Improvement method for common defects of thin wall injection molding

Time:2019-07-08 08:34:49 / Popularity: / Source:

Thin wall injection molding technology is also known as thin-walled plastic injection molding technology. It is a small branch of modified plastic injection moulding process.
There are three types of definitions:
Ratio of flow length to thickness L/T, that is, injection moulding with a ratio of L (flow length from melt into mold to the farthest point of cavity that melt must be filled) to corresponding average wall thickness T of 100 or more is a thin wall injection molding;
An injection moulding method in which thickness of plastic moulded parts is less than 1 mm, and projected area of plastic moulded parts is 50 cm 2 or more;
Wall thickness of mold parts is less than 1 mm (or 1.5 mm), or injection moulding of t/d (plastic thickness t, plastic part diameter d, for disc-shaped plastic moulded parts ) below 0.05 is defined as thin wall injection molding.
It can be seen that critical value defined by thin wall injection molding will also change, it should be a relative concept.
Thin wall injection molding 
Thin wall injection moulding requirements for raw materials: large flow length, high impact strength, high heat distortion temperature, high thermal stability, low directionality and good dimensional stability; also consider low temperature impact stiffness, flame retardancy, mechanical assembly and appearance quality of plastic raw materials.
Currently used thin wall injection molding materials are: polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PC / ABS blends and PA6. As wall thickness decreases, plastics with better physical properties need to be used to maintain strength of article.
Although thinning of plastic moulded parts has many advantages, it reduces formability of plastic moulded parts, so that these thin-walled plastic moulded parts cannot be formed by conventional injection moulding methods. When forming thin-walled plastic moulded parts, following common problems exist:

Short shot

Short shot refers to a quality defect in which plastic moulded parts is incomplete due to incomplete filling of mold cavity, that is melt has condensed before filling is completed.
Filling process and cooling process of conventional injection moulding are intertwined. When polymer melt flows, melt front encounters a relatively low temperature core surface or cavity wall, which forms a condensation layer on its surface. Melt continues to flow forward in condensing layer. As thickness of condensing layer increases, actual cavity flow path narrows and thickness of condensing layer has a significant effect on flow of polymer.
Because thickness of plastic moulded parts during conventional injection moulding is relatively thick, influence of condensation layer on injection moulding at this time is not so great. However, in thin wall injection molding, when ratio of thickness of condensing layer to thickness of plastic moulded parts gradually increases as thickness of plastic moulded parts is gradually increased, influence is large. In particular, dimensions of these two can be more prominent when compared to each other.
When thickness of plastic moulded parts is reduced, influence of condensation layer on flow will increase exponentially, which further illustrates influence of condensation layer in thin wall injection molding. If only from injection molding, injection moulding machine is required to have a high injection rate, so that rate at which plastic melt fills cavity exceeds rate at which condensation layer grows (or growth rate of condensation layer is slowed), so that filling action can be completed before flow section is closed, and injection moulding of thin-walled plastic moulded parts is performed.
When flow length is 300mm and wall thickness of plastic moulded parts is 3.0mm, L/T is 100 at this time, which is easily achieved by conventional injection moulding technology; however, when wall thickness of plastic moulded parts drops below 1.0mm, flow length ratio (100) which has been easily achieved is very difficult to achieve.

Warping deformation

Warpage deformation is a plastic part defect caused by uneven internal stress. Causes of warpage deformation are uneven shrinkage, orientation, and cooling.
Improvement method: It can improve warpage deformation defects of plastic moulded parts by balancing cooling system, adjusting cooling time, holding pressure and holding time.

Weld line

Weld line is boundary formed when two or more melt flow fronts are fused in cavity. Stress concentration is easy to occur at weld line, which weakens mechanical strength of plastic part, and is particularly detrimental to mechanical properties of plastic moulded parts, especially thin-walled plastic moulded parts. After external force, plastic moulded parts is easily cracked at weld line.
Improvement method: Design position can be reduced, changed by reducing number of gates or changing gate position to meet design requirements of plastic moulded parts.

Lack of materials

Small parts and corners of finished product cannot be completely formed. Because mold processing is not in place or exhaust is not smooth, design defects (insufficient thickness) due to insufficient injection dose or pressure.
Improvement method: It can correct mold at missing material, adopt or improve exhausting measures, add thickness, improve gate (increase gate), increase injection dose, increase injection pressure and other measures to improve.


Often occurs in wall thickness of molded product or uneven thickness, due to different cooling or curing shrinkage of hot melt plastic. Such as back of rib, edge of side wall, back of BOSS column stealing thickness, but at least keep 2 / 3 of thickness.
Improvement method: It can be improved by thickening flow channel, increasing gate, adding exhaust gas, increasing material temperature, increasing injection pressure, and prolonging dwell time.

Surface mark

Often occurs on the back of BOSS column, or back of rib, or because core and thimble are designed too high, causing stress mark to decrease.
Improvement method: It can be treated by modifying core, thimble, and mother mold surface blasting, etc., by reducing brightness of mold surface, reducing injection speed, and reducing injection pressure.

Gas lines

Occurs at the gate, mostly because mold temperature is not high, injection speed, pressure is too high, inlet is not properly set, and plastic hits spoiler structure when pouring.
Improvement method: It can be solved by changing inlet gate, runner lighting, increasing cold runner area and inlet gate, adding texture (by adjusting machine or repairing mold), increasing mold temperature, reducing injection speed and injection pressure.
Thin wall injection molding 

Combination line

Occurs at the confluence of two streams, such as flow of two inlet gates meets, flow of bypass core is due to decrease of material temperature and poor exhaust gas.
Improvement method: It can also increase material temperature, increase mold temperature by changing inlet gate, adding cold well, opening exhaust groove or biting male mold surface.

Raw edges

Often in the joint of male and female molds, due to poor mold clamping, or improper processing of corners of mold surface, molding is often due to insufficient clamping force, material temperature, pressure is too high.
Improvement method: mold correction, re-mold, increase clamping force, reduce material temperature, injection pressure, dwell time, and holding pressure.


Slender parts, large-area thin-walled parts, or large-scale products with asymmetrical structure are caused by uneven cooling stress or ejector stress.
Improvement method: Correct thimble, set tension pin, etc. If necessary, add texture to adjust deformation of male mold, adjust temperature of male and female molds to reduce pressure, etc. Adjustment of small deformation depends mainly magnitude and time of pressure, and adjustment of deformation of large part is generally dependent on mold temperature.

Dirty surface

It is due to rough surface of mold.
Improvement method: For PC materials, sometimes mold temperature is too high, and mold surface has residual glue and oil stains. It is necessary to clean mold surface in time, polish, and reduce mold temperature.


It is easy to occur at the corner of thin wall of molded product or at the root of thin-walled RIB. It is caused by poor force during demolding, improper setting of thimble or insufficient draft angle.
Improvement method: increase R angle at the corner, increase angle of demolding, increase  thimble or increase cross-sectional area, stencil surface, thimble or oblique pin, reduce injection rate, injection pressure, holding pressure and time.

Draft mark

It is characterized by poor mold release or mold injury. Molding conditions are also affected mainly due to insufficient draft angle or rough surface.
Improvement method: increase draft angle, mold surface lighting, can increase or change pull pin when sticking mold surface, pay attention to diameter of horn when feeding horn, add texture to male mold, reduce injection pressure, holding pressure and time.


Transparent finished PC material is easy to appear when it is formed. Due to lack of gas in  plastic injection moulding process, improper mold design or improper molding conditions have an impact.
Improvement method: increase exhaust gas, change gate (increase in gate), PC material flow channel must be polished, strict drying conditions, increase injection pressure, reduce injection speed and so on.

Gap difference

Occurs in joints of male and female modules, sliders, skew pins, etc., which are manifested by unevenness joint surface, etc. due to improper clamping or problem of mold itself.
Improvement method: Correct mold or re-make mold.

Size tolerance

Problems with mold itself, or improper molding conditions, result in improper mold shrinkage.
Improvement method: usually change dwell time and injection pressure (second stage) have greatest influence on size. For example, increasing injection pressure and pressure-holding, shrinking effect can significantly increase size, reduce mold temperature. Increasing inlet gate  can improve adjustment effect.

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