16 kinds of thin-walled injection molding defects improvement method full explanation

I. What is thin-wall injection molding

Thin-wall injection molding technology is also known as thin-wall plastic injection molding technology. There are three definitions of it as follows.

The ratio of flow length to thickness L/T, that is, from the melt into the mold to the farthest point of the cavity that the melt must fill the flow length L and the corresponding average wall thickness T ratio of 100 or 150 or more injection molding for thin-walled injection molding.
The injection molding method in which the thickness of the molded part is less than 1 mm and the projected area of the molded part is more than 50 cm².

Injection molding where the wall thickness of the molded part is less than 1 mm (or 1.5 mm), or where t/d (part thickness t, part diameter d, for disc-type parts) is less than 0.05 is defined as thin-wall injection molding.
It can be seen that the critical value of the definition of thin-wall injection molding will also change, and it should be a relative concept.
 

II. Selection of raw materials

Requirements for raw materials: large flow length, high impact strength, high heat deflection temperature, high thermal stability, low directionality, and good dimensional stability; also consider the low-temperature impact rigidity of plastic raw materials, flame retardancy, mechanical assembly, and appearance quality.

Currently commonly used thin-walled injection molding raw materials are: polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PC / ABS blends, and PA6, etc. As the wall thickness decreases, it is necessary to use plastics with better physical properties to maintain the strength of the product.
 

III. Analysis of common defects

Although thin-walled plastic parts have many advantages, it reduces the moldability of plastic parts so that these thin-walled plastic parts cannot be molded by conventional injection molding methods. When molding thin-walled plastic parts, the following common problems exist.


1). Short shot
A short shot refers to the quality defect of incomplete plastic parts caused by the incomplete filling of the mold cavity, i.e., the melt condenses before the filling is completed.

The filling process and cooling process of conventional injection molding are intertwined. As the polymer melt flows, the melt front encounters the relatively low-temperature core surface or cavity wall, and a condensation layer forms on its surface. The melt continues to flow forward within the condensation layer, and as the thickness of the condensation layer increases, the actual cavity flow path becomes narrower, and the thickness of the condensation layer has a significant effect on polymer flow.

The thickness of the condensation layer has a significant effect on the flow of the polymer. Since the thickness of the plastic part is thicker in conventional injection molding, the influence of the condensation layer on the injection molding at this time is not significant. However, in thin-wall injection molding, when the ratio of the thickness of the condensation layer to the thickness of the plastic part increases gradually as the thickness of the plastic part becomes thinner, the effect is significant. Especially when the dimensions of the two can be compared with each other, it is more prominent.

When the thickness of the part decreases, the influence of the condensation layer on the flow increases exponentially, which further illustrates the great influence of the condensation layer in thin-walled injection molding. If we consider injection molding only, we need a high injection rate, so that the melt filling rate exceeds the growth rate of the condensation layer (or slow down the growth rate of the condensation layer), so that the filling action can be completed before the flow section is closed, and the thin-walled plastic parts can be injection molded.

When the flow length is 300mm and the wall thickness of the part is 3.0mm, the L/T is 100, which is easily achieved by conventional injection molding technology; however, when the wall thickness of the part decreases to

However, when the wall thickness of the plastic part drops below 1.0mm, the flow length to thickness ratio (100), which was once easy to achieve, becomes very difficult to achieve.


2). Warpage
Warpage deformation is a defect of plastic parts caused by uneven internal stress. Warpage deformation is caused by uneven shrinkage, uneven orientation, and uneven cooling.

Improvement method: The warpage deformation defect of plastic parts can be improved by balancing the cooling system, adjusting the cooling time, holding pressure and holding time, and other measures.


3). Fusing line
The fusion line is the boundary formed when two or more melt flow fronts are fused in the cavity. In the fusion line is easy to produce stress concentration, weakening the mechanical strength of the plastic parts, especially the mechanical properties of thin-walled plastic parts is particularly unfavorable after the external force plastic parts are very easy to crack at the fusion line.

Improvement method: The design can be reduced or changed by reducing the number of gates or changing the location of the gates to reduce or change the location of the fusion line to meet the design requirements of the plastic parts.


4). Missing material
The small parts and corners of the finished products cannot be fully formed, because the mold processing is not in place or the exhaust is not smooth, and the design defects (insufficient flesh thickness) are caused by insufficient injection volume or pressure on the molding.

Improvement methods: can amend the lack of material at the mold, take or improve the exhaust measures, add meat thickness, gate improvement (increase the gate, increase the gate), increase the amount of injection, increase the injection pressure, and other measures to improve.


5). Water shrinkage
Often occur in the molding wall thickness or uneven thickness of the meat, due to cooling or curing of hot melt plastic shrinkage caused by different. Such as the back of the rib, the edge of the sidewall, the back of the BOSS column steal meat, but at least 2/3 of the meat thickness is retained.

Improvement method: can be improved by thickening the runner, increasing the gate, adding exhaust, increasing the material temperature, increasing the injection pressure, extending the holding time, and other measures.


6). Surface image
It often occurs on the back of the BOSS column or bar after stealing meat, or due to the core and ejector pin design being too high to cause stress marks to reduce.

Improvement methods: can be dealt with by correcting the core, ejector pin, mother mold surface sandblasting, etc., using the way to reduce the brightness of the mold surface, reduce the injection speed, reduce the injection pressure, etc.


7). Air pattern
It occurs at the inlet gate, mostly because the mold temperature is not high, the injection speed and pressure are too high, the inlet gate is not set properly, and the plastic hits the disturbance structure when pouring.

Improvement methods: can be solved by changing the gate, runner lightening, runner cold material area increase, into the gate to increase, the surface plus nibble (by adjusting the machine or repairing the mold to catch up with the line can also be), increase the mold temperature, reduce the injection speed, reduce the injection pressure and so on.


8). Combination line
It occurs at the confluence of two material flows, such as the intersection of the material flows of two inlets and the intersection of the material flows around the core, which is caused by the decrease of material temperature and poor exhaust.

Improvement method: can be changed into the gate, add cold material well, open the exhaust slot or male mold surface bite flowers, etc., can also raise the material temperature, raise the mold temperature, etc.


9). Raw edge
It often occurs at the combination of male and female mold, due to poor mold closing, or improper processing of mold surface edges, often due to insufficient clamping force, material temperature, pressure, etc.

Improvement methods: Mold correction, mold re-closing, increase clamping force, reduce the material temperature, reduce injection pressure, reduce the holding time, reduce the holding pressure, etc.


10). Deformation
The thin and long parts, thin-walled parts with large areas, or large finished products with asymmetrical structures are caused by uneven cooling stress or uneven ejector force during molding.

Improvement methods: correction of the ejector pin, setting the tensioning role of the pulling pin, etc., if necessary, the male mold plus the nibble to adjust the deformation, adjust the male and female mold temperature to reduce the holding pressure, etc., the adjustment of the deformation of small pieces mainly depends on the pressure size and time, the adjustment of the deformation of large pieces generally depends on the mold temperature.


11). Surface uncleanliness is due to the rough surface of the mold.
Improvement method: For PC material, sometimes due to high mold temperature, there are residual glue and oil stains on the mold surface, it is necessary to clean up the mold surface, light treatment, reduce the mold temperature, etc.


12). Pulling white
It is easy to occur in the corner of thin-walled molded products or the root of thin-walled RIB, which is caused by the poor force during mold release, improper setting of the ejector pin or inadequate die pulling slope.

Improvement methods: increase the R angle at the corner, increase the molding angle, increase the ejector pin or increase its cross-sectional area, the die surface is polished, the ejector pin or slant pin is polished, reduce the injection speed, reduce the injection pressure, reduce the holding pressure and time, etc.


13). Mold pulling
The performance is poor mold release or mold injury, pulling. Mainly due to insufficient die slope or mold surface roughness, molding conditions also have an impact.

Improvement methods: increase the angle of mold pulling, mold surface polishing, increase or change the pulling pin when sticking to the mother mold surface, pay attention to the diameter of the bull's horn when feeding the bull's horn, add the biting flower to the male mold, reduce the injection pressure, reduce the holding pressure and time, etc.


14). Pore
Transparent finished PC material is easy to appear when molding. Due to the injection process gas is not exhausted, improper mold design or improper molding conditions have an impact.

Improvement methods: increase the exhaust, change the gate (into the gate to increase), PC material runners must be light, strict baking conditions, increase the injection pressure, reduce the injection speed, etc.


15). Broken difference
It occurs at the joints of male and female modules, sliders, bevel pins, etc. It is manifested by the uneven level of the bonding surface, etc. It is due to improper mold closing or the problem of the mold itself.

Improvement method: correct the mold, or re-close the mold.


16). Dimensional out of tolerance
The problem of the mold itself, or improper molding conditions caused by molding shrinkage rate is not appropriate.

Improvement method: Usually changing the holding time and injection pressure (second section) has the greatest effect on the size. For example: increasing the injection pressure, increasing the holding pressure to compensate for the shrinkage can significantly increase the size, reducing the mold temperature can also, increase the gate, or increasing the gate can improve the adjustment effect.