7 common injection molding defects in the plastic manufacturing
by Lushi-Molding  | May 26 2020  |  Tags: Injection molding manufacturing

7 common plastic injection molding defects and feasible solutions

In the process of plastic injection molding, we often encounter some common product defects, which will seriously affect the product quality and the progress of production. Therefore, how to solve these injection molding defects is an important issue for every injection engineer. In this article, we will discuss the seven common injection molding defects in practice and their possible solutions.

Sink marks

We all know that the shrinkage formed in the thick location of plastic parts, such as ribs or protrusions, is more severe than the adjacent positions. This is because the thicker area cools much slower than the surrounding area. Different cooling speeds lead to the formation of depressions at the connection surface, which are familiar shrinkage marks. This kind of defect severely limits the molding quality of injection parts, especially large thick-walled products such as beveled cabinets and display cabinets for TV sets. The main causes of shrinkage marks due to thickness of the rib structure and the mold temperature

● The temperature of the molten material need to be appropriately reduced by adjusting the temperature of the barrel;

● Changing the mold cooling system to reduce the temperature of the cooling water;

● Adjusting the screw speed to obtain the correct screw surface speed;

● Controlling the mold temperature according to the characteristics of the plastic and the product structure;

● Reasonably design the wall thickness and rib thickness of the parts, and the fillet of the connection.

Weld line

Welding marks of plastic products refer to the marks formed where two different molten materials are in contact, which are completely different from other parts due to their morphological structure and mechanical properties. There are two basic types of weld marks in injection molded parts: One is due to the structural characteristics or big size of the plastic parts. Two or more gates are used to reduce the melt flow and filling time. The weld marks will be shaped when the melt fronts of different gates enter the cavity, which are called cold weld marks; when the wall thickness of the part is too big, the resistance of the fluid flowing through the cavity is different, and they merge at different flow rates. , Eventually forming a weld mark at the junction.

Influencing factors mainly include melt temperature, injection pressure and holding pressure, injection speed and injection time, and mold design defects, etc. If the adjustment is not reasonable during injection molding production, that will cause weld marks.

To eliminate or reduce the influence of weld lines, we can also start by selecting the right setting of injection machine to control the filling time and injecting speed. Because there is often stress concentration at the weld line, heat treatment can be used to eliminate or reduce the internal stress formed in the molding process for the injection molded product and improve the mechanical properties of the product.

how to avoide the injection molding defects

Warping

The shape of the plastic parts will be rotated and twisted after demolding resulting in local and overall deformations, which called warping. Such as the straight edges are facing inwards or outwards, the flat parts are undulating.

The main reasons of this injection molding defect are the unreasonable design of the pouring system, the improper control of the mold cooling system and mold temperature, and the difference in the orientation of the flowing molecular chains, resulting in different shrinkage rates.

● The gate location should be selected to minimize the flow distance of the plastic;

● The cooling system design should be beneficial to strictly control the temperature balance of the core and cavity;

● The cross-sectional area of the ejector should not be too small to prevent deformation of the plastic parts per unit area due to excessive force;

● When designing mold gating, the flow characteristics of the melt should be attention to keep the flow as parallel as possible during the filling process.

Burn marks

When the filling speed is too fast, the gas is compressed and heated up in a very short time, which may cause the defect of burning plastic. Many dark black stripes or black spots appear on the surface or inside of the plastic part, known as paste or burnt. It is possible that due to poor exhaust, small gate or improper gate position, and large local resistance of the cavity, the material flow will converge slowly to cause exhaust difficulty.

● Use simulation software, such as moldflow, to predict the draught, trapped air position, and strengthen the exhaust in trapped air area;

● Regular mold maintenance, and the exhaust groove need to clean to prevent clogging;

● If necessary, use the mold cavity exhaust device to force the exhausting;

● Properly increase the mold gate and control the pouring speed to allow sufficient time for the gas to be discharged.

Vacuum voids

There are bubbles inside the injection-molded parts. Bubbles are some small gaps or series of pores inside the product after molding. Dark bubbles are vacuum voids generated inside the plastic. In the barrel, there is air between the plastic particles. When the plastic is plasticized, it enters the barrel with air together. When the injection speed is too fast or too slow, it may cause vacuum voids in the injection molded parts.

● Increasing the back pressure, so that the melt in front of the screw is compressed during the plasticization to expel the air;

● Opening the exhaust slot to exhaust the gas sufficiently;

● Making a vacuum point at an appropriate place to evacuate the air in the mold cavity during injection.

Chromatic aberration

The chromatic aberration of the injection molded part is different from that of the single standard color sample with naked eyes. Chromatic aberration is a common defect in injection molding. There are many factors affecting color difference, including raw resin, color masterbatch, mixing of color masterbatch and raw materials, injection molding process, injection molding machine, etc. To reduce the chromatic aberration we generally carry out color the following five aspects.

● Adjusting the parameters of injection molding machine correctly;

● Choosing the raw resin with exactly color masterbatch;

● Reducing the effect of barrel temperature on the chromatic aberration;

Short Shot

Short Shot means that the injection molding process is incomplete, and the cavity is inadequate with plastic. Usually lack of glue means that the glue is not uniform, that is, the cavity is not full. There are many reasons for the lack of glue, such as insufficient materials, poor material flow performance, lacking of injection pressure or insufficient time.


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