Explanation of visible flow-lines
On nearly all products which are produced by the injection moulding process, a line is visible. This line can be found on the opposite site of the injection point and is called the "flow-line". There are products where this line is better visible then on others.
To declare this flow-line a short description of the injection moulding process will be given:
1. The raw material (plastic grain -granule) is brought into the cylinder of the injection-moulding machine.
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 homogeneous enough, 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 its shape 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 which means that a line will be visible at this point.
This is comparable to when two fresh pieces of chewing gum are pressed together and a good sticky connection will take place. The connection will still be visible as a line.
The connection will still be visible as a line. When the pieces of chewing gum become older, or in the case of injection moulding when the plastic has been cooled down, the adhesion on this point will be less.
6. It is obvious that the adhesion of this flow-line is checked on a regular base. This is being done twice a day with a so-called heating-test.
As seen in the description above, the flow-line is not a quality problem, but just inevitable when injection moulding a product.
