Failure analysis and troubleshooting skills of mold sticking and demoulding of injection molding parts
Die failure
There are many reasons for poor die sticking and demoulding, and die failure is one of the main reasons. The causes and treatment methods are as follows:
First, the surface of the mold cavity is rough. If the mold cavity and the runner are left with chisels, marks, wounds, sunken and other surface defects, the plastic parts are easily adhered to the mold, resulting in difficulty in mold release. Therefore, the surface finish of the mold cavity and flow channel should be improved as much as possible. The surface of the mold cavity should be chrome plated. During polishing, the action direction of the polishing tool should be consistent with the mold filling direction of the melt.
Second, the die wear is too big for scratches or inserts. When the molten material produces flash in the scratched part of the die or the gap of the insert, it will also cause difficulty in demoulding. Therefore, the damaged parts should be repaired and the gap between inserts should be reduced.
Three, mold rigidity is insufficient. If the mold cannot be opened at the beginning of injection, it indicates that the mold is deformed under the action of injection pressure due to insufficient rigidity. If the deformation exceeds the elastic limit, the die can not be restored to its original state and can not continue to be used. Even if the deformation does not exceed the elastic limit of the mold, the melt cools and solidifies under high conditions in the mold cavity to remove the injection pressure. After the mold recovers the deformation, the plastic part is clamped by the rebound force, and the mold still cannot be opened.
Therefore, when designing the die, enough rigidity and strength must be designed. During mold test, it is best to install a dial indicator on the mold to check whether the mold cavity and mold base are deformed during mold filling. The initial injection pressure during mold test should not be too high. While observing the deformation of the mold, slowly increase the injection pressure to control the deformation within a certain range.
In case of clamping failure caused by too large rebound force, it is not possible to increase the opening force alone. The mold shall be removed and disassembled immediately, and the plastic parts shall be taken out after heating and softening. For the mold with insufficient rigidity, the frame can be embedded outside the mold to improve the rigidity.
The demoulding slope is insufficient or moving, and the parallelism between fixed formwork is poor. When designing and making the mold, sufficient demoulding slope shall be ensured, otherwise the plastic parts are difficult to demould. When forcibly ejecting, the plastic parts are often warped, and the ejected parts are white or cracked. The moving and fixed templates of the mold should be relatively parallel, otherwise it will lead to the deviation of the cavity and poor demoulding.
5、 The design of gating system is unreasonable. If the sprue is too long and too small, the strength of the connecting part between the main sprue and the sub sprue is not enough, the main sprue has no cold material cavity, the gate balance is poor, the main sprue diameter is not matched with the nozzle hole diameter, or the gate sleeve is not consistent with the spherical surface of the nozzle, it will lead to poor mold adhesion and demoulding. Therefore, the length of sprue should be properly shortened and its cross-sectional area should be increased to improve the strength of the connecting part of main runner and shunt runner, and cold material cavity should be set on the main runner.
When determining the gate position, the filling rate of each cavity in the multi cavity mold can be balanced and the pressure in the cavity can be reduced by increasing the auxiliary gate. In general, the small end diameter of the sprue shall be 0.5 ~ 1mm larger than the nozzle aperture, and the concave radius of the gate sleeve shall be 1 ~ 2mm larger than the spherical radius of the nozzle.
6、 Unreasonable design or improper operation of ejection mechanism. If the stroke of the ejector is insufficient, the ejector is uneven or the top plate acts poorly, the plastic parts cannot be demoulded.
When conditions permit, the effective ejection area of the ejector rod shall be increased as much as possible to ensure sufficient ejection stroke. The ejection speed of plastic parts shall be controlled within an appropriate range, not too fast or too slow. The main reason for the poor action of the top plate is the viscosity between the sliding parts.
For example, when the sliding core is pushed by the top plate, because there is no cooling device at the sliding core and its temperature is higher than that of other cores, during continuous operation, the gap between the column body and the sliding core is very small, which often leads to poor core pulling action. For example, when the parallelism 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 set in the pushing mechanism, when there is foreign matter between the top plate and the mounting plate, the top plate tilts, and then the action of the top plate is poor. In medium and large molds, if only one ejector rod acts, the top plate cannot be pushed evenly, and poor action will also occur.
7、 Poor mold exhaust or no air inlet of the mold core will also cause poor mold adhesion and demoulding. The exhaust conditions of the mold shall be improved, and the air inlet shall 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, the mold setting temperature is too high, which will also lead to poor demoulding. When the mold cavity is made of porous soft material, die sticking will be caused. In this regard, it should be replaced with hard steel or surface electroplating treatment.
Mold defects such as poor sprue pulling out, no pulling and fishing mechanism at the gate, low concave below the parting surface, and the edge line of the cavity exceeding the clamping line will affect the demoulding of plastic parts to varying degrees. Attention shall be paid to this and it shall be trimmed.
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 large, and the injection pressure holding time is too long, excessive filling will be formed, making the molding shrinkage smaller than expected, which makes it difficult to demould.
If the temperature of the barrel and melt is too high and the injection pressure is too high, the hot melt can easily enter the gap between the die inserts to produce flash, resulting in poor demoulding.
In addition, too low nozzle temperature, too short cooling time and cut-off of injection material will cause poor demoulding. Therefore, when removing the defects of die sticking and demoulding, it is necessary to appropriately reduce the injection pressure, shorten the injection time, reduce the barrel and molten material temperature, prolong the cooling time, and prevent the molten material from cutting off.
Improper use of release agent
The purpose of using release agent is to reduce the adhesion between the surface of plastic parts and the surface of mold cavity and prevent them from adhering to each other, so as to shorten the molding cycle and improve the surface quality of plastic parts.
However, because the release effect of release agent is affected by both chemical action and physical conditions, and the molding raw materials and processing conditions are different, the best variety and dosage of release agent must be determined according to the specific situation. If used improperly, it often can not produce good demoulding effect.
In terms of molding temperature, the effective working temperature of fatty oil release agent should not exceed 150 degrees generally, and should not be used during high-temperature molding; The working temperature of silicone oil and metal soap release agents is generally 150 ~ 250 degrees; The working temperature of PTFE release agent can reach more than 260 ℃, which is the best release agent under high temperature.
In terms of raw material varieties, soft polymer plastic parts are more difficult to demould than hard polymer plastic parts. As for the use method, the paste release agent shall be brushed with a brush, and the sprayable release agent shall be sprayed with a spraying device. Since it is difficult to form a regular and uniform mold layer when applying paste release agent, and there will be wave marks or stripes on the surface of plastic parts after release, it is possible to use sprayable release agent.
