How to determine a reasonable injection pressure holding time?

1. What exactly happens during the stress phase?
After the molten adhesive fills the mold cavity, polymer molecules begin to crystallize, and the molecular chains are interconnected and arranged to produce a higher combination density of the product. This process starts from the outer surface of the wall thickness and ends at the center. During the production of POM parts, the resulting volume shrinkage can reach up to 14%. During the pressure maintaining stage, the volume shrinkage is supplemented by additional melt adhesive. If the holding pressure time is too short, it will cause the formation of small voids, which will have a negative impact on the quality of the formed products.
2. How to discover that the injection holding time is too short?
Parts produced with a shorter holding time usually have obvious shrinkage, warping, dents, voids, etc. In some cases, it seriously reduces the mechanical properties of the product and causes dimensional changes. In injection molding production, increasing cooling time is often mistakenly used to solve these problems, leading to prolonged market cycles and reduced production efficiency.
Method for confirming insufficient holding time of crystalline engineering plastics:
1) The best method for determining the holding pressure time in actual injection production is to confirm the most effective holding time by weighing the on-site injection molded parts multiple times.
2) Cut the maximum wall thickness position of the formed product, and then polish it to check whether there are voids and pinholes in the polished wall thickness section.
3) Use a magnifying glass or light reflection microscope to inspect the maximum wall thickness section of the formed product for any voids or pinholes, and make a preliminary judgment.
4) A more careful and precise method: The maximum wall thickness section of the formed product is made into a standard section group for observation and inspection under a light transmission microscope. At this point, even the smallest defects can be seen.
Method for confirming insufficient holding time of reinforced engineering plastics:
1) Weigh the product to determine the most effective holding time.
2) Check the section at the maximum wall thickness and observe if there are any defects in the section. When the pressure holding time is insufficient, foam structure will form in the section area, and the enlarged section micrograph shows that the exposed fiber is not embedded in the polymer.
3) The prepared and polished thick wall section section of the product can be examined through a microscope to observe whether pinholes or holes are formed.
In addition, for products with fixed wall thickness, testing the elongation at break and making actual charts for comparison can also obtain the optimal holding time. However, it cannot be ruled out that other factors such as temperature, crystallization additives and pigments, mold filling time, etc. have an impact on the testing. For thinner wall thickness products, the graph curve shows a decreasing trend while the graph curve shows an increasing trend.
elongation
Holding time
Holding time and cooling time
3. Essential conditions for obtaining the best formed parts
1) Determine the correct holding time by weighing, reducing the cooling time to the minimum necessary time (usually just exceeding the plasticizing time).
2) Design the correct mold runner system and gate position and quantity.
3) Correct holding pressure. According to different materials, the correct setting range of pressure maintaining pressure should be between 60MPa and 100MPa.