Stage 1: Making a Mandrel Tool
A mandrel tool is a shaping tool that is made of metal and is hollow. It is used to bend silicone rubber in the shape and diameter of the desired silicone hose. It is commonly used in many other applications like blowing glass. The first stage is to make mandrel tool(s) in the shape of the silicone hose(s). This is done by manipulating steel in the desired shape of the hose. These tools are usually stored by manufacturers in thousands of shapes and sizes. The dimensions are available on their website or product catalog
Mandrel material choice: Steel with chroming; Stainless Steel; Aluminium.
Stage 2: Choosing the Raw Materials
The raw materials are chosen specifically for making silicone hoses. These include silicone rubber, color pigments and reinforcement fabrics. Hoses are multi-layered and reinforcement fabrics are used to give them strength. A checking list of raw material:
- Silicone rubber compound: It needs to add cross-linking agent and heat stabilizer or coloring agent)
- Silicone color pigments: color powder is mixed with 110 silica gel (methylvinyl silica gel: 110-2) to form a pigments
- Curing agent: Chocies: DCBP, commonly known as ‘‘Double 2 4’’, peroxide bis(2,4-dichlorobenzoyl), a dispersion of DCBP-based DCBP and silicone oil; BP (benzoyl peroxide). Paste BP is BP and 110-2 silicone rubber (1/1) masterbatch; DBPH, is a domestic peroxide vulcanizing agent, 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, commonly known as ‘‘Double 2,5’’ ; DCP, Dicumyl peroxide, Paste DCP is DCP and 110-2 silicone rubber (1/1) masterbatch
- Defrosting agent: In order to neutralize the insoluble acidic products produced by high-temperature vulcanization decomposition of peroxide curing agents and solve the problem of blooming after the silicone tube is parked, the anti-corrosion agent for silica gel can be properly added in the mixing process, and its main component is dimethyl silicon. Oxane polymers, silicone oils and metal oxides
- Reinforcment fabric: aramid , polyester and glass fiber, etc.
- Hose outer surface treatment auxiliary raw materials: cellophane or nylon cloth type
- Logo printing: heat transfer logo tape
Stage 3: Milling
Mixing compound ， masterbatch vulcanizing agent and defrosting agent according a specific formula
The temperature of the control roll of the open mill is below 50°C. It is better to use a suitable tool for thinning it to promote the compatibility and uniformity between various compounding agents and silica gel.
Machine used: Open mill
Stage 4: Calendering
The silicone rubber compound and the color pigments are mixed together in a mill. This process is called ‘milling’. It takes around half an hour in the mill to produce a color pigmented silicone rubber compound. The colored silicone rubber compound is placed into a calendaring machine. The machine presses the compound between rollers to make it into thin sheets. The calendaring machine then presses the flattened compound on to the reinforcement fabric. Now the material is ready to hand build the hoses.
In order to meet the different performance requirements of the silicone tube for different environments and conditions of use, such as strength and heat resistance, and to ensure the adhesion and compactness between the silica gel and the cloth, it is necessary to wipe the used fabric and silicone rubber before the hose is molded. Glue processing. In order to ensure sufficient adhesive strength between the fabric and the silicone rubber, it is also necessary to dip the fabric before calendering and then perform special calendering treatment
Machine needed : 4- or 3-roll calenders.
Calender operation process brief
- start the calender;
- Put the fabric and plastic film into the corresponding gantry and set the main body of the calender machine to rotate forward. Pass the fabric flow through the fabrics to the take-off position. The plastic film passes through the film guide roller. Contrary to the roller after rolling, stop the calender;
- The temperature of the drying roller should be strictly controlled (115±5°C for No. 1 roller, 115±5°C for No. 2 roller, 110±5°C for No. 3 roller, 110±5°C for No. 4 roller) to ensure that the roller is before rolling The moisture content reaches the specified (1%) requirement. If it is operated intermittently, the drying curtain should not be parked for more than 4 hours in order to prevent the curtain from absorbing moisture;
- Adjust the roll length of the calender according to the produced product.
- Properly grasp the amount of glue supplied. Supply less amount of glue (≤ 10kg per roll) to increase, in order to maintain a certain amount of glue between the roll gap, reducing the thickness fluctuations;
- Roller roll temperature: Silicone rubber adheres to the chill roll. During the rolling process, the temperature of the roll in the calender should be lower than that of the upper and lower rolls. When the surface of the rubber material is corrugated or sticky, the roll temperature should be increased;
- The calendering speed should be controlled properly to reduce the stress relaxation of the rubber and maintain the uniformity of the thickness;
- The expansion of the cord is controlled by a tension roller to ensure the width of the cord;
Stage 5: Hose shaping
The thin sheets are cut and piled to roll. The builders roll the thin sheets in tubes. It is necessary to maintain the correct thickness of the tubes. A finished tube needs to have the correct number of reinforcement plies. Now these tubes are ready to be used for building hoses. The silicone tubes are carefully pushed on the mandrel tools to give the desired shape. Any air bubbles or joints are smoothed out. Silicone hoses that have extra joints or bends need extra care to push the tubes into shape.
Stage 6: Oven Curing & Cooling
The silicone hoses are then cured in the oven and then cooled down. This gives them the desired shape, There are 4 types of curing or vulcanization ways.
- The vulcanization of steam vulcanization tanks: It has the disadvantage that the environment is messy, and a large number of products must be put in at the same time to increase the usage of the hose molds.
- Hot water vulcanization: It has the same disadvantage as steam vulcanization tanks.
- Electric heating oven vulcanization: traditional open-closed oven or recirculating track oven, advantages: rapid mold flow, high vulcanization efficiency, ready to put the product into the equipment
- Electric heating oven vulcanization, traditional open-closed oven or recirculating track oven, advantages: rapid mold flow, high vulcanization efficiency, ready to put the product into the equipment
Stage 7: Tool removal
Demoulding method: Human demoulding; air pressure stripping; hydraulic stripping.
Stage 6: Trimming & Inspection
Before trimming the hose is marked where the edge has to be cut. The edges are then cut using a machine for fine edges. After trimming the silicone hoses are sent for quality inspection. Once they are approved they are ready for shipping to the client.