Design Tip - 2

Let’s talk about fillets.

Some people say fillets have to be added to all edges of a plastic part. This sentence is not 100% correct, it is necessary to add fillets to most of the edges but not everywhere.

The picture shows a hole on a plastic part. The hole could have fillets at both sides or just at one side of the hole.

The first and second sketches show a hole with fillets at both sides, from the CAD point of view it is a small difference but from the mold point of view things are so different. To have fillets at both sides means the parting line has to be placed at the middle of the hole, the mould will be difficult to construct and to adjust. The third sketch shows a hole with only one fillet. The parting line could be placed at the not filleted edge, so the mould construction and adjustment will be easier. Easy and robust moulds are necessary to have good quality parts.

Before to add fillets everywhere, we have to think how it will affect the mould and if it is really necessary. In some parts only one of the sides is visible, so it is not necessary to add fillets at both sides. In other cases, the fillet (or chamfer) is done to help the assembly of another part, but usually the other part is only assembled by one of the sides, so again not so necessary to add fillets at both sides. Unfortunately sometimes it is a must to add fillets at both sides and the molds become more difficult, but if possible has to be avoided.

Fillets YES but NOT EVERYWHERE