Analysis and troubleshooting of adhesive mold and mold release failure of injection molded parts

There are many reasons for the sticking and mold release of injection molded parts , and mold 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 there are surface defects such as chisels, nicks, scratches, and depressions in the cavity and flow path of the mold, the plastic parts are easily adhered to the mold, which makes the mold release difficult. Therefore, the surface finish of the cavity and the runner should be increased as much as possible. The inner surface of the cavity is preferably chrome-plated. When polishing, the direction of the polishing tool should be consistent with the direction of filling of the melt.

Second, the mold wears scratches or the gap at the insert is too large. When the melt is generated in the scratched portion of the mold or in the gap of the insert, it is also difficult to release the mold. In this regard, the damaged area should be repaired and the gap of the insert should be reduced.

Third, the mold rigidity is insufficient. If the mold does not open when the injection is started, it indicates that the mold is deformed under the action of the injection pressure due to insufficient rigidity. If the deformation exceeds the elastic limit, the mold cannot be restored and cannot be used any more. Even if the deformation does not exceed the elastic limit of the mold, the melt cools and solidifies under the high condition in the cavity, and the injection pressure is removed. After the mold is restored and deformed, the plastic part is clamped by the resilience and the mold cannot be opened.

Therefore, when designing the mold, it is necessary to design sufficient rigidity and strength. When testing the mold, it is best to install the dial gauge on the mold to check whether the cavity and the mold frame are deformed during the filling process. The initial injection pressure at the initial test injection is not too high, and the deformation of the mold should be observed while observing the mold. While slowly increasing the injection pressure, the amount of deformation is controlled within a certain range.

When the resilience is too large to cause the clamping failure, it is not enough to increase the mold opening force. The mold should be removed and decomposed immediately, and the plastic parts should be heated and softened and taken out. For molds with insufficient rigidity, a frame can be placed on the outside of the mold to increase rigidity.

4. The draft of the demoulding is insufficient or dynamic, and the parallelism between the templates is poor. When designing and manufacturing the mold, sufficient drafting angle should be ensured, otherwise the plastic parts are difficult to demould. When forcibly ejected, the plastic parts are warped, and the ejector parts are white or cracked. The movement of the mold, the template should be relatively parallel, otherwise it will cause the cavity to shift, resulting in poor mold release.

5. The design of the gating system is unreasonable. If the runner is too long, too small, the joints of the main runner and the sprue are not strong enough, the main runner has no cold pockets, the gate balance is poor, the main runner diameter is not properly matched with the nozzle hole diameter or the sprue bush and nozzle If the spherical surface does not match, it will result in poor mold clamping and demolding. Therefore, the runner length should be shortened and the cross-sectional area should be increased, and the strength of the connection between the main channel and the runner should be increased. A cold pocket should be placed on the main channel.

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 adding an auxiliary gate or the like. Under normal circumstances, the diameter of the small end of the main channel should be 0.5~1mm larger than the nozzle aperture, and the concave radius of the sprue sleeve should be 1~2mm larger than the spherical radius of the nozzle.

Sixth, the design of the ejection mechanism is unreasonable or improperly operated. If the ejector unit has insufficient stroke, the ejector is unbalanced or the top plate is not working properly, the plastic parts cannot be demolded.

In the case of sufficient conditions, the effective ejection area of ​​the ejector should be increased as much as possible to ensure sufficient ejection stroke, and the ejection speed of the plastic parts should be controlled within a suitable range, not too fast or too slow. The main reason for the malfunction of the top plate is due to the stickiness between the sliding members. For example, when the top plate pushes the sliding core, the temperature is higher than other cores because there is no cooling device at the sliding core. During continuous operation, the gap between the column body and the sliding core is extremely small, and the wicking is often caused to cause the core pulling action. Poor, for example, when the parallelism between the top pin hole and the top plate guide pin is poor or the pin is bent, the top plate will malfunction. If the pin is not provided in the jacking mechanism, when there is a foreign object between the top plate and the mounting plate, the top plate is inclined, and the operation of the top plate is poor. In the large mold, if only one ejector is used, the top plate cannot be balanced and pushed, and malfunction can also occur.

Seven, poor mold exhaust or mold core without air intake can also cause sticking and demoulding. The exhaust condition of the mold should be improved, and the intake hole should be provided at the core.

8. Improper control of mold temperature or length of cooling time is not appropriate. If it is difficult to demold at the parting surface, the mold temperature can be appropriately increased and the cooling time can be shortened. If it is difficult to demold at the cavity surface, the mold temperature can be appropriately lowered or the cooling time can be increased. In addition, the temperature of the mold is too high, which may result in poor mold release. When the mold cavity is made of porous soft material, it will cause sticking. For this, it should be replaced with hard steel or surface plating.

The sprue is pulled out badly, the gate has no pulling fishing mechanism, the lower part of the parting surface is concave, and the mold edge line exceeds the mold line and other mold defects will affect the demoulding of the plastic parts to varying degrees. In this regard, attention should be paid and trimmed.

Improper control of process conditions

If the size of the injection molding machine is large, the screw speed is too high, the injection pressure is too large, and the injection holding time is too long, excessive filling will be formed, so that the molding shrinkage rate is smaller than expected, and it is difficult to demold.

If the temperature of the barrel and the melt is too high, the injection pressure is too large, and the hot melt easily enters the gap between the mold inserts to generate a flash, resulting in poor mold release.

In addition, the nozzle temperature is too low, the cooling time is too short, and the injection is interrupted, which may cause mold release failure. Therefore, in the case of eliminating the problem of poor mold and mold release, the injection pressure should be appropriately reduced, the injection time should be shortened, the temperature of the barrel and the melt should be lowered, the cooling time should be prolonged, and the melt flow should be prevented.

Raw materials do not meet the requirements for use

If the raw materials are mixed with impurities during packaging and transportation, or mixed with different grades of raw materials during pre-drying and pre-heat treatment, and foreign matter is mixed into the barrel and hopper, the plastic parts will stick. In addition, the uneven or excessive particle size of the raw material also has a certain degree of influence on the sticking mold. Therefore, the purification and screening work should be done for the molding materials.

Improper use of release agent

The purpose of using the release agent is to reduce the adhesion between the surface of the plastic part and the surface of the mold cavity, preventing the two from sticking to each other, in order to shorten the molding cycle and improve the surface quality of the plastic part. However, since the release effect of the release agent is affected by both the chemical action and the physical conditions, and the molding materials and processing conditions are different, the optimum variety and dosage of the selected release agent must be determined according to the specific conditions. to make sure. If used improperly, it often does not produce a good release effect.

In terms of molding temperature, the effective working temperature of the fatty oil release agent should not exceed 150 degrees, and should not be used at high temperature molding; the working temperature of silicone oil and metal soap release agent is generally 150 to 250 degrees; The working temperature of the vinyl fluoride release agent can reach 260 degrees or more, which is the best release agent for high-release conditions.

In terms of raw material varieties, soft polymer plastic parts are more difficult to demold than hard polymer plastic parts. In terms of the method of use, the paste release agent is brushed, and the sprayable release agent is sprayed using a spray device. Since the paste release agent is difficult to form a regular and uniform mold layer during painting, there may be wavy marks or streaks on the surface of the plastic part after demolding, so it is possible to use a sprayable release agent.