Many of us in the manufacturing world know all too well – those pesky burrs and flashes on plastic parts. They're like the tiny gremlins of production, popping up and causing all sorts of headaches. But don't worry, we're here to tackle this issue head-on and find some fast, effective solutions.
Today, I would like to share why there are flash/burrs on plastic samples, and how to improve the problems on Mould.
Burrs and flashes mainly appear on contact surfaces. while the melt flows through the space between the moving mold and the fixed mold, the sliding part of the slider, as well as the gap of the insert and the ejector pin, and so on, excessive flashing or burrs are formed on the Molded product. Such flashing or burrs act as leverage during the plastic injection Moulding process to further increase the flashing or burrs, which leads to local depression on the Mould, causing flashing or burrs to enter a vicious circle. Therefore, if flashing or burrs are found in the very beginning, the Mould must be modified at the earliest possible time.
1.What Causes Flash Defects in Injection Moulding and How to Fix Them?
1.1 Old Mould
Old Mould can cause a flash on the plastic part surface. It doesn’t fit tight together, and plastic escapes out due to this looseness. Old Mould or worn-out Mould issues can solve with Mould repairing or Mould redesigning.
1.2 An Improper Balance Of Mould Flow & Pressure
Pressure, temperature, and flow are some normal reasons of flash. The packing will not be uniform if cavity filling is not uniform.
1.3 Mould Clamping Issues
Compared with projected area of molded part, if clamping force is small, a gap will appear between moving and fixed molds due to injection pressure, and burrs will inevitably appear. Especially when side gate is placed on the hole near center of part, because this molding gate requires a large injection pressure, it is extremely prone to burrs.
Reducing injection pressure or increasing clamping force can eliminate this defect. It is sometimes very effective to switch to low-pressure molding using a fluid plastic according to specific situation.
1.4 Mould Damage Problems
Parting line mismatches and any obstructions to proper clamping can be resolved by cleaning the mold surface and slides. You should also check Mould tool damage, misalignment of the clamp, and machine level. With the help of pressure-sensitive paper, you can check and solve these problems.
1.5 Mould Design Issue
Pressure or flash issues on the part surface can be caused by poor design and excessive space in the mould, especially when dealing with complex geometries and multiple parts. To solve this problem, consider retooling and redesigning the mould.
1.6 Fluidity of plastic is too good
Simple reason is that plastic is too fluid, which is not the cause of burrs in theory. However, when fluidity of plastic is too good, even fine gap plastic can be drilled in, so burrs are extremely easy to appear. To eliminate this burr, melt temperature or injection pressure can be reduced, mold temperature or injection speed can also be reduced.
1.7 Too Low Viscosity Problems
Flashing or burring can create if viscosity pressure will be too low. In this issue, the resin becomes too runny. Some reasons for becoming resin too runny are:
** The presence of moisture due to improper dryness.
** Excess amount of colorant consisting of lubricant vehicle.
** High melting temperature.
** Excess time of residence that creates degradation.
You can test melt temperature and melt flow. It will be according to the supplier’s instructions. Injection Moulding companies check temperature and Mould flow in the Injection Moulding process. Regarding to unfilled resin, you can change a 30% change in melt flow. For filled resin, you also change melt flow at least 40% or more.
1.8 Injecting too much melt
This is not direct cause of burrs. It is wrong to inject too much molten material in order to prevent shrinkage, and it should be formed by increasing injection time or holding time (refer to shrinkage).
1.9 Foreign matter on mold surface
Foreign matter on mold clamping surface will inevitably produce burrs. To solve this problem, it should obviously be cleaned on the surface of clean mold, and clamping surface can be closely adhered.
1.10 Caused by mold deflection
If originality of mold is insufficient, deflection will occur due to injection pressure. If there is a hole near center, burrs will be generated around hole. When center gate is used to open side gate, burrs may also occur around hole and runner. Burrs caused by this cause is caused by poor mold making, so it is difficult to repair. Reinforced molds can reduce burrs.
1.11 Mold is not tightly local
First of all, problem of loose clamping of moving and fixed molds is discussed. Although mold itself is tightly closed, when an injection machine using an elbow-type clamping mechanism is molded, it is often caused by poor mold parallelism or poor adjustment of clamping device, phenomena such as imbalanced clamping of left and right sides, that is, only one side of left and right sides are locked, and the other side is not closely attached. At this time, tie rods (two or four pull cymbals) must be adjusted to make them stretch uniformly. Secondly, there is also a lack of close adhesion due to poor fitting of mold itself, especially when there is a molding hole in the center of part. Due to supporting effect of this part, burrs are also prone to occur when clamping force is insufficient.
In addition, it is a sliding core. Since follow-up core is an action mechanism, burrs are often generated, so cooperation of sliding core is very important. Especially for left and right parted Huff molds, projected area on the side is also affected by molding pressure. If design is not able to fully withstand this pressure, burrs often occur. With regard to gap between inserts and ejector pin, not only vicious cycle of burrs increases, but also ejection resistance will increase.
2.What are the definitions of deflashing and deburring techniques?
Deflashing and deburring are the main steps in the plastic injection Mould process. Product cost can increase after deflashing and deburring. Its cost depends on the size of plastic parts, type of material and parts, and the product’s precision. Deflashing is a process of removal of unwanted parting lines. It removes the residue and debris leftover in a Moulding or casting process.
Deburring is a method of finishing and smoothing the edge. It occurs after the cutting and welding procedure.
3.Some tips for improvement of Flash/Burrs
3.1 Slow down the injection rate
Lowering the injection rate in flash injection molding can help control the formation of flash and burrs by increasing the material viscosity and cycle time, although it may not completely eradicate the flash. Flash may appear during the packing step or the filling stage. You can see flash with thin-walled parts and conventional parts. By minimizing the pressure in the packing, the stage may reduce the flash. It should be less than 500 psi plastic pressure.
During the first step, injection should fill at least 92% to 99.9% of the parts. The cavity needs to be filled completely. Minimal pressure is effective for inspecting short shots in the initial stage. If there is flash on the short shot, pay attention to the parameters that could affect the first stage injection. If there is no shot at all, adjust the transfer point to prevent cavity overfilling.
3.2 Improve flash by cleaning off dirt and plastic
In the event of a parting line mismatch in the Mould tool, there may be flashing on parts with an average wall thickness. To avoid problems with the full closure of the Mould, it is necessary to clean the Mould of any plastic, dirt, and contaminants. Careful inspection of the Mould is required to ensure proper functioning, as plastic could be located behind slides or in the pocket pins. If flash appears, you can check part line mismatch with pressure-sensitive paper. It can identify the clamping of the Mould with the parting lines.
An improper balance of molten flow is one of the reasons for flash in the multi-cavity. Imbalance filling leads to imbalance packing. This is the reason for flash on one cavity and sinks marks during the same shot. There are many methods for the determination of Mould balance. Lack of Mould support is also responsible for flash. So you must know the injection Moulding machines
3.3 Checking of clamp
Too little clamp tonnage is also a cause of flash. This type of flash can remove by increasing clamp tonnage. You may need 35,000 to 60,000 psi plastic pressure in the nozzle area. It will be a high-speed filling process in a thin-walled usage. In case the machine is not on the correct level, flash will also happen. Enhancing flash moulding defects due to clamp misalignment can be achieved with some effort. One of the suggested yet costly methods is a new laser approach. Injection moulding companies employ this technique to detect the absence of clamp parallelism. Excessive clamp pressure leads to flash and burrs at the center of the mould.
4.Conclusion
Dealing with burrs and flash on plastic parts can be a major hassle, but with the right techniques and preventative measures, you can tackle these issues head-on. Whether you're using manual methods for small batches or advanced technologies like thermal or cryogenic deburring for high-volume production, there's a solution out there for you. By slowing down the injection rate, cleaning off dirt, and checking of clamp, you can minimize these problems and keep your production line running smoothly.
