Solutions to common molding defects of injection molding

Posted on : Aug 25 , 2022 By  GREFEE

Defects are common when using molds to process plastic injection molding parts, and this largely affects the processing efficiency. The following are common molding defects and solutions for plastic injection mold parts.

Short shots

Short shots refer to the products made are incomplete due to the molds are not fully filled.

This defect usually appears on the farthest spot away from the gate or the parts that only can be reached through the narrow areas on the mold  because the narrow areas may affect the flow of melts.

Short shot may cause micro flow marks or result in a large part of product missing obviously.

Cause:

Causes for short shots include:

The raw material injected into mold is not enough.

The resistance of melts is large, resulting in the mold cannot be filled completely.

The venting of mold is poor and cause the generation of cavitation that blocks the melt, making the melt cannot flow to some areas of mold.

Burrs

Burrs are generated from the adhesion of excess raw materials extruded from the mold cavity to the product.

This defect will on the edges on the product or each composed part of mold. The raw material may be overflowed from the mold, or the bonding sites of the moving and fixing molds.

Burrs can also be found on the mold core, which is due to hydraulic pressure or angular pin.

The severity of burrs varies, sometimes thin, sometimes thicker.

Cause:

Causes for burrs include:

Clamping mold surface is damaged or is heavily worn.

The moving mold and fixing mold are dis-located when they are locked.

The pressure of raw material in mold is higher than mold clamping force.

The third condition mentioned above would be resulted from various reasons. In the following situations, the pressure of raw material is higher than mold clamping force.

At the first stage of injection mold (mold filling stage), too much raw material is filled, which increases the pressure inside the mold.

During the mold filling process, large resistance of melt flow will raise the pressure inside the mold, too.

The mold cavity pressure is too high during the pressure holding stage.

Mold clamping force is not enough.

Degradation

The decomposition could lead to many results. The extent and severity of the problem vary, too. In the most serious case, it may cause complete discoloration of the product and poor mechanical properties. Local degradation will only cause dark stripes or spots.

Cause:

The degradation is caused by the raw material being damaged. Long chain molecules that form plastics will decompose under the action of excessive heat or excessive shear stress. During the decomposition of molecules, the volatile gas will speed up the degradation process, which will cause the de-coloring of the raw material. The decomposition of large amount of molecules will finally break the content of the raw material and cause negative influence on the mechanical properties.

Local degradation may be resulted from uneven temperature of the material barrel.  

Degradation might occur in the following situations:

The raw material is being over heated in the material barrel or hot runner system.

The raw material stay in the barrel for too long.

During the injection molding process, the shear stress exerted on the raw material is too large. If the nozzles are blocked, or gates and runner are too narrow, it will increase the shear stress.

Deformation

In the normal situations, the shapes of products should be in line with that of molds. The deformation refers to the deformity of products.

When the condition gets worse, the products will be completely deform when it is ejected from the mold. When the condition is not serious, the product shape will appear small irregularities.

Long but without support edges or large planes are areas that are the most prone to deformation.

Cause:

Causes for deformation:

The temperature is too high when the mold is released.

As the cooling time is different in thick and thin areas, or the mold temperature difference in moving mold and fixing mold, the shrinkage inside the products is different.

Mold flow is not smooth when filling (the so called “Freezing orientation”) or the pressure inside the mold cavity is too high at pressure holding stage.

Impurities

The impurities is often appear in the form of spots in different colors, patches or stripes. The most common one is the black spot.

Impurities may be just tiny spots, but could also be obvious stripes or large part of de-coloring when it is serious.

Cause:

The impurities are caused by the sundries mixed with raw materials, such as:

The raw material mixed with sundries when it is transported into the barrels.

The decomposition of raw material might be fall from any cutting mechanisms and mixed into raw materials, such as machine bolts, inner wall of drying drum, joints / nozzles.

Lamination

The lamination will generate the “skin effect” on the surface of products, which is caused by the difference in properties and textures of the surface of products and other raw materials, and it forms a peeling skin that can be removed.

When lamination is serious, the whole cross section area is made up of different layers, and has not been melted together. When the defects are less obvious, the appearance of products might meet the requirements, but will break the mechanical properties of products.

Cause:

There are two main causes for lamination. The first one is that when two different kinds of raw materials mixed together incorrectly. The two raw materials will be transported into the barrel at the same time under the pressure. However, when the mold cannot be melted together when it is cooled, just as different layers are forcibly pressed together to form products.

Second: if the cold melt is forced to pass through the narrow gate, the shear stress will be generated. Too high shear stress will cause the melt layer melted in advance cannot be completely fused.

Risk of mixing:

One thing that should be aware of is that some raw materials mixed together will result in strong chemical reaction, such as PVC and Avetal must not be mixed.

Silver linear

Sliver linear might just be local phenomenon, but could be expand to the whole surface when it is serious.

Silver linear will affect the appearance of products and also damages the mechanical properties of products.

Cause:

The following two points cause the silver linear:

The raw material is wet and some of them will absorb the steam in the air. If the raw material is too wet, the pressurized vapor might be generated under the high temperature and high pressure of barrel. These vapors break through the surface of the product and form silver stripes.

The melt got thermal damaged and generates local degradation. The volatile gas generated will be blocked on the surface on mold and generate stripes on the surface of products.

This is not as worse as the degradation. As long as the temperature of melt is high or it is subjected to shear stress during plasticization or inject into the mold, this could happen.

Gloss/shadow

The surface finish of products should be the same as that of molds. When the surface finish of two are different, the gloss/shadow defects occurred.

The surface will be gloomy when defects occurred, and the rough surface is smooth and glossy.

Cause:

Causes for gloss/shadow include:

The melt flows un-smoothly or the temperature of the mold surface is low, resulting in the mold surface finish cannot be duplicated when material molding.

During the pressure holding, the pressure in the cavity is not high enough to make the material cling to the surface of the mold in cooling process, leaving shrinkage marks.

Flow marks

Flow marks can be found on the surface of products in multiple forms. Generally, it will form a shadow area.

Flow marks do not produce any embossing or depression on the surface of products, which cannot be felt with fingers. This defect is also called drag marks, ghosting, and shadows.

When flow marks are obvious, it will generate grooves, and leave defects like marks on the surface of products.

Cause:

Flow marks can be found when :

The flowability of melt is poor or the surface temperature of mold is low, resulting in a large flow resistance of plastic in the mold filling process.

In the mold filling, the melt flow with resistance, which may be caused by the uneven surface of the die, the marks or patterns printed on the die surface, or the change of melt flow direction during the filling process.

Jointing line

The joining line is generated when two melt fronts meet during mold filling, and will appear on the surface of the product like a line.

Jointing line is like the cracking line on the surface of products, which is not obvious to detect.

When designing molds, some visible jointing lines are unavoidable. In this case, shortens the jointing line as much as possible to prevent the strength and appearance of products being damaged.

Cause:

There are many reasons for the generation of melt front. The most possible reason could be the melt flow along the edges of the mold core. When the two melts meet, it produces the jointing lines. The temperature of two melts front should be high enough to allow them to be fuse together successfully, and do not affect the strength and appearance of products.

When the two melts cannot fuse together completely, the defects will be produced.

Causes for defects:

The mold has thicker and thinner parts, and the flow speed of melts is different, when the melt flows through the thin part of mold, the temperature is low.

The length of each runner is different. Loner runners will be easy to cool.

The mold cavity pressure is not sufficient enough to allow the melt to fuse completely during the pressure holding stage.

The remaining bubbles make the melt front unable to fuse, which will also lead to burning.

Burning

The burning is similar to that of short shot, but with irregular fading edges and slight burning smell. The carbon black areas will appear on the product, when the condition is serious, accompanied by the smell of plastic burning.

If the defects are not eliminated, there is often black deposition on the mold. If the gas or oil substances produced by burning are not checked immediately, they may block the air holes. Burning is generally found on the end of the pathways.

Cause:

Burning is caused by the internal combustion effect. When the pressure in the air increasing sharply in a very short time, the temperature will raise and cause burning. According to the collected data, the internal combustion effect in the injection molding process can produce high temperature up to 600 degrees.

Burning may be produced when:

The mold filling speed is fast so that the air cannot be voided from the mold cavity, and generates air bubbles due to the block of the incoming plastic, and lead to internal combustion effect after being compressed.

The air holes are blocked or the ventilation is not smooth.

The air in the mold should be voided from the air holes. If the ventilation is affected by the position, number, size or functions, the air will be stay in the mold and lead to burning. Large mold clamping force will also lead to poor ventilation.

Shrinkage

Shrinkage refers to the slight hollows on the surface of products.

When the defects are slight, the surface of products is uneven. When it is serious, the large area of products will collapse. Products with arches, handles and protrusions often suffer from shrinkage defects.

Cause:

Shrinkage is caused by large-area shrinkage of raw materials during cooling.

In the thick area of products (like arch), the core temperate of material is low, so the shrinkage will occur later than that of surface, which will produce a contraction force inside the raw material, and pull the outer side into the inward depression to produce the shrinkage.

Shrinkages occur in the following situations:

The pressure in mold cavity is lower than the force generated from the shrinkage of raw material in cooling process.

Insufficient pressurization time of mold cavity during the cooling process, resulting in the raw material flow out of the cavity from the gate.

The raw material does not have sufficient buffering capacity during the molding and pressure holding stage as the screw is completely  withdrawn before the excessive raw material is injected.

The cross section areas of gates and runners are far smaller than the thickness of products, which means the gates are already frozen before the products extrusion process.

Bubbles

The vacuum bubbles is presented in the form of air bubbles, which can be found easily on the transparent products. It can also be seen on the cross section of opaque products.

Cause:

Air bubbles is the vacuum part of products, which are produced when the raw material shrinks during the cooling process.

Similar as the shrinkage, the inside of raw material produces the contractile force. What is difference is that the outer appearance of products has been solidified when the bubbles are formed, and there is no collapse, so the hollow bubbles are generated.

The causes of bubbles are the same as those of reduction, including:

Inefficient mold cavity presssure

Insufficient cavity pressurization time

The size of runner and gate are too small

Spraying marks

Spraying marks refer to the threaded area opposite the gate. The spraying marks not only affect the appearance of products, but also affect the strength of products.

Cause:

Spraying marks are caused by the melt flow out of control during the mold filling process.

The molten plastic gets into the mold under the huge pressure. If the mold filling speed is too high, the plastic will eject from the open gap of the mold cavity, and quickly spring back and cool. At the time, the threads are formed, which blocks the molten plastic entering into the gates.

The main cause for spraying marks are the incorrect position of gates or the design of gate. The following two situations will worsen the situation of defects:

High mold filling speed

Poor melt flow during mold filling

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