Design Guidance
The quality and cost of an injection moulded component depends on many factors most of which are driven by the original component design.
Design Guidance
Whilst Amies offer our clients support to optimise components via the DFM process, its always saves time and is better if basic guidelines are followed during initial component design.
Even if the initial design is intended for prototyping via 3D printing where some injection moulding guidelines do not need to be followed it is always wise to design the part to suit injection moulding as closely as possible. This reduces delays and frustration caused by changing parts after initial prototype verification and testing is completed.
Design Guidelines
Wall Thickness – In a perfect world, injection moulded components have a uniform thickness throughout which is neither too thin or too thick.
In reality this is seldom possible and consequently the best designed components we encounter at least avoid abrupt changes in section and/or thick sections that follow thin ones in the flow path.
Draft Angles – It is a fundamental requirement that any face in the line of draw for demoulding has a draft angle applied to aid release. Faces should not be left parallel in line of draw except in the most exceptional unavoidable cases. Typically any face in the line of draw will have a draft angle between 1°-2° and be draw polished to aid release.
Surface Finish – The most typical finish on the faces of an injection moulded component other than polished are an EDM VDI finish created by the Spark Erosion machining process or an Acid Etched Texture applied to the tool by a specialist subcontractor.
Typically these are applied to faces that have aesthetic aspects often referred to as ‘A Surfaces’.
Where such finishes are applied to the mould, the faces they are applied to require greater draft of at least 3° and sometimes much more. This very much depends on the depth of the texture, with deeper textures requiring greater release angles.
Undercuts – Injection moulds have one main opening plane and all faces that permit the component to be released when the press opens in this plane are described as ‘being in the line of draw’. Any areas of the component that do not permit the release in line of drawer are described as ‘Undercut’. These areas require additional mechanical elements in the mould such as Side Movers which remove the steel areas that will hold the component in the tool during tool opening.
Whilst this is a common feature in many moulds it should always be remembered that it adds significant cost and should be avoided if not necessary. Many features that are undercut can instead be created by ‘cross’ coring in the mould. This basically means opening a window in a face below the undercut feature making it possible to extend a local steel core and make the feature ‘Line of Draw’.
Rib Thickness Ratio – Where there is any feature such as a wall, support rib, screw-post etc beneath another face it will cause what is known as sink. This is where the local section of plastic formed by the two features meeting is too thick and causes the surface to ‘sink’ following the form of the feature beneath. Ideally the wall thickness of the feature below the main face should not exceed 50% of the main surface if sink is to be absolutely minimal and not more than 60% in any circumstance.