Small tips to improve injection molding: 5 defects for 5 solutions

In the articles on the portal, we have already dealt with some topics concerning the defects that are created during the molding phase and the solutions suggested both for the management of the molds and for the defects that occur can be found on the finished product.

As you know, the molding of plastics is an imperfect science since the success of the molding work is influenced by a really important quantity of factors. We can start from the type of machine, its components and their state of wear, from the design of the mold and its maintenance in good condition, from the raw materials used, with particular attention to the regenerated ones, to the adjustment of the machine and mold parameters.

All these things put together go, in the end, to influence the success of the work and, with it, the financial success of your work. In fact, even this aspect, which perhaps the pure technicians neglect, must be taken into consideration as much as the quality and the technique in production.

Let's look briefly at some faults that can occur frequently:

Streaks on product walls

They are also referred to as flow lines which include streaks, spots and different colors which can be caused by the cooling profile and the path that the plastic makes to reach the parts of the mold. There are some ways to overcome this drawback regarding the increase of the injection speed to improve the filling of the cavities, or the damping of some angles of the mold where the flow passes.

Small cavities on product surfaces

Small depressions or depressions are normally formed on the walls of the products, especially in areas where the thickness is greater or where the flow is restricted. In these cases it is advisable to reduce the mold temperature, increase the injection pressure and the molding time to improve polymerization and cooling. If it is possible, in order to improve cooling, it is advisable to reduce the thickness of the walls where the product allows it.

Empty

If previously we talked about small cavities or small depressions that can be found on the surfaces, here we can talk about real gaps characterized by an incomplete finish of the wall.

The reason why these voids are created can be multiple. Normally they originate from air pockets that oppose the good flow of the mixture in the mold, whose causes can be found in an excessive presence of gas inside, formed for example by the degradation of the polymer in the screw of the machine, which it is not properly vented.

his can easily happen if compound polymers like PP / PE are used and the relationship between extrusion temperature and material composition is not taken into proper consideration.

Another cause could be the excessive viscosity of the raw material used on which one can intervene with a suitable choice of the fluidity of the product. Even maintaining the correct alignment of the mold during injection can help reduce these problems.

Delamination of surfaces

It may happen that the walls of the molded product have small layers that can be peeled off. The causes are to be attributed, among others, to certain factors such as the excessive presence of humidity or the use of regenerated polymers in which polymers with degrees of fusion different from the principal are present. It is always advisable, especially if using regenerated granules, to dry them in silos before putting them in the car.

Furthermore the correct choice of recycled granules passes from the knowledge of the DSC of the polymers that are used, since, the greater the percentage of polymers with different degrees of fusion, the greater the possibility of creating thermal stresses in polymers. Another tip is to increase the mold temperature.

Welding lines

Near the welding lines you can create aesthetic imperfections due to the different directions of movement of the liquid plastic. First of all the correct closure of a mold allows a good performance not only of the polymer inside but also a better finish of the products.

Furthermore, the correct choice of the fluidity of the product and the tonnage of the machine, depending on the final product to be produced, allows a correct production without straining the molding and the polymer, to the benefit of the quality of the molded element.

Finally, the design of the mold that takes into account the correct polymer flows in the presence of the welding of the product must be kept in mind when there are aesthetic problems on the weld and, in this case, evaluate an intervention on the mold that can solve the problem.