To declare this flow-line a short description of the injection moulding process will be given:

 


1. The raw material is brought into the cylinder of the injection-moulding machine in the shape of plastic grain (granule).

2. Due to the raise of the temperature and the transport of the grain in the cylinder, the grain melts and becomes homogeneous. This can be compared with chewing on chewing gum. 
 

3. When the plastic is enough homogeneous it is injected into the mould. The mould is a metal block where the shape of the product has been cut-out.
 

4. The product gets it shapes by filling the hole in the mould with plastic. This means that if the product is hollow the plastic has to flow in 2 directions around the core.
 

5. At the place where the two different material streams meet, the plastic will merge. In the mould no homogenisation will take place at the point where the two streams meet. This means that a line will be visible at this point. Comparable to: When two fresh pieces of chewing gum are pressed together a good stick connection will take place. The connection will still be visible as a line. When the pieces of chewing gum are older, or in the case of injection moulding when the plastic has been cooled down too much, the adhesion will be less.

6. It is obvious that the adhesion of this "flow-line" is checked on a regular base. This is twice a day with a so-called "oven-test."

As seen in the description above, the "flow-line" is not a quality problem, but just inevitable when injection moulding a product.