12 Injection Molding Defects and How to Prevent Them

There are several?problems manufacturers face when it comes to injection molding. Quality issues?in injection molded products can range from minor surface defects to more serious ones?that can affect the safety, performance,?and function of the product.

They can be either caused by problems related to the molding process, material use, tool design, or a combination of all three.

Some of the most common quality defects associated with injection molding are listed here.

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1.?Jetting

2.?Burn marks

3.?Warping

4.?Vacuum voids

5.?Sink marks

6.?Flash

7.?Discoloration

8.?Delamination

9.?Short Shot

10.?Black Spot

11.?Water Splash

12.?Brittleness

Let us elaborate on the major causes that trigger these defects?and what you can do to prevent them.

1.?Jetting

A fusion line is a surface defect that occurs when two streams of material meet and melt. It often occurs on the fusion surface of the molten resin flowing in the mold cavity, and mainly affects the appearance of the product. This defect also has an impact on painting and plating.

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Causes

These melt marks produce if there are holes, inserts,?or multi-gate injection patterns in the parts or due to uneven wall thickness.

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Improvement method:

●?Increase the flow of plastic melt

●?Change the location of the gate

●?Add exhaust slots

●?Process aspects.

●?Increase injection pressure and holding pressure

●?Increase melt temperature

●?Reduce the amount of mold release agent used

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2.?Burn marks

?Burn marks usually appear as black or rust discoloration on the edges or surfaces of molded plastic parts. These marks?generally do not affect the integrity of the part unless the plastic is?burned to the point of degradation.

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Causes

Common causes of burn marks in injection molded parts are

●?Lack of venting of the tool

●?Rapid injection rate

●?Decomposition of materials such as flame retardant

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Improvement Methods

●?Reduce the injection temperature

●?Decrease?the injection speed to limit the risk of trapped air in the mold

●?Enlarge the vent and gate, or shorten the gate to allow trapped air to escape the mold

●?Reduce injection pressure

●?Reduce the last stage of injection speed

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3.?Warping

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Warpage is a deformation that can occur in an injection molded product when different parts of the component shrink unevenly. Uneven shrinkage can place excessive stress on different areas of the molded part. This excessive stress can cause the finished part to bend or twist as it cools.

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Causes

The main causes of warpage in injection molded plastics and similar materials are?too short a cooling time, hot material, lack of cooling around the tool, incorrect water temperature (part bends inward toward the hot side of the tool), and uneven shrinkage between part areas.

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Improvement Methods

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●?Ensure that the cooling process is gradual and long enough to prevent uneven stresses on the material

●?Reduce the temperature of the material or mold

●?Try switching to materials that shrink less during cooling (for example, particle-filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades)

●?Redesign the mold with uniform wall thickness and part symmetry to ensure greater stability of the part during cooling

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4.?Vacuum voids/air?pockets

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Voids are air trapped in the cavity and cause the part to bubble. These?are usually located where the melt is last filled.

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Causes

●?Lack of holding pressure (holding pressure is used to pack the part during the holding period).

●?Filling too quickly and not allowing the edges of the part to set.

●?The mold may also not be aligned when the two halves are not properly centered and the part wall thickness is different.

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Filling too fast does not cause this. When the part shrinks, the resin separates from itself because there is not enough resin in the cavity. Voids can occur in any area or part not limited by thickness, but by resin flow and thermal conductivity. These?are more likely to occur in thicker areas such as ribs or tabs. Other root causes of voids are unmelted melt pools.

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Improvement Methods

●?Increasing injection pressure to expel trapped cavities

●?Selecting a lower viscosity grade of material to limit the risk of bubble formation

●?Placing the gate close to the thickest part of the mold to prevent premature cooling in the areas of the material most prone to voids

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5.?Sink marks

Sink marks are surface depressions in molded parts at the wall thickness. They are usually found in reinforcement bars, sinkholes, or internal grids.

Causes

Shrinkage is caused by

●?Low holding time/pressure

●?Cooling time is?too short

●?High?gate temperature

●?Too much material or too thick walls

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Improvement Methods

●?Increase the holding pressure and time to allow the material near the part surface to cool

●?Increase cooling time to limit shrinkage

●?Design molds with thinner component walls to allow for faster cooling near the surface

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6.?Flash

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Flash is the appearance of excess plastic in the mold parting surface or top bar and other parts.

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Causes

●?Insufficient clamping force

●?If the clamping force is too small to offset the injection pressure during the injection process, it is easy to produce flying/burrs on the parting surface.

Defects in the mold

●?Unreasonable molding conditions

●?Improper design of exhaust system

Improvement method.

●?Reasonable design of the mold, to ensure that the mold can be closed tightly when closing the mold, without creating gaps

●?Set the proper size of clamping force

●?Increase the injection time and reduce the injection speed

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7.?Discoloration

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Discoloration or "color streaks" occur when the color of a molded part differs from the expected color. Usually, discoloration is limited to a localized area or a few abnormal color streaks on a molded part. This defect usually affects the appearance of the part without reducing its strength.

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Causes and Prevention of Discoloration

Common causes of discoloration are:

●?Residual particles in the hopper or residual resin in the nozzle or mold from previous production runs.

●?Poor heat stability of colorants

●?Improper mixing of masterbatch

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Improvement Method

●?Ensure that workers properly clean hoppers, nozzles,?and molds between production run?to eliminate any residual particles or base material

●?Consider using cleaning agents to remove excess color from the machine

●?Make sure that you or your supplier uses a colorant with adequate heat stability

●?Ensure that masterbatches are mixed uniformly for consistent color output

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8.?Delamination

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Delamination is the surface of the part that can be peeled off layer by layer.

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Causes

The causes of delamination are as follows

●?Mixing incompatible polymers

●?Using too much release agent during molding

●?Low melt temperature in the cavity

●?Too much moisture

●?Sharp corners on the gate and runner

●?Inconsistent resin temperature

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Improvement Methods

●?Avoid mixing incompatible impurities or contaminated return material into the raw material

●?Chamfer all runners and gates with sharp angles to achieve a smooth transition

●?Increase barrel and mold temperatures

●?Proper drying of the material before?molding

●?Avoid the use of excessive release agents

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9.?Short shot

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The lack of glue is a phenomenon that indicates that the?mold cavity isn’t?filled.

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Causes

Any factor that prevents the flow of the polymer melt or makes the polymer injection volume insufficient can lead to a lack of glue, such as

●?Low melt temperature, mold temperature or injection pressure, and speed

●?Uneven plasticization of the material

●?Poor venting

●?Insufficient flow of raw materials

●?The part is too thin or the gate size is too small

●?Premature hardening of the polymer melt due to poor structural design or failure to mold on time.

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Improvement Methods

●?Increase the fluidity of the melt

●?Fill the thick wall before the thin wall to avoid stagnation and premature hardening of the polymer melt.

●?Increase the injection pressure

●?Enhance?the number of gates and reduce the flow ratio

●?Increase the flow channel size to reduce the flow resistance

●?Proper location of venting ports to avoid poor venting

●?Increase the number and size of exhaust ports

●?Raise?the injection speed to enhance the shear heat

●?Increase the injection volume

●?Increase the barrel temperature and mold temperature

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10.?Black spots or black lines

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Black spots or black stripes refer to the presence of brownish-black spots or stripes or brown stripes on the surface of the part.

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Causes

The causes are expected to be of two kinds.

●?Material degradation:?Decomposition of the plastic by overheating will cause black spots or streaks. If the plastic stays inside the closed barrel or on the surface of the screw for too long, it will lead to carbonization and degradation, thus producing black spots or streaks during the injection molding process.

●?Material pollution: The presence of dirty recycled materials, foreign objects, other colors of materials,?or easily degradable low molecular materials in plastic may cause the above phenomenon. The dust in the air also causes black spots on the surface of the parts.

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Improvement methods

●?Use non-polluting raw materials

●?Place the material in a relatively closed storage bin

●?Increase the thermal stability of the material

●?Clean the top bar and slider

●?Improving the exhaust system

●?Clean and polish any dead spots in the runners to ensure no material buildup

●?Clean mold surfaces before?injection

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11.?Water splash

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Splash is a surface defect in which water, air,?or carbide is distributed in the direction of flow on the surface of the part in an emissive manner.

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Causes

●?High water content in the raw material

●?Air entrapment in the raw material

●?Degradation of polymer

●?Contamination of the material

●?The barrel?temperature is too high

●?Insufficient injection volume

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Improvement method.

●?Dry the material before injection according to the data provided by the material supplier

●?Increase the size of the main flow channel, manifold, and gate

●?Check for adequate venting locations

●?When changing material, clean the old material completely from the barrel.

●?Increase back pressure

●?Improve venting system

●?Reduce melt temperature, injection pressure,?and injection speed

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12.?Brittleness

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Brittleness refers to the easy cracking and fracture of the part in certain areas. It?is mainly due to the degradation of the material resulting in the fracture of the macromolecules, which reduces the molecular weight of the macromolecules and thus the overall physical properties of the polymer.

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Causes

●?Unsuitable drying conditions

●?Wrong temperature setting for injection molding

●?Improper setting of gate and runner system

●?Improper screw design

●?Poor strength of dissolution marks

●?Using the high quantity?of the recycled?material

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Improvement method

●?Set proper drying conditions before injection

●?Increase the size of the main flow channel, manifold,?and gate

●?Select a good screw for the injection molding machine

●?Reduce barrel and nozzle temperature

●?Reduce back pressure, screw speed,?and injection speed to reduce excessive shear heat generation and avoid polymer decomposition.

●?If the brittleness is caused by insufficient dissolution trace strength, the dissolution trace strength can be improved by increasing the melt temperature and injection pressure.

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Conclusion

Manufacturing with injection molding usually requires a significant upfront investment in the mold. This is why it is especially important to design the mold the first time, rather than starting over after serious defects are discovered.

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Defects related to the molding process or material are often easier and less costly to fix. But no matter what the cause, defects in molded products can significantly hurt your product sales.

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As ZHXTOYS with over 30 years of experience in the injection molding industry, we have ample experience in these areas and will certainly do our best to take steps to prevent them in the future and ensure that your products meet quality standards.