What is Die Adhesion?

Die Adhesion, also known as Sticking Springs or Parts, is caused by poor contact surface between the injection port and the nozzle arc, failure of the sprue material to demould with the product, and abnormal filling. Usually, the diameter of the main runner needs to be large enough to ensure that the sprue material has not yet fully solidified during demolding.

Whether it is sticking to the mold runner or the product sticking to the mold cavity, poor demolding is the most important killer in plastic injection molding. The root cause of demolding defects may be different parts of the injection molding equipment, or it may be caused by improper injection molding process.

Although there are no burrs, insufficient draft angle, or reverse draft angle caused by errors in part design and mold design, sometimes formed parts may also experience poor demolding. When forcibly ejected, it often causes the workpiece to warp, turn white or crack after being ejected. Especially when the formed parts are stuck on one side of the static mold, sometimes they cannot be ejected.

Analysis and Solution of DieAdhesion Defects

1. Mold failure

The reasons for poor adhesion and demolding are multifaceted, and mold failure is one of the main reasons. The causes and handling methods are as follows:

1) The surface of the mold cavity is rough. If there are surface defects such as gouges, scratches, scars, and dents left in the mold cavity and flow channel, the plastic parts are easily adhered to the mold, causing difficulty in demolding.

Therefore, the surface smoothness of the mold cavity and flow channel should be improved as much as possible. The surface of the mold cavity should preferably be chrome plated. When polishing, the direction of the polishing tool should be consistent with the direction of the molten material filling.

2) The mold is worn, scratched, or the gap at the insert is too large

When the molten material produces flash in the scratched area of the mold or the gap of the insert, it can also cause difficulty in demolding. In this regard, the damaged area should be repaired and the gap between the inserts should be reduced.

3) Insufficient mold rigidity

If the mold cannot be opened at the beginning of injection, it indicates that due to insufficient rigidity, the mold undergoes deformation under the action of injection pressure. If the deformation exceeds the elastic limit, the mold cannot be restored to its original state and cannot continue to be used. Even if the deformation does not exceed the elastic limit of the mold, the molten material cools and solidifies under high conditions in the mold cavity, removes the injection pressure, and after the mold restores deformation, the plastic part is clamped by the rebound force, and the mold still cannot be opened. Therefore, when designing molds, it is necessary to design sufficient rigidity and strength.

When testing the mold, it is best to install a dial gauge on the mold to check whether the mold cavity and mold base are deformed during the filling process. The initial injection pressure during mold testing should not be too high, and the deformation of the mold should be observed while slowly increasing the injection pressure to control the deformation within a certain range.

When a clamping failure occurs due to excessive rebound force, simply increasing the opening force is not enough. The mold should be immediately removed and decomposed, and the plastic parts should be heated and softened before being taken out. For molds with insufficient rigidity, a frame can be embedded on the outer side of the mold to improve rigidity.

4) Insufficient or dynamic demolding slope, difference in parallelism between fixed templates

When designing and manufacturing molds, sufficient demolding angles should be ensured, otherwise it is difficult for the plastic part to demould. Forcible ejection often leads to warping of the plastic part, whitening or cracking of the ejection part, etc. The movement of the mold should be relatively parallel to the fixed template, otherwise it will cause the cavity to shift and cause poor demolding.

5) The design of the pouring system is unreasonable

If the sprue is too long or too small, the strength of the connection between the main sprue and the sub sprue is insufficient, there are no cold material pockets in the main sprue, the balance of the sprue is poor, the diameter of the main sprue is not matched properly with the diameter of the nozzle hole, or the spherical surface of the sprue sleeve and the nozzle does not match, all of these can lead to poor mold adhesion and demolding. Therefore, it is necessary to appropriately shorten the length of the sprue and increase its cross-sectional area, improve the strength of the connection between the main flow channel and the diversion channel, and set up cold material cavities on the main flow channel.

When determining the gate position, methods such as adding auxiliary gates can be used to balance the filling rate of each cavity in a multi cavity mold and reduce the pressure inside the mold cavity. In general, the small end diameter of the main flow channel should be 0.5-1mm larger than the nozzle aperture, and the concave radius of the gate sleeve should be 1-2mm larger than the spherical radius of the nozzle

6) Unreasonable design or improper operation of the ejection mechanism

If the ejection device has insufficient stroke, uneven ejection, or poor top plate movement, it can lead to the inability of the plastic part to demould.

Under sufficient conditions, the effective ejection area of the ejector pin should be increased as much as possible to ensure sufficient ejection stroke. The ejection speed of the plastic part should be controlled within a suitable range and should not be too fast or too slow. The main reason for poor top plate movement is due to adhesion between sliding components.

For example, when the top plate pushes the sliding core, due to the lack of cooling devices at the sliding core, its temperature is higher than other cores. During continuous operation, the gap between the column body and the sliding core is very small, often causing stickiness, which leads to poor core pulling action. For example, when the flatness between the top pin hole and the top plate guide pin is poor or the top pin is bent, the top plate will act poorly.

If no stop pin is installed in the pushing mechanism, when there is a foreign object between the top plate and the installation plate, the top plate will tilt and the movement of the rear top plate will be poor. In medium and large molds, if only one top rod acts, the top plate cannot be evenly pushed, which can also cause poor movement.

7) Poor mold exhaust or lack of air inlet in the mold core can also cause poor adhesion and demolding

The exhaust conditions of the mold should be improved, and air inlet holes should be set at the mold core.

8) Improper mold temperature control or improper cooling time

If it is difficult to demould at the parting surface, the mold temperature can be appropriately increased and the cooling time can be shortened. If it is difficult to demould at the cavity surface, the mold temperature can be appropriately reduced or the cooling time can be increased. In addition, too high a fixed mold temperature can also lead to poor demolding. When the material of the mold cavity is porous soft material, it can cause mold sticking. For this, hard steel or surface electroplating treatment should be used instead.

9) Mold defects such as poor sprue pulling out, lack of pulling mechanism for the sprue, low concavity below the parting surface, and cavity edge lines exceeding the clamping line can all affect the demolding of plastic parts to varying degrees

Attention should be paid and adjustments should be made to this.

2. Improper control of process conditions

If the specification of the injection molding machine is large, the screw speed is too high, the injection pressure is too high, and the injection holding time is too long, it will form excessive filling, making the molding shrinkage smaller than expected and difficult to demould.

If the temperature of the material barrel and melt is too high and the injection pressure is too high, the hot melt material can easily enter the gap between the mold inserts and produce flash, leading to poor demolding.

In addition, the nozzle temperature is too low, the cooling time is too short, and the injection flow is interrupted, all of which can cause poor demolding. Therefore, when troubleshooting poor sticking and demolding faults, it is necessary to appropriately reduce the injection pressure, shorten the injection time, lower the temperature of the material barrel and melt, extend the cooling time, and prevent melt flow interruption.

3. The raw materials do not meet the usage requirements

If impurities are mixed into the raw materials during packaging and transportation, or different grades of raw materials are mixed during pre drying and preheating treatment, as well as foreign objects are mixed into the material barrel and hopper, it can cause the plastic parts to stick to the mold. In addition, the uneven or excessively large particle size of the raw material also has a certain degree of impact on the adhesion of the mold. Therefore, purification and screening work should be done well for the forming raw materials.

4. Improper use of release agent

The purpose of using release agents is to reduce the adhesion between the surface of the plastic part and the surface of the mold cavity, prevent the two from adhering to each other, in order to shorten the forming cycle and improve the surface quality of the plastic part. However, due to the influence of both chemical and physical conditions on the release effect of release agents, and the varying raw materials and processing conditions, the optimal variety and dosage of release agents must be determined based on specific circumstances. If used improperly, it often fails to produce a good demolding effect.

In terms of molding temperature, the effective working temperature of fatty oil release agents should generally not exceed 150 degrees, and should not be used during high-temperature molding; The working temperature of silicone oil and metal soap release agents is generally between 150 ° C and 250 ° C; The working temperature of polytetrafluoroethylene type release agents can reach over 260 degrees, making them the best release agents for high temperature conditions.

In terms of the variety of raw materials, soft polymer plastic parts are more difficult to demould than hard polymer plastic parts. In terms of usage, the paste release agent needs to be brushed with a brush, and the sprayable release agent needs to be sprayed using a spraying device. Due to the difficulty in forming a regular and uniform mold layer during the application of paste release agents, there may be wave marks or stripes on the surface of the plastic parts after demolding. Therefore, it is possible to use sprayable release agents.

5. Overfill

When molding with excessive injection pressure, the molding shrinkage is smaller than expected, making demolding difficult. At this point, if the injection pressure is reduced, the injection time is shortened, and the temperature of the melt and mold is reduced, it becomes easier to demould. In this situation, using a release agent that reduces the friction between the plastic and the mold is more effective. For molds, methods such as improving the smoothness, eliminating concave and convex sidewalls, grinding, and adding top rods are also effective. When forming deeper parts, blowing compressed air between the mold and the part is more helpful for demolding (refer to overfilling in "Cracking, Cracking, Microcracks, and Whitening").

6. The workpiece is adhered to the static mold

There are two reasons for this, namely, there are stuck areas between the nozzle and the mold cavity, or the demolding resistance of the static mold is greater than that of the dynamic mold, which causes the workpiece to stick to the static mold.

The situation of sticking to the static mold due to the resistance between the nozzle and the mold cavity includes: the fillet radius R of the nozzle is larger than the corresponding fillet radius R of the mold, the nozzle is not concentric with the mold when clamping the mold, or there is plastic leakage between the nozzle and the mold. Any of these situations will cause the workpiece to jam and stick to the static mold. To avoid this situation, the mold should be installed correctly. The reason for the high demolding resistance of static molds is due to low smoothness or concave and convex side walls.

At this point, a Z-shaped material pulling rod should be set on one side of the moving mold to pull the workpiece. In mold design, it is necessary to fully consider avoiding this phenomenon. It is also effective to have a fixed temperature difference on both sides of the dynamic and static molds of the workpiece.

(1) Excessive filling of plastic in the mold reduces injection pressure; Reduce injection molding volume, injection cylinder temperature too high

(2) The injection pressure is maintained for too long, reducing the time for the screw to move forward

(3) Scratches, porosity, or scratches on the surface of the mold, removing stains and polishing the surface of the mold

(4) The ejection angle of the mold is insufficient. Use a minimum ejection angle of 0.5 degrees on each side (the larger the angle, the easier it is to eject and the faster it operates)

(5) Improper design of the setback ensures that there are no sharp corners in the setback

(6) The injection molded parts are adhered to the highly polished surface of the mold, and an exhaust valve is used to remove the vacuum generated when pressing the injection molded parts out of the highly polished surface; Beer emptying assisted ejection

(7) Improper ejection equipment, increasing the number of ejection rods or replacing with different systems

(8) Insufficient lubrication of plastic, if allowed, use a molding agent; Add external lubricants such as zinc stearate

Reasons and troubleshooting methods for mold sticking in the main channel:

(1) The cooling time is too short and the main channel has not yet solidified.

(2) The slope of the main channel is not enough (3-5 degrees), and its demolding slope should be increased.

(3) The fit size between the main channel liner and the nozzle is improper (usually 0.5mm larger than the radius used for the nozzle) or misaligned, resulting in adhesive leakage.

(4) The main channel is rough, check for burrs that may cause collapse; The reason for the burrs is that the nozzle sleeve is hardened in the nozzle placement area, or the incorrect radius is used during the nozzle sleeve processing, and there is no cooling well in the main channel.

(5) If the nozzle temperature is too low, increase the nozzle temperature or use an independent temperature controller to heat the nozzle..

(6) Excessive filling of plastic in the injection sleeve reduces injection pressure; Reduce the forward time of the screw.

(7) Plastic is not fully cured in the nozzle, especially for larger diameter nozzles, which increases cooling time. However, a better method is to use a nozzle sleeve with a smaller nozzle instead of the original one.

(8) The diameter of the nozzle sleeve is too small, increase the diameter of the nozzle

(9) Using improper injection needles, check if the correct injection needle type is used wherever possible. Use an inverted cone type injection needle design or a "Z" type needle to check if the "Z" type needle left the mold during the ejection process, and add an arc to the corner of the "Z" type needle or before the flow channel and injection port.