Design rules for injection molding prototypes

In order to produce injection molded prototypes within a few working days, we make an injection molding tool and then fill it with the specified thermoplastic, silicone or powder material. For this purpose, some design specifications have to be observed. On the one hand, these serve the manufacturability, but on the other hand they also ensure an optimal result.

Maximum size of the injection molded parts

Every manufacturing process has its limits when it comes to the size of the components to be produced. Prototype tools produced additively are generally only suitable for small components. There are three main reasons for this:

• Tool life: The larger the component, the shorter the tool life. Even if Injex can produce replacement tools quickly and cheaply, the process is not cost-effective if a new tool is needed for each component.
• Costs: The costs of additive manufactured tools increase with their volume. That means, if a component is scaled in all dimensions by factor 2, the volume of the tool increases by factor 8. This is directly transferred to the costs.
• Tolerances: We manufacture very hard and mechanically resilient tools. Nevertheless, they are not as stiff as steel molds. Therefore larger surfaces lead to more deformation and tolerances are more difficult to maintain.

Conclusion: The larger the component, the more expensive the tool and the more expensive the components. At the same time precision is lost.

Therefore in order to keep costs practical, small components are the most worthwhile. The maximum shot weight depends on the processing temperature and the selected material:

Demolding angles (Draft Angles)

Demolding angles (Also known as draft angles) help to make it easier to remove the finished injection molded part from the mold. These angles can vary depending on the material and application. However, the basic rule at Injex is that there should be at least 1° angle of demolding.

For special applications, such as gears, the demolding angle can be omitted. If this is the case, it is essential that this is noted in the inquiry and/or on the technical drawing.

Wall thicknesses

The injection molding process is very well suited for the production of thin-walled components. Wall thicknesses of up to one millimeter (1mm) rarely pose a problem. If thinner walls are required, the flowability of the material must be taken into account. The length of the flow path that the material must cover from the gate to the furthest corners of the component is also a decisive factor.

Many materials have a very high flowability or special modifications are available to increase the flowability. With the correct design of the injection molded part and the right choice of material, wall thicknesses of up to 0.5 mm and less are possible.

Welding lines (also known as knit lines) tend to occur when flow is incorrect and sometimes when wall thicknesses are too low. We can work with you to find the best way to avoid this and also optimize part flow thereby maximizing part strength.

Tolerances

Basically we work according to ISO-2768-m. Individual functional dimensions can also be manufactured to finer tolerances by prior arrangement.

However if you prefer a cheaper method than strictly adhering to tolerances, for example, fit to a counterpart is required, the counterpart can be provided and we can test the fit directly on site and make changes to tooling if necessary.

Undercuts

Undercuts can be easily reproduced with our flexible tool construction. In terms of costs, however, it always makes sense to avoid them if possible. If desired, we can offer assistance in the design of such components and provide inputs to avoid undercuts.

For complex geometries, however, undercuts are often unavoidable. For these cases we have a whole arsenal of tool components in stock. Not only sliders can be used - but more complex tools such as core pullers or folding cores can also be developed with ease and precision.