Injection molding

During the injection molding process, various defects often occur due to factors such as materials, molds, process parameters, and equipment. Understanding the characteristics, causes, and solutions of these defects is of great significance for improving product quality. The following lists several of the most common injection molding defects and their analysis points.

The first is short shot, also known as insufficient filling. Short shot is characterized by incomplete filling of the end or local area of the product, resulting in missing meat and incomplete contours. The main causes include low melt temperature leading to poor fluidity, insufficient injection pressure or speed, low mold temperature, improper gate position or too narrow flow channel. Solving the short shot problem usually requires increasing melt temperature and mold temperature, as well as increasing injection pressure and speed.

Flash, flash refers to the overflow of molten material from the parting surface of the mold or the pin, slider, etc., forming thin sheet-like excess plastic. The direct cause of flash is that the clamping force is insufficient to resist the high pressure in the cavity, or the mold itself has wear, poor fit, etc. From a process perspective, excessive injection pressure and melt temperature can also exacerbate the tendency of flash. Handling flash requires checking if the mold is aged or deformed, confirming if the clamping force is sufficient, and appropriately reducing the injection pressure.

Melt lines appear at the junction of two or more material streams, forming a visible line, which not only affects the appearance but also reduces the mechanical strength of the product at that location. Poor mold exhaust, excessive number or improper position of gates, and material additive precipitation can all aggravate melt lines. Improving melt lines can be achieved by increasing melt temperature, adding exhaust grooves, adjusting gate position, or adding local heating, etc.

Shrinkage and voids. Shrinkage is manifested as depressions on the surface of the product; voids are cavities formed inside the product. Both are essentially caused by insufficient pressurized replenishment at the cooling contraction points of thick walls during cooling. Uneven wall thickness design of the product, insufficient pressurization, or too short time, and too small gate size leading to poor pressurization transmission are the main causes of these defects.

Warping deformation refers to the phenomenon where the product becomes bent, twisted, or turned, deviating from the designed shape after being formed. This is mainly due to the uneven internal contraction of the product, resulting in residual stress. Excessive wall thickness difference and uneven cooling of the mold cause different contraction rates in various parts, which are common causes of warping. To reduce warping, it is necessary to ensure a reasonable design of the mold cooling system, optimize the uniformity of the product wall thickness, and reasonably set the ejection position.

Surface flow marks and spray lines belong to appearance defects. Flow marks appear as wavy or spiral patterns on the surface of the product, usually caused by unstable flow of the molten material in the mold cavity or the entry of low-temperature material into the mold cavity. Spray lines are formed when the molten material is injected into the mold cavity at high speed through a narrow gate, flowing in a serpentine manner and folding, eventually leaving a winding trace on the surface. Improving the mold temperature, optimizing the shape of the gate，and using a graded injection speed can effectively improve these problems.

Top white and poor ejection are often classified as the same category. Top white refers to the whitening or protrusion of the area where the ejector works due to stress concentration； poor ejection is manifested as the product being difficult to separate from the mold， even causing scratches or cracking. These problems arise from excessive ejection force， unreasonable ejection position， insufficient ejection slope， or rough mold surface. Appropriately increasing the ejection slope， optimizing the ejector layout， reducing the holding pressure， and extending the cooling time can help improve the ejection condition. 

There are also such undesirable features as burnt marks, silver streaks, black spots and impurities. These all need to be paid special attention to during the production of molds, and the raw materials must be kept clean and the production environment must be kept clean as well, so as to avoid the occurrence of problems as much as possible. In general, poor injection molding is often the result of the interaction of multiple factors. In actual production, one should carefully observe the form and distribution pattern of the defects, combine the material properties, mold structure and process parameters for systematic analysis, in order to accurately identify the root cause and formulate effective improvement plans.