Defects + Cause Analysis + Solutions for Injection Products

Time:2020-04-08 09:36:38 / Popularity: / Source:

1. Bubbles:

Bubbles are defects caused by gas being wrapped by raw material during final filling process of injection, which cannot be discharged from mold vents, inlets or gaps of mold and remain inside or on the surface of product.
product defects 


(1) Reasons for product design. Thickness ratio in product design is too large, which makes it difficult for compound to flow.
(2) Reasons for mold. Mold is designed with too few exhaust holes or there are no exhaust holes in some places where gas blocking is likely to occur. Gas cannot be discharged smoothly during filling process. At the same time, runner design is too long, which will cause material flow to be blocked.
(3) Reasons for raw material humidity. Moisture is not dry or drying time is not enough, shrinkage is too large.


Communicate with product designers to improve smooth transition of product thickness. Increase injection pressure, sufficient holding pressure, sufficient holding time, and appropriately reduce temperature of melt. Improve exhaust, reasonable design of gate location, and increase appropriate gate size. Air bubbles will be reduced after raw materials are fully dried.

2. Short shot: 

Short shot is caused by lack of filling of cavity in mold due to various reasons during injection process, resulting in partial product shortage.
short shot 


(1) Temperature of gate is too low, runner is too small and long, which causes raw material to flow slowly and blocked. Injection pressure is too low, and raw material cannot be injected into any cavity of mold, resulting in product defects.
(2) Reasons for raw material: insufficient or slow supply of raw material, poor fluidity leading to excessive temperature loss and solidification
(3) Causes of mold: Mold temperature is insufficient, and material cannot flow normally; mold is not properly designed with too few or no vents in local area, gas cannot be removed in time, resulting in uneven flow, resulting in missing materials and insufficient filling.


Increase plasticizing temperature, nozzle temperature, increase injection time, increase injection speed, mold temperature; increase runner size, gate size, improve exhaust; increase internal temperature of mold (install heating blanket), and increase the vents appropriately.

3: Flash:

Remaining material edge that overflows into gap such as mold clamping line or release pin and remains on plastic part during process.


In injection molding process, excessive injection pressure exceeds clamping force causes contact surface between male mold and female mold to increase, amount of material is too much, and remaining material leaks at weak seal, which is common on parting surface. In particular, resin products that require high-pressure molding have many flashes, because resin has good fluidity and possibility of flowing into gap is high.


If possible, appropriately reduce mold deformation caused by injection pressure or improve error in matching size of core and cavity, mold plate combination is not parallel and tightly closed, exhaust groove is too deep will cause flashing or increase pillar to reduce size of contact surface; increase amount of additives to reduce rubber fluidity.

4. Shrink marks:

Also known as shrinkage, plastic products are thicker in local areas or where ribs are slower in cooling than thin ones, heat is released slowly, which causes product surface or appearance to be sunken. This phenomenon is called shrink marks or shrinkage.
Shrink marks
Shrink marks 


Increase size of gate and runner, increase pressure during dwelling process, extend dwelling time, reduce mold temperature, so that thicker parts of plastic part can be replenished in a timely and rapid manner before cooling and solidifying, shrinkage can be avoided.

5. Water ripple:

Water ripple refers to leaving of traces of molten colloid flow on product surface after cooling and solidification of plastic products. It often appear on the surface of plastic parts injection molded by smooth molds.
water ripple 


Reason is that because raw material flows into cavity to cool quickly, high-temperature rubber material injected later pushes gel body with excessive temperature loss in the front to cause slippage, and finally a water ripple path is formed. We can improve or eliminate water ripple by increasing barrel temperature, increasing screw speed and temperature in mold, accelerating injection pressure and slowing temperature loss of molten colloid. If it is not improved, it may be residual cold material left over from last solidification, then a cold material well can be added at the end of main runner to eliminate residual cold material.

6. Welding mark:

Refers to line-like marks on the surface caused by two streams meeting and welding.
Welding mark 

Cause analysis:

Welding mark are due to existence of multiple glue inlets at the same time. However, during injection or extrusion, multiple streams of material are split and merged in mold, or they are merged at the gap of product. Due to loss of temperature, temperature difference caused by confluence cannot be welded to one another, resulting in fusion marks, which affects appearance quality and mechanical properties of plastic parts.


Increase temperature of melt, injection pressure, injection speed, increase temperature in mold, increase lubricant moderately, increase flow speed, reduce temperature loss before raw material merges, and reduce occurrence of welding marks.

7. Crack:

Refers to surface cracks that occur when mold opening and during use of plastic product. We call this phenomenon cracks. There are many reasons for cracks, let's analyze them one by one!

Design flaws

Product design should avoid acute angles, right angles, and sudden thickening or thinning of wall thickness. Edges of plastic parts must be transitioned with rounded corners. Wall thickness should not be changed abruptly and you can use a gradual transition! If it is a rubber-covered product, a material with metal expansion coefficient similar to that of plastic should be selected. If difference is larger, internal stress caused by it will be greater. The first thing that should be solved is design flaw, and move forward in process of constant exploration!

Reason for machine modulation

1. During injection, injection pressure is too high, injection speed is too fast. The longer holding time, the lower temperature of material in barrel and the lower temperature in mold, which will cause uneven plasticization and higher viscosity. During process of filling cavity with colloids, intermolecular orientation effect is large, residual internal stress will be relatively large after plasticizing, especially during holding process, colloid temperature rapidly decreases and viscosity increases at this stage, which causes orientation of molecular chains to be forcibly changed. The larger holding pressure and the longer holding time, the larger internal stress will be.

Design of mold:

Gate should be designed with a large wall thickness, which can reduce injection pressure and holding time. For large products, multiple gates should be designed to reduce flow distance of rubber and reduce orientation internal stress caused by too long distances. Cooling system should be designed far away from gate. Speed of ejection system should also be appropriate. After plasticizing, plastic parts will have internal stress to some extent. If speed is too fast or too slow, it may cause cracking.

Supply of raw materials:

Release agent and colloid are poorly miscible, too much amount will easily lead to cracks, raw materials should be kept dry and pure, can not contain impurities.

Black dot

black dot 

Molding impurities

Black spots may be formed due to foreign matter mixed in material, cleanliness of all aspects of production, packaging, storage, transportation, opening, mixing to barrel must be strictly controlled.

Reason for mold:

Full inspection of oil removal before production can effectively prevent oily, rusty products and runners from being mixed into crushed material and used (especially during production process instead of initial trial) ;
If gates of mold are too small or rough, a large amount of friction heat will be generated. If mold exhaust is not good, it will cause obvious black stripes (black spots) in gas combustion.
Motor of dryer runs for a long time, dirt are hidden, which must be cleaned regularly;

Clean and replacement

Filter screen of dryer must also be cleaned and replaced regularly, otherwise, foreign body will enter or material will change color and agglomerate due to poor ventilation.

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