Thin-wall injection molding technology is also called thin-wall plastic injection molding technology. There are three types of definitions:
Ratio of flow length to thickness L/T, that is, ratio of flow length L from melt entering mold to the furthest point of cavity where melt must be filled to corresponding average wall thickness T is 100 or more than 150 is thin-walled injection molding;
Injection molding method where thickness of molded plastic part is less than 1 mm and projected area of plastic part is above 50 c㎡;
Injection molding where wall thickness of molded plastic part is less than 1 mm (or 1.5mm) or t/d (plastic part thickness t, plastic part diameter d, for disc-shaped plastic 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, and it should be a relative concept.

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

 

Although thin-walled plastic parts have many advantages, it reduces formability of plastic parts, so that these thin-walled plastic parts cannot be molded by conventional injection molding methods. When forming thin-walled plastic parts, there are following common problems:

Short shot is incomplete quality defect of plastic part caused by incomplete filling of mold cavity, that is, melt has condensed before filling is completed.
Filling process and cooling process of conventional injection molding are intertwined. When polymer melt flows, melt front meets relatively low temperature of core surface or cavity wall, and a condensation layer is formed on the surface. Melt continues to flow forward in condensation layer. As thickness of condensation layer increases, actual cavity flow path becomes narrower. Thickness of condensation layer has a significant effect on the flow of polymer.
Because thickness of plastic part is relatively thick during conventional injection molding, influence of condensation layer on injection molding is not very large at this time. However, in thin-wall injection molding, when ratio of thickness of condensate layer to thickness of plastic part gradually increases as thickness of plastic part becomes thinner, influence is great. Especially when size of the two can be compared with each other more prominent.
When thickness of plastic part decreases, influence of condensate layer on the flow will increase exponentially, which also explains great influence of condensate layer in thin-wall injection molding. If only considering injection molding, injection molding machine needs to have a high injection rate so that rate at which plastic melt fills cavity exceeds growth rate of condensation layer (or slows down growth rate of condensation layer), so that filling action can be completed before flow section is closed, and injection molding of thin-walled plastic parts is carried out.
When flow length is 300mm and wall thickness of plastic part is 3.0 mm, at this time L/T is 100, which can be easily achieved by conventional injection molding technology; but when wall thickness of plastic part drops below 1.0mm, flow length to thickness ratio (100) that was once easy to reach becomes very difficult to achieve.

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

Weld line is boundary formed when two or more melt flow fronts fuse in cavity. Stress concentration is likely to occur at weld line, which weakens mechanical strength of plastic part, which is particularly unfavorable to mechanical properties of plastic part, especially thin-walled plastic part. After external force, plastic part is very easy to crack at weld line.
Improvement method: In design, you can reduce or change position of weld line by reducing number of gates or changing gate position to meet design requirements of plastic parts.

Small parts and corners of finished product cannot be completely molded, because mold is not processed in place or exhaust is not smooth, and molding is due to insufficient injection dose or pressure, which causes design defects (insufficient meat thickness).
Improvement method: It can correct lack of material mold, take or improve exhaust measures, increase thickness, improve gate (enlarge gate, increase gate), increase injection dose, increase injection pressure and other measures to improve.

It often occurs in uneven wall thickness or thickness of molded product, which is caused by different cooling or solidification shrinkage of hot melt plastic. Such as back of rib, edge of side wall, and back of BOSS column steal thickness, but at least retain 2/3 of thickness.
Improvement method: It can be improved by thickening runner, increasing gate, adding exhaust, increasing material temperature, increasing injection pressure, and extending pressure holding time.

It often occurs on the back of BOSS pillars or ribs that have been stolen, or stress marks are reduced due to high core and thimble design.
Improvement method: It can be processed by modifying core, ejector pin, sandblasting of female mold surface, etc., using methods such as reducing brightness of mold surface, reducing injection speed, and reducing injection pressure.

Occurs at gate, mostly because mold temperature is not high, injection speed and pressure are too high, gate is set improperly, and plastic encounters turbulence structure during pouring.
Improvement method: It can be changed by changing gate, runner lighting, increasing cold material area of runner, increasing gate, adding biting on the surface (also by adjusting machine or repairing mold to catch line), increasing mold temperature, reducing injection speed, and reducing injection pressure.

Occurs at confluence of two streams, such as intersection of two inlet streams, and intersection of streams bypassing core, due to drop in material temperature and poor exhaust.
Improvement method: It can be used to change gate, add a cooling well, open a vent, or a pattern on male mold surface, also increase material temperature and mold temperature.

 

Joints of male and female molds often occur due to poor mold clamping, or improper processing of corners of mold surface, and often due to insufficient clamping force, high material temperature and high pressure during molding.
Improvement method: mold correction, re-clamping, increase clamping force, lower material temperature, reduce injection pressure, reduce holding time, reduce holding pressure, etc.

Slender parts, thin-walled parts with large areas, or large finished products with asymmetrical structure are caused by uneven cooling stress or uneven ejection force during molding.
Improvement method: it can be modified thimble, set tensioning pin, etc., if necessary, male mold can be bitten to adjust deformation, adjust male and female mold temperature to reduce holding pressure, etc. Adjustment of deformation of small parts mainly depends on pressure and time, and adjustment of deformation of large parts generally depends on mold temperature.

It is due to rough surface of mold.
Improvement method: For PC materials, sometimes due to high mold temperature, there are residual glue and oil stains on mold surface, mold surface needs to be cleaned in time, polished, and mold temperature is reduced.

It easily occurs at the corners of thin-walled molded product or root of thin-walled RIB. It is caused by poor force during demolding, improper thimble setting or insufficient draft angle.
Improvement method: increase R angle at the corner, increase demolding angle, increase ejector pin or increase its cross-sectional area, polish mold surface, polish ejector pin or inclined pin, reduce rate of fire, reduce injection pressure, and reduce holding pressure and time.

It is manifested as poor demolding or mold damage, and garlanding. Mainly due to insufficient draft angle or rough mold surface, molding conditions also have an impact.
Improvement method: increase draft angle, polish mold surface, add or change pull pin when sticking female mold surface, pay attention to horn diameter when feeding horn, add biting to male mold, reduce injection pressure, reduce holding pressure and time and so on.

This phenomenon is prone to occur when transparent finished PC material is formed. Because gas is not exhausted during injection molding process, improper mold design or improper molding conditions have an impact.
Improvement method: increase exhaust gas, change gate (increase the gate), PC material flow channel must be polished, strict drying conditions, increase injection pressure, reduce injection speed, etc.

Occurs at joints of male and female modules, sliders, diagonal pins, etc. It is manifested as uneven levels of joint surface, etc., due to improper mold clamping or problems with mold itself.
Improvement method: correct mold, or reclose mold.

Problem of mold itself, or improper molding conditions caused molding shrinkage to be inappropriate.
Improvement method: usually changing holding time and injection pressure (second stage) have the greatest impact on size. For example: increasing injection pressure and increasing pressure-holding and shrinking effect can significantly increase size, reduce mold temperature, and increase gate to improve adjustment effect.