely increases the rigidity of the mold.

Plaster like lime powder for line drawing

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The plaster we used was Sakura brand "Baking Plaster" Class A. Professional use products generally come in 20kg bags, but there are also 5kg bags that are great for Sunday mechanics. There are some tricks to kneading plaster, but the thin plastic containers used to make FRP resin seem to be easy to use. Now it's time to start kneading.

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When the plaster appears on the surface of the water and the surface of the water and the plaster almost match, stop pouring the powder into the cup. This is the balance point of water and powder. I put my bare hand deep into the cup and spin my fingertips to mix the plaster and water to make a gel. The trick is to thoroughly mix the water and powder by rubbing with your fingertips so that there are no lumps of powder.

Pour the plaster into the mold carefully and slowly.

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The mixed plaster is slowly poured into the mold. Make sure there are no lumps of powder as you work. It's best to rub the plaster liquid with your fingertips as you pour it in. Don't just pour it on top of the master, but start from the perimeter of the formwork. Tear off the sisal that is sold pressed into a plastic bag and pull it out, then pull and stretch the denser chunks. If you don't make the density shallow, it's hard to get used to the plaster liquid. Anyway, let's pull and stretch it!!! It is important to soak the sisal in the plaster liquid by sinking it into the plaster liquid leftover in the cup. After rubbing them together with your fingertips, sink them into the mold. In this way, the plaster liquid poured into the mold and the sisal can easily get used to each other, and a plaster mold with high strength can be made with less air bite.

 

POINT
  • Point 1: Tickled and cured rubber parts are not reused, but have great value as master molds.
  • Point 2: The adhesive part of the aluminum tape is not a protective paper, but a product that directly adheres to the skin of the aluminum tape, which is overwhelmingly easy to use and has high strength.
  • Point 3: The proportion of gypsum and water mixed together determines the success or failure of the project. Too much water or too much plaster won't cut it.
  • Point 3: Use hemp fiber (sisal) to increase the strength of plaster. If the density of the sisal is too high, it will bite the air and reduce the strength of the mold, so it is best to loosen the sisal and let it blend with the plaster solution before submerging it in the mold.

 

The overwhelming majority of one-off rubber parts are made by filling the master mold with liquid silicone rubber, but the molding method varies depending on the size of the finished part. If you are making fingertip-sized small parts, "mold-making nendo," which is softened by heating it with hot water, would definitely be recommended. If you want to mold large parts and make a few products (about 4 or 5 depending on the mold making), silicone rubber is generally used. If the master part is large, you may have to apply force or twist the part when you pull out the master after mold making.

Plaster is well known as a mold for bending FRP parts and resin plate materials, but why did we choose to use a plaster mold for making rubber parts? We asked Mr. Itabashi, the leader of Model Create Maxi, who is a professional parts maker, about that.

As far as the master shape is concerned, it seems to be able to be taken out smoothly, so I thought it would be best to challenge with the plaster mold. I felt it would be best to challenge with a plaster mold. If you expect to make a certain number of pieces, making a silicon mold is definitely the way to go, but since the part shape is flat and simple, I felt that the plaster mold was more common. It is not suitable for the number making, but it is good if two pieces are completed this time. The parts are curved, but the shape is almost flat, so the removal should be smooth," says Maxi.

Here we have seen the first half of the mold making process, the pouring of the plaster liquid and the reinforcement of the mold using a sisal, and it is clear that a master mold of a simple shape can be made with a plaster mold, and that it is possible to one-off rubber parts quite well. Plaster is also an art material that has been used in many ancient Roman times. As for ease of use, it's clearly better than silicone rubber, what do you think?

Photography cooperation: Model Create Maxi

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Plaster Mold after Complete Drying is Resistant to Heat! DIY One-off Rubber Parts Challenge [Part 2]
As far as the shape of the original master part is concerned, it seems to be possible to take out smoothly, so it might be best to challenge with "plaster mold". This time, I decided to make DIY parts...
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