Injection Molding of Thin-Walled Plastic Parts

Definition of Thin-Wall Injection Molding Technology

Thin-Wall Injection Molding Technology, also known as Thin-Wall Plastic Injection Molding Technology, has three definitions:

(1) The ratio of flow length to thickness, L/T, where the ratio of the flow length. L from the point where the molten material enters the mold to the farthest point in the cavitythat the molten material must fill to the average wall thickness (T) is above 100 or 150, is considered thin-wall injection molding.

(2) The thickness of the molded plastic part is less than 1 mm, and the projected area of the part is over 50 cm², defines thin-wall injection molding.

(3) The wall thickness of the molded plastic part is less than 1 mm (or 1.5 mm), or the t/d ratio (plastic part thickness t, part diameter d, for disc-shaped parts) is below 0.05, defines thin-wall injection molding.

It can be seen that the critical value for defining thin-wall injection molding varies, making it a relative concept.

Injection Molding of Thin-Walled Plastic Parts

Prospects of Thin-Wall Injection Molding

As consumer awareness of food safety continues to rise, polypropylene products are being increasingly applied in fields such as food packaging.

It is understood that disposable meal boxes, due to their convenience, affordability, and a consumption rate exceeding 15 billion units annually in China, have a vast market potential.

Among them, polypropylene disposable meal boxes offer advantages such as hygiene, environmental friendliness, aesthetic appeal, practicality, and cost-effectiveness, gradually replacing meal boxes made from other materials.

High-flow injection-molded polypropylene, with its short processing cycle and good profitability, has also become one of the trends in high-end polypropylene development.

The demand in China for specialty materials for thin-wall polypropylene is expected to grow, offering promising market prospects and economic benefits.

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Applications of Thin-Wall Injection Molding

According to research, thin-wall injection molding is currently widely used in various fields in China, including food containers, pharmaceutical boxes, cosmetics, stationery, and even multimedia packaging.

For example, it is used in disposable packaging boxes and cosmetic bottles, among other everyday items.

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Common Issues and Solutions in Thin-Wall Injection Molding


Causes of Defects

Mold Correction

Molding Improvement

Short Shot

Small parts and corners of the finished product cannot be completely formed due to inadequate mold processing or poor ventilation. Insufficient injection volume or pressure during molding may also be attributed to design flaws (insufficient thickness).

Modify the mold at the inadequate material filling location, take or improve venting measures, increase thickness, and enhance the gate (enlarge the gate, add more gates).

Increase the injection volume, and raise the injection pressure, among other measures.


It often occurs at uneven wall thickness or thickness of the molded product, due to differential cooling or solidification shrinkage of the molten plastic, such as the backside of ribs, edges with sidewalls, and the backside of main columns.

Thin out, but at least retain 2/3 of the wall thickness; thicken runners, enlarge the gate; improve ventilation.

Raise the material temperature, increase injection pressure, prolong holding time, etc.

Surface Roughness

Commonly occurs on the backside of main columns or ribs that have undergone material reduction, or due to stress marks caused by the excessive height of cores or ejector pins.

Lower the sprue; modify the core and ejector pins; sandblast treatment on the female mold surface; reduce mold surface brightness.

Reduce the injection speed and decrease the injection pressure, etc.

Air Marks

Occurs at the gate, often due to low mold temperature, high injection speed, excessive pressure, improper gate design, or when plastic encounters flow disruption during gating.

Change the gate location, polish the runner, increase the cold slug area in the runner, enlarge the gate, and add surface embossing (can also be achieved by adjusting the machine or modifying the mold to eliminate weld lines).

Elevate mold temperature, decrease injection speed, reduce injection pressure, and so on.

Weld Lines

It occurs at the convergence point of two material flows, such as the merging of two gate flows or the merging of flows bypassing the core, due to decreasing material temperature and poor venting.

Change the gate location, add cooling wells, open vent slots, or create a pattern on the parting line of the mold.

Increase the material temperature and raise the mold temperature.


It often occurs at the junction of the male and female molds, typically due to poor mold alignment or improper machining of mold surfaces. In the molding process, it is often caused by insufficient clamping force, excessive material temperature, or high pressure.

Modify the mold, and then close it again.

Increase the clamping force (CHECK if the injection molding machine tonnage is sufficient), reduce material temperature, decrease injection pressure, reduce holding time, and lower holding pressure.


Narrow, large-area thin-wall parts, or relatively large products with asymmetric structures can experience uneven cooling stresses or inconsistent ejection forces during the molding process.

Adjust the ejector pin; install a material-pulling pin with tensioning function, if necessary; adjust deformation by adding trim on the core side.

Adjust the mold temperature for the male and female molds, reduce holding pressure, etc. (adjustments for small part deformation mainly rely on pressure magnitude and time, while adjustments for large part deformation typically depend on mold temperature).

Surface Contamination

The mold surface is rough; for PC material, sometimes due to excessive mold temperature, there may be residual adhesive or oil stains.

Clean the mold surface, apply a polishing treatment.

Reduce the mold temperature, etc.

Silver Streaks

It often occurs at thin-walled corners or the base of thin-walled RIBs, due to poor force distribution during demolding, improper ejector pin settings, or insufficient mold draft angles. 

Increase the radius of the corner R; enlarge the demolding draft angle; increase the size of the ejector pin or its cross-sectional area; polish the mold surface; polish the ejector pin or angled pin.

Reduce the injection speed, decrease the injection pressure, lower the holding pressure, and time, etc.

Cold Slugs

It manifests as poor demolding or mold damage, flashing, mainly due to insufficient draft angle or rough mold surface, with molding conditions also having an impact.

“Increase the draft angle; polish the mold surface; when sticking to the female mold surface, you can increase/change the ejector pin; pay attention to the diameter of the horn when using horn feed; add a parting line to the male mold.

Reduce injection pressure, lower holding pressure and time, etc.


When transparent PC materials are molded, it is prone to occur due to insufficient gas venting during the injection molding process, improper mold design, or incorrect molding conditions.

Increase ventilation, modify the gate (enlarge the sprue), and the PC material flow channel must be polished.

Strict drying conditions, increase injection pressure, reduce injection speed, etc.

Other defects such as pin marks, scorching, flow lines, silver streaks, etc.

Dimensional Out of Tolerance

Problems with the mold itself or inappropriate molding conditions causing improper shrinkage.

Typically, changes in holding time and injection pressure (second stage) have the greatest impact on dimensions. For example, increasing injection pressure and enhancing the holding pressure packing effect can significantly increase dimensions. Lowering mold temperature can also have an effect, and increasing the size of the gate or adding more gates can improve the adjustment effect.

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