Case Study

As we take new steps in moving ahead as leaders of ABS applications, we put our customers at the core of our organization; enhancing our long-term growth as company. At BEPL, we’re growing and spreading our technical acumen. We’ve documented some cases

Moulding Defects

20 August, 2019

BEPL has highly skilled Technical service personswho are ever ready to serve our customers all the time. We are committed to assist our customers to rectify various moulding related issues and give them the best we can. Some of the tactics used to handle the problems are as follow.

  1. Flow Marks
    A flow mark or halo is a surface defect in which circular ripples or wavelets appear near the gate.

    Causes

    - The molten polymer cools rapidly within the mold and becomes highly viscous. Once that molten polymer starts to coagulate, it gets pushed by the molten polymer injected afterward and forms miniature bands.
    - Low melt temperature, injection speed, injection pressure
    - Low mold temperature
    - Small runner stem and gate

    Troubleshooting

    IM Parameters

    - Increase the nozzle diameter. Increase the nozzle temperature
    - Increase injection speed, injection pressure.
    - Raise the cylinder temperature.
    - Reduce the cushion. (Reduce the charging amount.)

    Mold & Material
    - Increase the mold temperature.
    - Provide a cold slug well to trap the cold material during the filling stage
    - Increase the cross sectional area of gate.
    - Create more distance between the gates and the location of the mold coolant.

  2. Silver Streaks
    Silver streaks are the splash appearance of moisture, air, or charred plastic particles on the surface of a moulded part, which are fanned out in a direction emanating from the gate location.

    Causes

    - Material is absorbing water or aeration is occurring during moulding.
    - Material is contaminated with another type of material.
    - Improper barrel temperature setting may degrade polymer molecules, and it will begin to char.
    - Too much additive is present.
    - If the shot size is below 20 percent of the machine injection capacity, the melt resin will remain in the barrel too long and will begin to degrade.

    Troubleshooting

    IM Parameters

    - Select a proper injection moulding machine for a specific mold.
    - Fully purge the previous material from the barrel, if required to switchto other material
    - Apply back pressure to generate a kneading effect.
    - Decrease the melt temperature, injection pressure, or speed.
    - Thoroughly clean moulding machine cylinder.

    Mold & Material

    - Increase (or decrease) the size of the runners and gates.
    - Improve the venting system.
    - Increase the mold temperature.
    - Change the gate locations.
    - Pre-dry thoroughly.
    - Install aultra-red light inside the hopper.
    - Investigate for potential contamination.

  3. Sink Marks
    A sink mark is a local surface depression that typically occurs in mouldings with thicker sections, or at locations above ribs, bosses, and internal fillets. A void is a vacuum bubble in the core.

    Causes

    - Low injection and packing pressure, low short hold time or cooling time
    - High melt temperature or mold temperature
    - Localized geometric features

    Troubleshooting

    IM Parameters

    - Increase the amount of material feed.
    - Increase the cavity internal pressure (secondary pressure).
    - Lower the cylinder temperature setting.
    - Increase the injection rate.
    - Increase the injection speed.
    - Lengthen the injection time.

    Mold & Material

    - Lower the mold temperature.
    - Make the mold temperature uniform.
    - Change the gate locations.
    - Make the gates larger.
    - Make the runners shorter.
    - Choose a highly viscous grade

  4. Discoloration
    Discoloration is a colour defect characterized by a moulded part's colour having changed from the original material colour.

    Causes

    - The material staying in the barrel too long time.
    - The barrel temperature being too high, causing the colour to change.
    - Contamination caused by reground material, different colour material, or foreign material.

    Troubleshooting

    IM Parameters

    - Use a different size injection moulding machine.
    - Clean the hopper, nozzle and mold completely.
    - Purge the injection unit completely if there is any material changing.
    - Reduce the barrel temperature and nozzle temperature.

    Mold & Material

    - Add an adequate venting system. (25~38㎛)
    - Maintain proper housekeeping for origin materials and regrind materials storage to avoid contaminated materials.
    - Ensure you or your supplier is using a colour agent with adequate thermal stability.
    - Ensure that the master batch is evenly mixed for consistent colour output.