what to do if plastic part is not filling completely ？

The adjustment of process in the production process is a necessary way to improve the product quality and output. Because the injection molding cycle itself is very short, if the process conditions are not well mastered, there will be an endless stream of waste products. When adjusting the process, it is best to change only one condition at a time and observe more times. If the pressure, temperature and time are all adjusted together, it is easy to cause confusion and misunderstanding. If there is a problem, I don't know the reason.

There are many measures and means to adjust the process. For example, there are more than ten possible solutions to the problem of dissatisfaction with product injection. Only by selecting one or two main solutions to solve the crux of the problem can we really solve the problem. In addition, attention should also be paid to the dialectical relationship in the solution. For example, if the product is sunken, sometimes the material temperature should be increased and sometimes the material temperature should be reduced; Sometimes it is necessary to increase the material quantity, and sometimes it is necessary to reduce the material quantity. We should recognize the feasibility of reverse measures to solve the problem.

What if the plastic part is not filled completely?

This is a common problem, but it is also relatively easy to solve. When it can not be solved by technological means, it can be improved from the perspective of mold design and manufacturing, which can generally be solved.

1、 Equipment:

(1) The plasticizing capacity of the injection molding machine is small.

When the product quality exceeds the actual maximum injection quality of the injection molding machine, it is obvious that the supply quantity is beyond the income. If the product quality is close to the actual injection quality of the injection molding machine, there will be a problem of insufficient plasticization. The heating time of the material in the barrel is insufficient, and as a result, the appropriate melt can not be provided to the mold in time.

In this case, the problem can be solved only by replacing the injection molding machine with large capacity. Some plastics such as nylon (especially nylon 66) have narrow melting range and large specific heat. An injection molding machine with large plasticizing capacity is needed to ensure the supply of materials.

(2) The temperature displayed by the thermometer is not true, and the apparent high and actual low result in low material temperature.

This is due to the failure of temperature control devices such as thermocouples and their lines or temperature difference millivoltmeters, or the aging or burning of electric heating rings far away from the temperature measuring points, heating failure and no discovery or timely repair and replacement.

(3) The diameter of nozzle inner hole is too large or too small.

If it is too small, the specific volume of the strip increases due to the small flow diameter, which is easy to cool, block the feed channel or consume the injection pressure; If it is too large, the flow cross-sectional area is large, the pressure per unit area of plastic molding is low, and the injection force is small.

At the same time, non-Newtonian plastics such as ABS can not reduce the viscosity because they do not obtain large shear heat, which makes it difficult to fill the mold. The nozzle is poorly matched with the inlet of the main flow channel, which often leads to overflow outside the mold and insufficient filling in the mold. The nozzle itself has great flow resistance or is blocked by foreign matters, plastic carbonization deposits, etc;

The spherical surface at the inlet of nozzle or sprue is damaged and deformed, which affects the good cooperation with each other; The mechanical failure or deviation of the injection seat causes the tilting displacement of the nozzle and the axis of the main flow channel or the separation of the axial compression surface; The nozzle ball diameter is larger than the inlet ball diameter of the main flow channel. Due to the gap at the edge, the axial pushing force of the nozzle will gradually increase under the pressure of overflow, which will cause dissatisfaction of the product injection.

(4) Plastic frit blocks the feeding channel.

The local melting and caking of the plastic in the hopper dryer, the high temperature of the barrel feeding section, the improper selection of the plastic grade, or too much lubricant contained in the plastic will make the plastic enter the dryer

The diameter reduction position of the feed port or the deep groove at the screw starting end of the screw melt prematurely, and the granular material and molten material bond with each other to form a "bridge", block the channel or wrap the screw, and make circumferential sliding with the screw rotation, which can not move forward, resulting in interruption of feed or irregular fluctuation. This situation can be fundamentally solved only after the channel is chiseled and the block is removed.

(5) Nozzle cold material into the mold.

Injection molding machines are usually equipped with straight through nozzles due to pressure loss. However, if the temperature at the front end of the barrel and the nozzle is too high, or there is too much material stored at the front end of the barrel under high pressure, it will produce "salivation", so that the plastic will accidentally enter the main channel inlet first and harden under the cooling effect of the template without starting injection, which will hinder the smooth entry of molten material into the mold cavity.

At this time, reduce the temperature of the front end and nozzle of the barrel, reduce the material storage of the barrel, reduce the back pressure, and avoid excessive melt density at the front end of the barrel.

(6) The injection cycle is too short.

Due to the short cycle, the lack of material will also be caused if the material temperature is too late to keep up, especially when the voltage fluctuation is large. The cycle shall be adjusted accordingly according to the power supply voltage.

Generally, the injection and pressure holding time are not considered during the adjustment, but the time from the completion of pressure holding to the return of the screw is mainly considered, which not only does not affect the mold filling conditions, but also prolongs or shortens the preheating time of the material particles in the barrel.

2, mold aspects:

(1) Defective mold gating system.

The flow channel is too small, too thin or too long, which increases the fluid resistance. The diameter of the main flow channel shall be increased, and the flow channel and shunt channel shall be round.

The runner or nozzle is too large and the injection force is insufficient; The runner and gate are blocked by impurities, foreign matters or carbonization; The runner and gate are rough with scars or acute angles, and the surface roughness is poor, which affects the poor material flow; The channel is not provided with a cold well or the cold well is too small, and the opening direction is wrong;

For multi cavity molds, carefully arrange the balance of runner and gate size distribution, otherwise only the cavities near the sprue or with thick and short gates can be filled, but other cavities cannot be filled. The diameter of the runner should be appropriately thickened to reduce the molten material pressure drop flowing to the end of the runner, and the gate of the cavity far from the main runner should be increased to make the injection pressure of each cavity basically consistent with the material flow speed.

(2) Unreasonable mold design.

The mold is too complex, there are many turns, the feed port is improperly selected, the flow channel is too narrow, the number of gates is insufficient or the form is improper; If the partial section of the product is very thin, the thickness of the whole product or part shall be increased, or an auxiliary runner or gate shall be set near the insufficient filling place;

It is common to see dissatisfaction of parts caused by poor exhaust measures in the mold cavity. Most of these defects occur at the corners of products, deep depressions, thin-walled parts surrounded by thick wall parts, and the bottom of thin bottom shell formed by side gate. The design to eliminate this defect includes opening an effective exhaust duct, selecting a reasonable gate position so that the air can be easily discharged in advance, and deliberately making a part of the air trapped area of the cavity into an insert when necessary, so that the air can overflow from the gap of the insert;

For multi cavity molds, gate distribution imbalance is easy to occur. If necessary, reduce the number of injection cavities to ensure that other cavity parts are qualified.

3、 Process:

(1) Improper feed adjustment, lack of material or excessive material

Inaccurate feeding metering or abnormal operation of feeding control system, abnormal injection cycle caused by injection molding machine or mold or limited operating conditions, small pre plastic back pressure or small particle density in the barrel may cause material shortage. For granular materials with large particles and large voids and plastics with large specific volume change of crystallinity, such as polyethylene, polypropylene Nylon and other plastics with high viscosity, such as ABS, shall be adjusted to a higher amount. When the material temperature is high, the amount shall be adjusted to a higher amount.

When there is too much material stored at the end of the barrel, the screw consumes extra injection pressure to compress and promote the excess material stored in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the mold cavity and makes the product difficult to fill.

(2) The injection pressure is too low, the injection time is short, and the plunger or screw returns too early

Molten plastics have high viscosity and poor fluidity at low working temperature, so they should be injected at high pressure and speed. For example, when making ABS color parts, the non high temperature resistance of colorants limits the heating temperature of the barrel, which should be compensated by higher injection pressure and longer injection time than usual.

(3) Slow injection speed

Injection speed is of great significance for some products with complex shape, large thickness change and long process, as well as plastics with high viscosity, such as toughened ABS. When the product cannot be filled with high pressure, high-speed injection should be considered to overcome the problem of insufficient injection.

(4) Feed temperature too low

The temperature at the front end of the barrel is low, and the viscosity of the molten material entering the mold cavity rises too early to flow due to the cooling effect of the mold, which hinders the mold filling at the far end; The plastic with low temperature and high viscosity in the rear section of the barrel is difficult to flow, which hinders the forward movement of the screw. As a result, it seems that the pressure displayed by the pressure gauge is sufficient, but in fact, the molten material enters the cavity at low pressure and low speed;

If the nozzle temperature is low, it may be that the nozzle loses heat by contacting with the cold die for a long time during fixed feeding, or the material temperature is low due to insufficient heating or poor contact of the nozzle heating ring, which may block the feeding channel of the die; If the mold does not have a cold charging well, use a self-locking nozzle and adopt the post charging procedure, so that the nozzle can maintain the necessary temperature; The nozzle is too cold at the beginning of startup. Sometimes, the flame gun can be used for external heating to accelerate the temperature rise of the nozzle.

4、 Raw materials:

(1) Poor fluidity of plastics

Recycled crumbs are often used in plastic factories, and recycled crumbs often reflect the tendency of increasing viscosity. The experimental results show that the unit volume density of molecular chain breaking generated by oxidative cracking increases, which increases the viscosity of flow in the barrel and mold cavity. The regenerated scrap contributes to the generation of more gaseous substances, increases the injection pressure loss and makes it difficult to fill the mold.

In order to improve the fluidity of plastics, it should be considered to add external lubricants such as stearic acid or its salts, preferably silicone oil (viscosity 300 ~ 600cm2 / s). The addition of lubricant not only improves the fluidity of plastics, but also improves the stability and reduces the gas resistance of gaseous substances.