Sticky mold, mold release failure analysis and troubleshooting techniques

Time:2019-09-23 09:13:42 / Popularity: / Source:

plastic moulded parts 

Mold failure

There are many reasons for sticking and mold release failure, and mold failure is one of main reasons. Reasons and methods for handling are as follows:

First, surface of mold cavity is rough.

If there are surface defects such as chiseling, nicks, scars, and depressions in cavity and flow path of mold, plastic moulded parts are easily adhered to mold, which makes demoulding difficult. Therefore, surface finish of cavity and runner should be increased as much as possible. Inner surface of cavity is preferably chrome-plated. When polishing, direction of polishing tool should be consistent with direction of filling of melt.

Second, mold wears scratches or gap at insert is too large.

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

Third, mold rigidity is insufficient.

If mold does not open when injection is started, it indicates that mold is deformed under action of injection pressure due to insufficient rigidity. If deformation exceeds elastic limit, mold cannot be restored and used any more. Even if deformation does not exceed elastic limit of mold, melt cools and solidifies under high condition in cavity, remove injection pressure. After mold is restored and deformed, plastic part is clamped by resilience and mold cannot be opened.
Therefore, when do plastic injection mould design, it is necessary to design sufficient rigidity and strength. When testing mold, it is best to install a dial gauge on mold to check whether cavity and mold frame are deformed during filling process. Initial injection pressure at mold trial is not too high, and deformation of mold should be observed. While slowly increasing injection pressure, amount of deformation is controlled within a certain range.
When resilience is too large to cause clamping failure, it is not enough to increase mold opening force. Mold should be removed and decomposed immediately, and plastic moulded parts should be heated, softened and taken out. For molds with insufficient rigidity, a frame can be placed on outside of mold to increase rigidity.

Fourth, draft of demoulding is insufficient or parallelism between moving and fixed templates is poor.

When designing and manufacturing mold, sufficient drafting angle should be ensured, otherwise plastic parts are difficult to demould. When forcibly ejected, plastic parts are warped, and ejector parts are white or cracked. Movement of moving and fixed template should be relatively parallel, otherwise cavity will be displaced, resulting in poor mold release.

Fifth, design of gating system is unreasonable.

If runner is too long and too small, joints of main and split runner are not strong enough, main runner has no cold pockets, gate balance is poor, main runner diameter is not properly matched with nozzle hole diameter, or sprue sleeve does not match spherical surface of nozzle, resulting in poor sticking and demolding. Therefore, runner length should be shortened and cross-sectional area should be increased, strength of connection between main channel and runner should be increased. A cold pocket should be placed on main channel.
When determining gate position, filling rate of each cavity in multi-cavity mold can be balanced and pressure in cavity can be reduced by adding an auxiliary gate. Under normal circumstances, diameter of small end of main channel should be 0.5~1mm larger than nozzle aperture, and radius of concave circle of sprue sleeve should be 1~2mm larger than radius of spherical surface of nozzle.
plastic injection mould design 

Sixth, design of ejection mechanism is unreasonable or improperly operated.

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

Seventh, poor mold exhaust or mold core without air intake can also cause sticking and demoulding.

Exhaust condition of mold should be improved, intake hole should be provided at core.

Eighth, improper control of mold temperature or length of cooling time is not appropriate.

If it is difficult to demold at parting surface, mold temperature can be appropriately increased and cooling time can be shortened. If it is difficult to demold at cavity surface, mold temperature can be appropriately lowered or cooling time can be increased. In addition, temperature of mold is too high, which may result in poor mold release. When mold cavity is made of porous soft material, it will cause sticking. For this, it should be replaced with hard steel or surface plating.
Sprue pull out badly, gate has no pulling fishing mechanism, lower part of parting surface is concave, mold edge line exceeds mold line and other mold defects will affect demoulding of plastic moulded parts to varying degrees. In this regard, attention should be paid and trimmed.

Improper control of process conditions

If size of injection molding machine is large, screw speed is too high, injection pressure is too large, injection holding time is too long, excessive filling will be formed, so that molding shrinkage rate is smaller than expected, and demolding will be difficult.
If temperature of barrel and melt is too high, injection pressure is too large, and hot melt easily enters gap between mold inserts to generate a flash, resulting in poor mold release.
In addition, nozzle temperature is too low, cooling time is too short, injection is interrupted, which may cause mold release failure. Therefore, when eliminating problem of poor mold and mold release, injection pressure should be appropriately reduced, injection time should be shortened, temperature of barrel and melt should be lowered, cooling time should be prolonged, and melt flow should be prevented.

Raw materials do not meet requirements for use

If 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, foreign matter is mixed into barrel and hopper, plastic moulded parts will stick. In addition, uneven or excessive particle size of raw material also has a certain degree of influence on sticking mold. Therefore, purification and screening work should be done for materials.
plastic injection mould design 

Improper use of release agent

Purpose of using release agent is to reduce adhesion between surface of plastic moulded parts and mold cavity, preventing these two from sticking to each other, shorten molding cycle and improve surface quality of plastic part.
However, since release effect of release agent is affected by both chemical action and physical conditions, molding materials and processing conditions are different, optimum variety and dosage of selected release agent must be determined according to specific conditions. If used improperly, it often does not produce a good release effect.
As far as molding temperature is concerned, effective working temperature of fatty oil release agent is generally not more than 150 degrees, and it is not suitable for use at high temperature molding; working temperature of silicone oil and metal soap release agent is generally 150 to 250 degrees; Working temperature of vinyl fluoride release agent can reach 260 degrees or more, which is the best release agent for 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 moulded parts. In terms of method of use, paste release agent is brushed, and sprayable release agent is sprayed using a spray device. Since 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 plastic moulded parts after demolding, so it is possible to use a sprayable release agent.

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