What are the common defects of injection molded parts and how to solve them -2

After the plastic parts are molded, some defects may occur, which will affect the assembly efficiency or the performance of the whole machine, and there will be certain differences from the predetermined quality standards (inspection standards), which cannot meet the company's quality requirements. In addition to strengthening the quality awareness training of employees and timely handling of defective accessories (such as selection, processing, etc.); these defects may be caused by the following aspects: molds, raw materials, process parameters, equipment, environment, and personnel. The defects are summarized following article -1.

8. Short shot

A short shot means that the pressure at the end of the cavity is zero. There is no packing phase in the cycle. If there are no pressure and speed changes on the injection molding machine, it means that the viscosity has changed. The viscosity has increased and the plastic is not flowing.

9. Warp

uFrom plastic perspective

uWarpage is the inconsistent deformation of the part and the inconsistent shape of the part and the cavity. This is usually caused by stresses generated during filling, shrinkage, and cooling. Warpage is a complex phenomenon caused by many factors, some of which are the main factors.

uFor amorphous plastics, the effect of crystallinity does not exist. Therefore, crystalline or semi-crystalline plastics have one more factor. Parts with long, thin fibers, such as glass fibers, have another factor; with unfilled plastics, fiber orientation does not exist.

uCrystalline plastic

uFor crystalline plastics, most warpage is caused by uneven crystallinity due to uneven cooling. For amorphous plastics, compressive stress gradients caused by uneven feeding pressures are the main factor. In addition, orientation stresses caused by stress release during flow and cooling cause uneven stresses in the part. When analyzing warpage problems, it is important to classify: crystalline or amorphous, fiber filled or unfilled, and then move on. For semi-crystalline plastics, compare the first mold (cold mold or mold temperature is uniform) and the part after the mold is heated up to find out if this is the problem. If the first mold part does not warp, uneven cooling is the main problem.

uAnother way is to use amorphous plastics for inspection. Most of the time, ABS with good fluidity is used instead of PP. If the ABS part does not warp, but the PP part warps, it means that the crystallinity problem causes the warping.

uCrystalline Materials

To analyze the warpage of a plastic part containing long fibers, compare the part with the fiber-containing material to the part without the fiber-containing material to determine the degree of change. During filling flow, the fiber mesh is oriented in the direction of the flow, and during cooling, it is not oriented like many molecules. Usually, warpage caused by fiber orientation can only be solved by changing the flow direction or changing the part design.

uAmorphous unfilled material

Usually, warpage of amorphous plastics is caused by a combination of molecular orientation stress and compressive stress. The stress gradient in feeding can be reduced by reducing the viscosity, generally by increasing the filling speed or temperature.

If the Decoupled III process is used, the feeding speed can be optimized to reduce the feeding stress gradient.

Orientation stress can be reduced by increasing the plastic temperature, slowing down the filling speed and cooling rate. If shrinkage stress gradient is the primary cause, increasing the filling speed can reduce warpage. If orientation stress is the main cause, increasing the speed will worsen the warpage.

Another important factor is whether the gate is frozen. Many times, gate freezing is intentionally avoided and reflow is allowed to reduce compressive stress gradients and flatten the part. This is especially useful for center gated parts, semi-crystalline and amorphous plastics.

10. Flow marks

uSplay is plastic road kill!

Flow marks are traces left by the flow of gas or liquid on the surface of plastic parts. The gas or liquid that causes punch marks comes from the moisture in the undried plastic, the gas trapped in the plastic, the degraded polymer molecules, and the degraded additives. Water vapor condensed on the mold surface and stains left on the mold surface by the flow of plastic can also cause punch marks.

uPossible corrections

uDry material.

uIncrease back pressure to eliminate trapped air.

uChange screw type to eliminate trapped air.

uCheck plastic temperature and residence time to eliminate molecular and additive degradation.

uKeep the mold surface dry.

uEliminate water outlet breakage.

uReduce or eliminate returns.

uCheck the screw's L/D. If it is less than 16:1, use a screw with a higher aspect ratio or a screw with a different construction.

11. Charred ends of filling

Burning at the fill tip is usually caused by trapped gases creating a diesel effect, igniting the plastic causing the burn. This indicates a poor or clogged exhaust.

uPossible corrections

uClean the exhaust.

uIncrease the venting. Remember, there is never too much venting, only too deep venting.

12. Dark spots

There are several sources of black specks. Dead spots or hanging spots in the barrel. Old material not being removed or cleaned during material changes. Contamination of material before it enters the barrel.

uPossible corrections

uClean properly.

uClean the screw and barrel thoroughly.

uCheck dead spots and repair them.

uFor materials that oxidize easily, adding nitrogen to the barrel may eliminate the problem.

uCheck whether the heating belt is overheated.

uLook for dimples on the barrel.

uCheck the screw and barrel for wear, which may cause plastic retention or excessive shearing.

uCheck for contamination from material handling systems.

uInspect materials received from suppliers to ensure there is no contamination.

13. The gate is red

uFrom plastic perspective

Gate reddening is caused by the incoming plastic eroding the area around the gate during filling. Since it is an erosion problem, reddening near the gate is easy to solve. The best solution is to reduce the speed or cool the reddened area to reduce the erosion. Perhaps neither is very successful. Another measure is to change the angle at which the plastic hits the part or move the gate.

uPossible corrections

uReduce injection speed.

uReduce the mold temperature in this area.

uChange the angle of impact of the plastic.

uChange gate location.

14. Jetting

Jetting is a creeping or serpentine pattern. This is caused by the plastic flying a long distance without encountering any obstacles before it hits the cavity after entering the gate. Sometimes, reducing the injection speed and plastic viscosity can solve the problem. Usually, there is no perfect solution. The gate structure should be modified or inserts should be added to the cavity to allow the plastic to encounter obstacles. This forms a classic fountain flow. Many times, just a slight modification of the gate can solve the problem.

uPossible solutions

uModifying the gate allows the plastic to impact the cavity wall.

uTemporary: Reduce flow rate, raise plastic temperature, or adjust mold temperature to allow plastic to stick to cavity. (Usually not a good long-term solution).

15. Other molding issues

Blisters and air bubbles

Blisters and bubbles are trapped air or gas inside, and are the opposite of sink marks and cavitation caused by lack of plastic due to cooling. To solve, it is important to identify the foreign matter in the plastic. Usually it is one of the following:

uMoisture

uTrapped air

uDegraded polymer molecules

uDegraded additives

uUsually, blisters and bubbles are together with punch marks.

uPossible solutions

uDry plastic.

uLower the temperature.

uReduce residence time.

uIncrease back pressure to remove trapped air.

uChange plastic additives.

uIf there is water inlet material mixed in, use new material.

uIncreasing the feeding pressure - is a last resort as it can mask the problem.

uEliminate water outlet breakage or reduce feed back.

16. Plastic becomes brittle

Plastic parts become brittle, usually due to polymer degradation. Degradation can occur due to high temperatures, too long, or chemical corrosion that causes molecular chain breakage. It can also occur due to water in the material, such as hydrolysis of polyester, nylon, PC, etc. Degradation that usually causes brittleness is shown by changes in the plastic's viscosity. Other factors include material contamination that causes viscosity changes, and too much sprue material mixing.

uPossible corrections

uPolish gates to eliminate rough spots.

uFor PVC or other shear-sensitive plastics, reduce the injection rate.

17. Breaks and cracks

Fractures and cracks are mechanical failures of the material, usually in areas of stress concentration during the final cooling. The solution to this problem is to first determine if it is a mechanical problem caused by the demolding or post-processing of the part, or if it is caused by physical degradation of the material. It may also be caused by contamination of the material with additives or foreign matter.

However, this is usually due to over-packing causing sticking or ejection problems. It is important to determine what the cause is.

In case of material contamination, cracks are usually accompanied by a decrease in viscosity.

uPossible corrections

uCheck material viscosity (fill integral).

uReduce the amount of feeding.

uDetect inappropriate post-processing techniques if possible.

uIf the viscosity becomes lower, use new material.

uCheck the viscosity of the sprue material to determine if it is contaminated.

uCheck the demoulding angle of the parts and eliminate mold sticking.

uUse chemicals or soap to check the chemical resistance of the plastic.

To be continuted...