As buyers approach a plastic injection moulding service, they often wonder about unit price, tooling cost, and lead time. These factors are often overlooked but it is of utmost importance that they need to be prioritized. The simple reason being the numbers vary so much from one project to another.
In most cases, the answer comes down to a single design detail that is undercuts. Undercuts almost always require slides which fundamentally change how a mould reacts over its entire lifespan. For instance, mechanical and economic factors need to be considered if your part requires water transfer printing or a metal-plastic coating.
Now, let’s break it down for a better, in-depth understanding.
An undercut is an important feature that prevents a moulded part from being ejected straight out of the mould in the opening direction. Let’s say you pull an ice cube out of a tray. In case of straight walls, it’s an easy release, whereas a hook or ledge might not be a smooth way to pull the ice. Here, the hook is an undercut.
Slides aka side actions, are moving mould components that form undercut features and then retreat sideways before part ejection. Some common examples include side openings for connectors, snap-fit hooks used in assemblies, side-facing logos and internal retaining grooves.
Uses/applications of slides:
In major cases, slides are the solution when redesign is not possible. Slides act as a mechanical system inside your tool for extra motion, alignment requirements, wear points and maintenance.
Generally, slides increase tooling cost because they multiply complexity. In this scenario, the interactive decision point is whether the undercut delivers real functional value or just convenience in design.
What are the major cost drivers?
Slides shift every single cycle and every move and every shift creates friction, heat and wear. For example, a housing tool with four slides ran smoothly for the first 20,000 cycles but a similar tool without slides ran past 1 million cycles with minimal intervention.
Major areas of maintenance required:
1) Worn shut-off surfaces
2) Reduced sealing between the slide and the cavity
3) Flash forming at side features
4) Gradual dimensional drift
In case of vacuum metalising, finding guidance is necessary, which includes using consistent surface textures, avoiding slide shut-offs on visible faces and minimising undercuts in cosmetic areas.
While slides don’t necessarily add cost up front, they slow down the production over time. How? They need more time to fully extend and retract, an elongated mould opening sequence and the addition of safety delays.
| No. of slides | Time consumed |
| 0 | 20-22 seconds |
| 2-4 | 25-28 seconds |
Please note: Injection moulding machines are billed by time. There are some quality risks which are introduced by slides such as the following:
Moreover, processes like water transfer printing, plastic coating and foil printing magnify surface inconsistencies and have a multiplier effect on downstream finishing.
The best way to tackle undercut and slide situations is to question them. Below are some proven alternatives for the best results:
Pro tip: Focus should be on better functionality, not a specific geometry.
A plastic injection moulding service company should be able to think long-term. Some of the key features to keep in mind are comparing quality cost with benefits, explanation of maintenance and lifecycle impact, finishing and cosmetic requirements and identification of undercuts during DFM review. Rustagi Polymers is here to support every design, step and every detail with accuracy and precision.
Undercuts and slides add machining time, components, and complexity which often increases the overall tool cost by 30-60%.
In most cases, slides wear faster than static components which further increases maintenance frequency and reduces tool lifespan if not managed properly.
Some of the ideal design alternatives include reorientation of features, adding drafts, redesigning snaps and using inserts or splitting the parts.
While slides don’t increase cost directly, they increase cycle time which further raises machine-hour costs and per-part pricing over high volumes.
Major quality risks involved with slides are witness lines, flash, dimensional variation and cosmetic defects occurring after secondary finishing.
As buyers approach a plastic injection moulding service, they often wonder about unit price, tooling cost, and lead time. These factors are often overlooked but it is of utmost importance that they need to be prioritized. The simple reason being the numbers vary so much from one project to another. In most cases, the answer […]
