Common injection-molding defects and their countermeasures

Short shot or short molded

Ⅰ. Description

A molded product that is incomplete because the mold cavity was not filled completely.

Ⅱ. Causes

1. Insufficient injection-molding machine performance (shot capacity, plasticizing capacity, etc.)
2. Poor material flowability
3. Gate cross-section surface area is too small. Molded product thickness is too thin.
4. Poor gas venting.

Ⅲ. Countermeasures

<Molding equipment>

1. Increase the amount of material feed. If material feed is still insufficient at maximum material feed capacity, change to a larger capacity machine.
2. Install a screw with a back-flow check valve.
3. Increase injection pressure
4. Raise the cylinder temperature setting. Raise the nozzle temperature, too.
5. Make sure there are no severed lines to the heater.
6. Make sure the nozzle is not clogged. If the nozzle clogs frequently, raise the mold temperature or shorten the cycle time.
7. Increase injection speed

<Mold>

1. Raise the mold temperature.
2. Increase the mold gas release.
3. Increase gate cross section surface area.
4. Increase the molded product thickness.
5. Add ribs to the molded product design to improve flowability.

<Materials>

1. Choose a low-viscosity high flow material type.
2. Apply surface lubricant. (Add 0.05-0.1% by weight.)

Flow marks

Ⅰ. Description

Ring shaped miniature bands appear on the molded product surface around gates or narrow sections.

Ⅱ. Causes

The molten polymer cools rapidly within the mold and becomes highly viscous. Once that molten polymer starts to coagulate, it gets pushed by the molten polymer injected afterward and forms miniature bands.

Ⅲ. Countermeasures

<Molding equipment>

1. Increase the nozzle diameter. Increase the nozzle temperature
2. Increase injection speed.
3. Increase injection pressure.
4. Raise the cylinder temperature.
5. Reduce the cushion. (Reduce the charging amount.)

<Mold>

1. Increase the mold temperature.
2. Provide a cold slug well.
3. Increase the gate cross section surface area.
4. Create more distance between the gates and the location of the mold coolant.

<Materials>

1. Apply surface lubricant.

Silver streaking

Ⅰ. Description

Streaks of a silvery white color appear in the material flow direction.

Ⅱ. Causes

1. Material is absorbing water.
2. Material is contaminated with another type of material.
3. Aeration is occurring during molding.
4. Too much additive is present.

Ⅲ. Countermeasures

<Molding equipment>

1. Apply back pressure to generate a kneading effect.
2. Slow down injection speed.
3. Thoroughly clean molding machine cylinders.

<Mold>

1. Increase (or decrease) the size of the runners and gates.
2. Increase the mold temperature.
3. Change the gate locations.

<Materials>

1. Pre-dry thoroughly.
2. Install an ultrared light inside the hopper.
3. Investigate for potential contamination.

Poor weld line

Ⅰ. Description

The material splits off and flows in two or more directions. When the divergent flow fronts converge again, hair-like lines appear where they meet. This results in reduced strength, as well as burn-mark and bubble defects.

Ⅱ. Causes

1. Material splits and flows in separate directions, then converges again.
2. Polymer temperature is too low. Polymer is not fully melted.
3. Difficulties in venting gas and air.

Ⅲ. Countermeasures

<Molding equipment>

1. Raise the resin temperature
2. Increase the injection pressure.
3. Increase the injection speed.

<Mold>

1. Provide a cold slug well where weld lines occur.
2. Provide a gas vent.
3. Change the gate locations. Or increase the number of gates.
4. Stop using mold parting agents.
5. Change the gate locations so that the weld lines occur in locations less susceptible to external forces if weld strength is an issue.

<Materials>

1. Choose a low-viscosity high flow type material.

Voids

Ⅰ. Description

The presence of holes inside molded products

Ⅱ. Causes

1. Change in density as the material changes from a molten to a coagulated state.
2. The surface of the molded product loses heat rapidly through the mold wall and the polymer hardens. As a result, the polymer is pulled outward, causing all shrinkage to concentrate in the thickest parts of the product leaving holes within the product. In molded products with a thickness greater than 6 mm, voids are very difficult to avoid.
3. Air gets trapped within the molten polymer, leaving bubbles within the product

Ⅲ. Countermeasures

<Molding equipment>

1. Increase the injection pressure. (Increase the secondary pressure, too.)
2. Extend the injection time.
3. Lower the cylinder temperature setting.

<Mold>

1. Change the product design so that the thickness is no greater than 6 mm. To enhance strength, add several thick, uniform reinforcement ribs to the product design. Avoid thickness where possible.
2. Increase the size of the fates. Enable supplemental re-filling.
3. Make the runners shorter.
4. Lower the mold temperature. Make the mold temperature uniform.
5. Change the gate locations so as to prevent air intake.

<Materials>

1. Dry the material thoroughly.
2. Choose a highly viscous grade.

Sink marks

Ⅰ. Description

The surface of the molded product collapses somewhat-something that is particularly common in thicker products.

Ⅱ. Causes

Same as what causes voids, but sink marks arise when the surface cools too slowly.

Ⅲ. Countermeasures

<Molding equipment>

1. Increase the amount of material feed.
2. Increase the cavity internal pressure (secondary pressure).
3. Lower the cylinder temperature setting.
4. Increase the injection rate.
5. Increase the injection speed.
6. Lengthen the injection time.

<Mold>

1. Lower the mold temperature.
2. Make the mold temperature uniform.
3. Change the gate locations.
4. Make the gates larger.
5. Make the runners shorter.

<Materials>

1. Choose a highly viscous grade.

Warpage

Ⅰ. Description

The molded product warps immediately following or at some time following molding where the mold cavity should be straight.

Ⅱ. Causes

Residual stress from the molding process is released, causing deformation.

Ⅲ. Countermeasures

<Molding equipment>

1. Decrease the injection pressure.
2. Lower the cylinder temperature setting.
3. Lengthen the cooling time.

<Mold>

1. Make the thickness of the molded product uniform.
2. Eliminate undercuts.
3. Operate the knock out pins in a uniform fashion.
4. Provide multiple knock out pins.
5. Change the gate locations.
6. Lower the mold temperature.

Screw engagement failures

Ⅰ. Description

When feeding material for measurement, the screw rotates but does not engage the screw thread, or the screw does not turn at all.

Ⅱ. Causes

Occurs when the coefficient of friction between the material and another material, the material and the screw, or the material and the heating cylinder is unusually low or unusually high.

Ⅲ. Countermeasures

<Molding equipment>

1. Install a screw equipped with a back-flow check valve.
2. Choose a short compression type screw.
3. Change the cylinder temperature setting. Lower if the screw turns without engaging, raise if the screw is not turning.
4. Check to make sure no lines to the cylinder heater have been severed.
5. Perform an empty shot at least five times.

<Materials>

1. Apply a surface lubricant.
2. Make the granularity of the material uniform.

Sticking of parts in the mold

Ⅰ. Description

Ordinarily a molded product is removed from the mold by having the product stick to the moving side of the mold when the mold opens, after which the product is ejected using ejector pins. Sticking occurs when the part fails to separate from the stationary mold or from the moving mold. In the case of the latter, the problem could result in broken ejector pins.

Ⅱ. Causes

Possible causes include: the mold draft is too small; there is too much residual pressure from the molding process; too much shrinkage occurred; or the mold finish is perpendicular to the part release direction.

Ⅲ. Countermeasures

<Molding equipment>

1. Decrease injection pressure.
2. Lower the cylinder temperature setting.

<Mold>

1. Increase the mold draft.
2. Improve the mold finish.
3. Eliminate undercuts.
4. Use a more appropriate mold temperature. Be mindful of that fact that, in the case of box-shaped parts, the part will likely stick to the stationary mold if the stationary mold temperature is low.
5. Apply a mold parting agent.

Burn marks

Ⅰ. Description

Discoloration or dark reddish brown streaks in the molded product.

Ⅱ. Causes

Heat decomposition of the material.

Ⅲ. Countermeasures

<Molding equipment>

1. Lower the cylinder temperature setting.
2. Shorten the intermediate time (dry cycle time).
3. The ram is recessed for too long of a time duration.
4. Slow down injection speed.

<Mold>

1. Provide a larger gas vent.
2. Increase the gate sizes.