First, what is thin-walled injection molding
Thin wall injection molding is also known as thin wall plastic injection molding. There are three definitions of it:
The ratio of flow length to thickness, L / T, is the injection molding of a thin-walled injection molded body having a ratio of flow length L to the respective average wall thickness T in the farthest point of the cavity into which the melt must be filled ;
The thickness of the molded part is less than 1 mm, while the projection area of the plastic part is more than 50 c㎡.
Molded parts with a wall thickness of less than 1 mm (or 1.5 mm), or t / d (part thickness t, part diameter d, for disc parts) below 0.05 are defined as thin-wall injection molding .
It can be seen, the definition of thin-wall injection molding will also change the critical value, it should be a relative concept.
Second, the choice of raw materials
The 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 the low temperature impact rigidity of plastic raw materials, flame retardancy, mechanical assembly and appearance Quality and so on.
Currently used thin-walled injection molding materials are: polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PC / ABS blends and PA6, etc. As the wall thickness decreases, Physical properties of plastic to maintain product strength.
Third, common defect analysis
Although plastic thinning has many advantages, but it reduces the plastic moldability, so that conventional injection molding methods can not be molded thin plastic parts in the thin plastic parts molding, there are the following common problem:
1, short shot
Short shot refers to incomplete mold cavity quality defects caused by incomplete mold cavity filling, ie, the melt has already been coagulated before it is filled.
Conventional injection molding filling and cooling process is intertwined with the polymer melt flow, the front of the melt encountered relatively low temperature core surface or cavity wall, it will form a layer of condensation on its surface The melt flowed forward in the condensate layer. As the thickness of the condensate layer increased, the actual cavity flowpath narrowed and the thickness of the condensate layer had a significant effect on the polymer flow.
Because of the thick plastic parts in the conventional injection molding, the influence of the condensing layer on the injection molding is not great at this time, but in the thin-wall injection molding, when the ratio of the thickness of the condensing layer to the thickness of the plastic part increases, This effect is significant when the thickness is getting thinner, especially when the two are more comparable in size.
When the thickness of the plastic part is reduced, the influence of the condensate layer on the flow will increase exponentially, which also shows the effect of the condensate layer in the thin-wall injection molding .If only from the injection molding considerations, you need injection molding machine A high injection rate causes the plastic melt to fill the cavity at a rate that exceeds the rate of growth of the condensate layer (or slows the rate of growth of the condensate layer) so that the fill action can be completed before the flow cross-section is closed, Injection molding.
With a flow length of 300 mm and a plastic wall thickness of 3.0 mm, the L / T at this time is 100 and is very easy to achieve with conventional injection molding techniques; however, this was once easy when the wall thickness of the molded part dropped below 1.0 mm The resulting flow length to thickness ratio (100) becomes very difficult to achieve.
2, warping deformation
Warpage is a plastic part defect caused by uneven internal stress, which is caused by non-uniform shrinkage, uneven orientation and uneven cooling.
Methods to improve: By balancing the cooling system, adjusting the cooling time, packing pressure and dwell time and other measures to improve the plastic parts of the warp deformation defects.
3, welding line
The weld line is the boundary formed when two or more melt flow fronts in the cavity are fused, and the stress concentration is easily generated in the weld line, weakening the mechanical strength of the plastic part, especially to the plastic parts, especially the thin- Adverse, plastic parts by external force is very easy to crack in the weld line.
Method of Improvement: The design requirements of plastic parts can be satisfied by reducing the number of gates or changing the position of gate to reduce or change the position of welding lines.
4, lack of material
Finished small parts of the corner can not be fully formed, due to mold processing is not in place or poor exhaust, due to the injection molding dose or pressure is not enough and other reasons, resulting in design flaws (lack of meat).
Methods to improve: You can correct the missing material at the mold, to take or improve the exhaust measures, plus meat thickness, gate improvement (increase the gate, increase the gate), increase the injection dose, increase injection pressure and other measures to improve.
5, shrinking
Often occurs in the wall thickness of products or uneven thickness, due to shrinkage of the heat-shrinkable plastic cooling or curing due to such as the back of the ribs, the edge of the sidewalls, BOSS steal meat back, but at least keep 2/3 Thick flesh.
Methods to improve: can be bold through the runner, increase the gate, plus exhaust, raising the temperature, increase injection pressure, prolonged dwell time and other measures to improve.
6, the surface image
Often occurs after stealing meat BOSS column, or tendon on the back, or due to the core, thimble design caused by high stress reduction.
Methods to improve: can be modified core, thimble, the mother mold surface sandblasting and other ways to deal with, reduce the mold surface brightness, reduce injection speed, reduce injection pressure and other means.
7, air stripe
Occurs in the gate, mostly due to mold temperature is not high, injection speed, pressure is too high, into the gate set properly, into the pouring plastic encountered spoiler structure.
Improvements: By changing into the gate, runner light, runner cold area increased, into the gate to increase the surface plus bite flowers (by adjusting the machine or repair the line can be combined with the line), the rising mold Temperature, reduce injection speed, reduce injection pressure and other ways to solve.
8, with the line
Occurred in the convergence of the two streams, such as the two into the gate of the flow of mixed materials, bypassing the core of the flow of material mixed, due to the material temperature dropped, due to poor exhaust.
Methods to improve: You can change into the gate, plus cold material wells, open the exhaust groove or the public face bite flowers, etc., can also increase the material temperature, increase the mold temperature.
9, burrs
Often occur at the junction of the male and female mold, due to poor mold clamping caused, or die corner improper processing, molding often due to lack of clamping force, temperature, high pressure and so on.
Methods to improve: mold correction can be carried out, re-clamping, increase clamping force, reduce feed temperature, reduce injection pressure, reduce dwell time, reduce packing pressure.
10, deformation
Elongated pieces, large areas of thin-walled parts, or the structure of the larger asymmetric product due to molding uneven cooling stress or due to uneven force.
Improvement methods: can be modified thimble, pull the role of the tension set from the pin, if necessary, add mold bite flower deformation control, adjust the male and female mold temperature to reduce packing and so on, the regulation of small deformation depends mainly on the pressure and size Time, large deformation of the general regulation by mold temperature.
11, the surface is not clean
It is due to rough mold surface.
Methods to improve: For PC material, sometimes due to mold temperature is too high, the mold surface residual glue, grease, timely cleaning mold surface, light processing, reducing mold temperature and so on.
12, white
Easily occur in the thin-walled molded products corners or thin-walled RIB root, due to poor stress caused by demoulding, improper thimble setting or draft is not enough.
Methods to improve: increase the corner R angle, increase the release angle, increase the thimble or increase its cross-sectional area, die polishing, thimble or oblique pin lighting, reduce the rate of fire, reduce the injection pressure and reduce packing And time and so on.
13, drawing die
Poor performance as a mold release or mold injury, drawing flowers. Mainly due to draft or rough surface roughness, molding conditions are also affected.
Methods to improve: Increase the draft angle, the mold surface light, viscose mold surface can increase or change the pull pin, horn pay attention to the horns when feeding diameter, male mold plus bite, reduce the injection pressure and reduce the pressure and Time and so on.
14, stomata
Transparent finished PC material easy to appear when forming due to the gas injection process is not exhausted, improper mold design or molding conditions have an impact.
Methods to improve: increase exhaust, change the gate (into the gate increases), PC material flow must be polished, drying conditions, increase injection pressure, reduce injection speed.
15, off bad
Occurred in the male and female modules, sliders, oblique pins and other joints, the performance of the surface level of uneven surface, due to improper clamping or the mold itself.
Methods to improve: correct the mold, or re-clamping.
16, the size of tolerance
Mold itself, or improper molding conditions caused by improper molding shrinkage.
Improve the method: usually change the dwell time, the injection pressure (the second paragraph) has the biggest influence on the size, for example: to increase the pressure and increase the dwelling and shrinkage effect can obviously increase the size and reduce the mold temperature, Mouth or increase into the gate can improve the regulation effect.