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Emilian Popa Injection Molding Design Guide Manual 2012

3 Plate Mold

4- 3 plate structure

When a gate is designed on the upper surface of the product like pin point gate, a

runner stripper plate is required for runner separation in addition to 2 plate mold. This iscalled 3 plate mold.

3 plate mold has one more plate on the 2 plate mold, but the structure and theoperation is quite different from the 2 plate mold. The big difference is in open/close ofthe plate. 3 plate mold requires a big open/close stroke to open runner parting (PL2) fortaking runner out in addition to open/close of main parting line (PL1) for taking productout. Therefore the mold cycle is slower than 2 plate molding. Another disadvantage incomparison with 2 plate mold is the higher cost due to increased parts required for itscomplex structure. However 3 plate mold is popular as it has advantage over 2 platemold in molding comparatively largeproducts maintaining a good balance in the shape.

Open/close of PL surface of 3 plate mold is worked by transferring a force of movableside mold, initiated by open/close of the injection machine, to fixed side through moldmechanism. Concern must be on the sequence of PL surface opening.

Factors to determine sequence of the opening are separation resistance of product andrunner, and operation resistance from runner stripper plate. In the normal mold, eithermain PL surface or runner PL surface opens first and runner stripper plate opens next.Fig. 4.1 shows the relationship.

There is a mechanism to lock PL surface (PL1) and let the runner PL surface (PL2) openfirst. This is applicable for a thin product of which releasing resistance is small on thecore side. This enables the product to stay on the core surface. (Fig. 4.2)

Basic structure of the movable side is the same. Thus various types of the moldexplained for 2 plate mold are applicable to the case of 3 plate mold. In this way 3 platemold can cope with the requirement of various shapes of the products.

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4-1 Support pin

Support pin is a unique part in the 3-plate mold. It is to guide fixed side mold plate andrunner stripper plate in the process of the mold opening. Therefore both support pin andguide pin/bushing are used for standard mold base for 3-plate mold. Both share afunction of guiding fixed and movable plate. But this structure limits a space for pullerbolts and other components in the mold. Thus there is other type of standard mold baseto enable support pin to perform double purpose by adding guiding function of movablemold plate (Fig. 4-1.1).

This double purpose structure provides more space for other components but be awarethat support pin collar cannot be used here. The support pin collar is a safety device tostop the fixed mold plate from coming off from the support pin when a head of the stopbolt happens to be broken. Therefore if the double purpose structure is adopted, youshould either consider other method for safety consideration or use stop bolts and pullebolts of higher strength to ensure high reliability.

Be advised that the double purpose structure is inferior to the functionally shared

structure in terms of the positioning performance of the mold plate because there arepossibility of pin deflection and clearance problem with guide bushing as the support pifixed on the fixed side clamping plate has to guide the movable side mold plate.Considering all above, following design criteria can be presented for designing guide ansupport pin structure of 3 plate mold.

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Emilian Popa Injection Molding Design Guide Manual 2012 In the case of a mold with movable side guide pin structure, like stripper plate

ejector, the basic structure should be that support pin and guide pin/bushing arelaid out separately.

In the case of a mold with a thick fixed side mold plate or with a large openingstroke in PL2, select a basic structure to assure better performance for guidingmold plate because support pin deflects and positioning performance should drop

In the case of double purpose mold that requires accurate positioning of themovable and fixed side mold plate, increase positioning accuracy of the cavity anthe core by applying a taper pin on the PL surface.

4-2 Control parts for the extent of opening.

4-2-1 Stop bolt

A stop bolt is a part to limit the stroke of runner stripper plate. As Fig. 4-2-1.1 shows,various types of stop bolts are made available.

Type a in Fig. 4-2-1.1 is the most popular kind of stop bolt and it can be located in thesame location as puller bolt. Thus it is good in space utilization and in addition it savesmachining on various plates.

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Emilian Popa Injection Molding Design Guide Manual 2012Other types are more or less the same shape as puller bolt, but a puller bolt is too longfor the stop bolt. It is rather recommended to find a stopper bolt among stripper boltswhich have better possibility to be used for a stop bolt and are available as standardpress die parts.

4-2-2 Puller bolt

A puller bolt is commonly used as a control part to control opening extent betweenrunner stripper plate and cavity plate, in other wards opening extent of runner partingline (PL2). Also it is used for controlling opening extent of main parting line (PL1). Thismechanism is preferable when space is widely available for a mold where a support pinhas a dual function to guide movable side mold plate. One thing you should not forget ito pay enough consideration on the workability and interference with automated take-out device in the scope of product take-out.

Puller bolt has two types. One is male screw and another is female screw type. Femalescrew type is widely used because it can be used as a pair with stop bolt. Even it is usedindependently, female screw type has better reliability for repeated load applicationbecause it can be fastened from runner stripper plate side (Fig. 4-2-2.1).

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4-2-3 Tension link

Tension link is installed on the plate side of the mold to control the limit of moldopening. It is used for controlling opening of the main parting line (PL1) (Fig. 4-2-3.1). Aexplained, puller bolt is used for controlling opening of runner parting line (PL2), buttension link can be used for PL2 if layout space is not given enough to apply puller bolt.

Like above example, tension link gives more flexibility in layouting parts for mold

opening control, but close attention should be paid not to interfere with couplersmounted on the plate side face for temperature control. Also attention should be paid othe workability of product take-out, particularly of manual take-out, because tension linis installed on the side face of the mold plate. Thus the layout of the tension link shouldbe carefully made in consideration of take-out method of product and runner as well.

4-2-4 Chain

Tension link and puller bolt are normally installed in the range of mold thickness, butonce in a while there is a case in that the range of mold thickness is not wide enough fotheir installation when the extent of the opening is large relative to the mold thickness.In this case chain can be effectively applied (Fig. 4-2-4.1).

As it is flexible, the chain can be used for most of the cases. When the mold is closed,the chain can be folded and stored within the mold plate thickness. However a chainmay hang down when the mold is closed and may interfare with couplers fortemperature control device. Thus a chain should be only used for special cases as in thecase explained above.

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4-3 Parts for opening sequence control

4-3-1 Parting lock

Parting lock is to control mold-opening sequence providing operational resistance in theopening mechanism through various methods. Generally it is used to lock main partingline (PL1) and let the opening of runner parting line (PL2) operate in advance.

For example, in the case of shallow product, which has not, much difference in thereleasing resistance of fixed side and movable side, the product tends to remain on thefixed side due to gate cutting resistance. In such case it is effective if PL1 is locked by aparting lock by eliminating a chance of the product to remain on the fixed side. In award, the parting lock stabilizes releasing balance of the fixed and movable side.

Types

Creation of operational resistance can be made by mechanical system, friction,magnetic force, etc.. Parting locks associated with various methods are classified asfollows.

Features and application

Mechanical Lock

Mechanical lock is installed on the plate side face of the mold and controls the sequencof the mold opening. There are various types of mechanical locks available in themarket.

There is a common mechanism. A portion is kept closed by means of combination oflatches and springs until a certain stroke is achieved by another parting device (Fig. 4-31.1) example a locking device on the PL1 can be set not to release PL1 to open until PL2

opens to a certain position.

As you see, this kind of mechanical locking device can eliminate mold parts, such as atension link to control mold opening of PL1. The most important thing is the propertiming of the locking release. Parting line opens in the sequence of PL2 PL3 PL1.

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Emilian Popa Injection Molding Design Guide Manual 2012Release bar of the mechanical locking device should be adjusted to release the latchsomehow earlier than the opening extent set by stop bolt or puller bolt.

If the release if too early, it will result in insufficient opening of PL3 (PL between fixedside clamping plate and runner stripper plate). If the release is too late, it will result inthe damage of stop bolt or puller bolt. Mechanical locking device can be said the mostreliable parting locking device though it is costly.

Spring Locking Device

Spring locking device is to control sequence of the mold opening being installed on theplate side similar to mechanical locking device. It has a structure in which a plate spring

or a roller falls into the undercut portion to resist mold opening, driven by the plate ordish spring force (Fig. 4-3-1.2).

Traditionally the plate spring has been commonly used but adjustment of the springforce could not be made. In these days a roller to enable both locking and adjustment ba dish spring with a screw is made available in the market. This type is available in themarket and the reliability has much improved.

Spring locking device cannot control a stroke of the mold opening but is easier to behandled comparing to mechanical locking device. In the durability aspect, it is muchbetter than plastic locking device to follow.

Plastic locking device

Plastic locking device has a structure to lock the parting line by the friction force of aextruded part of a plastic cylinder pressed in a hole on the mold plate by the moldclamping force. This plastic cylinder is installed on the PL surface of the fixed or movablside mold plate.

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The center of the extruded part of the plastic cylinder is made by a tapered bolt. Thusthe outside diameter of the extruded part can be changed by screwing in the taperedbolt. Thus the friction force can be adjusted. (Fig. 4-3-1.3)

This parting lock device is less costly and easy for installation but due to the stressrelaxation of plastic material tapered bolt it tends to be loosened comparing to other

options. Thus evaluate how to stop loosening of the bolt and be ready for spares for thereplacement whenever needed.

Magnet locking device

Magnet locking device has a structure in which a permanent magnet is installed on theside face of the mold plate and is to pull steel block by magnet force (Fig. 4-3-1.4). Thisdevice has no better reliability comparing to mechanical locking device and noadjustment mechanism comparing to spring locking device and plastic locking device.But it can contribute to an efficient molding cycle because no mold clamping force iswasted, and you need not slow down the speed of mold closing. In addition the devicecan be made to standard parts because it can be easily installed and removed to andfrom the mold plate.

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4-3-2 Runner lock pin

Basic functions of the runner lock pin are as follows.

Functions

To cut off a gate

Main function of the runner lock pin for 3 plate mold is to create a cutting force to cutoff a gate along with the mold opening. Undercut part at the tip of the runner lock pin isto hold the runner on the runner stripper plate against an integral force of partingresistance of the runner and the 2nd sprue and cutting resistance of the gate. Due tothisholding force, the gate is cutted off and at the same time the runner and the 2nd spruecan

be taken out from the fixed side mold plate. Accordingly the runner can be taken out.

Mold opening sequence control

The secondary function of the runner lock pin is to control sequence of the moldopening. Opening of PL3 is for pushing off the runner. Thus PL3 should open after PL2is opened. As undercut part of the runner lock pin locks PL3, the desired sequence can

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Emilian Popa Injection Molding Design Guide Manual 2012be maintained.

Design considerations

Design considerations to assure above functions are explained as follows.

Shape and amount of undercut

Shape and amount of undercut at the tip of the runner lock pin should be appropriate tosuit resin and runner size. If the amount is too small, the gate cannot be fully cut off andthe runner may stay on the fixed side mold plate. If the amount is too big or corner edgportion is still remained, undercut portion will stay in the ring form and the gate cuttingforce cannot be created for the next injection (Fig. 4-3-2.1).

Layout

Runner lock pin cannot be located at any place on the runner. It should be located at thsame position of the gate or near the gate in order to effectively cut off the gate. If it islocated far from the gate, the gate may not be cut off due to bending deflection of therunner when mold opens. (Fig. 4-3-2.2)

Structure and layout in terms of resin flow resistance

When a runner lock pin is laid out on the gate following the principle 2 above, undercutportion of the runner lock pin may be subject to flow resistance. In such a case, a resinpool on the runner stripper plate can be prepared. Do not make it too deep. If too deep,injection cycle becomes long (Fig. 4-3-2.2). Or you may shift the position of the runner

lock pin 2~3mm against resin flow direction.

Galling

When there are many runner lock pins, you may face galling trouble. In order to waivegalling, the best solution is to match the taper of the runner lock pin with that of therunner stripper plate. But taper fitting is a difficult work. For easier solution it is advisedto use heat-treated bushings on the runner stripper plate (Fig. 4-3-2.3).

To protect from galling, another consideration is to give a certain flexibility for fixingmethod of the runner lock pin. For example, in the case that the runner lock pin is fixed

by a screw bolt from the brim side, it is suggested to apply a collar, which is a bit thickethan the brim, and fasten the collar so that the brim can be designed to have a certaindimensional freedom on the both radial and axial directions (Fig. 4-3-2.3).

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4-3-3 Push pin

Push pin is a supplementary part for PL2 and PL3 opening. As long as runner lock pinoperates satisfactorily, it will perform supplementary function for gate cut-off and

releasing of sprue and runner. Particularly for a mold to apply parting lock device, PL2should open before PL1. Push pin enables sure opening sequence and gate cut-off aswell.

The similar function can be attained by installing a spring between the runner stripperplate and the fixed side mold plate being guided by a puller bolt. Generally thismechanism is used more frequently but this does not have a function to support openinof PL3. (Fig. 4-3- 3.1)

4-3-4 Runner ejector

Runner ejector is an ejector installed on the runner stripper plate. This is to assure arunner to drop by a spring force after the runner separates from the sprue bushing.Therefore a runner ejector is effective when the product is taken out by the gravityforce. If automated device is used for product take-out, this device is better not beinstalled.

The runner ejector is sold in the market as a set including runner ejector pin, spring andhousing. This can be easily installed if the specification meets the requirement (Fig. 4-34.1).

When it is applied, the runner ejector should not use a pin smaller than runner width. A

part of the pin should be in contact with PL2 surface of the fixed side mold plate,otherwise the pin will be inside of the groove of the runner and may be in touch with thsurface of the draft angle or the bottom of the runner. In either case, function of ejectorcannot be worked. (Fig. 4-3-4.2)

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4-4 Pinpoint gate

Pinpoint gate may be called pin gate. This is of 3 plate mold structure and the gate is cu

off automatically. Because of 3 plate structure, the manufacturing cost is higher thansubmarine gate, the same automated cut-off gate, but made of 2 plate.

Advantage is that runner layout is flexible, thus this gate can be applied to smallproducts and to big products as well. Disadvantage is that this gate does not suit to aresin of poor flowability such as acryl resin because the gate is small and nor to a resinto include fiberglass, which erodes the gate quickly.

Automated cut-off in the pinpoint gate works like a pulling action generated by the moldopening force. Therefore the cut length varies. To cope with this problem gate areasometimes is designed to be 0.3~0.5mm in concave shape. To compensate decreased

thickness of the product, it is advised to make the core side the same amount in convexshape if possible (Fig. 4-4.1).

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5 Corrective Actions for Defective Molding

Defective molding may be derived from improper setting of the injection parameters ormaterial conditions.

Corrective action or improved procedure should be in practice by finding out the causeof the defective molding.

Typical phenomena of the defective molding, possible causes and actions forimprovement shall be explained as follows:

5-1 Short filling (short shot)

A short shot is a condition in which resin cannot be fully filled into the mold. This willcause poor appearance and poor function of the product.

Possible causes of the short shot are as follows:

Resin fluidity is poor. Amount of resin measured by the machine is too less.

Product wall thickness is too small. Air pocket due to flow pattern of the resin. Air ventilation of the mold is poor. Resin temperature is too low. Temperature of the cavity surface is too low. Injection speed is too slow.

Injection pressure is too less.

5-2 Flashes or burrs

Flashes are thin filmy resin adhered to edges of the product. Flashes may causedifficulty in assembly of the molded parts and may be hurting when handled.

Flashes can be resulted from following conditions:

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Resin viscosity is too low. Clamping force of the mold is not enough. Dimensional problem of the cavity. Clearance on cavity parts is too much. Rigidity of the mold plate is not enough. Resin temperature is too high.

Cavity surface temperature is too high. Injection speed is too fast. Injection pressure is too high.

5-3 Sink markSink mark is a concavity on the product surface. It causes poor appearance and poorfunction of the molded product.

Sink marks can be resulted from following conditions:

Contraction factor of the resin is too high. Wall thickness of the product is too thick. The gate is too small. Runner is too small. The gate location is too far. Cavity temperature is partially too high. Cooling capacity of the mold is too less. Holding pressure is too low. Time for holding pressure is too short. Cooling time is too short.

5-4 Void

Void is a bubble in the product. There can be classified to two causes.

One is caused by water or air mixed into the material and another is caused by vacuumcavity created when material is contracted. Void results in weak strength of the productor poor appearance if the products need to be transparent.

Void can be resulted from following reasons.

(In the case of mixed air)

Preheating of the pellets is not enough. Rotation speed of the screw is to fast. Staying time of the resin is too long. Resin temperature is too high. Injection speed is too fast.

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Emilian Popa Injection Molding Design Guide Manual 2012 Air ventilation in the mold is not enough.

(In the case of vacuum void)

Holding pressure is too low. Time for holding pressure is too short. Gate is too small.

Runner is too small. Amount of cushion is too less. The wall of the product is too thick.

5-5 Flow mark

Flow mark is a wavelike mark of the resin flow left on the surface of the product.Appearance and surface quality should be affected.

Flow marks can be caused by following reasons:

Resin flow is poor. Resin temperature is too low. Gate is too small. Injection pressure is too low. Holding pressure is too low. Surface temperature of the cavity is too low. Injection speed is too low.

5-6 Weld line

Weld line is a line mark on the surface of the product appeared around merging area ofthe resin flow. Appearance and strength of the product should be affected.

Weld line can be caused by following reasons:

Resin flow is poor. Resin includes additives such as fiber.

Surface temperature of the cavity is too low. Staying time of the resin is too long. Location of the gate is not proper. Air ventilation is not enough. Injection pressure is too low. Injection speed is too slow. Holding pressure is too low. Time for holding pressure is too short.

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Emilian Popa Injection Molding Design Guide Manual 2012 Preheating of the pellets is not enough.

5-7 BurnBurn is a phenomenon in that resin burns when air is heated upon compression aroundfinal filling area and air pocket area. Burn should affect appearance and quality of theproduct.

Burn can be caused by the following reasons:

Air pocket can be created. Air ventilation is not enough. A certain gas has plugged the vent line. Injection speed is too fast. Resin temperature is too high. Gate is too small.

5-8 Jetting

Jetting is a snaking mark appeared on the product surface, this tends to appear whenthe gate is too small.

Jetting can be caused by the following reasons:

Gate is too small. Gate is located around the thick wall of the product. Injection speed is too fast. Injection temperature is too low. Surface temperature of the cavity is too low.

5-9 Silver streak

Silver streak is a fine silvery line on the surface of the product.Silver streak can be caused by the following reasons: Preheating of the pellets in not enough. Air ventilation is not enough. Gate is too small. Resin temperature is too high. Rotation speed of the screw is too fast. Injection speed is too fast.

5-10 Poor luster

Poor luster is a phenomenon in that luster on the product surface is not enough orinconsistent. It affects appearance of the product.

Poor luster can be caused by the following reasons:

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Emilian Popa Injection Molding Design Guide Manual 2012 Resin flow is poor. Too much variation in the wall thickness of the product. Surface finish of the cavity is too rough. Preheating of the pellets is not enough. Injection speed is too slow. Injection pressure is too low. Holding pressure is too low.

Time for holding pressure is too short. Surface temperature of the cavity is too low.

5-11 Inclusion of foreign matters

Foreign matters such as dusts, dirt, bugs, etc. may be included in the material.Appearance and material strength may be affected.

Causes may be as follows:

Preheating management is poor. Foreign matters tend to be in at the crashing process. Cover of the hopper dryer is left open. There may be a fluorescent light above the machine. 5 S is not in practice in the factory. Management in keeping material bags is not proper. Oil and separation agent are spread too much on the mold. Air ventilation tube of the mold is plugged. Cleaning of the mold is not sufficient. Color change is not proper. Staying time of the resin in the cylinder is too long. Foreign matters are on the screw.

5-12 Poor releasing of the mold

Poor releasing of the mold is a phenomenon in that the product sticks to the cavity.There are three patterns for this problem. One is that the product sticks to the cavitywhen the mold is opened. The second is that the product sticks to the core when producis to be ejected. The third is that the product sticks to the slide core. When poorreleasing happens, naturally continuous production is interrupted and product surfacemay be damaged.

Poor releasing can be caused by the following reasons:

Releasing resistance of the resin is too big. Draft angle is too small. Cavity surface is too rough. Direction of the cavity surface finishing is perpendicular to draft direction. Cavity is machined in the shape of under cutting.

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Emilian Popa Injection Molding Design Guide Manual 2012 Measured resin is too much. Amount of cushion is too much. Injection pressure is too high. Holding pressure is too high. Time for holding pressure is too long. Cooling time is too short. Cooling capacity of the mold is not enough.

5-13 Bending and deformation

Bending and deformation is a phenomenon of the bent and deformed product.Bending and deformation can be cause by the following reasons:

Contraction rate of the resin is too big. Wall thickness of the product varies too much. Gate location is not proper. Filling pattern is not even. Surface temperature of the mold varies from one place to another. Holding pressure is too low. Time for holding pressure is too short. Surface temperature of the cavity is too high. Cooling time is not enough. Layout of the ejector pins is imbalanced. Draft angle is too small.

5-14 Dimension defect

Dimension defect is a phenomenon is that the product dimensions fall outside ofspecified tolerances.

Dimension defects can be caused by the following reasons:

There is a fluctuation in the quality of the material by the lot. Machining dimensions of the cavity may be in error. Contraction rate of the material is out of scope. Flow pattern of the resin is not stable. Molding condition is not stable. Surface temperature of the mold is not stable. In the case of multi-molding, balance of the runner and gate is not appropriate.

Holding pressure is released before gate sealing.

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