FCS has been specializing in injection molding machines for nearly 50 years, and has accumulated a wealth of practical experience and energy in the field of injection molding based on continuous innovative research and development in the industry. However, the process of injection molding is easy to be influenced by "people, machine, material, method and environment" (i.e., the operator, injection molding machine/mold, plastic material, injection molding conditions and production environment), which results in various injection molding defects. 12 common types of defects are listed in the table below.
 

There are many types of defects in injection molding, so we will first discuss the problem of bubble defects and propose solutions.

Causes of bubble formation
During injection molding of plastic parts, various types of molding defects such as crazing or bubbles occur due to the mixing of air, moisture or gas into the melt, and these bubbles cause apparent and physical property defects that are often difficult to resolve. When troubleshooting bubble defects, many operators make the mistake of guessing what the bubbles are, and then immediately embark on the process magic of adjusting the injection molding conditions to eliminate the bubbles. It is strongly recommended that process magic is not the best way to start bubble removal. Instead, it should start with defining bubbles.

Defining bubble types
Bubbles occurring in transparent products can be observed directly, while bubbles occurring in opaque products are sometimes not visible from the outside and can only be detected by cutting them open or by other means. Before troubleshooting bubble defects, the following types of substances contained in the bubble should be identified.
(1)  Trapped gases: These include air, moisture, volatiles from plastic additives, or decomposed gases.
(2)  Vacuum bubbles:
Bubbles caused by the shrinkage of the plastic in the presence of cold. Note: It is important to determine what type of bubble is present, and to identify the root cause and the source of the bubble before proceeding to the next step in eliminating the problem.

Methods for identifying bubble types
Slowly heating the part containing the bubble with a heat gun, infrared oven or something similar allows the heat to have enough time to penetrate the thick layers of the part's flesh and begin to diffuse until the bubbled area of the part softens, at which point the bubble should change form. If the bubble is formed by trapped gas, the gas will heat up and expand the surface of the part or even burst. If there is no air in the bubble, and it is a vacuum bubble, the pressure will act to push the surface to collapse after the part has softened.
 

Note: The softening test shall not be performed for more than 2 hours as especially the water-absorbing material will absorb the moisture in the air and fill the pores of the vacuum bubble, resulting in misjudgment.

Suppose the test results show that the trapped gas produces bubbles, the trapped gas may be:
(1) Moisture: The moisture attached to the surface of the particles is not removed due to insufficient drying of water-absorbing plastics or moisture in the air.
(2) Thermal decomposed gas: It is often the benefit of melt temperature and residence time, or the decomposition and volatilization of plastic additives, which usually occurs in the process of drying and plasticizing.
(3) Air: The original space gas has not been excluded in the molding process.

Identify the type of the trapped gas, and then discuss the reasons for the formation:
(1) Problem of moisture in gas drying: The choice of wrong hot air/dehumidification drying equipment leads to incomplete drying of the plastic.
(2) Thermal decomposed gas during dehumidification /plasticization: Thermal cracking is a characteristic of all thermoplastic materials, and is a function of temperature (including shear effect) and time. Screw design, barrel/drying temperatures, speed settings, or excessively long cycle and drying time can all make it difficult for by-product gases from thermal cracking to escape.
(3) Melt flow problems: The seam convergence, convergence of part geometries, abnormal melt injection flow; non-exhausted lifting pin in the mold, inefficient exhaust systems, etc. can lead to the fact that trapped gases cannot be discharged smoothly out of the mold cavity, resulting in trapped gases.
(4) Venturi effect: During the melt flow process, the poorly matched positions of the ribs, ejector pins, nozzle tips and runner bushings, the non-alignment of the nozzles, and the manifolds in the hot runner may generate a Venturi effect that draws air into the melt flow.
 

Note: Determining where the gas comes from is more important than knowing the composition of the gas, and a simple test can be used to find this out.
 

Read More
Causes and Countermeasures of Injection Molding Defects - Bubbles(2)