What Should I Do If The Injection Molding Product Has Bad Demoulding?

Poor demoulding, also known as sticky mold runner, is due to poor contact between the injection port and the nozzle arc, the gate material is not demolded together with the product, and abnormal fillers. Usually, the diameter of the sprue should be large enough so that the gate material is not fully cured when the mold is released.

Regardless of the sticky mold runner or the product sticking to the mold cavity, poor demoulding is the most important killer in plastic injection molding. The root cause of poor demolding 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, and reverse draft angle caused by mistakes in part design and mold design, sometimes the molded parts will have poor demoulding. When forcibly ejected, the workpiece is often warped, whitish or cracked after ejection. In particular, the molded part sticks to the side of the static mold and sometimes cannot be ejected.

Analysis of the causes of bad demoulding defects and solutions

1. Mold failure

There are many reasons for mold sticking and poor mold release, and mold failure is one of the main reasons. The causes and treatment methods are as follows:

1) The surface of the mold cavity is rough. If there are surface defects such as chisel lines, nicks, scars, and depressions in the mold cavity and runner, the plastic parts will easily adhere to the mold, resulting in difficulty in demolding.

Therefore, the surface finish of the cavity and the runner should be improved as much as possible, and the inner surface of the cavity should preferably be chrome-plated. When polishing, the action direction of the polishing tool should be consistent with the filling direction of the molten material.

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

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

3) Insufficient mold rigidity

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 mold cannot return to its original shape and cannot be used further. Even if the deformation does not exceed the elastic limit of the mold, the molten material is cooled and solidified under high conditions in the mold cavity, and the injection pressure is removed. After the mold recovers its deformation, the plastic part is clamped by the elastic force, and the mold still cannot be opened. Therefore, when designing the mold, sufficient rigidity and strength must be designed.

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

When the rebound force is too large to cause the clamping failure, it is not enough to only increase the mold opening force. The mold should be disassembled 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 improve rigidity.

4) Insufficient or dynamic demoulding slope, poor parallelism between fixed templates

When designing and making molds, a sufficient demoulding slope should be ensured, otherwise the plastic parts will be difficult to demould, and when forcibly ejected, the plastic parts will often be warped, and the ejection part will be white or cracked. The movement of the mold and the fixed platen should be relatively parallel, otherwise the cavity will be offset, resulting in poor demoulding.

5) The design of the gating system is unreasonable

If the runner is too long or too small, the strength of the connection between the main runner and the sub-runner is insufficient, the main runner has no cold slug cavity, the gate balance is poor, the diameter of the main runner and the diameter of the nozzle hole are not properly matched, or the sprue sleeve and nozzle If the spherical surface does not match, it will lead to mold sticking and poor mold release. Therefore, the length of the runner should be appropriately shortened and its cross-sectional area should be increased to improve the strength of the connection between the main runner and the branch runner, and a cold slug hole 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 adding auxiliary gates and other methods. In general, the diameter of the small end of the sprue should be 0.5~1mm larger than the diameter of the nozzle, and the concave radius of the sprue sleeve should be 1~2mm larger than the spherical radius of the nozzle

6) Unreasonable design or improper operation of ejector mechanism

If the stroke of the ejector device is insufficient, the ejection is unbalanced or the action of the top plate is poor, the plastic parts will not be demolded.

When conditions allow, the effective ejection area of the ejector rod should be increased as much as possible to ensure sufficient ejection stroke, and the ejection speed of plastic parts should be controlled within a suitable range, not too fast or too slow. The main cause of poor operation of the top plate is the sticking between the sliders.

For example, when the top plate pushes the sliding core, since there is no cooling device 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 extremely small, which often causes sticking and leads to the core pulling action. Bad, for example, when the parallelism between the ejector pin hole and the top plate guide pin is not good or the ejector pin is bent, the top plate will malfunction.

If there is no stop pin in the push mechanism, when there is a foreign object between the top plate and the mounting plate, the top plate will be inclined, and the movement of the rear top plate will be poor. In medium and large molds, if only one ejector rod acts, the ejector plate cannot be pushed evenly, and it will also cause poor action.

7) Poor mold exhaust or no air inlet in the mold core will also cause mold sticking and poor mold release

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

8) Improper mold temperature control or improper cooling time

If it is difficult to release the mold at the parting surface, the mold temperature can be appropriately increased and the cooling time can be shortened. If it is difficult to release the mold at the cavity surface, the mold temperature can be appropriately reduced or the cooling time can be increased. In addition, the temperature of the fixed mold is too high, which will also lead to poor mold release. When the mold cavity material is a porous soft material, it will cause sticking. In this regard, it should be replaced with hard steel or surface electroplating.

9) Mold defects such as poor runner pulling, no fishing mechanism at the gate, concave below the parting surface, and the edge of the cavity exceeding the clamping line will affect the demoulding of plastic parts to varying degrees.

This should be noted and corrected.

2. 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 high, and the injection pressure holding time is too long, excessive filling will be formed, making the molding shrinkage smaller than expected, and it will be difficult to demold.

If the temperature of the barrel and the melt is too high and the injection pressure is too high, the hot melt can easily enter the gap between the mold inserts and cause flash, resulting in poor demolding.

In addition, the nozzle temperature is too low, the cooling time is too short and the injection is cut off, which will cause poor demolding. Therefore, when eliminating the failure of sticking and bad demoulding, the injection pressure should be appropriately reduced, the injection time should be shortened, the temperature of the barrel and the melt should be reduced, the cooling time should be extended, and the melt flow should be prevented.

3. The raw materials do not meet the requirements for use

If the raw materials are mixed with impurities during packaging and transportation, or if different grades of raw materials are mixed during pre-drying and preheating, and foreign materials are mixed in the barrels and hoppers, it will cause plastic parts to stick to the mold. In addition, the uneven or too large particle size of raw materials also has a certain degree of influence on mold sticking. Therefore, the purification and screening work should be done well for the molding raw materials.

4. 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 and prevent the two from sticking to each other, so as 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 chemical action and physical conditions, and the molding raw materials and processing conditions are different, the optimal variety and dosage of the release agent must be selected according to the specific situation. to make sure. If used improperly, it often cannot produce good demoulding effect.

As far as the molding temperature is concerned, the effective working temperature of fatty oil release agents should generally not exceed 150 degrees, and it should not be used during high temperature molding; the working temperature of silicone oil and metal soap release agents is generally 150 degrees to 250 degrees; polytetrafluoroethylene The working temperature of vinyl fluoride mold release agent can reach more than 260 degrees, and it is the mold release agent with the best mold release effect under high temperature conditions.

In terms of raw material varieties, soft polymer plastic parts are more difficult to demold than hard polymer plastic parts. As far as the method of use is concerned, the paste release agent is applied with a brush, and the sprayable release agent is sprayed with a spray device. Since it is difficult to form a regular and uniform mold layer when the paste release agent is applied, there will be wavy marks or streaks on the surface of the plastic part after mold release, so it should be possible to use a sprayable mold release agent.

5. Overfill

When molding with an excessive injection pressure, the molding shrinkage rate is smaller than expected, and mold release becomes difficult. At this time, if the injection pressure is reduced, the injection time is shortened, and the melt and mold temperature are lowered, it becomes easy to demold. In this case, it is more effective to use a release agent that reduces the friction between the plastic and the mold. For the mold, it is also effective to improve the light control degree, cancel the side wall convex and concave, grind, and increase the ejector pin. When molding deeper parts, blowing compressed air between the mold and the part may help release the mold (see overfilling in "Cracks, Cracks, Cracks, and Whitening").

6. Parts are glued to the static mold

There are two reasons for this, that is, there is a stuck place between the nozzle and the cavity, or the demolding resistance of the static mold is greater than that of the moving mold, so that the part sticks to the static mold. The cases of sticking to the static mold due to the resistance between the nozzle and the cavity are: the fillet radius R of the nozzle is larger than the corresponding fillet radius R of the mold, the nozzle and the mold are not concentric when the mold is clamped, or the nozzle is And there is leakage of plastic between the molds.

Either of these situations will jam the part, causing the part to stick to the static mold. In order for this not to happen, the mold should be installed correctly. The reason why the demolding resistance of the static mold is large is due to the low finish or the convex and concave side walls. At this time, a Z-shaped pull rod should be set on the side of the movable mold to pull the workpiece. In the mold design, it is necessary to fully consider that this phenomenon does not occur. It is also effective to have a fixed temperature difference on both sides of the moving and static molds.

(1) The plastic in the mold is excessively packed, reducing the injection pressure; reducing the injection volume and the temperature of the injection cylinder is too high

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

(3) The surface of the mold is scratched, porous or scratched, remove the stain and polish the surface of the mold

(4) The ejection angle of the mold is insufficient, use the minimum ejection angle of 0.5 degrees on each side (the larger the angle, the easier the ejection and the faster the operation)

(5) Improper design of the collapse to ensure that the collapse has no sharp corners

(6) The injection molded parts are stuck to the surface of the highly polished mold, and the exhaust valve is used to remove the vacuum generated when the injection molded parts are pressed out from the highly polished surface; the beer is empty to assist the ejection

(7) Inappropriate ejector equipment, increase the number of ejector rods or change to a different system

(8) Insufficient lubrication of plastics, use plasticizers if allowed; increase external lubricants, such as zinc stearate