（32） Spillage of Pifeng airtrap (flash, flash) Injection Molding Process Defect Handling Manual

"Flaps, also known as fins, overflows, fins, and overflows, mostly occur at the mold opening and closing positions, such as the mold opening and closing surfaces, the sliding parts of the slider, the gaps in the inserts, and the holes in the ejector pins.". If the overflow is not resolved in a timely manner, it will further expand, resulting in partial collapse of the imprinting mold, causing permanent damage. Overflow of the insert gap and ejector pin hole can also cause the product to get stuck on the mold, affecting mold release. Although the precision is very high when making molds（ μ Grade m), and high-pressure molding is used during molding, but due to the high filling pressure of the resin, there are actually very small gaps left. The flash is formed by the resin entering this gap. Flashes can occur at the PL surface, casing, sliding core interface, and exhaust ports.
A flash is a phenomenon in which resin is squeezed into the PL surface of the mold (the parting surface of the mold) and the product is coated with excess film. This occurs when the PL surface is not separated by resin pressure, or when there are gaps in the PL surface.
In terms of mechanical equipment:
(1) The actual clamping force of the machine is insufficient. When selecting an injection molding machine, the rated clamping force of the machine must be higher than the tension formed by the longitudinal projected area of the injection molded product during injection, otherwise it will cause mold expansion and flash.
(2) The mold closing device is not adjusted properly, the elbow mechanism is not straightened, resulting in uneven mold closing on the left, right, or up and down sides, and the mold parallelism cannot be achieved. This causes one side of the mold to be tightly closed while the other side is not tightly attached, resulting in flash during injection.
(3) The poor parallelism of the mold itself, or its installation is not parallel, or the template is not parallel, or the force distribution and deformation of the pull rod are uneven, all of which will cause the mold to be not tightly closed and cause flash.
(4) The check ring is severely worn; Spring nozzle spring failure; Excessive wear of the barrel or screw; "Bridging" due to failure of the cooling system at the feed inlet; Insufficient injection amount set for the barrel, too small buffer pads, etc. may cause flash to occur repeatedly, and it is necessary to repair or replace accessories in a timely manner.
II. Mould
(1) The accuracy of the mold parting surface is poor. Deformation and warpage of movable formwork (such as middle plate); There are foreign matters on the parting surface or protruding skid marks and burrs around the mold frame; The periphery of the cavity of the old mold was fatigued and collapsed due to earlier flash extrusion.
(2) The mold design is unreasonable. The opening position of the mold cavity is too biased, which can cause tension on one side of the mold during injection, causing flash; Plastics with too good fluidity, such as polyethylene, polypropylene, and nylon, have a low viscosity in the molten state and are prone to enter movable or fixed gaps, requiring a high manufacturing accuracy of the mold; On the premise of not affecting the integrity of the product, it should be placed on the center of mass symmetry as much as possible, and feeding materials into the thick parts of the product can prevent the situation of missing materials and having flash edges at the same time; When there are molding holes in or near the center of the product, it is customary to open side gates on the holes. Under high injection pressure, if the clamping force is insufficient and the supporting force of this part of the mold is insufficient, slight warping can occur, resulting in flash. For example, if there are movable components on the side of the mold, the projected area of its side is also affected by the molding pressure, and if the supporting force is insufficient, it can also cause flash; Poor fitting accuracy of the sliding core or deviation of the installation position of the fixed core and the cavity resulting in flash; Poor venting of the mold cavity, failure to open an exhaust groove on the parting surface of the mold, or the exhaust groove being too shallow or too deep or too large, or being blocked by foreign objects, will cause flash; For multi cavity molds, attention should be paid to the reasonable design of the runner gates, otherwise uneven mold filling forces will result in flash. Even in a simple 2-piece mold, the mold can sometimes be damaged due to improper ejection of the molded product, and flash occurs at the damaged area. When using a sliding core, special attention must be paid to the fit and the gaps in the sliding surface.
In addition, the mold is made of steel, and molding pressure belongs to high pressure, while resin pressure is also equivalent to high pressure. Therefore, in almost all injection molding, mold deformation generally occurs. Especially in the case of large molded products. At this time, the presence or absence of a pillar also has an impact on the flash (if there is no pillar, the deformation → gap will increase, and the flash will also increase).
III. Process
(1) Injection pressure is too high or injection speed is too fast. Due to high pressure and high speed, the opening force on the mold increases, resulting in overflow. The injection speed and injection time should be adjusted according to the thickness of the product. Thin products should be quickly filled with high-speed mold, and no injection will be conducted after being filled; Thick products need to be filled with a low speed mold and the skin generally fixed before reaching the final pressure.
(2) Excessive feeding causes flash. It is worth noting that too much molten material should not be injected to prevent sags, which may not necessarily "fill out" the sags, but flash edges may appear. This situation should be resolved by extending the injection time or pressure holding time.
(3) Too high temperature of the barrel and nozzle, or too high mold temperature, can reduce the viscosity of the plastic, increase its fluidity, and cause flash during smooth mold entry.
Four aspects of raw materials
(1) Flashing can occur if the viscosity of the plastic is too high or too low. For plastics with low viscosity, such as nylon, polyethylene, polypropylene, etc., the clamping force should be improved; Plastics with strong water absorption or sensitive to water can significantly reduce the flow viscosity at high temperatures, increasing the possibility of flash. These plastics must be thoroughly dried; If too much recycled material is added to the plastic, the viscosity will also decrease. If necessary, it is necessary to supplement the retained components. If the viscosity of plastic is too high, the flow resistance increases, resulting in a large back pressure that increases the mold cavity pressure, resulting in insufficient clamping force and flash formation. When the resin pressure is too high, the mold separates and creates flash. On the contrary, when the mold pressure is low, it is also prone to flash. The main reasons for the increase in resin pressure are as follows: PSS resin has a strong fluidity in the low shear zone, so it has the disadvantage of being prone to flash due to its own properties. Therefore, more attention must be paid to preventing flash when using PPS resin than when using other materials. At this time, the requirements for the accuracy level of the mold are also more stringent than when using other materials.
(1) Injection speed is too fast
(2) Injection pressure is on the high side
(3) High pressure retaining
(4) V-P switching is slow
In general, when you want to achieve a good appearance, you may sometimes set the pressure retaining setting too high, especially to prevent dents and use a higher than standard setting. This can sometimes lead to flash. The better the fluidity, the easier it is for the resin to enter the gap, and therefore the greater the flash. Generally speaking, the higher the resin temperature and mold temperature, the greater the flash; Conversely, the lower the temperature, the smaller the flash.
(2) When the particle size of plastic raw materials is uneven, the feeding amount may vary, resulting in unsatisfactory parts or flash edges.
Troubleshooting
If the plastic temperature is too high, lower the plastic temperature and lower the mold temperature
The glue injection speed is too high to reduce the glue injection speed
The injection pressure is too high to reduce the injection pressure
Excessive filling reduces injection time, speed, and dosage
Repair the mold due to poor clamping line or mating surface
Insufficient mold locking pressure Increase mold locking pressure or replace an injection molding machine with a higher mold pressure
If the slider and positioning block are worn, they are prone to burrs.
When foreign matters are attached to the surface of the mold, burrs may also appear.
Insufficient mold locking force, the mold was opened during injection, resulting in burrs.
If the temperature of the raw material and the mold is too high, the viscosity will decrease, so it is also easy to produce burrs on the mold with only gaps.
Excessive material supply leads to excessive raw material injection, resulting in burrs.
Burr edge
Injection molding - Injection molding is also known as injection molding or injection molding. This method of molding involves placing plastic (usually granulated material) into injection molding
The material barrel of the machine is heated and melted. When in a flowing state, the molten plastic is compressed and moved forward under the pressure of a plunger or screw, thereby passing through the material
The nozzle at the front end of the cylinder is quickly injected into a closed mold with a relatively low temperature. After a certain period of cooling and shaping, the mold is opened to obtain the product.
Injection molding is developed based on the principle of metal die casting. Due to the fact that injection molding can produce complex shapes and precise dimensions in one step,
"Or products with metal inserts, which are widely used, currently account for more than 20% of the total molding process.".
The injection molding process usually consists of five stages: plasticization, mold filling (i.e., injection), pressure maintaining, cooling, and mold release.
Generally, injection molding products have waste materials such as gates and runners that need to be trimmed and removed. This not only consumes labor, but also wastes raw materials.
The development of sprueless injection molding in recent years not only overcomes the above drawbacks, but also helps to improve production efficiency