Lab Hydraulic Press

When the hydraulic system is working, the ram is pushed upward. It will first drive the lowermost cooling plate to move upward, and then drive the middle cooling plate and heating plate in turn until it finally contacts the uppermost heating plate. The plates gradually apply pressure until it reaches the target value and then maintains it constant for a defined period of time. During this process, the pressure is automatically increased and decreased to hold the pressure. If there is insufficient pressure, the system will automatically compensate for the pressure. Note that if the rubber vulcanization is controlled by PLC, the machine allows for multiple stages of pressurization. It can give first pressurization, exhaust according to the preset exhaust number and exhaust time, and then pressurize the material for many times automatically. When the pressurization time is reached, the system automatically depressurizes, the oil from the hydraulic pump is drained, and all movable platforms and attached plates will be lowered to their original positions with the help of the equipment’s own weight.

For vulcanizing fine rubber products with complex shapes, we recommend using a vacuum vulcanizer.

When the plate vulcanizer is working, the most power-consuming parts is the heating tubes inside the two heating platens. When the machine is started, especially during the platen heating stage, a large current input needs to be maintained, and the entire machine will be maintained in a high power state. The main function of the ammeter is to monitor the current changes of the heating devices to determine the operating status of the equipment, including whether the current exceeds the standard and whether the equipment is operating normally. This is crucial to ensure the stability of production and the long-term operation of equipment.

The rubber vulcanization process is a process that is processed at high temperatures. From low temperature to high temperature, it possible that the material volatilizes, the rubber material is not dense, there are gaps, excess rubber material is extruded and discharged, and vulcanization gas is released. Let’s imagine that the mold cavity is filled with air before filling the raw material. When the mold is pressurized, the raw material flows into the mold cavity. Because we set a lower pressure value for the exhaust, the lower pressure cannot allow the raw material to fill the mold cavity before exhaust, leaving a space for air. The air in the mold cavity is squeezed into the depth of the mold cavity as the raw material is filled, and is compressed to generate a higher air pressure. At this time, if we exhaust the air again, the air inside will be easily discharged out of the mold cavity due to the higher air pressure.
Sometimes the fluidity of the raw materials is not ideal, or when the secondary feeding method is used, if the pressure is in place at one time, it is easy for the raw materials to fail to completely fill the mold cavity, causing the product to be short of material, but with large burrs. Setting low pressure exhaust can solve this problem very well. The raw material will first flow into the entire mold through low pressure, and then fill all the mold cavities through high pressure.

Before vulcanization, various parameters of the vulcanizer need to be adjusted, among which the number of exhausts is a very critical one. The principles for adjusting the exhaust frequency of the vulcanizer include:
1）Appropriately change the number of exhausts according to the size of the stroke.
2）The specific number of exhausts should be determined according to the needs of the process and the actual situation, but it must be stable and consistent.
3）Avoid too much or too little exhaust to avoid affecting the vulcanization effect and vulcanization quality.
4）Clean the exhaust pipe in time to avoid blockage.

1. Rubber materials contain moisture during mixing, storage, and use. The moisture is not removed, resulting in bubbles or moisture due to weather changes.
2. The mixing operation is not standardized, resulting in uneven rubber mixing and incomplete and even dispersion of the compounding agents.
3. The mold design is unreasonable. Improper setting or blockage of the vents will also produce air bubbles. When the raw materials are mixed, the air will be trapped inside the material, causing the material and air to enter the mold together during the processing. If the mold does not have the exhaust groove so that air can easily be trapped inside the mold, causing bubbles to form in the product.
4. There is no exhaust process during the vulcanization process.
5. Insufficient vulcanization and the immature rubber material appears to have bubbles.
6. Insufficient vulcanization pressure.
7. The vulcanizing agent contains many impurities. Small molecule impurities are decomposed in advance and bubbles remain in the product.
8. The product is too thick. If there is too little rubber material, the rubber will transfer heat slowly. After the surface is vulcanized, the fluidity of the rubber will decrease, resulting in a shortage of material, so bubbles may occur.
9. Formula problem, and the vulcanization system needs to be improved.

It is easy to understand that the upper layer of the flat plate vulcanizer is used for heating. The plastic polymer particles are heated and melted to form thin sheets under a certain pressure. So what is the purpose of using water cooling in the lower layer? After the sample is molded on the upper layer of the flat vulcanizer, some materials have a large shrinkage rate and cannot be opened immediately, but the process from the high temperature in closed mold state to room temperature is very slow. In order to speed up the production of test pieces and avoid large-area shrinkage of the material, we need to quickly transferred the mold from the upper layer to the lower layer, and the lower plate is supplied with circulating water to reach cooling purpose, which can greatly improve work efficiency.
This type of flat vulcanizer is generally used to process plastic materials, composite materials, polymer particles and other materials. Cooling is one of the common processes for making test pieces before tensile testing, impact testing, light transmittance and other tests.

Vulcanization is a process that the semi-finished products made from appropriately processed rubber materials (raw rubber, plasticized rubber, mixed rubber) with certain plasticity and viscosity are transformed into soft rubber or hard tough rubber products under certain external conditions through the action of chemical factors or physical factors to obtain desired performance. During the vulcanization process, external conditions such as heating or radiation cause a chemical reaction between the raw rubber and the vulcanizing agent or the raw rubber and the raw rubber in the rubber component, and the linear rubber macromolecules are cross-linked into a three-dimensional network structure. Through this reaction, various properties of rubber are greatly improved, allowing rubber products to obtain physical, mechanical and other properties that can meet the needs of product use.

Under a certain temperature and molding pressure, in order to change the rubber material from plasticity to elasticity and maximize the cross-linking density, the time it takes to optimize the physical and mechanical properties is called the rubber product vulcanization time. Usually it does not include auxiliary time for the operation process.
Vulcanization time is closely related to vulcanization temperature. During the vulcanization process, when the physical and mechanical properties of the vulcanized rubber reach or approach the optimal point, this degree of vulcanization is called optimum vulcanization. The vulcanization time required to achieve optimum vulcanization at a certain temperature is called optimum vulcanization time. A certain vulcanization temperature corresponds to a certain optimum vulcanization time. When the rubber compound formula and vulcanization temperature are constant, the vulcanization time determines the degree of vulcanization. Rubber products of different sizes and wall thicknesses control the degree of vulcanization by controlling the vulcanization time. Generally, the larger the size or thickness of the product, the longer the vulcanization time required.

Rubber vulcanization temperature is one of the three major vulcanization factors. It is the basic condition for rubber vulcanization reaction (cross-linking reaction) and directly affects the rubber vulcanization speed and product quality. Like all chemical reactions, the vulcanization reaction accelerates as the temperature increases, and more low-sulfur cross-links are easily generated. when the vulcanization temperature is low, the speed is slow, the generation efficiency is low, and more polysulfide cross-links are generated. The vulcanization temperature generally applies to Van’t Hoff’s law, that is, for every 8~10°C increase in temperature, the reaction speed approximately doubles; or in other words, the reaction time is approximately reduced by half.

Vulcanization pressure refers to the pressure that the rubber compound bears per unit area during the vulcanization process. Generally speaking, except for some cloth and sponge rubber, other rubber products require a certain amount of pressure when vulcanized. Rubber vulcanization pressure is an important factor in ensuring the geometric dimensions, structural density, and physical mechanics of rubber parts. It can also ensure that the surface of the parts is smooth and defect-free, and meets the sealing requirements of rubber products.

When rubber products are vulcanized, pressure needs to be applied. The purpose is:

1. Prevent bubbles from forming in the rubber material and improve the density of the rubber material.
2. Make the rubber flow and fill the mold to produce products with clear patterns.
3. Improve the adhesion between each layer in the product (rubberlayer and cloth layer or metal layer, cloth layer and cloth layer), and improve the physical properties of the vulcanized rubber (such as flex resistance).

Generally speaking, the selection of vulcanization pressure should be determined based on product type, formula, plasticity and other factors. In principle, the following rules should be followed: for greater plasticity, the pressure should be smaller; for thicker products, many layers, and complex structures, the pressure should be larger; for thin products, the pressure should be smaller, and even normal pressure can be used.

The pressure F required for vulcanizing rubber products depends on the pressure area “S” of the vulcanized product or mold and the required unit pressure “P”.

F=S*P

In the formula,

F – the pressure required for vulcanizing rubber products, in N

S – Pressure area of the product or mold, in m²

P – The unit pressure necessary for the vulcanized product required by the process, in Pa

1) The plastic deformation of rubber is large before vulcanization. After external force is applied, the randomly curled rubber molecules are in a straightened state. If they are further stretched, slippage will occur between the molecules. After the external force is removed, the rubber returns to its original state, but molecular slippage cannot be restored, resulting in large permanent deformation of the rubber. After vulcanization, a network structure is formed between the rubber molecules and the permanent deformation is small.

2) Rubber is easy to dissolve before vulcanization. For example, natural rubber can be dissolved in gasoline, benzene, and toluene. Rubber swells after vulcanization. The size of swelling depends on the degree of vulcanization. The greater the cross-linking density, the smaller the degree of swelling.

3) The thermal stability of rubber is not good before vulcanization. It becomes sticky when exposed to heat and becomes hard when exposed to cold. After vulcanization, the heat resistance and cold resistance of rubber are greatly improved, and the stability against hot and cold is greatly improved.

4) The strength of rubber is low before vulcanization and becomes high after vulcanization. For example, the tensile strength of natural rubber NR is only 1.4-2.5 MPa, but after vulcanization processing, its tensile strength can reach 17-25MPa.