Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (2023)

Pineapple marks (scratches)

Pit marks are indentations in the surface.injection molded plastic partdid not cause the lastphase or step of the plastic injection molding processduring the cooling process. Thicker parts of the plastic cool more slowly than others, resulting in a higher percentage shrinkage in that local area. After the material on the outside cools and solidifies, the material on the inside begins to cool and its contraction pulls the surface inward, causing a cleft in the surface.

The occasional use of high compaction pressures causesacceptable brands of sinksreduction in volumetric shrinkage, although it cannot be completely eliminated. This is because the volumetric change from molten to solid plastic is around 25%, and the compressibility of plastic at typical injection molding pressure is only 15%, which means that it is impossible to compress molten plastic sufficiently. enough to compensate for cooling shrinkage.

The following design and engineering rules in mold making and injection molding can minimize this drawback:

  1. The thickness of the plastic part must be uniform.
  2. Avoid using gates that are too small, as they prevent proper compaction of the polymer.
  3. The thicker areas of the part should be filled first so that they are compacted before the thinner parts solidify.
  4. Increase the compression time.
  5. Increase the tamping pressure to push more material into the injection mold and reduce shrinkage.
  6. Increases cooldown time.
  7. Modify the geometry of the part so that the dimples appear in areas where they have the least impact on the design.
  8. Lower the injection temperature of the material and the mold. Although increased plastic injection temperature allows for greater compaction of the molten polymer, sag marks increase due to additional volumetric shrinkage.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (1)

incomplete charge

Incomplete charging occurs when ainjection molded part is missing materialto correctly generate its geometry. This occurs when the molten polymer does not fill the entire cavity (or cavities) in theInjection mold, usually the thinnest parts where the molten polymer cools before completely filling the mold. Any factor that increases the frontal resistance to flow of the molten polymer can result in incomplete filling. Some of these factors are:

  1. Existence of areas of the injection mold that excessively restrict the flow of molten polymer, such as feed channels, inlets, thin walls, etc.
  2. Low plastic injection temperatures and mold walls.
  3. The existence of trapped air inside the mold cavity.
  4. Insufficient plastic injection pressure from the machine.
  5. Premature solidification of the molten polymer due to:
    • The phenomenon of hesitation.
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Some of the practices that we develop inMoldblade Engineering Departmentto solve the problem of incomplete loading are:

  1. Fill the thickest parts of the workpiece first before the thinnest parts to avoid "wavering".
  2. Plastic collets should be placed away from thin areas of the part or areas with sudden changes in thickness.
  3. Increase the number of ports to reduce the length of the flow.
  4. Increase the size of the inlet ducts to reduce resistance to flow.
  5. Place vents in appropriate locations (in the areas of the mold that are ultimately filled) to allow trapped air to escape.
  6. Increase the size and number of ventilation openings.
  7. Increase the injection pressure. To prevent damage to the machine's hydraulic system, the operating injection pressure is generally limited to 70-85% of the maximum injection pressure.
  8. Increase injection rate. This causes the temperature to rise and reduces the viscosity of the molten polymer.
  9. Increase injection temperature and/or mold wall temperature. Higher temperatures reduce the viscosity of the molten polymer and make it easier to fill the mold.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (3)

trapped air

trapped air defectappears whena certain amount of air cannot escape from the mold during injection,a small area appeared without materialinjected part. correctlyinjection mold designWith each shot, air is forced out through mold openings, mold inserts, or even ejectors, which also act as vents.

locationsareas of air leakage in the moldsare found in the filled in areas at the endcycle or injection phase. A common cause of entrapped air defects is the mold. Another common cause is when a raceway occurs (tendency of molten polymer to flow mostly in thicker sections, leaving thinner areas with trapped air). Translated from (free version)

Trapped air will lead to voids and bubbles inside.molded plastic part, incomplete filling or surface defects such as stains or burns.

followdesign rulesdoesinjection moldcan be used to minimize air entrapment defects:

  1. Avoid large thickness changes.
  2. Locate the openings in the last areas of the mold to be filled.
  3. Place the inlets in areas that provide adequate ventilation for the forms that are filled at the end of the cycle.
  4. Make sure the vents are large enough to allow cavity air to escape during injection, but not so large that molten polymer escapes and creates flash. The recommended size of the vent zones is 0.025 mm for crystalline polymers and 0.038 mm for amorphous polymers.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (4)

– Reduce injection speed. High speeds of plastic injection can cause jetting, resulting in trapped air appearing right at the front door. Reducing the injection rate will give the air displaced in the door enough time to escape through the vent zones.

Welding and joining lines.

A weld line (also called a weld mark) is formed when two melt flow fronts traveling in opposite directions meet. Conversely, a bridge line occurs if those two fronts flow parallel to each other, creating a bridge line.

Traditionally, the angle of connection between two faces is useddistinguish weld lines from seam lines.A seam angle less than 135° gives a weld line, while a seam angle greater than 135° is defined as a seam line. Generally, the weld line mark disappears when the welding angle reaches 120° to 150°. INweld lines are considered more criticalinstead of seam lines in terms of joint aesthetics and mechanical properties. Translated at (free version)

Weld lines and seams can be caused by holes or inserts in the part, existencemore injection holes, or due to areas of variable wall thickness where hesitation or stroke following occurs.

If weld or seam lines cannot be avoided, it is good practice to ensure that they are generated in areas of low visibility or mechanically non-critical areas. often doesmodifying the plastic injection port, modifying the flow fronts and the areas where the welding/joining lines appear. Another practice is to try to get a good match between the two flows so that the resulting mechanical weakness is not excessive. To achieve this, the goal is to bring the two flow fronts closer to the highest possible temperature and pressure so that they are not far from the inlet. Translated at (free version)

Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (5)

followrecommendationscan be used to reduce impactweld lines and parting lines on injection molded parts.

  1. Adjust injection port position to produce weld/joint lines in areas of poor visibility or low mechanical demand.
  2. Allow weld lines to form at higher temperature and pressure. For this, it is recommended: o Increase the injection temperature. o Increase injection rate. o Increase injection pressure.
  3. Increase the wall thickness. This will facilitate the transfer of pressure and maintain a higher melt temperature.
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OFlash is a defect that occurs when part of the molten polymerflows through the gaps in theinjection moldsuch as the cutting plane, ventilation zones, ejectors, etc. Burrs occur for the following reasons:

  1. Low clamping force of mold halves. if this isInjection molding machine clamping forceis too weak to hold the mold plates together during the molding process, some of the polymer injected into the cavity will spill out of the parting plane and create bulges.
  2. spaces between halvesinjection mold. If the mold parting surfaces do not make full contact during mold closing (for example, the mold is deformed), a defect will quickly develop.
  3. inappropriateplastic injection conditions. Conditions such as too high an injection temperature (polymer is more fluid and flows more easily through the holes in the mold) or high injection pressures.
  4. Inadequate ventilation system. When the size of the air holes in the mold is too large, irregularities will appear in these areas.

considering onlyinjection Variables,followcorrections can be made to reduce the buildup problem.

  1. Increase the size of the injection molding machine. The irregularity may be the result of an insufficient clamping force problem of the machine.
  2. Adjust the closing force if the capacity of the machine allows it.
  3. Lower the injection temperature. The high temperature reduces the viscosity of the molten polymer, making it easier for the polymer to flow through gaps in the parting plane, ejectors, or aeration zones.
  4. Reduce injection and compaction pressures to reduce the clamping force required.
  5. Increase the injection time or decrease the injection rate to reduce the injection pressure and therefore reduce the required clamping force.

Dimensional shrinkage, bent and folded..

Odimensional shrinkageparts is inherentinjection molding process. Shrinkage occurs because the density of the polymer varies from processing temperature to room temperature (see, for example, the specific volume of a semi-crystalline polymer in Figure 5.46 – PVT Curve). Duringinjection molding process steps, shrinkage due to cooling produces a series of internal stresses in the part. These residual stresses act on the piece with effects similar to possible external stresses. If the residual stresses induced during molding are high enough, the part can warp/twist or distort after ejection, resulting in defective parts.

Deformation or torsion of ainjection molded plastic partthis is due, therefore, to the existence of a series of residual internal stresses in the piece, which in turn are derived from the differential contraction of the material during cooling. If the shrinkage is uniform throughout the part, the resulting part will not warp or twist, it will just shrink evenly and become smaller. INcrystalline polymers, For example. acetal, nylon,high density polyethylene,polyethylene terephthalatemipolypropylenecause the biggest problems with 1-4% shrinkage. Amorphous polymers, e.g. polystyrene, acrylic andpolycarbonatethey are easier to treat, with retractions of only 0.3 to 0.7%.

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However, the scopeuniform contractionis complicated by the presence and interaction of many factors, such as the orientations of the polymer molecules, temperature variations on the mold walls,compression variations in plastic parts(overcompacted areas and undercompacted areas, due to unbalanced flow paths), etc. Note that areas of higher compaction, such as injection ports, have less shrinkage, as this offsets some of the compaction of the molten polymer. On the contrary, the areas furthest away from the neck suffer less compaction and therefore tend to shrink more.

A basic rule to avoid excessive distortion of the part due totemperature differences after injection, is that the average temperature differences in any part of the piece after pressing should not exceed 15-20ºC.

Some corrective actions toreduce the problem of deformationmitwisting of injection molded partsThey are the following:

  1. Avoid thick parts. Thick parts of the part cause significant shrinkage on cooling. The use of uniform ribs and thicknesses produces a uniform contraction of the piece and reduces the cost and weight of the piece, maintaining its mechanical resistance.
  2. Balanced flows. The injection system must generate balanced material flows with a constant flow rate.
  3. Proper compaction pressure can partially offset shrinkage of the polymer on cooling.
  4. Locate the inlet openings in the parts with the largest cross section.
  5. Increases cooldown time.
  6. Increase the injection temperature. The use of low injection temperatures causes the material to solidify rapidly without time to compact sufficiently, resulting in high shrinkage. Some of the above factors and their effect on the shrinkage of the final part are shown in the following figure.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (8)

Poor surface finish (flow marks, wrinkles, dents, etc.).

A poor finish can be caused by folds or waves that form at the edges of the part or in the last fill areas during injection molding.

Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (9)

The formation of folds or waves is due to the fact that part of the flow front cools rapidly on the walls of the mold, creating folds in the flow front itself. INthe main factors that influence the formation of these wrinklesThey are the flow rate, the temperature of the mold walls, the temperature of the molten polymer, among others.

Some of the actions to takeimprove surface treatmentthey are associated with actions to increase the flow rate and temperature of the molten polymer and the walls of the mold. Therefore, the improvement of surface quality is achieved through measures such as:

  1. Increase the mold temperature.
  2. Increase the injection rate, which increases the temperature of the molten polymer due to shearing and reduces the viscosity of the polymer.
  3. Increased injection pressure.
  4. Increased injection temperature.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (10)

jet effect.

Oblast defectIt occurs when molten polymer is forced at high speed through a small area, such as a nozzle or injection port, to access a much larger area. A blast defect results in mechanical weakness of the part, surface imperfections, and multiple internal defects.

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Forminimize the effect of detonation,The following corrective actions can be taken:

  1. Reduce injection rate in areas of abrupt area change.
  2. The use of tongue gates or fan gates, which produce a smoother transition between the feed channel/funnel and the mold cavity.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (12)
  1. Increase the size of the inlet port and inlet conduit.
  2. Adjust the injection profile so that the rate decreases as the molten polymer enters the mold cavity and then speeds up. Figure 5.52 shows an example of an injection profile with these characteristics.
Plastic Injection Molding: Main Disadvantages of Injection Molded Parts - Moldblade (13)
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