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Emilian Popa Injection Molding Design Guide Manual 2012

Under-Cut Handling

3- Undercut Handling

Undercut has nature in which ejection of a product in the direction of mold open/close cannot be possible. As

affects mold cost and product cost as well, undercut should be avoided at the stage of product evaluation. On

the contrary, tendency is toward increased undercut to satisfy needs to reduce number of parts and to simplifyjoints by utilizing resin elasticity.

Undercut mechanism as well as ejection mechanism is one of a few mechanical operations in the injection

mold. It is expected to make the mechanism reliable.

Here we will discuss various methods in handling undercut, and then typical methods, slide core and inclinedcore, will be taken for details.

3-1 Classification

There should be various ways to classify undercut. Generally accepted way is to classify undercut to two,namely whether the undercut locates outside of the product or inside, in other words whether the undercut is tobe handled from outside of the product or from inside.

Through hole on the outside of a product may be classified to either way. It should be appropriate to be

classified to outside undercut because a handling from outside must be easier.

1) Outside undercut (Fig. 3-1.1)

Outside undercut is easier than inside undercut because enough space may be made available. In the most

cases, slide core mechanism, which will be explained later, is used for the outside undercut handling. The

weakness must be that parting line of slide core and cavity comes on the outside of a product. This mayinfluence appearance quality of the product.

2) Inside undercut (Fig. 3-1.1)

Parting line is hidden in the inside of a product because undercut is located in the inside (core side). But the

design is difficult due to limited space. Various undercut handling methods are proposed, in which inclined

core (loose core) mechanism is widely accepted.

3-2 Undercut Handling Methods

There are many handling methods are proposed depending upon shape and size of undercut.

Typical methods are as follows.

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Power source of undercut handling is from mold open/close, ejection, outside force, manual, etc.as showing in (Table 3.2.1).

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3-3 Characteristics and Application of Various Undercut Handling Methods

1) Slide core

Slide core is the most widely used undercut handling mechanism. Normally it is applied for outside undercut,

but can be used for inside undercut depending upon the product requirement. This mechanism is to release

undercut by means of transfer of a core, which forms undercut, in the parallel direction to the parting line. Thename comes from sliding transfer of the core.

Driving force for the sliding comes from either mold opening force, which is converted to horizontal force

through angular pin, and outsourced force such as hydraulic cylinder (Fig. 3-3.1).

Generally, angular pin or angular cam is mostly applied for a mechanism to utilize a mold opening force. In th

case slide core is located on the movable side and the slide stroke is relatively small. If the slide stroke is big,

mold design will be difficult due to restraints from angular pin, length of the pin, mold thickness, etc.. Inaddition loss in molding cycle due to big opening stroke will be invited. In this case you should consider

application of outsourced device to allow big sliding stroke. You may apply hydraulic cylinder, pneumatic

cylinder, actuator to convert motor rotation to linear force through rack and pinion gear, etc..

If a sliding core is installed on the fixed side, outsourced application makes the mechanism simple and gives

more flexibility for various product shapes with higher reliability. When mold opens, product will separate

from the fixed side mold (cavity) and transfer with the movable side mold (core). Thus it is required to releaseundercut from the cavity before mold opens.

When the driving force is sourced outside, you should design an electrical inter locking device on the injectionmachine not to start mold opening or ejection process until slide core comes to the designated position.

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2) Inclined core (loose core)

Inclined core, which may be called loose core, is typical inside undercut handlingmechanism. This may be called inclined ejector pin because this has a function ofejector pin as the core to form undercut is installed on the ejector plate with designatedangle.

The undercut handling is done in such a way that the inclined core transfers to the

direction to release undercut incorporating to ejection stroke. Ejector plate installed onthe inclined core should slide smoothly on the ejector plate not to cause galling.

Traditional inclined core has a structure in which ejection point and guide area arelocated too far a part. Then the ejection force works as a bending moment on theinclined core. Thus traditional structure was considered as troublesome undercuthandling mechanism (Fig. 3-3.2).

But in these days, improved design is proposed, in which the bending moment on theinclined core is eliminated by installing a slide guide rod diagonally on the receivingplate and the movable side mounting plate. This improved type is available in the

market (Fig. 3- 3.3).

If you adopt above reliable mechanism, you can extend the inclined core application towide range. You will have a benefit to minimize cavity pitch for multi cavity applicationwith outside undercut because it does not require a wide operation space that is needeif slide core method is adopted.

3) Dogleg Cam

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Dogleg cam is applied for a mold for multiple cavity in which the space is limited. In thiscase undercut should be relatively small (Fig. 3-3.4).

If undercut area locates in the movable side, you may apply it on the both outside andinside. But in practice inside undercut is more in cases. Core to form undercut is in theshape of a dogleg. Similarly to the inclined core, the core will slide on the ejector plate i

the releasing direction from undercut area incorporating the movement of ejectionstroke.

Accordingly, galling may be incurred if the core does not slide smoothly on the ejectorplate similar to the inclined core.

Another weakness may be that the core hole edge is subject to abrasion because thedogleg cam hits on the hole edge every time when ejector plate returns to the originalposition.

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3) Elastic core (collapsible core)

The undercut core is made of an elastic steel which is inclined if there is no load. Thecore, which is reformed by other parts, tries to be back to original position at theejection process. Undercut is handled by this motion. There are two types. One iscylinder type to take care of inside undercut of round parts. Another is bar type to takecare of small undercut. They are available in the market under the name of CollapsibleCore and Spring Core respectively (Fig. 3-3.5).

Advantage is that molding cycle can be made fast and reliable as there is no operationmechanism like sliding. But you cannot apply it to a product with a big undercut becausundercut is handled only by the inclined core.

Collapsible core is often applied to a product, which has female threads inside like abottle cap. Spring core is used similarly to dogleg cam and inclined core (Fig. 3-3.6). Asit is more reliable than dogleg cam or the inclined core, consider the spring core first ifsituation is allowed.

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3-3-5) Removable core

This is used for a small lot production. This is not operated through mold mechanism buto remove the core, placed in undercut area, manually after the product is ejected withthe core in it (Fig. 3-3-5.1).

Accordingly the core cost can be low, but productivity should be low, too. This kind ofcore is seldom used in Japan where labor expense is high. It is used only for a prototypemold before production mold is produced.

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Removable core is set at the undercut area in the mold like insert. Normally tworemovable cores are prepared per one undercut so that one can be removed whileanother is under molding processes.

Basically similar consideration to insert molding should be paid for the design ofremovable core.

To evaluate the method of the core removal at the time of mold design.

To design the removal core that can be easily inserted in the mold with a foolprooshaping.

To design the removable core to be positioned exactly in the mold. It should notbe dislocated by the mold clamping motion.

To layout an ejector pin for the removable core.

To use light metal with high thermal conductivity. Aluminum alloy isrecommended.

3-3-6) Enforcing

This is to take the undercut out just by enforcing the product manually or by ejectordepending on the elasticity of the material. The quality of the product will be very muchinfluenced by the shape and the kind of material. Thus a thorough evaluation is essentiat the design stage (Fig. 3-3-6.1).

Following considerations are needed in the design when enforcing method is chosen.

Undercut should be within a size so that distorted product could be elasticallyregained to the designed shape.

Edge of the undercut area should be designed to have smooth R corners.

If undercut is enforced by an ejector pin or a stripper plate, the structure shouldbe made so that the product can be elastically deformed.

Material should have enough elasticity to be distorted and to be regained to thedesigned shape. Gene rally, crystalline non-reinforced resin such as PE, PP, PA,etc. is relatively applicable for the enforcing.

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3-4 Design of Slide Core Mechanism

3-4-1 Driving mechanisms

As explained, driving mechanism of slide core has two ways depending upon how tosource the driving force. One is to utilize mold open/close force and another is to sourcethe force from outside. Here we will discuss the design criteria of the driving mechanismof an angular pin, which is most commonly applied.

One angular pin per one slide core

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Emilian Popa Injection Molding Design Guide Manual 2012The function of an angular pin is only to activate sliding core. It has neither function forbearing injection pressure, nor for positioning. Thus positioning between angular pin ansliding core is not severe, and in the same principle, high rigidity is not required.

Rough positioning between angular pins and sliding core is associated with difficulty tokeep relative position accurately in machining. This tells you that if 2 angular pins areinstalled for a sliding core, one is in contact but another is not. This will give a moment

to sliding core, then result in galling on the sliding area.

Thus, one angular pin should be prepared for one slide core in the normal practice, but you need to install 2 angular pins for some reason, you need to machine relativepositions accurately (Fig. 3-4-1.1).

To keep angle less than 25

Angle of an angular pin is better be 10~25. In practice, 15 or 20 are often used. Ifthe angle is set beyond 25 in the case of large stroke, initial resistance due to moldseparation becomes too big to risk damage of the mold.(Fig. 3-4-1.2).

If you need a large stroke, you should consider outsourced actuator or installation of anangular cam, which can change the angle in the ejection process.

The angle of a locking block should be 2 plus pin angle in principle.

To assist mold opening by a spring.

It is advised to apply a spring to assist mold opening not only for a mold to have asliding core on the top but also on any position.

In principle the main function of the spring is to push the sliding core against the stoppein order to position the sliding core accurately, and the secondary function is to assistseparation of the product from the mold. The spring should not be too strong not toinduce unstable movement of the sliding core.

There are 2 ways for spring installation. One is to install it between sliding core and macore. Another is to install it on the end of the sliding core by way of a stripper bolt. Theformar is popular because of its compact layout. In this case it is advised to prepare aspring cover so that any foreign materials cannot be pinched by the spring (Fig. 3-4-1.2

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3-4-2 Guide mechanisms

Generally 2 guide rails are to guide a side face of the sliding core. These design criteriaare explained as follows.

4) To design guide longer than a slide core width

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Emilian Popa Injection Molding Design Guide Manual 2012The longer the guide is, the better the guiding stability will be. It must be ideal to havethe length 1.5 times of the slide core width. If it is not possible, maintain the length atleast more than the slide core width. If the length is less than the width, you will have aback lashing movement similar to old drawers. Naturally this will cause galling. Thetendency toward a galling is evident if you try to handle multiple undercut areas by awide sliding core (Fig. 3-4-1.2). In this case prepare a narrow guide, which is a guide ofparallel key shape, on the bottom face of the sliding core and let the guide rails on the

side work in the direction toward brim thickness only.

To apply hardened metal for the guide

In wear consideration, hardened metal or other kind of metal such as brass is used onthe sliding surface. This general principle should be applied to guide area of the slidingcore. Or one of sliding core and sliding guide should be made of hardened metal.Normally sliding core of small and medium size mold is made of hardened metal, butmold bases are not. They may be of pre-hardened steel as it is. In this case you areadvised to attach partially hardened wear plate on the sliding part of the mold base toimprove wear resistance. If partial load is not expected, wear plates made of brass or oless metal canalso be useful (Fig. 3-4.1.2).

3-4-3 Positioning mechanisms

Sliding core should be positioned precisely at the time of mold clamping and moldopening as well. Otherwise mold can be damaged. Positioning criteria of the sliding corewill be discussed as follows:

To use a stopper block in the mold opening direction

Occasionally a sliding core, installed horizontally from operation side to non-operationside, relies on the positioning of the last stroke by means of ball plunger only withoutspring and stopper block. Even for only horizontal direction, this type of positioning isvery unstable and unreliable.

As a spring of the ball plunger is not made strong, there is always a risk of dispositioninthrough vibration and moment inertia of the sliding core. This risk isparticularly evidentwhen toggle-clamping mechanism with a quick motion is applied to the injectionmachine.

The final positioning of the sliding core stroke in the direction of mold opening should be

set by a contact of the sliding core against a stopper block with a help of a spring force(Fig. 3-4-1.1). Thus a ball plunger should be used as a supplemental means for possibledamage of the spring.

To select the surface for positioning except touching surface in the mold clampingdirection

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Emilian Popa Injection Molding Design Guide Manual 2012Positioning toward mold opening can be done by a stopper block. Do not position theend of mold clamping against touching surface. Particularly if you use a small touchingsurface as a stopper for positioning, the surface is likely to have marks or to beconcaved due to concentrated force on the surface.

Positioning toward mold clamping direction should be made against a large surface sucas core side, which does not affect product quality. The properly selected area should

function as a stopper to withstand touching of the sliding core together with a lockingblock, and the touching face should be protected (Fig. 3-4-1.1).

3-4-4 Layouts

Slide core should be laid out horizontally in the direction of operation side to non-operation side, and avoid a vertical layout (Fig. 3-4-4.1). If you need to select avertical layout for a product to require 4 directions sliding or for other reasons, safetyconsideration for the sliding core not to fall by its weight should be made carefully.

A safety consideration may be to install a spring having 1.5~2.0 times strength of thsliding core weight and in addition to install a ball plunger under the sliding core incase of the spring failure.

Similar safety consideration had better be paid for any kind of layout of the slidingcore.

3-5 Design of An Inclined Core

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Emilian Popa Injection Molding Design Guide Manual 2012It was explained that traditional inclined core has a weakness in mold failure andparting failure because of a bending moment on the inclined core derived from toofar a distance between ejection point and guiding area.

It should be ideal to develop a new inclined core to eliminate such bending moment,but here we will discuss some idea for modification or design points based ontraditional design concept of the inclined core.

To prepare a guide on the support plate or on the core plate

A bending moment on the inclined core can be reduced if the guiding area of theinclined core comes closer to the ejection point.

Specifically, a quenched guiding plate can be installed on the support plate or thelower core plate surface (the other side of PL surface). In this way two areas, core ansupport plate (or core plate), will take care of guiding. This will contribute toimproved operational stability of the inclined core (Fig. 3-5.1).

To minimize friction on the inclined core slide

Inclined core will slide on the ejector plate under an ejecting force applied to themounting part on the ejector plate. If the sliding is very smooth, the ejection forcecan be converted effectively to a pushing force along the inclined direction not to abending moment on the inclined core.

You may apply non-lubricant type sliding plate available in the market. But

modification by having a quenched plate on the ejector plate and a cam follower or aneedle bearing to provide small friction resistance are more likely effective (Fig. 3-5.1).

To minimize product movement along with inclined core

Undercut handling of the sliding core is to release the undercut part by pushing itparallel to the plate being derived by the ejection force on the inclined core.

Accordingly the product should not move angularly with the inclined core. Followingpoints may be useful to cope with this problem.

* If possible, to prepare a draft angle on the undercut area to reduce releasingresistance.* To have an ejector pin to cut into the product as much as 0.1~0.3 mm.* To make the height of the inclined core lower than the main core as much as0.1~0.2mm.

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Emilian Popa Injection Molding Design Guide Manual 2012 To minimize the angle of the inclined core

In order to minimize a bending moment, minimize the angle.Imagine an ejection stroke without undercut consideration and then set an minimumangle to operate the undercut with the stroke.

It is advised to limit the angle to maximum 15. If it exceeds 15, you are advised toconsider a design with no bending moment. This new type of core is subject to patenissue. Thus you cannot manufacture it in house, but can purchase it in the market

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