There may be a variety of solutions, especially for some specific situations, but the general solutions include:
1 Use injection machines with large clamping force, wherein the rated clamping force must be higher than the tension of injection molds during filling, otherwise, mold expansion and flash will occur.
2 Check the clamping system of injection machine, especially whether the toggle unit is straight under clamping state; dispel such factors as unbalanced clamping and fluctuated clamping force of melt during the process of injection and filling.
3 Optimize gate position and achieve balanced mold-filling; avoid unbalanced clamping and partial over-dwell for melt in parts where filling has been completed.
4 Properly reduce injection pressure and dwell pressure and switch pressure in time.
5 Optimize prescription of plastic raw materials, properly reduce dosage of lubricating agent, keep the viscosity of plastics within a proper range; appropriately reduce temperature of barrel, nozzle and molds and injection rate, properly increase section dimension of gate and thereby optimizing the flow and filling state of melt.
6 Check the attaching degree of parting line for deformation or foreign matters (if any), and polish the part of parting line with flash; check the places that have gliding or insert fit with the molding parts for discrepancy of assembly or abrasion clearance (if any); check whether rigidity of mold plate is sufficient; when necessary, properly improve the design and manufacturing precision of clearance at moving or fixed part of molds.
7 Check whether exhausting slot is overlarge or overdeep and properly arrange the clearance dimension of mold exhausting channel.
A common first response to reduce flash is to slow down the injection rate. While decreasing the injection rate eliminates flash by raising material viscosity, it also increases cycle time and still doesn’t address the original cause of flashing. Worse yet, flash may reappear during the pack/hold phase.
Flash can occur during filling (first injection stage) or during the pack/hold (second stage), and it can occur with conventional parts and with thin-walled parts. Discovering in which stage flash occurs helps to reduce the complexity of determining the cause. A simple test is to bring the second-stage (pack/hold) pressure to the minimum the machine allows, which should be below 500 psi plastic pressure. The aim is to see the effect of just barely filling the cavity. First-stage injection fills 92% to 99.9% of the part, but this stage must end before the cavity is full. At the minimal pressure level for the second phase it is easy to inspect whether the part is a short shot at the end of first-stage injection. If there is flash on the short shot, we can focus on parameters affecting first-stage injection. If you don’t get a short shot at all, then the answer may be to adjust the transfer point to avoid over-filling the cavity.
Keep in mind that with thin-wall parts, even short shots can generate enough pressure to blow the clamp open. But if you see flash in a part with an “average” wall thickness after a short shot of the first stage, the most likely cause is a parting-line mismatch in the tool. Clean off any plastic, dirt, or contaminants that could keep the mold from closing properly. Check the mold especially for bits of plastic behind slides and down in the pocket of the leader pins. After such housekeeping, if flash persists, check for parting-line mismatch with pressure-sensitive paper, which can reveal if the mold is clamping evenly along the parting line. A suitable pressure-sensitive paper will have a rating of 1400 to 7000 psi or 7000 to 18,000 psi.
In multi-cavity or family molds, flash is often caused by an improper balance of melt flow. If filling of the cavities is not uniform, the packing is not uniform. This is why multi-cavity molds may see flash in one cavity and sink marks in another during the same shot.
Inadequate mold support can also lead to flash. Molders should consider whether the machine has enough support pillars in the right places for the cavity and core plates.
The sprue bushing is another possible source of flashing. Nozzle contact forces can range from 5 to 15 tons. If thermal expansion causes the bushing to “grow” far enough past the parting line, the nozzle contact force can be enough to push on the moving side of the mold, trying to open it. For non-sprue-gated parts, molders should check the length of the sprue bushing while it is hot.
Flash is one of the most common defects in the injection molding industry. In the production of injection molded parts, the first process after demolding is to manually scrape off the flash, which will undoubtedly reduce the productivity and directly affect the appearance quality of the plastic part. In the plastic injection molding process, because the mold is subjected to the complex influence of various external factors, it often causes the distortion of the molded product. The most common manifestation of this distortion is the flash caused by the plastic overflow.
The followings are some possible cause flash in molding parts:
1）Parting line mismatch or mould damage： Clean mould surfaces and behind slides and inspect cavity for any obstruction to proper clamping. Also check for a clamp misalignment, tool damage, uneven tiebar stretch, or improper machine levelling. Use pressure-sensitive paper to check pressure on parting line in flash area and non flash area while the tool is in the press.
2) Mould improperly supported: Check number, placement, and length of support pillars in mould.
3) Uneven flow in a multi-cavity tool: Use Beaumont “5 Step Process” to check mould balance.
4) Clamp tonnage too low: Reduce pack/hold pressure. Use a press with higher clamp force.
5) Clamp pressure too high: Reduce clamp tonnage.
6) Sprue bushing too long: For non-sprue-gated parts, check thermal expansion of sprue bushing under production conditions. Shorter bushing may be needed.
7）Overfilling cavity during first-stage injection：Adjust first-stage cut-off position or reduce shot size. Also check machine response on switchover from filling to pack/hold stage.
8）Viscosity too low：Test melt temperature to confirm it is not above supplier’s recommended limit. Check melt flow rate before and after moulding for signs of degradation. Minimize melt residence time. If resin is hygroscopic, check moisture content. Try different lot of resin. Only as a last resort, reduce injection rate.
the cause of the overflow in the injection molding process:
1. In terms of injection molding equipment, factors such as the quality and installation of the injection molding machine itself will cause overflow.
2. In terms of injection molds, poor precision of mold parting surface, unreasonable mold design, etc.
3. In terms of injection molding process, the injection molding process is also a high-pressure, high-speed, and high-temperature process. If the details of the process are not handled properly, it will lead to overflow.
The phenomenon of flashing is a problem that injection molding factories often face during the production process, so every step from the beginning of product design to product molding needs to be carried out in strict accordance with the process.
These May Cause Flash In Injection Molded Parts:
Equipment
1. The real clamping force of the machine is insufficient. When selecting the injection molding machine, the rated clamping force of the machine must be higher than the tension formed by the longitudinal projection area of the injection molding product during injection, otherwise, the die will expand and flash.
2. The adjustment of the mold closing device is not good, the elbow bar mechanism is not straight, resulting in uneven mold closing or left and right or up and down, and the mold parallelism can not reach the phenomenon, resulting in the situation that one side of the mold is closed tightly while the other side is not close, and there will be flash during injection.
3. The poor parallelism of the die itself, or the installation is not parallel, or the template is not parallel, or the force distribution and deformation of the pull rod are uneven, which will cause the die closing not tight and produce flash.
4. Serious wear of check ring, failure of spring nozzle spring, excessive wear of barrel or screw, failure of cooling system at feed inlet, insufficient feeding amount of barrel, and too small cushion may cause repeated flash. Therefore, it is necessary to repair or replace accessories in time.
Mould
1.The uneven distribution of die cavity or insufficient parallelism will result in local flash and local dissatisfaction. Under the premise of not affecting the integrity of the part, the runner should be placed in the center of mass symmetry as far as possible.
2.The flash will be caused when the force of moving component and sliding core is unbalanced.
3.When the mold is not well vented, the compressed air will make the parting surface of the mold expand and flash. Therefore, it is necessary to set up a good exhaust system or dig an exhaust ditch on the parting surface.
Process
1. The injection pressure is too high or the 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 products. The thin products should be filled with high speed and fast, and no injection should be made after filling; the thick products should be filled with low speed and the skin should be fixed before the final pressure is reached.
2. Excessive feeding amount causes flash. It should be noted that too much molten material should not be injected in order to prevent the depression, so that the depression may not be “filled in”, but the flash will appear. This situation should be solved by prolonging injection time or holding pressure time.
3. Too high temperature of barrel and nozzle or too high temperature of mold will reduce the viscosity of plastic, increase the fluidity, and cause flash in the case of smooth injection.
Raw Materials
1. Flash may occur if the viscosity of plastic is too high or too low.
2. When the particle size of plastic raw material is not uniform, the feeding amount will change indefinitely, and the parts may be dissatisfied or flash.
3. If the fluidity of plastics is too large or too much lubricant is added, the pressure, speed and temperature should be appropriately reduced to reduce the amount of lubricant used. If necessary, plastics with low fluidity should be selected.
The followings are some possible causes and remedies:
Parting line mismatch or mould damage
Clean mould surfaces and behind slides and inspect cavity for any obstruction to proper clamping. Also check for a clamp misalignment, tool damage, uneven tiebar stretch, or improper machine levelling. Use pressure-sensitive paper to check pressure on parting line in flash area and non flash area while the tool is in the press.
Mould improperly supported
Check number, placement, and length of support pillars in mould.
Uneven flow in a multi-cavity tool
Use Beaumont “5 Step Process” to check mould balance.
Clamp tonnage too low
Reduce pack/hold pressure. Use a press with higher clamp force.
Clamp pressure too high
Reduce clamp tonnage.
Sprue bushing too long
For non-sprue-gated parts, check thermal expansion of sprue bushing under production conditions. Shorter bushing may be needed.
Overfilling cavity during first-stage injection
Adjust first-stage cut-off position or reduce shot size. Also check machine response on switchover from filling to pack/hold stage.
Viscosity too low
Test melt temperature to confirm it is not above supplier’s recommended limit. Check melt flow rate before and after moulding for signs of degradation. Minimize melt residence time. If resin is hygroscopic, check moisture content. Try different lot of resin. Only as a last resort, reduce injection rate.
As a common defect in injection molding, flash can have a significant impact on the level of scrap you are producing or cost of secondary processes as a manufacturer. While Automotive and Medical suppliers typically reduce scrap to near zero as part of the controlled process monitoring, many other components do not operate with as much oversight. Since scrap is the loss of usable material, you could be losing money with each injection run, as excessive flash continues to disrupt your ability to accurately create product…. What causes defects and flash and what can you do to prevent it? These are important questions to ask yourself if you are struggling to eliminate defects on your injection mold projects.
In many cases, the best way to prevent possible defects and mold flash is by ensuring that your process is set up and defined properly. This will help to determine whether outside variables may be impacting your processes without changing machine process parameters. Make it a point to check the materials that you are using to see if moisture may be an issue or if water temperatures for the mold or hydraulic pressures from the machine have caused the mold to “back up”.
If all outside variables are fine, you may need to investigate the process itself to determine what changes could be made that would help mitigate the flashing that is occurring. For example, cavity pressure that is being generated may be exceeding the tonnage of the clamp you are using and is allowing the plastic to flow into the parting line when it should not be. However, temperatures may also play a vital role in the amount of flash that is being created if the clamp tonnage is appropriate for the application that you have because it may reduce viscosity while creating a higher flow rate. With so many variables possible, would it be helpful to upgrade core actions and invest in a product that immediately provides results?
Design for manufacturability (DFM) is a key aspect of flash prevention and reduction. For example, plastic injection molding guidelines recommend placing the parting line along the part’s edge rather than on a flat surface to improve the part’s appearance and function. This not only helps to hide the seam, but it also reduces the chances of flash formation.
diagram of an injection molding design for a lego brick with proper parting line placement
During the design phase, your part drawing can indicate how much or how little flash you will accept. There are two measurements to consider. Flash extension measures how much it projects from the plastic part along the mold’s parting line. Flash thickness measures perpendicular to the parting line instead. The location of flashing is also important and can tell you when you designed a parting line in the wrong place.
Reducing flash extension and flash thickness improves part quality but increases your mold costs and part costs. If your flash removal requirements are strict, you can specify a flash-less mold so that your plastic parts have only minimal flashing. This type of mold can double or triple your tooling costs, however, so flash removal during post-processing may be more cost-effective.
If your injection molder is using existing tooling, mold maintenance and process improvements can reduce flashing. For example, in the case of parting line mismatches, the solution may be to correct a clamp misalignment, inspect a cavity, or clean mold surfaces. If the clamping tonnage is too low, an injection molder may need to reduce the pack/hold pressure or use a machine with a higher clamping force.
A common first response to reduce flash is to slow down the injection rate. While decreasing the injection rate eliminates flash by raising material viscosity, it also increases cycle time and still doesn’t address the original cause of flashing. Worse yet, flash may reappear during the pack/hold phase.
Flash can occur during filling (first injection stage) or during the pack/hold (second stage), and it can occur with conventional parts and with thin-walled parts. Discovering in which stage flash occurs helps to reduce the complexity of determining the cause. A simple test is to bring the second-stage (pack/hold) pressure to the minimum the machine allows, which should be below 500 psi plastic pressure. The aim is to see the effect of just barely filling the cavity. First-stage injection fills 92% to 99.9% of the part, but this stage must end before the cavity is full. At the minimal pressure level for the second phase it is easy to inspect whether the part is a short shot at the end of first-stage injection. If there is flash on the short shot, we can focus on parameters affecting first-stage injection. If you don’t get a short shot at all, then the answer may be to adjust the transfer point to avoid over-filling the cavity.