What is the plastic injection molding process?

by Adhecogen

A plastic injection molding machine

Another plastic injection molding machine

What is the plastic injection molding process?

The plastic injection molding process mainly includes four stages: filling, pressure maintenance, cooling and demolding. These stages make up a single process, and the final quality of the part depends on the proper development of both.

Filling begins with the closing of the mold and ends with approximately 95% of the cavity filled. It is the first step in the whole injection molding procedure.

The continuous application of pressure increases the density of the plastic to compensate for the possible shrinkage of the plastic. Injection pressure control is generally divided into one injection pressure, two injection pressures (holding pressure) or more than three injection pressure controls. Proper timing of the pressure switch is essential to avoid high pressure in the mold and to avoid excessive or missing materials.

The cooling time depends mainly on the melt temperature, wall thickness, the product’s cooling efficiency and the material’s hardness. More complex material will set faster in the mould than soft material. If cooling is carried out from both sides, the cooling time required per 0.100′ wall thickness will generally be approximately 10 to 15 seconds. Glue-coated products require a longer cooling time because they can cool efficiently through a smaller surface area. The cooling time required per 0.100′ wall thickness will be 15 to 25 seconds.

Demolding is the last step of a complete injection molding cycle. Although the products have been cooled, the demolding or quality has a significant impact. Improper demolding can cause the effects to demold with uneven force, resulting in product deformation and other defects.

What are the main problems of the plastic injection molding process?

Product shrinkage marks: These marks are defects that occur in molded parts due to raw material deficiency or a high thermal gradient within the part. It causes the material in the center to shrink and “pull” the surface material towards itself without compensation for this volume shrinkage.

Visible flow lines on the part’s surface while filling the cavity: These are commonly caused by poor dispersion of the resin colour concentrate. They are evident in black or transparent features, smooth surfaces, or metallic finishes. Another cause may be that the working temperature is too low because if it is not high enough, the corners of the flow fronts will not fully develop, causing a flow line to appear.

Sink marks and voids: Sink marks are caused by solidification and contraction of the outer surface before the inner surface. Once the exterior surface solidifies, an internal shrinkage of the material occurs, causing the crust to depress below the surface and cause sinking. Voids are also caused by the same phenomenon, manifesting with an internal opening.


Pack more plastic into the cavity: It may be that the amount of raw material available in the cycle is not sufficient. This is achieved by increasing the level or duration of post-pressure, improving the injection cushion, increasing the diameter of the injection channel or changing the position of the injection point of the part. It is always recommended to fill from the thickest to the thinnest end of the part.

Solutions for the flow lines: In this problem, three procedures are necessary to solve it. Increase the injection speed, injection pressure or maintenance. Reduce the mould or mass temperature by lowering the back pressure or drum temperature. Increase the gate size and, if possible, reposition it.

Polishing of the piece: Any imperfection that may remain in the piece can be solved by polishing it to achieve a smoother surface.