Key control elements in polyethylene rolling process

During the heating stage of the rolling process,chemical or physical bonding occurs on the interface between the polyethylene powder or melt and the inner surface of the mold due to surface oxidation.When there are local defects on the inner surface of the die,the polyethylene melt will flow into these defects and form local embeddings.This will make it difficult to remove the cooled product from the mold.In order to avoid the above situation,the inner surface of the mold needs to be coated with a layer of heat stable material to prevent adhesion,such material is called release agent.There are many kinds of industrial release agents.The rolling process of polyethylene has high requirements for release agents,mainly the requirements of heat resistance.Oil,wax and silicone oil are commonly used release agents,but they need to be coated once before each feeding,so they are called disposable release agents.This kind of release agent has low cost and good release effect,but it is easy to migrate to the surface of the product and affect its surface properties.

There is a special phenomenon in the rolling process of polyethylene:in the melting process of powder,air trapped between powder particles forms bubbles,and these bubbles disappear with the continuous heating process.Closer examination showed that the bubbles did not disappear because they moved towards the free surface of the melt under buoyancy,but because the air in the bubbles gradually merged into the molten plastic melt.The results show that bubbles of different sizes are formed in the polyethylene melt when the temperature rises to 150℃.Because of the high viscosity of the polyethylene melt,the buoyancy of the bubble is not enough to push the bubble toward the free surface.When the temperature rises to 200℃,all the bubbles disappear.Therefore,it is very important to control the heating process scientifically to eliminate the bubbles in polyethylene products and improve the quality of polyethylene products.

Because the heating time of rolling molding is sometimes longer,especially when the product wall is thick.It could last half an hour to over an hour.At this time,measures should be taken to prevent the thermal oxidation of the material and the degradation of the material properties during the heating process.The purpose of prevention can be achieved by adding antioxidants to polyethylene plastic.However,when the polyethylene material is heated to too high a temperature or for too long,antioxidants do not prevent oxidation of the material.When the product thickness is large and needs to be heated for a long time,the heating temperature must be reduced.If the heating time is shortened by increasing the temperature,the air in the bubble may be retained because it is not enough to disappear.When a polyethylene plastic is heated to a molten state,the material undergoes a transition from a crystalline state to a melt,which is what happens when the polyethylene particles begin to melt and become soft.It first appears in a layer of material in contact with the inner wall of the mold,forming a uniform layer of molten material.Then,it is gradually extended to the inner layer until the whole section is completely transformed into plastic melt.This is followed by further heating to gradually remove the bubbles.The temperature control and time control of this process need to be adjusted.

During the cooling process,the temperature of polyethylene melt will drop from 200℃ to near room temperature,and the molecule of polyethylene will change from a disordered state to a more ordered crystalline state.The crystallization process takes some time,and the crystallization rate is related to the viscosity of the polyethylene melt.When the polyethylene melt is cooled rapidly,the viscosity of the melt increases rapidly,which impedes the growth of its grains,thus affecting the crystallinity of polyethylene.When the crystallinity is different,the density of polyethylene products is different,and the physical properties will be different.Thus,the rapidly cooled polyethylene rotary-molded product has a lower density,while the slowly cooled product has a higher density.