14 injection molding defect solutions

Short shot refers to a situation in which the injected product is incomplete due to the mold being not completely filled.

This defect usually occurs at the point farthest from the gate, or in the part that needs to be reached through a narrow part of the mold, because the narrow part may hinder the flow of the melt.

Short shots may cause very small marks, and may also cause a significant loss of a large part of the product.

the reason:

The reasons that may cause short shots are:

Insufficient raw materials for injection into the mold;

The melt flow resistance is too large to allow it to flow to the mold;

Poor venting of the mold forms air pockets, which block the melt and prevent it from flowing to some parts of the mold.

2. Flash

The flash edge is formed by adhering excess material extruded from the cavity to the product.

This defect can occur on the edge of the product or between the various components of the mold. The material may overflow from the mold along the parting line and may also overflow from the joint of the static and dynamic mold.

Flashing may also occur at the core, which may be due to hydraulic or angled pins. The flash edge problem is different in severity, sometimes thin, and may be thicker in severe cases.

the reason:

The reasons that may cause flashing are:

The mold clamping surface is damaged or worn seriously;

Misalignment when the static and dynamic modes are locked;

The pressure of the raw material in the mold is greater than the clamping force;

The third case mentioned above can also be caused by many different reasons. In the following cases, the material pressure may be greater than the clamping force:

In the initial stage of injection molding (filling stage), too much raw material is injected, which increases the pressure inside the mold;

Excessive resistance of the melt flow during the filling process also increases the pressure in the mold;

The cavity pressure is too high during the pressure holding phase;

The clamping force is not enough.

3. Degradation

The results from degradation are varied and the extent and severity of the problem varies. In the most severe cases, the product may be completely discolored and cause poor mechanical properties. Local degradation only causes dark streaks or spots.

the reason:

Degradation is caused by damage to the raw materials. Long-chain molecules that form plastics decompose under excessive heat or excessive shear stress. The volatile gases produced by the molecular decomposition process can accelerate the degradation process, which causes the raw materials to discolor. A large amount of molecular decomposition will eventually destroy the composition of the raw materials and adversely affect its mechanical properties.

Local degradation may be caused by uneven temperature in the drying cylinder.

Degradation occurs under the following conditions:

Raw material is overheated in a drying cylinder or hot runner system

The material stays in the drying cylinder for too long

The shear stress applied to the material during the injection molding process is excessive. If the nozzle is clogged, the gate and runner are too narrow, or a similar situation occurs, the stress will be exacerbated.

4. Deformation

Under normal circumstances, the shape of the product should match the shape of the mold. Deformation defects refer to the product being deformed.

In severe cases, the product will be completely deformed when it is ejected from the mold; when the situation is not serious, the shape of the product will appear small irregularities.

Longer, unsupported edges and larger areas of the product are the areas that are most prone to deformation.

the reason:

The reasons for the deformation are:

The temperature of the product is too high when demoulding

Due to the different cooling time of the thicker and thinner areas of the product, or due to the difference in static and dynamic mold temperatures, the internal shrinkage of the product

In-mold pressure caused by poor mold flow during filling (so-called "frozen orientation") or excessive pressure of the cavity during the holding phase.

5. Impurities

Impurities are expressed in the form of spots, plaques or markings on the product. The most common situation is the appearance of black spots.

Impurities may be only tiny spots, and in severe cases they may show significant streaks or large fading.

the reason:

The reason for the impurities is that impurities are mixed into the raw materials, for example:

The raw material is mixed with impurities in the feed funnel.

The degraded material may fall from the bolts of the machine, the inner wall of the drying cylinder, the joint/nozzle assembly or any cutting mechanism, and be mixed into the raw material.

6. Layering

Layering defects create a "skin effect" on the surface of the product, ie the surface and the properties of the other parts of the product are different, forming a layer that can be peeled off.

When the delamination defect is severe, the entire cross section is composed of multiple layers and is not fused together. When the defect is light, the appearance of the product may meet the requirements, but the defect will damage the mechanical properties of the product.

the reason:

There are two main reasons for the formation of layered defects:

First, delamination can result when two different materials are mismixed together. The two raw materials may enter the drying cylinder under pressure, but they cannot be melted together when cooled in the mold, but are forced together as different layers to form a product.

Second, shear stress is generated if the cold melt is forced to pass through the narrow gate at high speed. Excessive shear stress can cause the melt layer that melts earlier to fail to fuse completely.

The risk of mixing:

Please note that some chemicals react with each other when a strong chemical reaction occurs. For example, PVC and Avetal must not be mixed.

7. Silver wire

Silver filaments usually appear as silver or gray stripes on the surface of the product.

The silver wire may only appear as a local stripe, and in severe cases the entire surface will be covered by stripes.

Silver wire can affect the appearance of the product and also destroy the mechanical properties of the product.

the reason:

The main reasons for the silver wire are as follows:

The raw materials are moist. Some hygroscopic materials absorb moisture from the air. If the raw material is too moist, pressurized steam is formed under the action of high temperature and high pressure of the drying cylinder. These steam breaks through the surface of the product to form a silver stripe.

The melt receives thermal damage and produces localized degradation. The resulting volatile gas is blocked at the surface of the mold, forming streaks on the surface of the product.

This situation is not as severe as the degradation mentioned earlier. This can occur as long as the melt temperature is too high or subjected to shear stress during plasticization or injection into the mold.

8. Gloss / shadow

The surface finish of the product should be as good as the finish of the mold. When the finish of the two is different, a gloss/shadow defect occurs.

When the defect occurs, the surface that should be smooth will appear dim, and the rough surface should be shiny.

the reason:

The reasons for the occurrence of gloss/shadow defects are:

Poor melt flow or low mold surface temperature makes it impossible to replicate the surface finish of the mold when the material is formed.

The cavity pressure generated during the holding phase is insufficient, so that the material cannot adhere to the surface of the mold during the cooling process and leaves a sink mark.

9. Flow marks

Flow marks can appear on the surface of the product and come in a variety of forms, usually forming a shaded area.

The flow marks do not appear uneven on the surface of the product, and they do not feel when they are crossed by fingers. This defect is also referred to as drag, ghost and shadow.

When the flow marks are severe, grooves are formed, leaving marks like marks on the surface of the product.

the reason:

Flow marks can occur under the following conditions:

Poor melt flow or low mold surface temperature makes the plastic flow resistance excessive during the filling process.

The melt flow is blocked during the filling process. This may be due to the uneven surface of the mold, the marking or pattern printed on the surface of the mold, or the change in melt flow during the filling process.

10. Bonding wire

The bond line is produced when two melt fronts meet during the filling process. It will appear on the surface of the product like a line.

Bonding lines may form lines like "cracks" on the surface of the product, or they may be fine lines that are not easily found.

When designing the mold, some bonding lines that can be seen by the naked eye are unavoidable. In this case, the wire needs to be shortened as much as possible to avoid damaging the product strength or affecting the appearance of the product.

the reason:

There are many reasons for forming a melt front. The melt front is most likely to form as the melt flows along the edge of one core. Bonding lines are formed when the two melt fronts meet. The temperatures of the two melt fronts must be sufficiently high so that they can be fused together without affecting the strength and appearance of the product.

Bonding line defects are caused when the two melt fronts are not sufficiently fused.

The occurrence of defects is:

The melt flow rate is different in the thicker and thinner portions of the mold. The temperature of the melt decreases as it flows through the thinner portion of the mold;

Each channel has a different length, and a longer channel is easier to cool;

The cavity pressure during the packing phase is insufficient to completely fuse the melt front;

The remaining bubbles prevent the melt front from fusing, which in turn leads to charring defects.

Scoring

Charred performance is similar to short shots, but with irregular fade edges and a slight burnt smell. In severe cases, carbon black areas appear on the product, accompanied by the smell of plastic burning.

If defects are not excluded, black deposits usually appear on the mold. They may also block the pores if the gases or oily substances produced by charring are not checked in time. Charging usually occurs at the end of the channel.

the reason:

Charring is caused by the internal combustion effect. When the air is pressurized for a short period of time, the temperature rises, causing combustion. Statistics show that the internal combustion effect during injection molding may produce high temperatures above 600 °C.

Charring defects may occur in the following cases:

The filling speed is too fast to allow air to be removed from the cavity. The air is blocked by the inflowing plastic to form bubbles, which generate an internal combustion effect after being pressed;

The air holes are blocked or the ventilation is not smooth. The air in the mold should be exhausted from the air holes. If the ventilation is not smooth due to the position, number, size or function of the air holes, air will remain in the mold, causing burnt. Excessive clamping force can also result in poor ventilation.

12. Print down

Shrinking refers to shallow depressions on the surface of the product.

When the defect is light, the surface of the product is uneven; when it is severe, it shows a large area collapse of the entire product. Products with arches, shanks and protrusions often suffer from shrinkage defects.

the reason:

The shrinkage is caused by a large area shrinkage of the raw material during the cooling process.

In thicker parts of the product (such as arches), the core temperature of the raw material is lower, so it will shrink later than the surface, which will produce a contraction force inside the raw material, and the outer surface will be pulled inward to form a shrinkage.

A reduction will occur in the following cases:

The cavity pressure during cooling is lower than the force generated by the shrinkage of the material;

The cavity pressurization time is insufficient during the cooling process, causing the raw material to flow back out of the cavity from the gate;

There is not enough buffering of the material during the forming and holding stages, which may be due to the complete withdrawal of the screw before the extrusion of sufficient additional material.

The runner and gate sections are much smaller than the wall thickness of the product. This means that the gate may have frozen before the product is completely squeezed out.

13. Bubbles

Vacuum bubbles appear in the form of bubbles, which are easily visible on transparent products and can be seen on the cross section of opaque products.

the reason:

Bubbles are the vacuum on the product that is produced as the material shrinks during the cooling process.

As with shrinking, the shrinkage force is generated inside the raw material. The difference is that when the bubble is generated, the outer surface of the product has solidified and does not collapse, thus creating hollow bubbles on the product.

The causes of bubbles are the same as shrinking, including:

Insufficient cavity pressure;

Insufficient cavity pressurization time;

The runner and gate size are too small.

14. Spray marks

The spray marks refer to the threaded areas that appear across the gate. Spray marks not only affect the appearance of the product, but also affect the strength of the product.

the reason:

The spray marks are caused by the uncontrolled flow of the melt during the filling process.

The molten plastic enters the mold under great pressure. If the filling speed is too fast, the plastic will eject from the open space of the cavity and quickly bounce back and cool to form a thread that blocks the molten plastic from entering the gate.

The main cause of the spray marks is poor gate location or poor gate design, but the following two conditions can make the defects bigger:

Filling speed is too fast;

The melt flow is not smooth when filling the mold.