Talking about plastic molding: the essentials and problems of plastic injection molding

Time:2023-01-07 08:39:49 / Popularity: / Source:

1 Seven Elements of Plastic Forming

A pressure B speed C time D temperature E measurement F raw material G mold

2 Programmable control effects

A. Improve appearance by controlling injection process

According to experience, appearance of finished product is determined by the first half of molding process, that is, closure of resin filling injection outlet, etc., and size of resin injection rate is closely related to appearance of finished product, which is well known.
Injection speed controls filling state of resin, in order to make quality of finished product excellent, especially to improve accuracy of appearance and size.
Especially in shooting range, injection method that arbitrarily controls injection rate of resin is representative work. For various requirements of different quality, In injection process, given different injection speeds, control of this shooting method not only prevent appearance of defects such as burrs, uncertain injection, bubbles, and cracks of finished product, but also improve precision of size, reduce internal stress, and increase strength of finished product.

B. Relationship between quality and injection conditions

Because surface layer of finished product is solidified during filling, defects on the surface of finished product depend on whether surface layer is formed correctly or not during filling. Therefore, proper control of injection speed has excellent effects on following undesirable phenomena.
1. Flow marks formed around injection port or sudden increase in cross-sectional area are due to jet phenomenon, which makes flow of resin into a turbulent flow, resulting in an irregular skin layer. In this case, reduce injection speed and set pressure to be able to flow as benchmark. Most of them can be solved.
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2. Blurredness, roughness, silver bars, scorching and other phenomena are generated around injection product, most of which are caused by thermal decomposition and gas generation due to friction between injection port and injection port in the early stage of injection. Because plastic has been filled all the time, exhaust effect of rear end is poor, and air volume pressure will be generated. In this way, reducing speed of starting injection and reducing injection pressure will significantly improve effect.
3. Bonding line mark is formed by bonding of skin layer after curing. It is very difficult to make mark disappear. Change flow mode, so that location of occurrence is changed in an invisible place, or when surface layer is joined, injection speed is accelerated to make surface layer thinner, and joint marks can also be reduced to a considerable extent.
Other reasons for occurrence of bonding lines are: during filling process, flow direction suddenly changes, flow of resin produces turbulent flow, or there is a temporary split. At this time, flow speed of this part is reduced. It will have different effects on preventing irregular flow rates.
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4. Burr: caused by poor mold or insufficient mechanical closing force, but controlling injection speed may also help, that is, resin flows to place where burr occurs, injection speed is reduced, so that skin layer at the place where burr occurs is thicker. Due to cured skin layer, although mold has some gaps, inflow of resin becomes more difficult.
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5. Gas scorching (discoloration): Air or resin in mold hole contains gas gas (GAS), and after being sealed, it is subjected to adiabatic compression, resulting in high temperature. Therefore, when air is sealed, if injection speed is reduced, it will be effectively prevented.
6. Dent: Shrinkage of plastic caused by cooling and hardening process is very difficult to completely avoid, but at the moment when filling is completed, injection speed is reduced, and it is difficult to produce burrs. When cooling, pressure holding is increased, and there will be a considerable improvement. For molds or finished products with pinholes, farther from injection port, effect of holding pressure is poor, so that dents are more likely to occur, reducing injection speed through thick part, making skin layer of this part thicker, when cooling and hardening, inner core layer causes concave nests , dents on the surface are less visible.
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7. Shrinkage: Among defective injection molding products, shrinkage is a common occurrence. For products with longer runners and thicker thicknesses, and products with uneven glue level, it often occurs. For injection molding, molten plastic starts to harden when it hits surface of mold, density of heating plastic and curing plastic begins to differentiate, so shrinkage begins to occur, especially product with irregular thickness of glue position passes through thin surface to thick body, and injection pressure cannot be replenished, and shrinkage is even more obvious.
If necessary, it can be filled with glass fiber to form a composite material, and shrinkage will be reduced. Otherwise, shrinkage rate of all crystalline plastics is quite large, shrinkage at the downstream end is easy to occur, so plastic is adjusted by holding pressure before hardening. However, if holding pressure is too large, cracks will occur, so holding pressure must be reduced with time. When runner is larger or thickness difference is reduced, it is also very helpful for shrinkage.
8. Blackening at the end: Air in cavity is compressed to generate high temperature, and filling of machine continues. If injection pressure is increased and injection speed is increased, blackening will occur. This situation can improve porosity or reduce injection pressure/reduce end injection speed and plastic temperature, avoid using recycled materials and use materials with better thermal stability. Of course, increasing runner, reducing rate of fire at the end or improving exhaust hole will help to improve blackening of end.
9. Silver strips: Silver stripes and white streaks on the surface of molded product due to flow of plastic during injection are caused by water in plastic being compressed into a gaseous state. Under pressure of nozzle, it cannot be evaporated and attached to hardening process, so moisture in raw material can be completely dried to prevent it from happening.
10. Welding line: When plastic flows through hole, it is divided and then combined, which is caused by air barrier, or there is too much release agent. Therefore, it is helpful to improve temperature of plastic or increase speed of plastic through holes, increase temperature of mold, stop using materials with good fluidity and release agents.

3 Injection molding machines often have problems

A. Supply of molding materials is not smooth

Under normal rotation of stored material, plastic cannot be supplied to screw normally and there is an idling situation.
Reason for this is that temperature of raw material does not match actual raw material, resulting in agglomeration of machine screw. Machine does not turn off electric heat for a long time, screw raw material is heated and dispersed, raw material gas (GAS) is serious, machine back pressure is too large, powder or toner caused by too much recycled material use too much viscous oil, etc., molding conditions are not appropriate, there are also reasons such as incomplete dry material or abnormal temperature setting, or excessive screw speed.

B. Temperature rise of material pipe is too high in normal production; back pressure is too high or screw material pipe is damaged by abnormality.

C. Insufficient plasticizing ability:

(Cycle time is too short), that is, heating is not enough to dissipate heat, wattage of heating element can be increased moderately, and thermal decomposition of raw material itself is insufficient.

D. Injection machine is unstable, temperature is abnormal or countercurrent system is broken, position of starting point of injection point switching and pressure maintaining is different, voltage at manufacturing site is not stable, and voltage parameters of transformer are adjusted

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