Plastic injection molding - Major defects and how to avoid them.

Maqsood Mohammed

Maqsood Mohammed

27th February 2021 · 9 min read

A detailed blog on different types of defects that arises during plastic injection molding and how you can avoid them.

Plastic injection molding - Major defects and how to avoid them.

If you are planning to manufacture or import the injection molded parts/ products, you need to learn about injection molding defects your products might have and ways to fix them. The defects can range from minor surface issues to significant shortcomings that hamper the functioning and may arise safety or performance issues as well. Injection molding defects can be related to the molding process, tool-design, material use, etc. Knowledge of common injection molding defects can help you to save time and money. It can also save you from the hassle of returning or loss of unsellable products. Let's have a brief understanding of the most common injection molding defects:

1. Warping

Once the product is molded into the desired shape, it is left to cool down. The rate of cooling can be different for various areas of the product; it entirely depends upon the shape and size. If few parts of the product cool down earlier than others, it may result in the bending or twisting of the product. This phenomenon of differential cooling rate and bending/twisting of products is known as warping. Warping not only causes aesthetic problems but also gives rise to structural issues and affects the strength of the product. You need to make sure that there must be gradual cooling of the product to inhibit the generation of stresses or warping. You should also ensure that mold must have uniformly thick walls and plastic should flow in one direction. It will help in uniform distribution of mold material and cooling rate.

Here are the key points to avoid warping:

  • Keep the cooling time and temperature rate controlled to ensure that complete material cools down evenly.
  • While injecting material, decrease the temperature of mold as well as of material.
  • Ensure to position the gate carefully so that cavities are filled evenly. The uniform filing will facilitate the even solidification, which will further prevent the warping.
  • Avoid using materials that are susceptible to warping. Semi-crystalline materials such as polyether ketone(PEK), polyphthalamide (PPA) and high-temperature nylon(HTN) are most likely to develop warping. You should choose plastic materials which are less susceptible to deformation or shrinkage.

2. Flow lines

Flow lines are completely aesthetic deformity. They appear as streaks or lines over the products' surface. They spoil the look of the final product. During injecting the material into the mold, if material flows into different directions at different speeds, it results in the development of flow lines. It also results in uneven distribution and differential rate of cooling. You should ensure to increase the speed, pressure and material temperature while injecting material so that molds get filled uniformly, and the material does not cool before filling up the molds properly.

Here are the key points to avoid flow lines:

  • Keep the corners rounded to ensure uniform filling in the mold.
  • Inject the material with high speed and pressure
  • Keep the material and mold temperature high during injecting
  • Ensure the optimum filling of mold by increasing the gate size

3. Sink Marks

Sink marks are opposite to warpage defects. It produces due to the prolonged cooling of some parts or lack of enough pressure to fill the mold properly, which gives rise to depressions or small recesses. You need to ensure that if there is any sink mark in the prototype, make sure to modify part of the mold with the thickest wall. You can also speed up the cooling in that particular area to prevent the sink mark.

Here are the key points to avoid sink marks:

  • Make sure to have a uniform wall thickness
  • Ensure to add injection in the thickest section; it will facilitate the filling of the thickest area on priority.
  • Prefer honeycomb or ribbed designs over large thick areas
  • Reduce the temperature of material and mold
  • Increase the pressure during injecting material as it will force the more material into the mold and thus prevent the shrinkage.

4. Vacuum voids:

Lack of sufficient pressure while injecting the material into molds results in quick cooling of the outer layer of material which is close to the wall. This rapid cooling also traps the air bubbles and leaves vacuum voids inside the products' material. To prevent deformity of vacuum voids, you should ensure to inject the material at high pressure; it will remove the air bubble by forcing more material in the mold. Controlled cooling and the use of low viscosity material would help in preventing vacuum voids.

Here are the key points to avoid Vacuum voids:

  • Make sure that product design doesn't contain unnecessary thick sections.
  • Ensure to have adequately large gate size to facilitate optimum mold filling
  • Decrease melt temperature or adjust injection speed whereas Increase Injection pressure, shot volume, screw forward time.
  • Make sure that material is dried completely and there is no moisture left to cause voids.

5. Burn Marks

Burn marks appear when the molten material is exposed to overheating or extreme fiction during the injection. It is caused when plastic material gets burned or degraded. Once the material solidifies, burn marks become visible. You should decrease the temperature of the material as well of mold to prevent friction and overheating. Shortening the cycle time of mold and increasing the gas vents, gates etc. can also be helpful to prevent burn marks.

Here are the key points to avoid burn marks:

  • Make sure there are adequate air vents or gates to release the trapped air out of the cavity.
  • Decrease the injection speed and injection pressure.
  • Decrease the plastic temperature during injecting.
  • Avoid keeping feather edges in your product's design.

6. Weld Lines

Weld lines often arise when the molten material splits in the mold. When a molten material is poured in a designed mold, it is supposed to take the given shapes, and then at merging points of designs, they are supposed to remerge. In some cases, they don't merge completely and give rise to weld lines by splitting in the molds. You can prevent weld lines by using a mold which facilitates single source flow pattern. Increased material temperature and injection speed will help material to bond effectively in the mold and prevent its splitting.

Here are the key points to avoid Weld Lines:

  • Prefer mold with the continuous and unidirectional flow of material
  • In case of unavoidable splitting of molten plastic, ensure that gate positioning will facilitate that both melt fronts come together uniformly at the same time.
  • Increase the temperature of mold as well as of material
  • Increase the injection speed

7. Jetting

This kind of deformity occurs due to excessive injection pressure while injecting material into the mold. It results in the appearance of wavy folds in the product. It is known as "Jetting". It doesn't only distort the product aesthetically but also affects its functionality. To produce an aesthetically as well as structurally sound product, keeping the optimum pressure level is the key.

Here are the key points to avoid Jetting:

  • Decrease the injection pressure to prevent the rapid squirting of molten material.
  • Increase the temperature of mold and material to prevent premature cooling
  • Ensure the right positioning of the gate in the design so that material gets Filled properly in the mold.

8. Short shot

The short shot is a deformity which refers to a shortage of material in the mold. By any reason, if a machine fails to pour enough material in the mold, it is known as "Short shot". Reasons for short shots can be lack of adequate heating of the material, use of highly viscous material or blockage of air vents, etc. To prevent short shots, you must ensure that machines are thoroughly checked for all blockages, air vents should be properly aerated.

Here are the key points to avoid Short shot:

  • Make sure to keep gate size large enough to allow adequate filling of molds
  • Ensure the gas vents are properly working to avoid the trapping of air
  • Raise the temperature of mold as well as molten plastic; it will help in filling the mold faster.
  • Ensure an adequate supply of material per shot.

9. Flash

In the process of injection molding when extra material solidifies on the edges of part is known as flash. It usually occurs due to leakage of material from the clamp & cavity during the production process. Once the production completes, these extra materials are removed by various processes like grinding or cutting etc. To prevent flashes, you must use high-quality molds with tighter tolerances. Optimize the injection pressure, air vents and clamp force to minimize the material leakage.

Here are the key points to avoid flash:

  • Increase the clamping pressure as well as maintain consistency during the production process.
  • Ensure that molds are clean and not damaged before loading the material
  • Decrease the injection pressure and temperature of molds as well as material
  • Optimize the pinhole tolerances and diameter of ejector pins

10. Discoloration

It is also known as "colour streaking". Discoloration refers to the defect when the manufactured product does not possess the desired colour. Generally discolouration limits to a localized area or in the form of streaks. It is entirely a cosmetic defect, and it doesn't affect the strength or other properties of the product.

Here are the key points to avoid discolouration:

  • Ensure to use an adequately thermo-stable colouring agent
  • Make sure that there is no residual pellet or base material left in the machine. The hopper, nozzle and molds etc. should be properly cleaned.
  • Remove the excess colour with the help of purging compounds.
  • Check the masterbatch carefully for the desired colour output.
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