September 2010 Design Tip



Pickouts for Interior Undercuts

We’ve written often about the way that Protomold handles undercuts which interfere with ejection from a straight pull mould. As long as undercuts are on the outside surface of the part, most of them can be formed by a side action that withdraws during mould opening to allow the part to be ejected. If, however, the undercut is on the inside surface of a core, mechanisms like lifts or collapsible cores are required but are currently unsupported in Protomold's process. Earlier this year, we started using pickouts to form such features.


A pickout is part of the mould during resin injection and cooling, but it is part of the moulded part during ejection, filling the undercut feature and providing an appropriately drafted surface to allow ejection from the mould. The pickout is then “picked out” of the part and then reinserted into the mould to form the next part.


A pickout is like a side action in two ways. First, neither is milled as part of the A- or B-side mould half. It is its own component of the mould. Second, like a side action, a pickout is removed from the finished part in a direction other than that in which the mould opens. In other ways, however, pickouts and side actions are very different.


First, using a cam, the side action that formed the undercut is withdrawn comlpetely from the part mechanically as the mould opens, and the part is then ejected from the mould. With pickouts, the order is reversed; the part is ejected with the pickout attached to the plastic part. Second, and perhaps more significantly, a cam operates automatically, while pickouts are removed from parts and reinserted into the mould manually.


For efficiency, a typical pickout operation will use a single mould but two copies of the pickout. Immediately after a part and pickout are ejected, the operator inserts the second copy of the pickout into the mould. Then, while that part is being moulded, the operator removes the pickout from the previously ejected part and is ready to repeat the process when the next part is ejected. Manual handling adds to the cost of moulding but may be justifiable if there is no other way to form the required feature.


There are alternatives to pickouts. For very shallow undercuts in parts made of flexible resin, a bump-off may be a better choice. However, if the undercut is too deep or the resin is too rigid to use a bump-off, then a pickout may be a better choice. The Protomold Torus sample part includes several features formed using pickouts (see Figure 1).

Figure 1: This section of the Protomold Torus shows an o-ring groove and mushroom stud. They are created using a two-piece pickout.

Figure 1: This section of the Protomold Torus shows an o-ring groove and mushroom stud. They are created using a two-piece pickout.


In the case of the Torus, the direction of mould opening would be vertical, so the three undercut features at the top of the part obviously need some additional treatment. Under some circumstances these features could have been formed, at least partially, using side actions depending on orientation, proximity to the parting line, and freedom from interference from other features. In this case, however, that wasn’t feasible, so we used pickouts. Figures 2 and 3 show the actual pickouts and mould half used to create the features in this part.

Figure 2: The two-piece pickout is manually removed from the part after ejection.

Figure 2: The two-piece pickout is manually removed from the part after ejection.

Figure 3: Highlighted areas hold the detachable pickouts, which are manually inserted into the mould prior to each injection cycle.

Figure 3: Highlighted areas hold the detachable pickouts, which are manually inserted into the mould prior to each injection cycle.


In Figure 3, you can see the straight, drafted surfaces on the A-side mould half that will allow the pickouts to fall free of the straight-pull mould when the mould is opened for the ejection cycle.


Of the thousands of parts submitted to Protomold, relatively few actually require pickouts. One example might be inward-pointing clips on a shell to engage mating slots (see Figure 4).

Figure 4: The protrusions at the pink highlighted corners are too large to be formed using bump-offs, so pickouts could be used here.

Figure 4: The protrusions at the pink highlighted corners are too large to be formed using bump-offs, so pickouts could be used here.


The following should be considered in determining whether your application is right for pickouts:


  1. Undercuts, in general, add to the complexity of moulding and should only be included in your design if absolutely necessary.
  2. Small undercuts in flexible resin may be suitable for bump-offs. These involve some extra work in mould milling but have no moving parts, so they are generally less expensive than side actions or pickouts.
  3. Because the moulding of pickouts is labor-intensive, if undercuts are on the outer surfaces of the part, side actions may be preferable, especially in longer production runs.
  4. Pickouts must exceed a minimum size, which is very dependent on geometry. A rough rule of thumb is 12mm square.
  5. Because plastic shrinks as it cools, a pickout may not be suitable if the cooled part will grip the pickout too tightly for easy removal. Features moulded by a pickout should be drafted.
  6. Finally, because the pickout must be manually removed, there can be no features in or in front of the pickout that would interfere with its removal (see Figure 5).
Figure 5: The circled feature would interfere with the manual removal of a pickout.

Figure 5: The circled feature would interfere with the manual removal of a pickout.


If you submit a 3D CAD model for a ProtoQuote and a pickout is required, that will be indicated in the quote and incorporated into the pricing. If you have any questions, as always, feel free to contact Proto Labs Customer Service Engineers at +44 (0) 1952 607447.


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