The two biggest determining factors of molding sink marks are product design and processing with the assumption that the tool is functioning correctly. With that being said there is a fine line between the two. You can over compensate for poor product design with molding parameters, and vise versa. If you over pack a part you can begin to effect it dimensionally or ride on the fringe of flashing your parting line. You can very easily make a part that is too big by trying to pack out molding sink marks that are caused by poor product design. The other down fall of over packing is a small processing window.
A slight variance in other areas of your process could cause your scrap rate to go through the roof. For one example, different lots of material. Sometime the difference from one lot of material to the next will flow different (viscosity of your material during injection). Your process should always be based on a good rheology and gate freeze study. RJG pressure transducers in the tool run through an Edart system will also help you monitor your process for repeatability and quality control of your product. Once you have established a good repeatable process with as big of an operating window as possible, one that you can measure and monitor then, adjust your product design. You will be much happier with your end results doing the plastic molding sink marks determining. After a few times of going through this you will begin to establish some lessons learned that you can start applying to your product before you ever get to the press.
Few simple steps you should follow:
- At first stage of product design you should consider about bos/rib construction there are rules for it. So perfected by design first.
- If you find molding sink mark there are two factors mold related and machine/process related.
- Mold related take a look the feeding system (gate, runner, sprue, and orifice diameter of machine nozzle. Restricted area at the gate, undersize runner, insufficient nozzle diameter could cause insufficient packing. The point is plastic material must flow as easy as possible. And don’t forget proper venting at the mold. If show us the product, the runner, and the mold might help us to decide.
- Machine/process related, you can avoid thick ribs and bosses, increase injection pressure, increase back pressure, or apply melt cushion to product. Holding pressure might help to improve quality.