The injection process is complex, affected by many factors, and has both process and mechanical equipment. They are connected and interact with each other. Several important process operating parameters of the injection process are introduced one by one.
1 temperature control
The rubber injection process temperature control is to adjust the barrel temperature, injection temperature and model temperature.
a. barrel temperature
The barrel temperature is important and should be as high as possible with the safety of the scorch. Increasing the barrel temperature within a certain range can increase the injection temperature, shorten the injection time and cure time, and increase the hardness of the vulcanizate. The choice of barrel temperature should consider the form, operation mode and formula of the injection machine. Generally, the plunger type is controlled at 70~80 °C. The reciprocating screw type is more uniform due to the temperature of the rubber compound, and the temperature of the barrel can be higher, about 80. ~100 ° C, some even up to 115 ° C.
b. Injection temperature
The injection temperature, ie the temperature after the rubber has passed through the nozzle, should be as close as possible to the cavity temperature with the safety of the scorch. If the injection temperature is too high, it is easy to scorch; otherwise, the vulcanization time is prolonged. In addition to the barrel temperature, the injection temperature is also related to many factors. The injection temperature can be increased by increasing the speed of the screw, back pressure, injection pressure and reducing the diameter of the nozzle. The temperature rise of the different rubbers after passing through the nozzle is different. The average temperature rise after passing the nozzle is as follows: isoamyl rubber 10 ° C; silicone rubber 18 ° C; neoprene 23 ° C; oil-filled styrene butadiene rubber 25 ° C; butyl rubber 26 ° C; natural rubber 35 ° C Ordinary low temperature styrene butadiene rubber 38 ° C; Ding Yue green glue 60 ° Co In order to prevent scorch, nozzles with separate temperature control system, or to expand the nozzle diameter and other measures.
c. Model temperature
The model has a low temperature, a long vulcanization time, and a low yield. However, if the mold temperature is too high, scorching will occur during filling. Therefore, from the viewpoint of increasing the yield, the maximum temperature at which the mold is not scorched should be used as much as possible.
The method of selecting the safest maximum mold temperature is to first slowly increase the mold temperature until the wrinkles indicating scorch appear on the edge of the molded article, and then select a temperature lower than the temperature by 3 to 5 ° C as the highest safety. Mold temperature. When the barrel temperature and mold temperature are determined, the temperature of other parts can also be determined.
2 injection pressure
Injection pressure is an important parameter. It has three meanings: one is the injection pressure written on the injection machine trademark, or the nominal injection pressure, that is, the maximum injection pressure of the machine; the second is the allowable injection pressure, usually 80% to 90% of the maximum injection pressure; It is the actual injection pressure, which is determined by the process conditions and product structure.
Within a certain range, the injection pressure is increased, the injection time is shortened, the injection temperature is increased, and the vulcanization time is also shortened. It is generally believed that the injection pressure is increased by an average of 6. 9 MPa, and the temperature of the rubber is increased by about 1 to 3 ° C. Lowering the injection pressure will increase the injection time, but there is a threshold. This critical value is related to the type of rubber, the size of the nozzle, and the like.
Screw back pressure means that when the screw is plasticized and gradually retreats, it must be pressed against the rubber with a certain pressure. This pressure is called back pressure and can be adjusted by hydraulic pressure. The role of back pressure: First, the bubbles and volatiles in the rubber compound are excluded, so that the rubber compound in the plasticizing chamber is compacted to ensure accurate measurement of the injection amount; the second is to improve the plasticizing and mixing effect, because the rubber compound is denser. The friction shearing effect is large, and the plasticizing and mixing uniform effect is good. By increasing the back pressure, a higher screw speed can be used and the temperature of the compound increases. As the back pressure increases, the temperature of the compound in the barrel rises and the injection temperature rises only slightly; there is no benefit from excessive back pressure.
3 injection speed
The injection speed can be seen from the injection plunger moving speed and injection time, etc. The injection speed is related to factors such as the injection pressure and the nozzle diameter and the properties of the rubber compound. As the injection plunger moves faster, the injection temperature and vulcanization rate increase. The injection temperature is increased and the injection time is shortened. However, the injection speed is too large, and there is no greater benefit to the improvement of the vulcanization rate. On the contrary, it will cause scorch due to frictional heat generation, and at the same time, it will easily cause internal stress, anisotropy and air bubbles. Of course, the injection speed is too low to improve the production efficiency, and it also causes the rubber to burn during the flow or wrinkles or lack of glue on the surface of the product.