During molding process that transforms plastic particles into plastic products, plastics are often subjected to high temperature, high pressure and flow molding at high shear rates. Different molding conditions and processes will have different effects on product quality. Injection molding consists of four aspects: plastic raw materials, injection molding machines, molds, and injection molding processes.

 

If plastic raw materials are correctly selected, technical parameters and mold structure of injection molding machine meet requirements of injection molding conditions, then quality of processed product is directly related to injection molding process.
Quality of product includes quality of internal material and appearance. Quality of internal material is mainly mechanical strength, amount of internal stress directly affects mechanical strength of product. Main causes for internal stress are determined by crystallinity of product and orientation of molecules in plastic molding.
Appearance quality of product is surface quality of product, but warpage of product caused by large internal stress will also affect appearance quality problem. Appearance quality of products are: insufficient products, dents, weld marks, flashes, bubbles, silver wires, black spots, deformation, cracks, delamination and discoloration, etc., all of which are related to molding temperature, pressure, flow, time and location.

It is melting temperature of plastic. Barrel temperature is set too high and viscosity of plastic is low. Under same injection pressure and flow rate, injection speed is fast, and molded product is easy to flash, silver wire, discoloration and brittleness.
Barrel temperature is too low, plastic is poorly plasticized, viscosity is high, injection speed is slow under same injection pressure and flow rate, molded product is easy to be insufficient, weld marks are obvious, size is unstable, and there are cold blocks in the product.

Nozzle temperature is set high, nozzle is easy to drool, causing cold filaments in product. Low nozzle temperature causes blockage of mold casting system. Injection pressure must be increased to inject plastic, but there is a cold block in molded product.

Mold temperature is high, injection pressure and flow rate can be set lower, but under same pressure flow rate, product is easy to flash, warp and deform, product is difficult to eject from mold. Mold temperature is low. Under same injection pressure and flow rate, product is insufficiently formed, bubbles and weld marks are formed.

Various plastics have different drying temperatures. ABS plastic is generally set to a drying temperature of 80 ~ 90℃, otherwise it will not be easy to dry, volatilize moisture and residual solvents, product is prone to silver wire and air bubbles, product strength will be reduced.

 

High back pressure, high storage density, high storage capacity in same storage volume. Low back pressure, low storage density and low storage. After setting storage position, adjust back pressure to a larger extent. You must pay attention to resetting storage position, otherwise product may be flashed or insufficient.

Different types of plastics have different melt viscosities, viscosity of non-crystalline plastics changes greatly with change of plasticizing temperature. Injection pressure is set according to welding viscosity of plastic and plastic process ratio. If injection pressure is set too low, product is insufficiently shot, dents, weld marks, and dimensions are unstable. Injection pressure is too high, product flashes, it is difficult to discolor and eject sticky mold.

It depends on projection area of mold cavity and injection pressure. If clamping pressure is insufficient, product is likely to flash and weight increases. Excessive clamping force makes it difficult to open mold. Clamping pressure setting is generally not more than 120par / cm2.

When injection is finished, screw is kept under a pressure called holding pressure. At this time, product in mold cavity has not been frozen. Holding pressure can continue to mold cavity to ensure that product is full. Holding pressure are set too high, which brings great resistance to supporting mold and drawing insert, product is easy to be whitened and warped. In addition, gate of mold is easily expanded by supplementary plastic, gate is broken in runner. Products with too low pressure have dents and unstable dimensions.

setting of supporting mold and neutron pressure is based on comprehensive situation of mold cavity area, core projection area of insert core, and geometric complexity of molded product. Generally, it is necessary to set pressure of supporting mold and neutron cylinder so that product can be pushed.

In addition to adjusting pre-plastic flow, it is mainly affected by pre-plastic back pressure. If pre-plastic flow is adjusted to be large and pre-plastic back pressure is high, as screw rotates, plastic has a large shear force in barrel, molecular structure of plastic is easily cut, product has black spots and black stripes, which affects appearance quality and strength of product, and barrel heating temperature is not easy to control. If pre-plastic flow is set too low, pre-plastic storage time will be extended, which will affect molding cycle.

Set a reasonable injection speed, otherwise it will affect product quality, injection speed is too fast, product has air bubbles, burnt, discoloration, etc. If injection speed is too slow, product is insufficiently formed, and there are welding marks.

It should not be set too large, otherwise ejection and core pulling action will be too fast, causing ejection and core inserting to be unstable, product will be easily ejected.

It is drying time for plastic raw materials. Various types of plastics have the best drying temperature and time. Drying temperature of ABS plastic is 80 ～ 90 ℃, time is 2 hours. ABS plastic generally has a water absorption of 0.2 to 0.4% for 24 hours, while water content that can be injection molded is 0.1 to 0.2%.

Control method of computer injection machine is equipped with multi-stage injection, which can be used to regulate pressure, speed and amount of plastic injected. Injection speed of plastic in mold cavity reaches a constant speed, appearance of molded product and quality of internal materials are improved.
Therefore, injection process is usually used for position control without time control. Holding time is controlled by time. If holding time is longer, product is dense, weight is heavy, internal stress is large, demolding is difficult, top is easy to white, and molding cycle is extended. If holding time is too short, product is prone to dents, and size is unstable.

It is to ensure stablility of product forming. It is to have sufficient cooling and setting time after molding plastic injected into mold cavity for product. Otherwise, product will easily warp and deform when mold is opened. Excessive cooling time extends molding cycle, which is uneconomical.

 

Mold moving position refers to entire moving distance from mold opening to mold closing and locking. The best position to move mold is to take out product smoothly. Mold opening distance is too large, and molding cycle is long.
As long as position of supporting mold is controlled from position of ejection from mold, product can be easily removed.

First of all, it is necessary to ensure amount of plastic injected at one time of molded product, and secondly to control amount of material stored in barrel. If storage position is controlled more than once, injection volume is too large, product is liable to flash, otherwise product is insufficiently formed.
If there is too much material in barrel, plastic stays in barrel for a long time, product is easy to fade, and strength of molded product is affected. On the contrary, it will affect plasticization quality of plastic, and it will be added to mold without any material during holding pressure.

Quality of injection molding products involves product design, plastic material, mold design and processing quality, choice of injection molding machine and process adjustment.
For normal production molds, to ensure and improve product quality, only injection process is adjusted; that is, temperature, pressure flow rate, time and position adjustment.
Process adjustment must be comprehensive and balanced. If size of product changes greatly, consider starting from plastic raw materials, whether it is crystalline plastic or non-crystalline plastic, size of molecular weight of plastic. For crystalline plastics and macromolecular plastic, crystallinity and molecular orientation of product must be considered. Shape of product changes to the flow during molding, which causes stress and size of product to change and causes product to shrink.
Solve small changes in product size, from process can increase temperature of materials, reduce temperature of mold, control amount of injection material, reduce injection pressure, increase injection speed, to fill mold cavity instantly, reduce holding time, extend some cooling time, reduce density of product under condition that product is shot enough, reduce internal stress of product and reduce shrinkage of product.
In short, adjustment of injection process cannot be started from a single point, but must be started from principle of injection process. Comprehensive consideration of issues can be adjusted one by one from multiple aspects or multiple issues at one time. However, adjustment methods and adjustment principles depend on quality and process conditions of products produced at that time.