Technological processes of plastic injection molding mainly include filling, pressurization, cooling and demolding.
The four steps are a complete continuous process.
Filling is the first step of the whole injection molding
. Time should be originated from mold closing to filling about 95% of mold cavity. In theory, the shorter filling time is, the better molding efficiency is. However, molding time or injection speed is limited by various conditions in fact.
High speed filling
High speed filling has higher shear rate. Viscosity of plastic may decrease due to shear thin and the whole flow resistance falls. Partial viscous heating also attenuates thickness of consolidated monolayer. During flow control, filling depends on the filled volume. During flow control, due to high speed filling, shear thinning of melt is huge while the cooling on thin wall not obvious. So, effectiveness of speed prevails.
Low speed filling
When heat conduction controls low speed filling, shear rate is lower, partial viscosity is higher and flow resistance is also higher. Due to lower supplement rate of thermal plastic, the flowing is also slower, making effect of heat conduction more obvious. Heat is taken away quickly by cold mold wall. With bits of viscous heating, thickness of consolidated monolayer is ticker, increasing the flow resistance on thin wall further.
Due to fountain flow, plastic polymer chain before travelling wave is almost parallel with travelling wave. So, polymer chains on contact face are parallel with each other. For different properties of two melts, confluence area of melts has poor structural strength in the micro. Under ray of light, place the part at a proper angle. Apparent seamline can be discovered by naked eye. That is mechanism of forming welding mark. Welding market cannot only have influence on appearance of plastic part, but also easily cause stress concentration because of incompact microstructure, resulting the strength reducing or the part even breaking.
In general, welding mark caused in high temperature region has high strength. Under high temperature, activity of polymer chain is better, which can penetrate and twine each other. Besides, in high temperature region, the two melts has the same temperature and the thermal properties of melts are almost the same, increasing the strength in welding region. On the contrary, welding strength is poorer in low temperature region.
Pressurization refers to that continue applying pressure, compacting meld and increasing density of plastics to compensate the shrinkage of plastic. During pressurization, because of the cavity being filled with plastic, backpressure is higher. During compaction, screw of injection molding machine only can move slightly. The flow velocity of plastic is also slower. The flow is called as pressurizing flow. During pressurization, solidification of plastic is accelerated by mold wall cooling. Viscosity increasing of melt is also rapid. Resistance inside mold cavity is high. In the end of pressurization, the density of materials is growing constantly and the plastic part is formed gradually. Pressurization is continued to solidification and sealing of sprue. Then, pressure inside mold cavity reaches the maximum value.
For pressurization, the pressure is quite high. plastic shows partial compressibility. In higher pressure region, the plastic is denser and the density is higher. In lower pressure region, plastic is looser and the density is lower. So, the density distribution varies with the position and time. Plastic has low flow velocity and the flow does not play a leading role any longer. The pressure is main factor having influence on pressurization. The plastic has already been filled with cavity. The, gradually solidified melt is used as medium to transmit pressure. The pressure inside cavity is transmitted to the surface of mold wall by plastic, having the tendency of strutting mold. So, appropriate clamping force should be used for mold clamping. Swelling force struts the mold slightly under normal circumstance, helpful for mold exhausting. If the swelling force is oversize, deckle edge and flash of products are easily caused, or the mold is even strutted. So, when choosing injection molding machine, injection mold machine having enough clamping force should be chosen in case of swelling mold and pressurization should be carried out effectively.
The design of cooling is important for plastic injection mold. The plastic products should be cooled and solidified into definite strength, the, the plastic products cannot deform when suffering from external force after demolding. The cooling accounts for about 70%~80% of the whole molding cycle. So, a good cooling system can significantly reduce forming time, improving productivity and reducing cost. Improper cooling system can increase forming time and cost. Uneven cooling can cause warping for deformation of plastic products further.
Based on experiment, heat of melt entering into mold is divided into two parts, 5% of heat is transmitted to atmosphere by radiation and convection. 95% of heat is transmitted from melts to mold. Because of cooling water tube, the heat is transmitted from the plastic inside cavity to cooling water tube through mold base by heat conduction. Then, the heat is taken away through cooled heat convection. A few of heat which is not taken away by cooling water is continue to being transmit in mold, overflowing to air until contacting with the outside world.
Molding cycle of injection molding consists of closing time, filling time, pressurization time, cooling time and demolding time. Among that, cooling time has the highest proportion, about 70%~80%. Therefore, cooling time has direct influences on forming cycle of plastic products and output. During demolding, the temperature of plastic products should be reduced to lower than heat deflection temperature of plastic products, in case of plastic loosening caused by residual stress and warping or deformation caused by demolding external force.
Factors have influences on cooling rate of products:
Design of plastic products
The thickness of wall of plastic products: the higher thickness is, the longer cooling time is.
Mold materials and cooling ways
Mold materials including materials used for mold core, cavity and mold base have great influences on cooling rate. The higher coefficient of heat conduction of mold materials is, the better conduction of heat from mold is better in unit time and the shorter cooling time is.
Collocation method of cooling water tube
If the cooling water tube is closer to mold cavity, pipe diameter is larger and quantity is more, the better cooling is and the shorter cooling time is.
Flow of cooling water
The higher flow of cooling water is, the better heat dissipation through thermal convection.
Properties of cooling water
Viscosity and coefficient of heat conduction of cooling water also have direct influence on heat conduction of mold.
Machine parameter set
The higher of materials temperature is, the higher of mold temperature is, the lower of ejection temperature is, the longer required cooling time is.
Design rule of cooling system
Designed cooling gallery should ensure homogenous quick cooling effects.
The cooling system is designed to maintain appropriate and efficient cooling on mold. Cooling hold should use standard sizes, convenient for processing and assembling.
When design cooling system, designers should set the following design parameters based on thickness and volume of plastic parts: location and dimension of cooling holes, hole length, categories of hole, configuration and connection of holes, flow rate and heat transfer characters of cooling water.
Demolding is the last step of plastic injection molding. Although the products have already been solidified, demolding still has impacts on the quality of products. Improper demolding may result in nonuniform stress and the product deforming during ejection. The methods of demolding are classified into two categories: ejector demolding and stripper demolding. When designing mold, appropriate demolding methods should be chosen based on structural features of products, ensuring the quality of products.
For the mold using ejector demolding, the ejector should be set as even as possible. The position should be set at the place where the demolding resistance is the highest and strength and rigidity of plastic parts are the highest, in case of plastic parts deforming or being damaged.
Stripper plate is generally used thin-wall container having deep cavity and transparent products which that push rod traces are not allowed. The features are the knockout press is large and even, steady movement without obvious remaining mark.