What are the reasons for the poor release of blow molded products?

Poor demoulding, also known as sticky mold runner, is caused by the poor contact surface between the injection port and the nozzle arc, the gate material is not demolded with the product, and the abnormal filler is caused. Today, let's talk about the demoulding of blow molding products. What are the reasons for the bad.

In the process of blow molding, we often encounter poor demoulding. Let's analyze the reasons for poor demoulding.

1. Mold failure

There are many reasons for mold sticking and poor mold release, and mold failure is one of the main reasons. The surface of the mold cavity is rough. If there are surface defects such as chisel lines, nicks, scars, depressions, etc. in the cavity and runner of the mold, 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 is preferably chrome-plated. When polishing, the action direction of the polishing tool should be consistent with the filling direction of the molten material.

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. The rigidity of the mold 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 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.

MEPER Machine

2. Insufficient demoulding slope

The parallelism between the fixed templates is poor. When designing and making a mold, a sufficient demoulding slope should be ensured, otherwise the plastic part will be difficult to demould, and when it is forcibly ejected, it will often cause the plastic part to warp, and the ejection part will become 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.

3. 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. In general, the diameter of the small end of the main runner 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.

Double station extrusion blow molding mould plate

4. 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 permit, 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 reason for the poor operation of the top plate is the sticking between the sliders. When the top plate pushes the sliding core, because there is no cooling device at the sliding core, its temperature is higher than that of 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 poor core pulling action. For another example, when the parallelism between the ejector pin hole and the guide pin of the ejector plate is poor or the ejector pin is bent, the ejector 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 installation plate, the top plate is inclined, and the action of the rear top plate is 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.

5. Poor mold exhaust or mold core without air inlet

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

Die head for MEPER blow molding machine

6. 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.

7. Poor runner pullout

The gate does not have a fishing mechanism, the parting surface is concave below the parting surface, and mold defects such as the edge of the cavity exceeding the parting line will affect the demoulding of plastic parts to varying degrees. This should be noted and corrected.

The above seven points are the main reasons for poor demoulding. I hope my article will be helpful to you.