What are the common defects of injection molded parts and how to solve them - 1
After the plastic parts are molded, some defects may occur, which will affect the assembly efficiency or the performance of the whole machine, and there will be certain differences from the predetermined quality standards (inspection standards), which cannot meet the company's quality requirements. In addition to strengthening the quality awareness training of employees and timely handling of defective accessories (such as selection, processing, etc.); these defects may be caused by the following aspects: molds, raw materials, process parameters, equipment, environment, and personnel. The defects are summarized as follows:
1. Flash
Flash is caused by pressure in the mold exceeding the clamping force or because of mold damage. Flash can occur during filling or feeding. When analyzing flash, it is very important to determine at which stage it occurs. The best way is to look at the filled part, determine if a separate filling and feeding method is being used, and determine whether it occurs during the filling or feeding stage.
uReasons of flash
uFlash occurs during the packing stage (conventional injection molding method).
uFlash occurs during the DECOUPLED MOLDINGSM stage
uFlash occurs during filling
uMaterial viscosity too high
uDuring filling, the mold deforms
uThe mold is damaged or the fractal surface does not match well.
uMaterial viscosity is too low;
uThe flash is in the area where the fractal surface is not tightly closed.
Flash during filling; filling too fast relative to the mold condition.
uPossible corrections to remove flash
uChange the switching position and adopt DECOUPLED MOLDINGSM technology.
uReduce the packing pressure, or reduce the packing rate, if possible. Check whether the clamping force is appropriate.
uCheck whether the clamping force is appropriate.
uCheck material viscosity (fill integral)
uWhen using MPNs (Mold Platen Normalizers), a thin sheet is usually placed in the center of one side of the fixing plate (use with caution).
uRepair the mold.
uCheck viscosity. Determine cause of viscosity change. Repair mold.
uCheck fill time and reset if necessary.
2. Flying edge and short shot occur at the same time
uFrom plastic perspective
Usually indicates that the pressure distribution during filling has changed due to changes in dynamic viscosity. It may also be due to insufficient clamping force.
3. Sink marks and voids
As the plastic cools and shrinks, insufficient packing can cause sink marks on the outside of the part and internal cavities. Sink marks and cavities are most noticeable in thick-walled parts at the last cooling point, or at locations far from the gate, or very close to the gate. Sink marks in thick walls and far from the gate are typically due to insufficient packing or increased viscosity. Sink marks near the gate are usually due to lack of gate freeze-up, possibly due to decreased viscosity, and most of the time, due to increased plastic temperature causing the gate to not freeze. Reducing packing will generally eliminate sink marks at the gate, increasing packing will cause sink marks at the gate, and increasing pressure will cause gate decompression after injection.
uPlastic viscosity increases
uFeed and holding pressures are too low (sink marks at cavity ends and thick walls).
uThe injection time is too short and the gate is not frozen.
uThe mold temperature is too high, which affects the gate freezing.
uVoids are sometimes misunderstood as bubbles. Bubbles are where there.
uUse the 30/30 method to check the plastic temperature.
uIncrease feed and/or hold pressure.
uIncrease the injection time/holding time.
uLower mold temperature.
4. Size changes
uFrom plastic perspective
The dimensional change is due to the change of pressure distribution in the cavity, and for crystalline plastics it is due to the change of cooling rate. For both plastics, the dimensional change may also be caused by post-processing, cooling, and environmental changes. In order to better analyze the dimensional change, the following specifically classifies the problem and solution:
The plastic pressure in the cavity is too low
The plastic pressure in the cavity is too high.
Reduce the feeding pressure to achieve the required cavity pressure.
uThe gate of the part is too small
The pressure at the gate is too low, but it is OK at other places. Usually it is because the gate is not frozen.
Increase the injection time/holding time, or find out the root cause of the problem, such as: increased plastic temperature.
uCavity end is too small
uCavity end is too large
This means that the pressure at the gate is OK, but the pressure at the end of the cavity is too high. This indicates that the viscosity has decreased.
uPossible corrections
uUse the 30/30 method to check the plastic temperature.
uCheck the filling time.
uCheck viscosity (fill integral). If viscosity is too low, reduce injection speed until viscosity is correct.
uFrom plastic perspective
Inconsistent dimensions mean inconsistent pressure distribution. This means that some gates are frozen and some are not. Otherwise, the cavity pressure distribution varies from mold to mold. If this is the case, check if it varies greatly from mold to mold, or if there is a trend over time. A trend indicates temperature changes or material batch changes, whereas mold-to-mold differences indicate changes in shrinkage that cause gate freeze-up changes, or pressure changes due to slip ring leakage, etc.
6. Ejector pin ejection mark
The ejection mark of the ejector pin is actually a mold sticking problem. Mold sticking should be considered, see mold sticking.
7. Defects or appearance problems of the bonding line, continued
uA knit line is essentially two flow fronts that come together without re-arrangement. This is not the case when crystals are above the melting point. A typical knit line is also a good bond. The material must be at a low enough viscosity, the flow front must be clean, and there must be enough pressure and time for the plastic to solidify. There is also trapped air, and good venting must be provided. If the above issues are resolved, the knit line will be well bonded.
