Serial No. 8 (Friends who are interested can follow Gud Mould and view previous serials from historical messages)

 

Impact of processing quality on product deformation is relatively small. It should be pointed out that processing quality, especially quality of exhaust system, will have a greater impact on deformation.
1) Depth and surface finish of exhaust groove. Processing quality of exhaust groove has a great influence on exhaust effect during injection molding process. Exhaust effect will affect flow resistance of various areas in mold cavity, thereby affecting flow and thus affecting deformation of product.
2) Surface quality of cooling water path. Processing quality of cooling water circuit, especially length of dead ends, leads to dead water (water that does not flow in cooling cycle). Stagnant water does not flow, which will affect transfer of heat and thus affect deformation.
3) Installation quality of water-proof sheet. Water barrier is installed in cooling water well and needs to be fixed to prevent rotation, otherwise it will affect flow and direction of cooling water and form stagnant water.
All in all, impact of processing quality on deformation is mainly reflected in exhaust and cooling effects, and poor processing affects exhaust effect. In addition, large deviations in mold cavity glue position processing will affect wall thickness distribution of product, thereby affecting product deformation.

Injection molding process conditions have a huge impact on deformation of molded products, mainly in following aspects:

There are two concepts of injection molding speed. One is speed of screw of injection molding machine, which represents amount of glue injected per unit time; the other is displacement speed of melted glue in flow channel in mold, which determines shearing effect experienced by melted glue. In same set of molds, the former determines the latter, and the two have different meanings. The latter determines surface quality of product, filling stress during filling process, frictional heat of melt flow, stagnation effect, flow pattern, filling pressure, etc. Geometric shape of product includes size and wall thickness. In product, it is carrier to realize function of product, and in mold, it is path for flow of melt. For same product, if gate position design is different, even if screw advancement speed of injection molding machine is same during injection molding, displacement speed of melt glue in mold cavity will be different; even if mold is same, even if injection speed remains unchanged, displacement speed of melt in each area of mold cavity is different. Since displacement speed of melt glue in mold cavity affects surface quality of product, if difference in displacement speed distribution of melt glue in mold cavity is too large, excessive shear stress differences will occur, thus affecting surface quality. This is one of reasons why the plastic injection function of injection molding machine is designed in segments.
The faster screw advances, the faster molten glue moves in mold cavity and the shorter filling time, so it can switch to pressure holding stage earlier. Since molten glue experiences a short time in mold cavity, solidification degree of molten glue is low, so it has a better pressure-holding effect, and pressure distribution in mold cavity is more uniform. From this point of view, it has a positive impact on product deformation. But from another perspective, due to excessive speed, it means that shear is intensified and more shear stress is generated. The higher degree of molecular alignment, the higher degree of uneven shrinkage from cross-section with same flow distance. It should be pointed out that effect of shear stress generated by flow on product deformation is very small and can almost be ignored.
The faster melt moves in mold cavity, the more friction heat is generated, and viscosity of melt decreases rapidly due to shear thinning. Generation of frictional heat rapidly increases temperature of melted glue, but because time is very short, melted glue generally does not undergo physical degradation (unless maximum shear rate of material is exceeded). In actual operation, it is better to increase injection speed by using its shear heating effect to improve flow than to increase temperature of material tube. Because temperature of material tube is too high, risk of material degradation is greater (especially for molds with a small amount of glue and a large size, because required injection amount is too small compared to maximum injection amount of machine, plastic remains in material tube for too long, and setting material temperature too high can easily lead to plastic degradation).

 

Increasing injection speed can largely overcome stagnation effect, thus improving filling of thin-walled ribs near gate.
Changes in injection speed have a great impact on flow pattern and will change position of bonding line to a certain extent. As mentioned above, high injection molding speeds favor increased molecular flow orientation and therefore influence flow patterns.
Injection speed has a great influence on injection pressure. In actual operation, during filling stage, high pressure is set to ensure speed, that is, sufficient pressure required to achieve speed. Relationship between actual injection pressure and speed is a U-shaped curve. High speed requires high pressure to ensure. On the other hand, due to intensification of high-speed shear, viscosity of plastic decreases and it flows more easily, so actual required pressure will decrease. If injection speed is too low, it means that it takes longer to complete filling. Due to time constraints, melt solidifies too high, making it more difficult to flow and requiring greater pressure to drive. Non-Newtonian fluid properties of plastic make its flow conditions extremely complex.
In short, to sum up, under premise that material properties and mold structure allow, setting injection molding speed as high as possible has a very positive impact on product quality. Using optimization method to find the best injection speed can obtain the lowest injection pressure, which is very beneficial to reducing internal stress of product.

Function of holding pressure is to make up for space released after molecules shrink, so holding pressure has a crucial impact on deformation. Essence of using holding pressure to overcome deformation is to reduce shrinkage difference in each area of product. When shrinkage difference is reduced, deformation is improved. Theoretically, before mold cavity is filled, injection pressure required is basically just to overcome filling resistance caused by viscosity of plastic. If exhaust is sufficient, pressure at flow front is equal to atmospheric pressure; if injection speed is reasonable, mold temperature is reasonable, and melt flow length is controlled within a reasonable range (that is, location of gate is reasonably selected), required injection pressure can be effectively reduced. Only when mold cavity is fully filled will pressure in mold cavity rise sharply.
Effect of pressure holding is affected by product structure, design of pouring system and cooling system, as well as injection time. Complexity of product structure naturally determines objective existence of uneven shrinkage in various areas of product (such as thickness difference between body and ribs); design of gating system naturally determines that pressure-holding effect is inversely proportional to flow distance, that is, the longer flow distance, the worse pressure-holding effect at the end of flow; uneven heat conduction of cooling system objectively exists in mold, which also affects pressure-holding effect. In order to improve effect of pressure holding, we must start from following aspects:
Increasing glue injection speed means switching to pressure holding earlier. The shorter time, the lower viscosity of melted glue and the more even pressure distribution in mold cavity, so pressure holding effect is better.
The larger gate size, the smaller pressure-holding resistance, and the slower gate solidifies, which can more effectively transmit holding pressure and more effectively transmit feeding plastic into mold cavity. However, gate size is too large and takes a long time to freeze. Therefore, pressure holding time needs to be adjusted appropriately to ensure that gate is frozen before pressure can be released. Otherwise, melted glue in mold cavity will flow back into runner. In addition, if gate is too large and holding pressure setting is unreasonable, over-holding pressure in gate area may easily occur.
Position of gate determines flow length of plastic, and flow length determines pressure-holding effect; at the same time, position of gate determines balance of filling of each gate and degree of over-pressure. If degree of local over-pressure is too high, it will increase shrinkage difference in various areas of product, which will be detrimental to product deformation. Therefore, location of gate, size and shape of gate determine effect of pressure holding.
Position of gate determines channel for plastic flow (that is, channel for pressure-holding transmission). Upstream solidification earlier than downstream is not conducive to downstream pressure-holding.
Cooling system design. Slow heat dissipation in heat accumulation area means that plastic molecules in this area have a longer time to shrink (or crystallize), so shrinkage is greater, more effective pressure holding is needed to compensate for shrinkage. Heat accumulation area of mold needs to be cooled to reduce holding pressure.
Selection of pressure-holding switching point affects pressure-holding effect. Usually, holding pressure is switched too early, and mold cavity is not filled sufficiently. Remaining part is filled with pressure-holding material and shrinkage. Pressure-holding effect is not good, which is not conducive to overcoming end shrinkage and overcoming small overall size. Switching holding pressure too early is detrimental to end rib filling. If you switch holding pressure later, holding pressure effect will be better, because generally speaking, plastic flows much faster during filling than during holding pressure, and time required to fill mold cavity is short, so melt can maintain a higher temperature and lower viscosity when switching pressure holding, so pressure holding effect is better. Holding pressure switching point is closer to the front (closer to nozzle), which means that it is easier to enlarge size, and it is easier to overcome end shrinkage; holding pressure switching point is further back (far away from nozzle), which means that more plastic needs to be filled into mold cavity during pressure holding pressure. Since filling speed is slower during pressure holding, filling time is longer. Due to heat loss, viscosity of plastic increases and flow becomes difficult. Therefore, a greater holding pressure is required to obtain an effective pressure holding effect. Therefore, from this perspective, it is beneficial for products that are too large.
Essentially, injection stage and pressure holding stage are same, both of which push plastic into mold cavity. Main difference is: when injecting, plastic flows very fast, aiming to achieve injection speed; when holding pressure, plastic flows extremely slowly, focusing on pressure, aiming to compensate for shrinkage, and speed is not important. Some injection molding machines such as Engel doesn't even have a holding speed setting function. In actual operation, if final injection speed is very slow, it can be understood as pressure maintaining.
If injection and pressure holding are clearly defined: injection is a process used to fill the entire mold cavity; and pressure holding is a process in which melt flow basically stops after the entire mold cavity is filled, plastic begins to shrink, and pressure drives melt to make up for space released by shrinkage. Fundamental difference is: first, melt flow rate in mold cavity is very fast when glue is injected, and melt flow rate is very slow when pressure is maintained; second, melted glue is in contact with mold wall during injection. During pressure holding, melted glue flows inside solidified layer and has no contact with mold wall; thirdly, melt mainly exhibits a viscous behavior during injection, due to high shear caused by flow, melt exhibits viscoelastic behavior to a large extent during pressure holding. Since flow rate is very slow during pressure holding, melt begins to solidify, molecules begin to shrink and wrap around, and viscosity rises. From this definition, real holding pressure begins when mold cavity is fully filled, not when holding pressure is set.