Flashing

Views: 0 Author: Site Editor Publish Time: 2020-07-25 Origin: Site

Flashing is defined as excess material that has exceeded the boundary of a mold beyond the mold cavity. Flash can be found in a number of areas. The parting line is one of the more frequent locations where flash is found. When a mold does not clamp fully, this provides a space for material to flow. Vent areas are another location of flash, especially if vents are cut too deep and/or wide. If not fully closed, movable mold sections such as slides can move back, due to the pressure of the melt front permitting material to flash into the space resulting from the relocation of the slide. The lists causes and solutions for flashing.

Causes:

1. The cause of flash on a part is inadequate drying of the

material,especially hygroscopic materials that can absorb

moisture.(1) The addition of water to many hygroscopic materials, such as nylon and polycarbonate, can reduce the viscosity of the material by breaking down the molecular weight of the polymer (2) Moisture in the form of bubbles creates an easy-flowing

2. The feed setting or shot size is too high, excessive material can flash around the mold

3. Excessive melt temperatures allow the material to flow easily, especially if a mold parting line is not fully clamped

4. Injecting the material at a high speed can cause excessive heat ue to shear, causing the material to flow easier

5. Flow properties of the material, such as viscosity and melt MFR), affect the amount of flashing found. Low- viscosity, high-MFR materials will have a tendency to flow easier, raising the potential of flashing

6. Holding pressure can cause flashing, especially when packing pressures are too high, forcing more material into the cavity

7. Too long a melt residence time in the barrel will overheat the material and reduce viscosity, allowing the material to flow easily

8. An insufficient number of gates can create high injection pressures and thus increase the chance of flashing in a part

9. If mold temperatures are too high, the material will flow more easily increasing the chances of part flashing

10. If an injection machine clamp is too low for the part being molded, the mold platen can actually be blown open, causing material to flow between the open halves of the mold

11. Projected areas of the parts and runner system are oversize

12. A wom-out area in a mold can cause localized flashing to be found on a part

13. When a tool is designed with not enough support to the mold surface, flexing of the mold surface may occur, which will flash the tool

Solutions:

1. Drying the material according to recommendations needs to be followed to avoid this phenomenon

2. To reduce injection pressure. Reducing the feed setting should reduce the amount of material being jammed into the cavity

3. Using recommended melt temperatures, specifically midrange recommended, can reduce the chance of flashing

4. Decreasing injection speed will reduce shear heating decreasing the melt temperature

5. High-viscosity, low-MFR also can resist flashing since they display stiffer flow proper Decreasing melt temperature will also prevent flashing since the viscosity of the material increases resisting flow

6. Reducing hold pressure will reduce the chance of flashing. Reducing the amount of screw feed or reducing the cushion will reduce the risk of flashing, due to less material being forced into the cavity

7. Using no less 30% of the barrel capacity the right-size barrel for the part being molded helps prevent overheating and ultimately flashing the mold

8. Increasing the number of gates to reduce injection pressure by distributing the pressure over more gates, or by using other gating technologies, such as sequential valve gating, which us hydraulically control pins that control the flow in each gate. This will make it possible to control how and when the material enters each gate

9. Reducing the mold temperature

10. Increasing the clamping force of the mold

11. Projected areas of the parts and runner system determin the size of the machine and clamp tonnage

12. Machining the tool to improve the flatness of the tool can eliminate flash

13. Support inside the tool to resist the high pressures applied is important in preventing flash. Sufficient support of the mold surface and cavity needs to be designed to prevent flexing of the surface