Introduction to the freezing and deburring of the

Introduction to the freezing and deburring of rubber products

I. overview

unvulcanized rubber is a viscous fluid under high temperature and pressure, and in the molding vulcanization stage, the rubber quickly fills the mold cavity, and the excess part (in order to prevent lack of rubber, the rubber filled in the mold cavity must maintain a certain excess) overflows and vulcanizes, forming rubber overflow (also known as waste edge and flash). Once the overflow is formed, it must be removed in order to make the appearance neat and beautiful. This process is often called trimming. The requirements for trimming are accurate size and neat appearance. In actual production. Trimming of products is often time-consuming and labor-consuming. For products with strict requirements, a little carelessness in trimming may lead to waste times, which must be treated with caution. Generally speaking, the smaller the size of the product, the more complex the configuration, the higher the difficulty of trimming, and the more waste products

classification of trimming methods

II. Trimming of rubber products is divided into manual, mechanical and frozen:

1. Manual trimming. The operator holds a knife and repairs the overflow edge step by step along the outer edge of the product. This is the most primitive method. The efficiency is low and the quality is difficult to guarantee, especially for products with complex configuration and high accuracy requirements, it is difficult to be thorough and clean, and it is easy to damage the connection between the product body and the overflow edge. Often leave tooth marks and notches, leaving oil leakage, air leakage and other aftereffects that affect the seal. In addition. The dependence of manual trimming on operation proficiency is also prominent

2. Mechanical trimming. In order to improve efficiency and quality, mechanical trimming has appeared. The common one is the special electric trimming machine with rotating blade. The blade used should match the size and height of the product. If the inner and outer edges of the product have overflow edges. It can be designed as double-edged and multi edged. To achieve one-time completion. The machining accuracy of mechanical trimming is higher than that of manual trimming, and the efficiency is also doubled. Especially for the first mock examination multi cavity products, matching cutters can be designed according to the arrangement and distribution of products. After the product is molded. It can be put on the whole plate to complete punching and cutting at one time. With the help of heating, dozens can be repaired at a time. A typical example is the punching, cutting and trimming of medical stopper. The key is that the punching temperature must be well controlled to prevent adhesion after being too high

3. Frozen trimming. Remove the edges of the vulcanized finished products together with the waste edges under freezing conditions. For decades, with the selection and replacement of freezing media and the improvement of mechanical action, freezing trimming has also experienced several generations of improvement, becoming more mature and perfect, and the working efficiency and processing quality have been significantly improved. The process is to cool the vulcanized products under freezing and dynamic conditions, so that the waste edge enters the embrittlement state. Then, through the mutual friction between them, or under the dynamic conditions of rotation, vibration, swing and so on, the waste edges are removed with the help of friction. Or, make the rigid pellet medium. Impact the repaired products at a certain speed. Remove the overflow edge

III. introduction to freezing trimming

1. Basic principle. Rubber becomes hard and brittle at low temperature, and the degree of embrittlement varies with the thickness. That is, under the same low temperature conditions, the thin part becomes brittle before the thick part. Therefore, the edge trimming is completed by using the brittle gradient caused by the thickness difference between the overflow edge and the body, that is, grasp the time difference between the brittle overflow edge and the non brittle body, and apply external forces such as friction, impact and vibration to the product to be repaired to remove the overflow edge. At this time, the product body is still in an elastic state without damage. Later. The effect of trimming is further improved through the application of spraying medium

2. Evolution of freezing trimming technology. Frozen trimming first appeared in the 1950s, and has roughly experienced four development stages since then

(1) the first generation of frozen trimming frozen drum trimming takes the drum as the working container, and initially chooses dry ice as the refrigerant. Load the parts to be repaired into the drum, or add some working media that can play a role of friction. The temperature in the barrel is controlled within the range where the overflow edge is brittle and the product body is not brittle. In order to achieve this goal properly, the thickness of overflow edge should be ≤ 0.15mm. The drum is the main part of the equipment, which is octagonal. The key point is to control the falling point of the spraying medium so that the tumbling can be carried out repeatedly

the drum rotates counterclockwise, and the material falls along line 1-2 under the action of gravity, and then circulates in sequence to achieve even turning. After a period of time, the overflow edge becomes brittle, and finally the trimming is completed evenly. The disadvantage of the first generation technology is that it is not complete, especially on both sides of the parting surface, it is easy to have residual overflow. The reason is that the mold design is improper, or the adhesive layer at the parting surface is too thick (greater than 0.2mm)

(2) the second generation of freezing trimming machine

has made three improvements on the basis of the first generation

① use liquid nitrogen as refrigerant. Because the gasification point of ice is -78.5 ℃, it is not applicable to some rubber types with low brittle temperature (such as silicone rubber). The boiling point of liquid nitrogen is only -195.8 ℃, which can cover all kinds of rubber

② improvement of containers for products to be repaired. Change from drum to trough conveyor belt as carrier. In this way, because the product to be repaired can roll around in the belt groove, the probability of dead corners is greatly reduced. It not only improves the work efficiency, but also greatly limits its scope of application and improves the precision of trimming

③ no longer rely solely on the friction between parts to be repaired to remove edges, but introduce fine-grained jet medium to cheer. Granular metal or hard plastic projectiles with a particle size of 0.5 ~ 2mm are used to shoot at the surface of the products to be repaired at a linear speed of 25 ~ 55m/s, causing a great impact force, which greatly shortens the cycle

(3) the third generation of freezing trimming machine

is improved on the basis of the second generation. The container of the parts to be repaired is changed into a hanging basket with four walls and holes. These holes are distributed on the four walls of the basket, and the hole diameter is about 5mm (larger than the diameter of the projectile to allow the projectile to fall smoothly through the hole), which is recycled to the top of the equipment for second use. This arrangement can not only expand the effective capacity of the container, but also reduce the impact medium (projectile) 7 Less sample storage. The key points of the internal structure design of the third generation freezing trimming machine include: the hanging basket is not placed vertically, but with a certain inclination (40 ~ 60). Its advantage is in the process of punching and repairing. The overturning is violent due to the superposition of the two forces. The first force is the rotating force provided by the basket chassis; The second force is the centrifugal force with the impact of the projectile. When these two forces converge, 360 ° omni-directional motion is generated, resulting in uniform and complete overturning, thus shortening the processing cycle

(4) the fourth generation frozen trimming machine

although the third generation frozen trimming machine has the advantages of even turning and fast processing, it also has two disadvantages. First, it is not suitable for large products with a diameter ≥ 200mm due to the limitation of the volume of hanging blue; Second, because of the limit of the hanging blue volume, only batch operations can be carried out, and each batch change and start-up will require repeated consumption of liquid nitrogen, which will increase the cost. Therefore, a continuous production line that can carry out flow operation has emerged. After the parts to be repaired enter the working area, they run forward with the help of the annular conveyor belt, and then pass through the liquid ammonia freezing area to cool down, and the impact area to receive the projectile spray, completing the impact edge removal, and then the finished products with edges repaired can be offline. If the large and small specifications are mixed, they can be sorted. After the impact medium is recovered, it returns to the storage after external circulation

IV. discussion of related issues

1. Cooling methods. Cooling is a necessary condition to realize freezing trimming. The alternative cooling ways are:

(1) refrigeration by refrigerator. Freon is used as refrigerant to provide suitable low temperature conditions. The disadvantage is that it consumes a lot of energy and is not conducive to environmental protection, so it is not considered

(2) refrigerant refrigeration. Available refrigerants are dry ice (solid carbon dioxide) and liquid nitrogen. Dry ice has been eliminated because of its high boiling point and poor refrigeration effect. Liquid helium is widely used at present because of its low boiling point (-195.8 ℃) and outstanding refrigeration effect. It is also chemically inert. It is safe to use and will not affect the performance of rubber. Evaporation of 1kg of liquid helium requires 201 kJ of heat from the outside, so the refrigeration effect is good. Its unit consumption is roughly controlled at 1 kg of liquid ammonia and 1 kg of rubber products. Another advantage is that the gasification expansion rate is large. 1 liter of liquid nitrogen can be gasified into 700 liters of nitrogen, which can produce a very high air velocity and achieve a good heat balance

due to the inevitable loss of liquid helium in long-distance transportation, in order to reduce the transportation cost, the distance between the user and the supply point is required to be no more than 300km

2. High carbon steel balls were used in the early stage of spraying medium. The diameter is between 0.5 ~ 2mm. No matter the introduction of materials or practice, it has been proved that the effect of using a single particle size is often not as good as using several particle sizes in a certain proportion. Those with particle size larger than 1.5mm are suitable for punching large and thick waste edges; The small particle size can be used for supplementary processing. Complete the cleaning of residual dead corners

in recent years, hard plastic (such as polyformaldehyde) pellets have also been used. Their advantages are No. 4. Aerogel gel will leave scars on the surface of products and will not rust

the disadvantage is poor durability and short service life. In addition, due to poor strength, each trimming. It takes a long time

3. Selection of process parameters

(1) freezing temperature. It should be determined according to the type of glue, the thickness of waste edge and the spraying medium. As for the rubber type, the lower the brittleness temperature is, the lower the temperature should be selected

in addition, the thicker the overflow edge and the larger the batch at a time mean that the probability of surface contact with refrigerant is smaller and the freezing temperature should be lower

(2) loading capacity. It depends on the container capacity and also on the product structure. If the product is pure rubber, the loading capacity (in kilograms) should be equal to 1/3~1/2 of the container volume by weight. For example, the suitable loading capacity of a 60 liter container should be 20 ~ 30 kg. However, if it is a rubber/metal combined product, the loading capacity can be calculated as 1:1, that is, 60 kg

(3) injection angle. Practice has proved that the ejection (or ejection) angle of the projectile (that is, the angle between the ejected medium flow and the container) is the best 70 ~ 8, so that the ejected medium and the parts to be repaired in the hanging basket are in the best opposite state, so as to ensure uniform overturning

(4) unloading and cleaning. The sprayed medium after use will be vibrated and separated along the downward inclined vibrating screen, together with the repaired overflow and rubber chips, and collected at the bottom to realize the separation of waste edges and finished products

v. quality (210) wood based panel quantity problem and control

in the process of freezing trimming, we will encounter some quality problems, some of which are caused by the previous process, and some are caused by improper condition control. Common problems mainly include:

1, fracture at the joint. This is generally caused by poor joints in the previous process (molding or injection molding), for a variety of reasons, including poor liquidity of rubber, local scorch of rubber, insufficient vulcanization pressure, etc. The weak joint will be disconnected after being impacted under freezing

2. Potholes and pockmarks on the surface. Generally, it is caused by too low temperature, too long freezing time or too fast spraying speed. Among them, large potholes are caused by large projectiles, while pockmarks are caused by small particle projectiles

3. Incomplete edge removal. Sometimes there are residual waste edges at the parting surface. It can be seen that there are many reasons for the formation of such defects, the most important of which is the improper proportion of large and small projectiles. Generally, too few small particles are easy to cause such problems. Another situation is that the waste edge is too thick. If the thickness exceeds 0.2mm, it is difficult to remove the waste edge. (end)