Things that can easily cause overflow and whitening but are not easy to notice
Flash and whiteout are common cosmetic defects in injection molding. Besides obvious causes like loose mold clamping and excessive ejection pressure, subtler details can also contribute, requiring a multi-faceted investigation. One often overlooked factor is improperly designed mold vents. Vents not only evacuate air from the cavity but also affect pressure distribution during melt filling. If the vents are too deep (exceeding the plastic flash threshold) in a non-critical area of the parting surface, melt can easily overflow from the vents, causing flash. A clogged or improperly positioned vent can lead to abnormally high cavity pressure, forcing the melt through the parting surface gap and causing flash. Furthermore, poor venting increases melt filling resistance, requiring greater ejection force during ejection, which can easily cause whiteout. Because vents are often invisible, these issues are often mistaken for insufficient clamping force. Confirmation requires checking the vent depth, cleanliness, and alignment of the vents with the final fill area.
Minor misalignment of the ejector mechanism is also a hidden cause of ejector blanching. If the contact position of ejector components, such as the ejector pin and ejector plate, with the core or part deviates, the ejection force can be unevenly distributed, leading to excessive localized stress and causing blanching. For example, if the coaxiality error between the ejector pin and the ejector hole exceeds 0.02mm during installation, the ejector will exert lateral force on the part during ejection, resulting in a whitish mark at the contact area. Deviations in the parallelism of the ejector plate with the core can cause the ejector plate to tilt, exposing the part to excessive localized stress and causing blanching. Because these misalignments are subtle and difficult to detect with the naked eye, a dial indicator is required to monitor the ejector component’s movement during ejection to ensure alignment with the core axis. Furthermore, if the contact area between the ejector pin and the part is too small (for example, an ejector pin diameter less than 3mm and no ejector plate is used), excessive pressure per unit area can also cause blanching, especially with brittle plastics such as PS and PMMA.
Trace impurities or degradation products in plastic raw materials can easily cause overflow, yet they are often overlooked. Impurities or degraded low-molecular-weight substances can reduce the viscosity of the plastic melt, enhance its fluidity, and make it more likely for the melt to overflow from mold gaps. For example, if the proportion of recycled material mixed into the raw materials is too high (over 30%), or if the recycled material contains impurities, this can lead to unstable melt viscosity and localized overflow under the same process parameters. Moisture released by inadequate raw material drying vaporizes during the molding process, disrupting melt uniformity and creating localized areas of low viscosity, which can also cause overflow. Because these issues do not present obvious visual abnormalities in the raw materials, it is difficult to directly link them to overflow. Verification requires testing the raw material melt index, screening the raw materials, or enhancing drying.
Minor damage or oil stains on the mold cavity surface can also cause overflow or whitening. Scratches or pits on the cavity surface can cause eddies to form in these areas during melt filling, leading to increased local pressure and overflow. Oil stains reduce friction between the melt and the cavity surface, making it easier for the melt to overflow along the parting surface. Oil stains can also affect the separation of the part from the core, increasing ejection resistance and causing whitening. These damage and oil stains are easily overlooked during routine inspections due to their small size and hidden location (such as at the bottom of a deep cavity or at the base of a rib). Therefore, the cavity surface should be illuminated with strong light and inspected by touch for any irregularities. The cavity should then be thoroughly cleaned with a specialized cleaning agent to eliminate any residual oil.
Even slight fluctuations in process parameters can lead to subtle flash and whitening, which can be difficult to detect. If pressure fluctuations during the injection and holding phase exceed 5% of the set value, the cavity pressure will instantly rise, exceeding the clamping force and causing flash. Excessive holding time can overfill the part, increasing internal stress and making whitening more likely during ejection. Changes in ambient temperature can also indirectly affect process stability. For example, a workshop temperature difference of more than 5°C between day and night can cause mold temperature fluctuations, affecting melt fluidity and potentially causing flash. Because these parameter fluctuations are small, they can only be detected through continuous recording of process curves (pressure, temperature, and time). This requires setting upper and lower limit alarms for process parameters and timely adjustments to equipment status. Furthermore, minor leaks in the injection molding machine’s hydraulic system can lead to unstable clamping force. Regular checks of hydraulic oil pressure stability and cylinder seal aging are necessary to prevent flash and whitening caused by pressure fluctuations.
