At first, when the resin enters the compression section (L2) from the feeding section (L1), it is compressed and degassed due to the reduction of the screw groove volume. Before entering the main measuring section (L3), the resin temperature has reached its melting temperature, so it has become a melt.
The metering section (L3) is also called the mixing section. Because the screw groove depth h2 is smaller, the resin will be mixed by strong shear force during the screw rotation, so the melting is more complete.
The values of the following three screws will completely control the degree of degassing and melting of resin. The more these three values exist, the more complete the melting of materials will be.
1. effective length of screw: l
2. Compression ratio of screw: h1/h2
3. Relative length of compression part of screw: h2/L
One Injection pressure
Generally, it is divided into two stages. The first stage is to inject molten materials into the mold at high speed, and the pressure at this time is called primary injection pressure, which is generally called injection pressure. The second stage is the pressure added after the material fills the mold, which is called secondary injection pressure or holding pressure.
Low primary injection pressure will lead to insufficient filling amount. High pressure can increase the density and decrease the shrinkage rate of the product, but high pressure will make the product burr or produce larger residual stress, and sometimes make it difficult to demould the product. Therefore, when debugging the product, we should start with low pressure and gradually increase it to determine the appropriate primary injection pressure.
Secondary injection pressure (holding pressure) refers to the pressure acting on the material after filling the mold cavity and cooling and curing. The whole time of holding pressure is called holding time. Its function is to continuously supplement the volume reduction caused by shrinkage from the nozzle on the basis of preventing burrs and overfilling, so as to prevent shrinkage marks (shrinkage) of the workpiece caused by shrinkage. Other pressure settings are generally lower than primary pressure.
Two Screw back pressure
Before entering the next injection, the screw will rotate to deliver the molten material to the front of the charging barrel for storage. At this time, the screw rotates while delivering the material to the front of the charging barrel, which generates back pressure. In order to adjust and control the way the screw retreats, a certain pressure opposite to the molten material can be applied to the screw, which is called back pressure.
Back pressure of screw can improve the melting effect of material, and at the same time, it can ensure that the molten material is completely filled in front of screw, so as to improve the accuracy of injection metering.
If the back pressure is too high, it will lead to the decrease of the material handling capacity and the increase of the temperature of the material due to the increase of friction heat. On the contrary, if the back pressure is too low, it will lead to inaccurate measurement of injection quantity.
Three Barrel temperature
When setting the temperature of the barrel, it is generally to keep a certain temperature gradient, that is, the nozzle from the rear to the front should be set to increase its temperature gradually. First, the temperature set in the feeding section is mainly to prepare the material for heating, and the temperature in the compression section should be higher than the melting point of the material. Finding and investigating the best temperature of the material can be adjusted in a small range of 2-3℃.
Four Mold temperature
The low mold temperature can cool the materials in the mold cavity quickly, which improves the molding efficiency. However, the low mold temperature can easily lead to quality problems, such as flow marks, shrinkage, fusion lines and so on.
The mold temperature is high, and the crystallinity can be increased due to slow cooling, which is beneficial to improve the dimensional precision and mechanical properties of its parts.
Five Injection speed
Injection speed can be a means of machine adjustment other than temperature and pressure. It can control and adjust the viscosity of materials. By controlling and adjusting the injection speed, the appearance of parts can be prevented and improved, such as burrs, spray marks, silver marks or coke marks.
