Prominent Defects Found In Molded Plastics

Prominent Defects Found In Molded Plastics

Plastic molding is a vast field, and there is a vast variety of products that are manufactured through molded plastics. The processes of plastic molding are basically extrusion of molten plastic resin through a mold or die.

 The process of plastic molding is one of the most delicate processes in the plastic industry which need a high level of perfection and should be performed with utmost care. If the manufacturers do not take proper care of the process and there is even a slight deflection of the process, it can result in prominent defects in the final products. Below are discussed some of the defects that are commonly exhibited by molded plastic products.

Silver Streak

The silver streak, the name given to the defects, has been introduced to the plastic products due to degradation, the inclusion of air or due to insufficient drying. This causes the appearance of gaseous components on the surface of molded products. This can ruin the look of the products and also affects the quality and strength of the product as well. In this case, the drying equipment should be checked immediately to avoid this problem. Additionally, the design of the dies should also be verified for any possibility of air inclusion.  Along with that, the properties and quality of the plastic should also be examined.

Short Shot

When a molten plastic resin is not able to reach certain parts of a die, it leaves a blank space at that place. This defect is called ‘short shot’ or ‘short mold.’  This is due to several reasons such as generation of gas during the process of molding, due to varying material thickness and due to the higher viscosity of fluids. In order to avoid this type of defects in molded plastic products, first of all, it should be ensured that there is no evolution of gases during the process. The plastic resin used for molding should be chosen according to the desired thickness of the final products. At last, one must determine the viscosity of plastic resin so that proper dies shapes and molding process should be used for molding of the plastics.

For determination of the exact viscosity of a plastic resin or granule, its melt flow index must be determined. A melt flow index tester is used to evaluate the melt flow index of the plastic resins and granules accurately.


Jetting is an important quality flaw that is found in molded plastics in which there are flow patterns visible on the surface of the final products. This is because of the fact that when plastic which is at a relatively low temperature, is injected through a nozzle, when it comes in contact with the mold walls and hence becomes a bit more viscous and causes swirling in the plastic products. In order to avoid such defects in molded plastic products, it is essential to understand the rheological behavior of plastic granules. The melt flow index tester is a major instrument that can help in the determination of the exact value of the melt flow index which further gives better understanding of the rheological behavior of the plastics. When you know the exact behavior of molten plastic resin, you can easily alter the temperature to increase or decrease the viscosity of plastics and avoid such defects from occurring in the final molded plastic products.

Weld Lines

Weld line is a particular defect that is found in molded plastics in the form of a V-shaped cavity where there is a meeting point of two different flows. When viewed with a naked eye, a weld line is often confused with a crack. This defect occurs due to several reasons and fluidity is one of them. If the estimation of the viscosity of the materials is not exact, this products will happen, guaranteed. Additionally, this effect is also caused due to the use of different injection holes and multipoint gates. Weld lines can cause problems with the strength when they occur at the stress concentration area of a product. Hence, countermeasures must be implemented to reduce the frequency of weld lines in the plastic products.

Although there is no theoretical remedy for such defects but still there is some measure that can be useful in reducing the frequency. The first step is an exact evaluation of the viscosity or fluidity of the plastic material to be molded. The melt flow index helps in that. The melt flow index tester is a great way of calculating melt flow index of a given plastic granule or resin which further gives an exact evaluation of the fluidity and viscosity. Other remedies include keeping the temperature of resin and fluids high. Additionally, the weld lines can also be minimized by applying a compressive effect on the products while molding.


Flash is the defect in plastics which occurs when an excess of plastic material enters the parting faces of a mold that and intends the plastic product. This defect causes a decrease in quality and strength of the final goods and is also aesthetically unacceptable. There are two possible reasons that may lead to this defect. One is insufficient clamping pressure on the molds, and the other is the low viscosity of the plastic resin. To prevent this defect from happening, the first thing is to apply proper clamping pressure on the plastic mold. The other remedy is to control the viscosity of the plastic resins. To control this attribute of plastics, first of all, the melt flow index of the plastic resin being used for the molding process.

Apart from these defects, there are also many other defects that occur in molded plastic due to many different reasons. In order to prevent these defects, one should first acquire complete knowledge about these defects, their causes and possible remedies for them. The quality testing instruments and procedures are very helpful in monitoring of the quality defects and prevent them from occurring. With proper knowledge and right instruments, it is really easy to ensure the best quality of plastic products with increased productivity and efficiency.