Getting your plastic product to market in the shortest time possible, with the expected quality, leads to greater customer satisfaction. When all aspects of design for manufacturing and assembly have been considered, you will enjoy greater profits too.
If you start the process as early as possible, you can determine much of the final cost of a part at the design stage.
Why design for manufacturing matters
For injection moulded parts, the size and complexity of the design and its features drive the overall manufacturing cost. Large and complex moulds require more machining and resources than small and simple ones. If you want optimum results, the same process should be followed for products at both ends of that spectrum.
The benefits of design for manufacturing include lower production cost, higher quality parts and a reduced product development cycle.
What the DFM report covers
- Part geometry and features
- Wall thickness evaluation, including 'B' surface appraisal
- Draft angles, split lines and undercuts
- Gate position, determined via Autodesk Moldflow analysis
- Ejection mechanisms
Starting from an idea rather than a model
You do not need CAD to start. A sketch, a sample or a description of what the part has to do is enough, we will do the CAD. It is much easier to design a part for injection moulding from scratch than to fix one that has already been designed without the manufacturing process in mind.
Why the physics makes this complex
The concept of heating plastic until it becomes fluid, then forcing it into a much colder metal mould to cool and take shape, seems fairly simple. There are many pitfalls a product designer must avoid if components are to be reliably manufactured at the lowest cost.
Production cost is a combination of the weight of the plastic part and the production rate, or cycle time. Put simply, the faster the parts are made, the lower the manufacturing cost, so the designer should minimise polymer content and enable fast production.
Thermoplastic polymers shrink in volume as they cool from being molten. If that change were consistent throughout the geometry of the part, all that would be needed would be to make the mould larger by the percentage the polymer shrinks. The biggest issue is that the polymer shrinks more or less in different regions of the same component.
These differences may combine to prevent production of a perfect part:
- Pressure needed to fill the mould
- Orientation effects of the polymer flow
- Packing time and pressure
- The mould's heat removal efficiency
One solution is to extend the cycle time so the polymer cools more before ejection. At lower ejection temperatures the polymer is stiffer, so it can freeze in internal stresses that would otherwise distort the product, but this increases manufacturing cost and may lead to distortion or failure later as those residual stresses work to comply with the laws of physics.
Some brief design guides
- Before creating any geometry, learn the characteristics of the chosen polymer. Is it amorphous or semi-crystalline? Does it contain a filler such as talc or glass fibre?
- Keep wall thicknesses as even as possible, so the polymer cools at the same rate throughout the part.
- Feed the mould cavity on at least one axis of symmetry.
- If you want 'A' surface perfection, eliminate or minimise 'B' surface detail such as ribs and bosses.
- Use predictive technologies to evaluate mouldability early, and develop the optimum geometry through an iterative process of analysis and change.
- Accept the fact that physics determines all outcomes.