Undercuts and side actions: cost impact

What counts as an undercut

An undercut is a feature that traps the part on the tool. A snap hook on the outside of a wall, a window with a lip, a side hole, a thread, a barb on a terminal pocket: each of these blocks the straight pull that opens the mould. The part cannot leave the cavity or the core without the steel moving out of the way or the plastic bending.

Designers often add an undercut for a function that matters, such as a latch that must hold a cover or a clip that must retain a wire. The problem is not the function, it is that the feature has been drawn without deciding how it will be released. That decision changes the tool, the cycle and the price, so it belongs in the design review rather than in the toolmaker's quote.

The first check is simple: look at the part in the direction of opening and ask whether any material sits behind another surface. If it does, the feature is an undercut, even if it looks small on the drawing.

The four ways to mould an undercut

There is no single answer. The right method depends on how much the feature must move, how often the tool will run, and how visible the release marks are allowed to be.

Stripping, also called bump-off, uses the flexibility of the plastic. The part is pushed over a steel feature and the material deflects enough to pass it. It is the cheapest method because the tool has no moving side member, but it only works for small undercuts in a ductile material, and it leaves a small witness mark.

A lifter is an angled pin or block that moves with the ejector system and slides sideways as the part is pushed off. It handles an undercut on an internal face, such as a clip inside a housing, without a separate slide. Lifters need space below the part and they wear at the sliding faces.

A side action, or slider, is a separate tool member that travels horizontally as the tool opens, driven by a cam, a hydraulic cylinder or an angled pin. It is the most capable method and the most expensive. A split cavity, where the mould opens in two directions, is a variant used for parts with undercuts on several faces.

The fourth option is redesign. Moving the parting line, adding a window, splitting the part into two mouldings or changing the latch to a separate clip can remove the undercut entirely, and it is usually cheaper than any moving steel.

What a side action costs

A side action adds four cost lines. The first is tooling: the slider, its guides, the cam or cylinder, the wear plates and the extra steel removal. The second is cycle time: the slide has to travel and return within the mould open time, and on a hydraulic slide the movement is part of every cycle. The third is maintenance: slides wear, their shut-offs flash and their guides need adjustment. The fourth is quality: the shut-off line leaves a mark, and the position of that mark is decided by the slide, not by the designer.

The relative cost depends on the part. On a small, simple tool, one slider can add a large share of the tool price because the slider is a fixed cost against a small base. On a large tool with many features, the same slider is a smaller share. That is why a supplier needs the geometry, the cavitation and the volume before quoting.

Design changes that remove the undercut

The most useful question in a DFM review is whether the undercut can become a window. A window is an opening through the wall, formed by a shut-off, with no trapped material. A clip that hooks over an edge can often be replaced by a tab that passes through a window and is secured by a separate part or a screw.

The second question is whether the parting line can move. A feature that is an undercut on the side of a part may be open if the tool splits along a different plane. This is often free at the design stage and impossible after the tool exists, because the parting line decides the whole tool layout.

The third question is whether the part can be split. Two mouldings that snap or weld together can replace one part with an undercut, especially on a housing where the seam falls on a natural line. The trade-off is another tool, another part number and an assembly step, which may still be cheaper than a slider on a high cavitation tool.

The fourth is whether the feature can be a separate insert. A metal clip, a stamped spring or a moulded plug can be assembled into a simple pocket, removing the undercut from the moulded part altogether.

• Convert the hook into a tab through a window.
• Move the parting line so the feature opens.
• Split the part and join it with a snap or a weld.
• Use a separate metal or moulded insert.
• Reduce the undercut depth so it can be stripped.

When a side action is the right answer

Sometimes the function cannot move. A connector housing with a retention barb, a breaker pocket with a return flange or a sealed housing with a continuous groove may all need a slide. In those cases the question is not whether to use one, but where to put the shut-off and how to keep the wear manageable.

Volume changes the calculation. A slider on a low volume tool costs money once and is then forgotten. The same slider on a tool running millions of shots becomes a maintenance item with a defined service interval, spare wear plates and a repair procedure. It also becomes a source of variation, because a worn slide changes the shut-off position and the part dimension with it.

Material matters too. A glass filled grade wears steel faster and is less tolerant of stripping, so a feature that could be bumped off in unfilled nylon may need a slider in a glass filled grade of the same polymer. The release method and the material should be chosen together.

What a side action does to tolerance

Dimensions formed by a slide carry the clearance of the slide in addition to the normal moulding variation. The slide has to move freely, so it cannot be a tight fit, and that clearance appears as a position variation on the feature it forms. Do not call a slide formed dimension as tight as a dimension formed in solid steel.

The shut-off line is also a witness line. If the mark falls on a sealing face or a visible surface, the design needs to move it. Sealing faces are the harder case, because a step at the shut-off can leak even when the part dimension is in tolerance.

For inspection, agree which dimensions are formed by moving steel and treat them as a separate group. Check them on the first article, then monitor them for drift as the tool wears. A dimension that moves with slide wear will not be held by adjusting the injection process.

Getting the cost into the quote, not the invoice

Ask for an undercut list in the DFM response. Each entry should name the feature, the release method proposed, whether it needs a slide, lifter or strip, and the effect on tool cost and cycle time. A supplier that quotes a price without this list has either missed the undercut or priced it without telling you.

Then ask what happens to the price if the undercut is removed. That single question often pays for the review, because it shows the cost of the feature in the tool and in the part. It also gives the designer a real trade-off to take to the product team.

Finally, agree the maintenance expectation for any slide. A tool with two slides needs a service plan, spare wear parts and an agreed definition of what counts as worn. That is cheaper to settle at order than during a production stop.

1. 01

   List the undercuts

   Mark every feature that blocks the opening direction, including small barbs and threads.

2. 02

   Test the alternatives

   Window, parting line, split part, insert, strip. Cost each one before choosing a slide.

3. 03

   Price the release

   Get the tool, cycle and maintenance effect in writing, with the shut-off position.

4. 04

   Plan the upkeep

   Agree wear parts, service intervals and how slide formed dimensions will be monitored.