Common injection moulding defects

Table of Contents

Injection molding is a process used to make plastic parts. Although it is a fairly simple process, some common pitfalls can occur.

This blog post will outline the most common injection molding process defects and how to prevent them.

One of the defects of injection molding: Cold Slug

Cold Slug

Appearance: This means that there is a piece of cold material stuck or stuck to the surface near the material head. The cold head will lead to traces on the surface of products, serious will reduce the mechanical properties of products.

Physical reasons

Cold heads often occur when the melt can be cooled near a machine nozzle or hot runner.

Defects can occur in this area because the molten material injected first always accumulates near the gate.

It is caused by unreasonable temperature control around the machine nozzle or hot runner nozzle.

Reasons related to processing parameters and improvement measures are shown below:

If the hot runner temperature is too low, increase the hot runner temperature.

The nozzle temperature is too low. Measure the nozzle temperature, raise the nozzle temperature, and reduce the nozzle contact area.

Reasons related to design and improvement measures are shown below:

The nozzle cross-section is too small to increase the nozzle cross-section.

Improper gate geometry changes gate geometry will leave the cold head in the passage.

Improper hot runner geometry changes the hot runner nozzle geometry.

Injection molding defects two: Gramophone Rippie

Gramophone Rippie

Deep grooves can be seen throughout the direction of the material flow, even to the end of the flow passage.

This phenomenon occurs in the production of products made of highly viscous (illiquid) materials and thick walls, where the grooves look like grooves on a record. It is very clear on PC material products, but it is larger on ABS products and is grey.

Physical reasons

If during injection molding, especially at low injection speeds, the melt in contact with the mold surface condenses too fast and the flow resistance is too high, distortion will occur at the fluid front end. The solidified outer material does not completely contact the cavity wall and forms a wavy shape. The wavy material freezes and holding pressure no longer flattens it out.

Reasons related to processing parameters and improvement measures are shown below:

The injection speed is too low: Increase the injection speed

If the melt temperature is too low: raise the temperature of the barrel and increase the screw back pressure

Increase the mold temperature: if the mold surface temperature is too low

If the pressure is too low, increase the pressure

Reasons related to design and improvement measures are shown below:

1. The gate cross-section is too small to increase the gate cross-section, shorten the gate

2. The nozzle hole is too small to increase the nozzle hole

Injection molding defects three: Weld lines

Weld lines

In the filling mode, the fusion joint refers to a line where the front ends of each fluid meeting. Especially where the mold design has a highly polished surface, the fusion joint on the product looks like a scratch or groove, especially on dark or transparent products. The position of the fusion joint is always in the direction of the material flow.

Physical reasons

Fusion joints are formed where streams of molten material bifurcate and are joined together, typically around the core of the molten flow or the use of multiple sprue products.

Where streams meet again, the surface forms fusion joints and streamlines. The larger the core around the melt or the longer the runner between the gates, the more obvious the fusion joint formed. Small fusion joints will not affect the strength of the product.

However, where the manufacturing process is very long or the temperature and pressure are insufficient, the improper filling will result in noticeable grooves.

The main reason is the weak spot caused by uneven fusion of fluid front end. Spots may occur where pigment is added to the polymer because there is a marked difference in orientation. The number and location of gates determine the number and location of fusion joints. The smaller the angle at which the fluid front meets, the more obvious the fusion joint.

In most cases, it is not possible to completely avoid fusion joints or flow lines during process commissioning. What can be done is to reduce their brightness or move them to an inconspicuous or completely invisible place.

Reasons related to processing parameters and improvement measures are shown below:

1. The injection speed is too low. Increase the injection speed

2. If the melting temperature is too low, increase the temperature of the barrel.

3. If the mold surface temperature is too low, increase the mold temperature.

4. If the pressure is too low, increase the pressure and switch the pressure as soon as possible.

Reasons related to design and improvement measures are shown below:

1. The gate location is not reasonable to relocate the gate and move it to an invisible place.

2. There is no exhaust hole in the material flow passage. The size of the exhaust hole should conform to the characteristics of the material.

Injection molding defects four: Moisture Streaks

Moisture Streaks

The surface watermark is a long silver wire on the surface of the product. The opening direction of the watermark is along the direction of the material flow. Where the product is not filled, the fluid front end is rough.

Physical reasons

Some plastics such as PA, ABS, PMMA, SAN, and PBT are easy to absorb water. If the plastic is not stored well, moisture can enter the particles or adhere to the surface. When the particles melt, the moisture turns into steam to form bubbles. During injection molding process, these bubbles are exposed to the surface of the fluid front, bursting and producing irregular patterns

Reasons related to processing parameters and improvement measures are shown below:

The residual moisture in the particles is too high to check the storage conditions of the particles, shorten the time of the particles in the hopper, and provide enough pre-drying for the material.

Injection molding defects five: Colour Streaks

Colour Streaks

Apparent color imbalance is the color of the surface of the product is not the same, can be near the head and far, occasionally in the sharp edge of the material flow area.

Physical reasons

Uneven color is caused by the uneven distribution of pigments, especially when adding color through masterbatch, toner, or liquid color.

When the temperature is lower than the recommended processing temperature, the master of color cannot be completely homogenized. When the molding temperature is too high, or the residue time of the barrel is too long, it is easy to cause the thermal degradation of pigment or plastic, resulting in uneven color.

When the material is plasticized or homogenized at the correct temperature, if injected too quickly through the cross-section of the head, friction heat may result in degradation of the pigment and color change.

Usually, when using color masterbatch, it is necessary to ensure the compatibility of chemical and physical properties of pigment and resin to be colored by its solution.

Reasons related to processing parameters and improvement measures are shown below:

1. The material is not evenly mixed to reduce the screw speed; Increase barrel temperature and screw back pressure.

2. If the melt temperature is too low, increase the barrel temperature and screw back the pressure.

3. Screw back pressure is too low to increase screw back pressure.

4. The screw speed is too high to reduce the screw speed.

Reasons related to design and improvement measures are shown below:

1. If the screw stroke is too long, the cylinder with a large diameter or long diameter comparison shall be used.

2. The cylinder with a large diameter or large length-diameter ratio shall be used for the short residence time of the molten material in the cylinder.

3. If the screw L: D is too low, use the barrel with a large diameter.

4. Screw compression ratio is low, high compression ratio screw.

5. No cut and mixed sections provide a cut and/or mixed sections.

Injection molding defects six: Charred Streaks

Charred Streaks

The surface of the apparent product shows very dark streaks of silver and light brown.

Physical reasons

Scorched dark marks are caused by excessive thermal degradation of the melt. The pale brown markings are due to oxidation or decomposition of the melt. Silver lines are generally caused by friction on the screw, check ring, nozzle, material head, narrow cross-section, or sharp edge area in the product.

In general, severe degradation or decomposition of plastics will occur when the machine is shut down and the barrel continues to be heated.

If stripes are found only near the head, the cause is not only inadequate optimization of hot runner temperature control but also related to the nozzle of the machine.

Even if the temperature of the melt is slightly higher, the residual time of the melt in the barrel is relatively long, which will lead to the mechanical properties of the product decline.

Due to the degradation chain reaction caused by the thermal movement of molecules, the fluidity of the melt will increase, so the phenomenon of mold overflow inevitably occurs. Be especially careful with complex injection molds.

Reasons related to processing parameters and improvement measures are shown below:

1. If the melting temperature is too high, reduce the temperature of the barrel.

2. If The hot runner temperature is too high to check the hot runner temperature, reduce the hot runner temperature.

3. Small diameter barrel is used when the molten material remains in the barrel for too long.

4. The injection speed is too high to reduce the injection speed: multi-stage injection: fast-slow.

Defects in injection molding: Glass Fiber Streaks

Glass Fiber Streaks

The surface of the molten plastic injection molding products with glass fiber appears a variety of defects: gray, rough, some of the metal bright spots, and other obvious features, especially the convex part of the material flow area, fluid convergence near the junction line.

Physical reasons

If the injection temperature is too low and the mold temperature is too low, the material containing the glass fiber tends to condense too quickly on the mold surface, after which the glass fiber is no longer embedded in the melt.

When two front lines of material flow meet, the orientation of the glass fiber is in the direction of each thin flow, resulting in irregular surface material at the intersection, resulting in the formation of joints or flow lines.

These phenomena are more obvious when the melt in the barrel is not completely mixed, for example, the screw stroke is too long, resulting in the uneven mixing of the melt is also injected.

Reasons related to processing parameters and improvement measures are shown below:

1. The injection speed is too low to increase the injection speed: consider using multi-stage injection: slow first and fast later.

2. The mold temperature is too low to increase the mold temperature.

3. If the melt temperature is too low, increase the barrel temperature and screw back the pressure.

4. Melt temperature change is high, such as melt uneven increase screw back pressure; Reduce screw speed; Use the longer barrel to shorten the stroke.

Injection molding defects eight: Overflow (Flash)

Overflow (Flash)

A thin flapping edge appears around the recess, along the parting line, or on the die sealing surface.

Physical reasons

In most cases, overflow occurs because the clamping force of the machine is insufficient to lock and seal the mold along the parting line during injection and pressure holding. If there is high pressure in the cavity, mold deformation here may cause mold overflow.

Under the condition of high molding temperature and injection speed, the melt can still flow fully at the end of the flow channel, if the touch is not locked, it will produce an overflow edge.

If an overflow is found at only one point on the mold, it indicates a defect in the mold itself: the mold is not completely sealed there. Typical overflow case: local overflow is due to mold defects, and spread to the whole surrounding is due to insufficient clamping force.

Attention must be paid! To avoid overflow, we should be careful when increasing the clamping force, because excessive clamping force is easy to damage the mold.

It is recommended that the true cause of overflow should be carefully identified. Especially before using a multi-cavity mold, it is a good idea to prepare some mold analysis data to provide the correct answer to all the questions.

Reasons related to processing parameters and improvement measures are shown below:

1.The clamping force is not enough to increase the clamping force

2. Injection speed is too fast to reduce the injection speed: multi-stage injection: fast – slow

3.The late pressure switch a little earlier pressure switch

4. Melt temperature is too high to reduce the temperature of the barrel

5. The mold wall temperature is too high to reduce the mold wall temperature

6. The pressure is too high to reduce the pressure

Reasons related to design and improvement measures are shown below:

1.The strength of the mold cavity is not enough to increase the strength of the mold

2.The mold is not sealed enough at the parting line or convex edge to redesign the mold

Defects in injection molding: Shrink (Sink marks)

Shrink (Sink marks)

There are dents in the material accumulation area on the surface of the plastic injection molded parts. Shrinkage water mainly occurs in plastic parts where the wall is thick or where the uniform wall thickness changes.

Physical reasons

When the product is cooled, shrinkage (volume reduction, shrinkage) occurs, at this time the outer layer of the mold wall first freezes, forming internal stress in the center of the product.

If the stress is too high, it can cause the outer plastic to plastic deformation, in other words, the outer layer to sag inward.

If the shrinkage occurs and the outer wall deformation is not stabilized (because there is no cooling) and the pressure holding is not replenished into the die, the settlement will form between the die wall and the solidified outer layer of the product.

These depressions are usually referred to as contractions. If the product has a thick section, such shrinkage may also occur after demoulding. That’s because there’s still heat inside, and it’s going to go through the outer layer and heat the outer layer. Tensile stress in the product causes the hot outer layer to settle inward, contracting in the process.

Reasons related to processing parameters and improvement measures are shown below:

1. Pressure is too low to increase pressure

2. The holding time is too short to prolong the holding time

3. The mold wall temperature is too high to reduce the mold wall temperature

4. The melt temperature is too high to reduce the melt temperature, reduce the temperature of the barrel

Reasons related to design and improvement measures are shown below:

1. The cross-section of the head is too small to increase the cross-section of the head

2. The material head is too long to shorten the material head

3. The nozzle hole is too small to increase the nozzle aperture

4. The material head is positioned in the thick wall at the thin wall

5. Excessive material accumulation to avoid material accumulation

6. The wall/bar section is not reasonable to provide a more reasonable wall/bar section ratio

Defects in injection molding: Short shot

Short shot

Appearance: The cavity is not filled, mainly away from the head or thin wall surface.

Physical reasons

The injection pressure and/or injection rate of the melt is too low and the melt cools at the end of the flow length. This is usually the case when high viscosity materials are injected at low melt and mold temperatures. It can also occur when a high pressuring pressure is required but the pressure holding Settings are disproportionately low.

When high injection pressure is required, the pressure holding should be increased proportionally: normally, the pressure holding should be about 50% of the injection pressure, but if high injection pressure is used, the pressure holding should be 70%~80%.

If the unsatisfactory injection molded is found near the head, it can be interpreted that the fluid front is blocked at these points and the thicker areas are filled first. Thus, after the cavity is nearly filled, the melt at the thin wall has condensed and there is a small amount of flow at the center of the fluid resulting in insufficient injection.

Reasons related to processing parameters and improvement measures are shown below:

1. Increase injection pressure if injection pressure is too low

2. Injection speed is too low. Increase the injection speed

3.The pressure is too low to increase the pressure

4. Pressure holding switch too early delays the switch from injection to pressure holding

5. If the melt temperature is too low, increase the barrel temperature and screw back the pressure

6. The holding time is too short to prolong the holding time

Reasons related to design and improvement measures are shown below:

1. The cross-section of the runner/head is too small to increase the cross-section of the runner/head

2. Insufficient exhaust of the mold improves the exhaust property of the mold

3. The nozzle hole is too small to increase the nozzle aperture

4. The thickness of the thin wall is not enough to increase the section thickness

Injection molding defects eleven: Warpage

Warpage

The shape of the apparent part rotates or twists after the part is demoulded or for some time after. Typically, the flat part of the product is undulating, and the straight edge bends or twists inward or outward.

Physical reasons

Product- because of its properties – the frozen molecular chain shifts internally under stress. In demoulding, according to the shape of different products, stress will often cause different degrees of deformation.

The internal stress makes the product shrink unevenly, the small particles shift, the cooling in the particles is not balanced or the excessive pressure in the particles.

In particular, products made of partially crystalline materials, such as PE, PP, and POM, are more likely to shrink and warpage than amorphous materials such as PS, ABS, PMMA, and PC.

Reasons related to processing parameters and improvement measures are shown below:

1. The pressure inside the mold is too high to reduce the pressure, the pressure switch in advance

2. The mold temperature is too low to increase the mold temperature

3. Fluid front, viscosity is too low to increase the injection speed

4. If the melt temperature is too low, increase the barrel temperature and screw back the pressure

Reasons related to design and improvement measures are shown below:

1. Mold temperature is not stable to provide cooling/heating balance of the mold release agents

2. Irregular section thickness according to the characteristics of resin redesign product shape size

Injection molding defects of the twelfth: Top white (Ejector marks) 

Top white (Ejector marks) 

Appearances Stress whitening and stress elevation are found on the side of the product facing the nozzle, i.e., where the ejector rod is located on the ejector side of the mold.

Physical reasons

If the necessary release force is too high or the surface of the ejector bar is relatively small, the surface pressure here will be high and deformation will occur, eventually causing whitening of the ejector area.

The causes and improvement measures related to processing parameters are shown below:

1. If holding pressure is too high Reduce holding pressure 
2. If holding time is too long shorten the holding time 
3. holding time switch too late to switch the holding pressure ahead of time 
4. The cooling time is too short Extend the cooling time

The design-related reasons and improvement measures are listed below:

1. Insufficient demoulding slope Select demoulding slope according to specifications 
2. Rough surface in the demoulding direction Polishing the mould in the demoulding direction 
3. Vacuum formed on the ejector side Air valve installed in the core

Summary

Defects in injection molding can be expensive and time-consuming to troubleshoot and correct. By understanding the most common defects, you can take precautions to avoid them in advance.

We hope this article has been helpful in outlining some of the most common injection molding defects. If you have any questions or would like more information on how to prevent these problems, you can contact ZetarMold.

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