Foreword:
When designing plastic molds, when we take over a product for a new project, we first simply analyze product size, number of cavities, pouring method, weight, material, distinguish between front and rear molds, undercut positions, whether there is draft, identify problem points, etc.
Today I would like to talk about case of lifter detaching from undercut that I used during mold design.

1. Lifter composition:
Lifter usually consists of an lifter head, an lifter rod, a lifter base and a guide block.

 

2. Movement process:
Ejector plate pushes product upwards and moves in lifter seat, generating two forces (ejector plate ejection force and force in direction of breaking away from undercut), which is a trigonometric function relationship.

 

3. Lifter angle:
Lifter is usually designed with an angle of 3-8°, and maximum is best not to exceed 8°. Because the larger angle, the greater torque generated, and lifter is easy to break, which does not meet needs of long-term production.
4. Disengagement distance:
Product should be extended based on undercut distance. Minimum should not be less than 0.5mm. If distance is too small, considering shrinkage rate of product and movement of product at ejection position, product will hang on lifter, resulting in product not being taken out; if distance is too large, ejection stroke will inevitably be increased, which is detrimental to mold manufacturing cost.
5. Consider direction:
① When designing lifter, you must consider fitter. Driven mold core passes through B plate and guide block to match mold. It is commonly known as: pulling lifter.

 

② Also consider whether it is easy to assemble and grind, and whether it is easy to process.
③ Whether mold will stick when used on a lifter, product cannot be taken out, or bright marks will appear on ejection, etc.
④Height of top surface of lifter is usually 0.02mm lower than mold core to prevent product from being shoveled during ejection.
⑤ A small flat surface is usually left under lifter to facilitate processing and repair, and it is also equivalent to a stop. In addition, it can play the role of sealing glue.

 

1. Smooth slope
This is the most commonly used lifter, which means that movement directions of glue surface and lifter are parallel.
2. Uphill lifter
When movement directions of glue surface and lifter are not parallel, upward angle is an uphill lifter.
3. Downhill lifter
When movement direction of glue surface and lifter is not parallel, downward angle is downhill lifter. Note: Only when downhill lifter is ejected, it is separated from product.

 

Applicable angle range is 3-12°. When angle of lifter exceeds 8°, lifter is easy to break. An additional auxiliary rod is required to increase strength of lifter. However, this structure occupies a very large position on mold base and is often used on large products such as automobile molds. In addition, export mold is used more often. Advantages: This structure is strong and durable, and service life of mold will also be extended.

 

This structure can realize products with buckles on the surface. On the basis of lifter of rear mold, it is necessary to consider what kind of mechanism is used to eject and retract lifter.
The most commonly used method is to use a spring to eject lifter, and to press back front mold back pin when mold is closed. It should be noted that height of return needle is greater than the ejection height of lifter. Is there a mold closing sequence requirement? If lifter is used incorrectly, angled ejector is pressed back through movable mold core.

 

Ejector pin is hung with an lifter. This method is often used on small batch test molds. Lifter is processed with a dovetail groove, and lifter moves on ejector pin during ejection. Advantages are simple processing and low cost. Of course, if you only make dozens of molds of products, you can cancel dovetail groove, push ejector pin to eject slanted ejector, and directly eject slanted ejector. It is also possible to take out product by hand.

 

This structure is composed of an inclined head, a guide sleeve, a straight gear rod, a helical gear rod, a fixed seat, and an extension rod.
It is suitable for large-angle lifters of 8~30°, both smooth and inclined sliding (uphill and downhill) can be achieved. It adopts method of gear matching, which is strong and durable and can realize mass production. Ejection stroke can be shortened to avoid short ejection distance of molding machine. And ejected product will not damage glue surface.

 

Structure I designed is that white helical gear rod and oblique head are inserted from mold core, and purple straight gear rod is inserted from B plate.
Fixed base is also locked from front of plate B of mold base, and then mold core is locked.
It should be noted that guide sleeve and fixed seat must be processed accurately. It is necessary to consider whether slow wire processing at a large angle can be processed accurately, otherwise structure cannot be installed. Newly ordered double-tooth lifter has a lot of margin and needs to be chamfered to penetrate mold base and mold core.

 

It should be noted that purple rod is in vertical direction, which is equal to ejection stroke of ejector plate. Mold core needs to be opened or thickness of mold core needs to be thickened, otherwise structure cannot be used.