Undercuts in plastic injection molding

When there are through holes, recesses and bosses, etc. on the side wall of a plastic product which make it impossible to release the mold, the molded product must be moved laterally.we call them undercuts. The entire mechanism that drives the lateral movement of the molded product is referred to as slider or lifter. There are many types of slider, but the most commonly used one is core slider.

According to the deployment of the guide pin and the slide in the mold core, the structure also comes with various forms.

Slide on core side

undercuts released by core slider

Slider on cavity side

undercuts release by cav sliders

Inner slider

The side core pulling mechanisms, such as the guide pin are only applicable to plastic products with short undercuts. When the undercuts is greater than 80 mm, the Gear driven mechanism is usually employed. In addition, hydraulic driven and spring driven are also very commonly applied.

undercuts released by gear driven sliders

Hydraulic driven sliders are more expensive, so its application should be minimized, so as to prevent it from affecting the pace and mold layout

long undercuts released by hydraulic slider

Spring driven Mechanism

undercuts released by spring slider

With the oblique movement of the lifter, the undercut is released during the product ejection process.

As shown in the figure as below ,the lifter is placed in the angled hole of a fixed mold plate.Give the lifter a push from the bottom up, which moves the lifter up for a certain distance to release the undercts. Now, the lifter not only moves upwards under the forcing of the angled hole and the push, but also moves a certain distance in the direction that the lifter inclines

Some way to remove undercuts in plastic injection mold

When the customer accept a design change,we have easy way to release the undercuts,For example:we may need a slider or lifter to release the hook as picture show:

but if we make a cut on top,then we can easily remove the undercuts,and we just need a insert.it will save a lot of time and money.


Please feel free to contact us if you are looking for a professional mold making company.

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