Designing for Two Shot

The process of designing a two shot part is extremely similar in many respects to that of a common injection moulded component. The same fundamentals apply, such as coring-out, wall thickness, draft etc. Here, I have provided some guidance, based on the designers experience level.

 Two Shot Moulding 01Introduction

Adding the 2nd shot element of a two shot part can be considered as a wrap or skin being added to the 1st shot (substrate). Try designing the 1st shot initially, then use the ‘offset surface’ feature (set to 0.00mm) across any surfaces which may need the 2nd shot applying to them.

Two Shot Moulding 02Next, use the ‘Thicken Surface’ feature to create the 2nd shot, making sure you do not ‘merge bodies’ and that the thicken feature is adding material to the correct side of the surface. You will now have a single part file containing two solid bodies. Two Shot Moulding 03From here, you might be confident to chop the two bodies around, resulting in a model you are happy with. If not and you have more complex requirements, the information below might help you progress.

Intermediate

This information should help those designers who have worked with injection moulding before and have a good idea of the general constraints and considerations that are associated with the process.

The 1st shot can be designed exactly like any common single shot moulded part and can include side actions, lifters and even more complicated tool functions such as collapsing or unscrewing cores.Two Shot Moulding 05In many respects, the 2nd shot requires more consideration and some issues may require you to revert back again to the 1st shot part design. For this reason, I believe it is a good idea to design both shots in unison. As described above, I always use a single part file, using two main bodies. This means you can use the existing geometry of both shots to influence the design, making the design process more streamlined than alternative methods (e.g. using an assembly and modelling each shot in unison). Depending on how you prefer to build the CAD model, it can be useful to start with a solid body and complete as much of the external geometry as possible, before using surfaces to split the bodies into two (1st and 2nd shots).Two Shot Moulding 07You can then concentrate on coring-out the 1st shot as you normally would for any other injection moulded part. As the core of the part will be common to both shots, it is very difficult to add 2nd shot material to the underside of the part, so try to keep all of the 2nd shot on the top side of the part.Two Shot Moulding 06An approach which I encourage designers to adopt, is to consider the 1st shot as part of the core (moving half) of the tool, when designing the 2nd shot. If you can image the core, with the 1st shot located in position prior to the 2nd shot being injected, this is exactly what is happening.

The 2nd shot will need to shut-off against the 1st shot. This is where many designers give up as it can seem complicated. A good thought process is to imagine how the 2nd shot cavity die work will be closed against the core (as well as the 1st shot moulding resting in position). As the steel is pressing hard against the core and 1st shot, the 2nd shot is injected under massive pressure, so if the shut-off (seal between steel faces and plastic) is not tight enough, the 2nd shot will escape across the 1st shot moulding, resulting in flash.

To ensure you have a good shut-off between the 1st and 2nd shots, you will need to think about how the steel is re-introduced into the 1st shot. Any side walls on the 1st shot where the 2nd shot shuts-off against will need plenty of draft angle (ideally 5˚ per side), to allow the steel to press hard against the 1st shot component. In an ideal scenario, the parting line between the 1st and 2nd shots will be perpendicular with the line of draw, so the steel faces can crush hard against the face of the 1st shot moulding, instead of scraping against a side wall profile. If this causes a problem for the designer, a slide may be used in the 2nd shot cavity to create the correct shut-off angle. Heavy sink caused by thick sections on the 1st shot can ruin the shut-off on the 2nd shot, so this is important to design-out where feasible.

Advanced

To get the most out of two shot moulding, the designer needs to be armed with as many of the complexities of the process as possible, to ensure the design is optimised. To put it simply, the best two shot designs are produced by the designers that take the time to understand, in depth, how the process works. For anyone starting a complex two shot design, the best step forwards will be to see a two shot tool in a press, running in the production environment. Seeing a tool which is split into the moving half (containing the 2 x identical cores) and the fixed half (containing the 2 x different cavities) is a great way to understand where the values and limitations of two shot mouldings lie.

A two shot injection mould tool has two identical cores and two different cavities per impression. It is the differing cavities that provide the form of the 1st shot (substrate) and the 2nd shot (overmould), while the cores are identical as they need to rotate from one cavity to the other. As the mould is closed, both of the cores align with each of the respective cavities.

After the 1st shot has been injected, the tool opens and instead of ejecting the part (as with conventional injection moulding), the moving half of the tool rotates 180˚ and is closed against the 2nd shot cavity. The 2nd shot is then injected over the top of the 1st shot which remains on the core.

Once the 2nd shot has been injected, the mould opens again and this time the part is ejected.

While the 2nd shot has been completed, the next 1st shot has also filled and is sitting on the opposite core, ready to be rotated and located back in position against the 2nd shot cavity.

For this reason, the two shot moulding process can be considered simultaneous. As a result, the price of finished components is not dissimilar to that of single shot parts from an equivalent tool with equal impressions.


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