After injection molding, some problems come into being which will affect the assembly efficiency or the performance of the whole machine. Given the certain discrepancy between the defective and the predetermined standard (inspection standards), those parts will not meet the company’s quality requirements. In that case, strengthening the quality awareness training of employees and dealing with defective parts in a timely manner (such as selection and processing) would be a good method. Besides, the following factors can also cause defects: molds, raw materials, technical parameters, equipment, environment, and personnel.
The followings are some of the common defects of injection molded products and the reasons for the defects.
1. Dissatisfaction (lack of glue)
Usually, the lack of glue means that the glue is neatly arranged, that is, the cavity is not fully filled. However, cases like bubbles, voids, holes, etc., which do not match the standard model, can also be called a lack of glue.
- Process problems: plasticizing temperature is too low, the nozzle temperature is too low, the injection time is too short, the injection speed is too slow, the mold temperature is too low;
- Mold problems: the runner is too small, the gate is too small, the gate position is unreasonable, the exhaust is poor, and there is debris in the cavity;
- Raw material problems: poor fluidity, mixed with debris.
Process problems: too high plasticizing temperature, too long injection time, too much feed, too high injection pressure, too high mold temperature, and debris between the templates;
Mold problems: mold deformation, core and cavity matching size error, template combination is not parallel, the exhaust groove is too deep;
Equipment problem: the template is not parallel, and the template is not tightly closed;
Raw material problem: excessive liquidity.
- The shape of injection molded parts is rotated and twisted after the plastic parts are demolded or after that for a while, such as the straight edge facing inward, the flat part curved outward, and the flat part bulging. Deformation can be classified into two basic types, which are local deformation and overall deformation.
Process conditions: the material temperature is too high, the mold temperature is too high, the holding time is too short, and the cooling time is too short to force demoulding;
In terms of mold: improper gate position, insufficient gate quantity, and improper ejection position cause uneven force.
4. Flow marks
Process conditions: material temperature is too low and not fully plasticized, injection speed is too low, the injection pressure is too small, holding pressure is not enough, mold temperature is too low, and injection volume is insufficient;
Mold aspect: the gate is too small, the gate quantity is too few, the runner gate is rough, surface finish is poor;
In terms of equipment: after temperature control, the system fails and the oil pump pressure drops;
In terms of raw materials: too much volatile matter, poor fluidity, mixed with miscellaneous materials.
Voids appear inside the injection plastic parts. Air bubbles are small volumes or clusters of pores shaped inside the product after molding. While density bubbles are vacuum holes generated inside the plastic.
Process conditions: low injection pressure, insufficient holding pressure, insufficient holding time, and high material temperature;
Mold aspect: poor exhaust, unreasonable gate location, gate size is too small;
In terms of raw materials: the moisture content is not dried or the drying time is not enough, and the shrinkage rate is too large.
Process conditions: insufficient feeding amount, too short injection time, too short holding time, too high material temperature, too high mold temperature, too short cooling time;
In terms of mold: the runner is too small, the gate is too small, and the exhaust is poor;
In terms of equipment: insufficient injection pressure and foreign matter in the nozzle;
In terms of raw materials: the shrinkage rate is too large.
7. Unstable size
Process conditions: injection pressure is too low, the barrel temperature is too high, pressure holding time changes, the injection cycle is unstable, mold temperature is too high;
Mold aspect: uneven gate size, inaccurate cavity size, loose core, too high mold temperature, or no water channel;
In terms of raw materials Variations in grades, uneven particle sizes, and volatile substances.
8. Difficult to demold
Process conditions: injection pressure is too high, the pressure holding time is too long, the injection volume is too much, mold temperature is too high;
Equipment: insufficient ejection force, insufficient ejection;
In terms of mold: no demolding slope, insufficient finish, improper ejection method, improper matching accuracy, poor exhaust, and template deformation.
9. Silver pattern
Process conditions: too high material temperature, too fast injection speed, too much injection pressure, uneven plasticization, too much release agent;
Mold aspect: the gate is too small, the mold finish is poor, poor exhaust;
Equipment aspect: back pressure is too low, the nozzle has cast material;
In terms of raw materials: moisture content is not dried, excessive lubricant.
Process conditions: the material temperature is too high, the injection pressure is too high, the speed is too fast, the downtime is too long, and the release agent is not clean;
Mold aspect: the gate is too small, has a poor exhaust, complex cavity, and poor cavity finish;
In terms of raw materials: there are sundries in the material and powder in the pellets.
In terms of surface finishing solutions.
Process conditions: the material temperature is too high, the injection pressure is too large, the molding cycle is long, the mold is not cooled, and the nozzle temperature is high;
Mold aspect: the gate is too small;
Equipment: there are obstacles in the barrel or nozzle, and the screw speed is high. The core is not concentric with the nozzle center;
In terms of raw materials: material pollution, colorant decomposition, and high volatile content.
12. Weld line
linear traces on the surface of injection plastic parts are formed when plastics come together in the mold. If the melts are not completely fused at the juncture and cannot be integrated as a whole, welding marks then appear, and mostly they are straight lines developing from deep to shallow. This problem influences the appearance and mechanical properties of the injection plastic parts. For example, the welding line at the bottom of plates for hanging hooks.
Process conditions: low injection pressure, short injection time, low material temperature, too much clamping force, inappropriate release agent;
In terms of mold: the mold temperature is too low, the runner is small, the gate position is wrong, poor exhaust;
Equipment: poor plasticization;
- In terms of raw materials: poor fluidity, too much lubricant, and foreign matter in the material.
13. Gloss defect
Process conditions: material temperature or mold temperature is too low, the injection pressure is too low, injection speed is too high or too small, and cooling time is too short;
Mold aspect: mold roughness becomes larger, the gate is too small, the runner is too thin, and exhaust is poor;
Equipment: insufficient material supply;
In terms of raw materials: the raw materials are not dried or contain volatile substances, the raw materials are easily degraded, the amount of auxiliary or release agent is too large or the quality is not good, and they contain foreign matter.
Other common problems:
- Color difference: under a standard light source, the color of injection plastic parts is different from its standard color sample when viewed with the naked eye.
- Overflow: Along the parting line of injection plastic parts or the air-tight surface of the mold, a thin layer of glue flows out.
- Corrugation: the spiral or cloudy wave-like bumps and holes appear at the surface of injection plastic parts or in the transparent products.
- Silver-white thread: Long fine lines in needle-like shape, and frost-like silver-white color show up on the surface of injection plastic parts. It is because of the incomplete filling of flowing glue. Therefore, this problem usually appears at the end where the filling material flows towards, and then it will produce a rough front end.