Dogleg cam is applied for a mold for multiple cavity in which the space is limited. In this case 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 and inside.
But in practice inside undercut is more in cases. Core to form undercut is in the shape of a dogleg. Similarly to the inclined core, the core will slide on the ejector plate in the releasing direction from undercut area incorporating the movement of ejection stroke.
Accordingly, galling may be incurred if the core does not slide smoothly on the ejector plate similar to the inclined core.
Another weakness may be that the core hole edge is subject to abrasion because the dogleg cam hits on the hole edge every time when ejector plate returns to the original position.
PITAC
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Fig. 3-3.2 Problem in Traditional Inclined Core Structure
Fig. 3-3.3 New Inclined Core Mechanisms (from catalog of タカオ設計事務所)
Fig. 3-3.4 Dogleg Cam Application
Inclined Core (Loose Core)
Guide Plate
Slide Base Guide Rod
(This if to cancel a bending moment on the inclined core.)
Holder Bushing (a) Inside undercut handling mechanism
(a) Inside undercut handling mechanism (b) Outside undercut handling mechanism
Dogleg Cam
Operational trouble will be incurred unless this part moves smoothly.
Edge of this part tends to be worn away.
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4) Elastic core (collapsible core)
The undercut core is made of an elastic steel which is inclined if there is no load. The core, which is reformed by other parts, tries to be back to original position at the ejection process.
Undercut is handled by this motion. There are two types. One is cylinder type to take care of inside undercut of round parts. Another is bar type to take care of small undercut. They are available in the market under the name of ‘Collapsible Core’ and ‘Spring Core’ respectively (Fig. 3-3.5).
Advantage is that molding cycle can be made fast and reliable as there is no operation mechanism like sliding. But you cannot apply it to a product with a big undercut because undercut is handled only by the inclined core.
Collapsible core is often applied to a product, which has female threads inside like a bottle cap. Spring core is used similarly to dogleg cam and inclined core (Fig. 3-3.6). As it is more reliable than dogleg cam or the inclined core, consider the spring core first if situation is allowed.
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Fig. 3-3.5 Elastic Core Application
Fig. 3-3.6 Various Undercut Handling Applications
Spring Core
Collapsible Core
Center Pin
Screw
Sleeve
(a) Spring Core (from日本金型産業catalog)
(b) Collapsible Core (from日本ディエムイーcatalog)
(a) Dogleg Cam (b) Inclined Core (c) Elastic Core
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3-3-5) Removable core
This is used for a small lot production. This is not operated through mold mechanism but to remove the core, placed in undercut area, manually after the product is ejected with the core in it (Fig. 3-3-5.1).
Accordingly the core cost can be low, but productivity should be low, too. This kind of core is seldom used in Japan where labor expense is high. It is used only for a prototype mold before production mold is produced.
Removable core is set at the undercut area in the mold like insert. Normally two removable cores are prepared per one undercut so that one can be removed while another is under molding processes.
Basically similar consideration to insert molding should be paid for the design of removable core.
① To evaluate the method of the core removal at the time of mold design.
② To design the removal core that can be easily inserted in the mold with a foolproof shaping.
③ To design the removable core to be positioned exactly in the mold. It should not be dislocated by the mold clamping motion.
④ To layout an ejector pin for the removable core.
⑤ To use light metal with high thermal conductivity. Aluminum alloy is recommended.
3-3-6) Enforcing
This is to take the undercut out just by enforcing the product manually or by ejector depending on the elasticity of the material. The quality of the product will be very much influenced by the shape and the kind of material. Thus a thorough evaluation is essential at the design stage (Fig. 3-3-6.1).
Following considerations are needed in the design when enforcing method is chosen.
① Undercut should be within a size so that distorted product could be elastically regained to the designed shape.
② Edge of the undercut area should be designed to have smooth R corners.
③ If undercut is enforced by an ejector pin or a stripper plate, the structure should be made so that the product can be elastically deformed.
④ Material should have enough elasticity to be distorted and to be regained to the designed shape. Gene rally, crystalline non-reinforced resin such as PE, PP, PA, etc. is relatively applicable for the enforcing.
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Fig. 3-3-5.1 Screw Forming by Removable Core
Fig. 3-3-6.1 Enforced Pullout of Tape Guide Roller
Removable
Ejector Pin
After product is taken out, apply a spanner to this part for removal.
Cavity Insert
Pull cavity insert out so that product can be elastically deformed.
Undercut Area
(a) Molding Completed (b) Before Enforcing (c) After 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 to source the driving force. One is to utilize mold open/close force and another is to source the force from outside. Here we will discuss the design criteria of the driving mechanism of an angular pin, which is most commonly applied.
① One angular pin per one slide core
The function of an angular pin is only to activate sliding core. It has neither function for bearing injection pressure, nor for positioning. Thus positioning between angular pin and sliding 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 to keep relative position accurately in machining. This tells you that if 2 angular pins are installed 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 if you need to install 2 angular pins for some reason, you need to machine relative positions accurately (Fig. 3-4-1.1).
② To keep angle less than 25°
Angle of an angular pin is better be 10°~25°. In practice, 15° or 20° are often used. If the angle is set beyond 25° in the case of large stroke, initial resistance due to mold separation 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 an angular cam, which can change the angle in the ejection process.
The angle of a locking block should be 2° plus pin angle in principle.
③ 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 a sliding 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 stopper in order to position the sliding core accurately, and the secondary function is to assist separation of the product from the mold. The spring should not be too strong not to induce unstable movement of the sliding core.
There are 2 ways for spring installation. One is to install it between sliding core and main core. Another is to install it on the end of the sliding core by way of a stripper bolt.
The formar is popular because of its compact layout. In this case it is advised to prepare a spring cover so that any foreign materials cannot be pinched by the spring (Fig. 3-4-1.2).
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Fig. 3-4-1.1 Proper / Improper Slide Core Design
Fig. 3-4-1.2 Points of Slide Core Design
Slide Core Quenched Plate
Slide Core Angular Pin Hole
(2places per slide core)
(a) Improper • l1 < l2
• 2 Angular Pins per slide core. (b) Proper • l1 < l2
• One Angular Pin per slide core.
Section A-A
Locking Block Slide Core
Extrusion-cut Surface
Stopper Block
Spring Cover Wear Plate
Ball Plunger
(Surface A plays a role of stopper, not extrusion surface.)
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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 criteria are explained as follows.
① To design guide longer than a slide core width
The longer the guide is, the better the guiding stability will be. It must be ideal to have the length 1.5 times of the slide core width. If it is not possible, maintain the length at least more than the slide core width. If the length is less than the width, you will have a back lashing movement similar to old drawers. Naturally this will cause galling. The tendency toward a galling is evident if you try to handle multiple undercut areas by a wide sliding core (Fig. 3-4-1.2). In this case prepare a narrow guide, which is a guide of parallel 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.
② To apply hardened metal for the guide
In wear consideration, hardened metal or other kind of metal such as brass is used on the sliding surface. This general principle should be applied to guide area of the sliding core.
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, but mold bases are not. They may be of pre-hardened steel as it is. In this case you are advised to attach partially hardened wear plate on the sliding part of the mold base to improve wear resistance. If partial load is not expected, wear plates made of brass or oil-less metal can also 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 mold opening as well. Otherwise mold can be damaged. Positioning criteria of the sliding core will be discussed as follows:
① To use a stopper block in the mold opening direction
Occasionally a sliding core, installed horizontally from operation side to non-operation side, relies on the positioning of the last stroke by means of ball plunger only without spring and stopper block. Even for only horizontal direction, this type of positioning is very unstable and unreliable.
As a spring of the ball plunger is not made strong, there is always a risk of
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positioning through vibration and moment inertia of the sliding core. This risk is particularly evident when toggle-clamping mechanism with a quick motion is applied to the injection machine.
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 possible damage of the spring.
② To select the surface for positioning except touching surface in the mold clamping direction
Positioning toward mold opening can be done by a stopper block. Do not position the end of mold clamping against touching surface. Particularly if you use a small touching surface as a stopper for positioning, the surface is likely to have marks or to be concaved due to concentrated force on the surface.
Positioning toward mold clamping direction should be made against a large surface such as 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 locking block, 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 a vertical layout for a product to require 4 directions sliding or for other reasons, safety consideration 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 the sliding core weight and in addition to install a ball plunger under the sliding core in case of the spring failure.
Similar safety consideration had better be paid for any kind of layout of the sliding core.
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Fig. 3-4-4.1 Layout of Slide Core
Platen of Injection Machine
Not preferable sliding direction.
Preferable sliding direction.
Mold
Top
Bottom
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3-5 Design of An Inclined Core
It was explained that traditional inclined core has a weakness in mold failure and parting failure because of a bending moment on the inclined core derived from too far 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 on traditional design concept of the inclined core.
① 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 the inclined core comes closer to the ejection point.
Specifically, a quenched guiding plate can be installed on the support plate or the lower core plate surface (the other side of PL surface). In this way two areas, core and support plate (or core plate), will take care of guiding. This will contribute to improved operational stability of the inclined core (Fig. 3-5.1).
② To minimize friction on the inclined core slide
Inclined core will slide on the ejector plate under an ejecting force applied to the mounting part on the ejector plate. If the sliding is very smooth, the ejection force can be converted effectively to a pushing force along the inclined direction not to a bending 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 a needle bearing to provide small friction resistance are more likely effective (Fig. 3-5.1).
③ To minimize product movement along with inclined core
Undercut handling of the sliding core is to release the undercut part by pushing it parallel to the plate being derived by the ejection force on the inclined core.
Accordingly the product should not move angularly with the inclined core. Following points may be useful to cope with this problem.
* If possible, to prepare a draft angle on the undercut area to reduce releasing resistance.
* 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 as 0.1~0.2mm.
④ 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 minimum angle to operate the undercut with the stroke.
It is advised to limit the angle to maximum 15°. If it exceeds 15°, you are advised to consider a design with no bending moment. This new type of core is subject to patent issue.
Thus you cannot manufacture it in house, but can purchase it in the market.
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Fig. 3-5.1 Design Points for Traditional Inclined Core
Draft Angle
Detail A Receiving plate
Guide Plate Ejection
Pin
Needle Bearing
Cam Follower
Wear plate
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