Plastic injection knowledge: 5 common undesirable phenomena and solutions

Plastic injection knowledge: 5 common undesirable phenomena and solutions
In the injection molding industry, the yield of the finished product is very important, because many plastic injection products are small parts, such as plastic gears, buckles, rotating shafts, etc. As long as these parts have problems such as cracks, shrinkage in size, and deformation, they will cause problems. Downstream manufacturers are unable to normally manufacture objects, which affects assembly and shipment and increases product production costs. How to avoid defects in the plastic product during the injection molding process? So we must first understand what are the common defects? How to solve it? Today's article takes you to understand the types and solutions of common injection molding defects! Let you know more about plastic injection!
Extended reading: What are the costs of plastic injection products?

The most common injection molding defects


When we understand the knowledge of plastic injection, we often mention the phenomenon of "cracking" in the finished product. It is a common defect in plastic products. It refers to the phenomenon of cracks on the surface of plastic products after cooling. Why do cracks appear? The reason is that there will be a residual internal pressure in the plastic manufacturing and forming process called "residual stress", we can imagine it as a response to changes in the external environment (the mold temperature becomes lower, the injection speed becomes higher) or the plastic solidifies. Reaction force. When the residual stress of the plastic part is too large, the pressure will be released, which may cause cracks on the surface of the plastic part.
  • Solution 1: Appropriately reduce the mold gate, shorten the filling time, and reduce the pressure in the mold during sealing.
  • Solution 2: Increase the mold temperature and reduce the residual stress caused by uneven cooling of the plastic material.
  • Solution 3: Increase the temperature of the plastic material, improve the fluidity, and can reduce the injection pressure and reduce the residual stress.
Extended reading: What are the causes of injection molding defects

Finished raw edges (Falsh,Burr)

Burr refers to the plastic material still attached to the joints of the product after the injection is completed. The reason is that the male mold and the female mold cannot be completely sealed due to the poor mold design during the injection molding process, resulting in the plastic material from the middle. The voids run out to form burrs; the high probability of occurrence of burrs in the finished product is a common problem in most injection factories. Such problems may occur if the mold design engineers and operators do not have enough knowledge about plastic injection.
  • Solution 1: Confirm that the clamping force of the injection machine is greater than the pressure in the film cavity, reduce the pressure in the film cavity, and reduce the chance of the male and female molds being stretched.
  • Solution 2: Make sure that the male and female molds are tightly closed during injection, because once the clamping surface is misaligned, uneven, or there is a foreign object in the middle, it will make the clamping surface unable to fully seal and cause burrs.
Extended reading: Principles and steps of plastic injection molding

Record Groove

Once the plastic material leaves the heating tube, the plastic at the front end of the mold that first touches the mold will begin to solidify and adhere to the mold. At this time, the plastic at the rear end will pass the solidified plastic and continue to flow forward, and then touch the next section of the mold and adhere. After repeated occurrences, fine lines like water waves will be formed; Record Groove are also one of the defects that we often mention when learning plastic injection knowledge, and are related to the mold temperature, raw material temperature, and injection speed of the injection machine. Closely related, Record Groove defect is a problem that many plastic injection products will encounter.
  • Solution 1: Strengthen the fluidity of the raw materials, such as increasing the temperature of the raw materials, or directly replacing the raw materials with better fluidity, so that the plastic materials can be uniformly solidified in a short time.
  • Solution 2: Increase the mold temperature, reduce the temperature difference between the plastic material and the mold, and reduce the solidification rate of the raw material. 
  • Solution 3: Increase the injection speed and pressure, so that the plastic material can quickly fill the entire mold, and there will not be too much difference in the solidification time. 
〈Extended reading: 5 key points to the stability of injection molding


When manufacturing thicker plastic products, the surface of the plastic material will first cool down and solidify, and then slowly shape the inside. At this time, if the injection pressure is too low and the internal plastic material solidifies and shrinks seriously, bubbles will be generated. This type of Bubbles are the "vacuum bubbles" mentioned in the reference book when we are learning about plastic injection knowledge; in addition, as long as the gas in the plastic material cannot be discharged normally, it is easy to form bubbles in the finished product.
  • Solution 1: Increase the injection pressure and increase the injection time to extend the product solidification time.
  • Solution 2: Reduce the injection speed to allow enough time for the gas in the mold to be discharged.
  • Solution 3: Increase the gate and runner, and leave a certain amount of supplementary injection material during the injection process.
Extended reading: What are low pressure injection machines?

Gas streaks, Silvery line

Plastic raw materials have different characteristics depending on the type. If the material has high hygroscopicity, tends to retain moisture, or the plastic is easily mixed with air, then after the water and gas enter the mold, it will be easy to stick to the mold. Above, after injection, a silver-white pattern is formed, which is the silver line and air sore we often hear. To reduce the production of silver line, the operator must have a certain understanding of plastic injection knowledge.
  • Solution 1: Adjust the temperature and time of drying the raw materials according to the properties of different raw materials, especially those with higher requirements, such as PMMA. As long as the material is free from moisture before injection, it can reduce the chance of silver lines on the surface of the product.
  • Solution 2: If there are different types of materials to be mixed, it is recommended to mix the plastic raw materials evenly.


After introducing several common defects in plastic injection, many people will ask a question: if the injection speed is too high, the finished product is prone to air bubbles, and the speed is too low, and the finished product may produce water ripples. So what should be done to adjust? Many companies will directly choose to hire a professional injection molding engineer to supervise, but we recommend that you first have at least a preliminary understanding of the injection machine settings, operating skills, and plastic injection, and figure out where the problem may occur. And ask the injection machine manufacturer's opinions, maybe you can solve it yourself, and reduce the cost of the company's operation. We are experts in injection molding machines, specializing in injection molding machine technology. If you have any questions about plastic injection knowledge, please contact us!