Troubleshooting for Injection Molding

Evaluation of the quality of plastic products

Quality includes appearance and internal quality.

Appearance quality includes integrity, color and finish.

  • Integrity refers to the plastic products obtained by injection molding of the mold, which are completely consistent with the structural shape in the design of the product, and it cannot have defects such as weld-lines, injection defects and shrinkage sinks.
  • Color means that the color of the molded plastic product must be consistent with the customer’s demands. For transparent plastic products, the transparency should be good, and there should be no defects such as whiting, black spots, and silver streaks.
  • Finish means that the roughness of the surface of the molded plastic product should meet the requirements of the customer. The embossing and blasting should conform to the customer’s specifications.
    The internal quality includes whether the tissue is loose or not, and whether there are defects such as bubbles, cracks, and silver spots on the inside.

The dimensions of the molded plastic product must meet the tolerance requirements of the design drawings, and the product must meet the functional requirements of the customer after assembly.
Mechanical properties include tensile and pressure resistance, impact resistance, etc.; chemical properties include acid and corrosion resistance, radiation resistance, and special environment resistance.

Causes of product defects

There are many types of defects in plastic products, and the reasons are also very complicated. There are mold problems, plastic problems and process conditions.

Plastic problems:

Plastic problems include plastic quality, ingredients and baking materials. If the plastic varieties are not impure, the blending ratio is improper, or the plastics of the baking materials (such as PC, ABS, PA, etc.) are not baked, it will cause problems such as loose internal structure, poor strength, internal bubbles, and silver streaks on the surface.

Molding process problems

The three elements of the molding process include injection pressure, temperature and cycle. Tuning is to find a suitable set of values ​​between the three, so that the mold can form a qualified product in the shortest time. But tuning is a meticulous and complex technology, many of which are caused by improper adjustments.

Mold problems

Even if the mold design is handy, plus the mold level master, it is difficult to ensure that the mold has no problems during the test. Mold issues include mold design, manufacturing and wear. One thing to note here is that the structure of the plastic part is unreasonable and can cause defects in the product.

However, the problem of product structure directly leads to the problem of mold. As a mold design engineer, it is necessary to have a foresight to the problems that may arise from the unreasonable structure of the product, and to communicate with the customer or product engineer in time to prevent the problem before the mold is manufactured.

Among all the above problems, the mold problem is often the main problem, and it is the most difficult problem to solve.

General Approach to Problem Solving

Most of the defects of the product appear during the test. Among these defects, although the problem of the mold is often the main problem, it is much easier and easier to change the plastic and adjust the molding process conditions than to repair the mold.

Therefore, the problem is often solved by changing the plastic and adjusting the injection process parameters. Repairing the mold is only considered when changing the plastic and molding conditions does not solve the problem.

Repairing the mold should be very cautious, not very sure not to act rashly. Because once the mold conditions are changed, no major modifications and restitution can be made. Therefore, before the mold is repaired, it should be based on the actual situation of the undesirable phenomenon of plastic products.

A detailed analysis of the research to find out the real cause of defects in plastic products, and then propose a detailed repair program.

1. Short shot The plastic parts made by plastic injection molding companies have irregular and incomplete edges. It usually occurs at the farthest point of the gate, the roots of thin and long ribs.
2, Shrinkage Occasionally, a place 1. when the material thickness is uneven, 2. thick section of injection molded part, 3. boss and rib section. Due to shrinkage, the plastic parts develop dent, uneven, and wavy under low light
3, Flash A part edge with an extra thin layer of plastic is often found in the following locations 1. Parting line; 2. Moving core; 3. Ejector pin position, boss position, hole position, snap location
4. Bubbles The bubbles on the plastic surface are of a different color from the surrounding colors and are typical 1. bubbles caused by gas, air, and water-gas that are not released in time. 2. bubbles caused by shrinkage. The bubbles in the transparent part are particularly distinct.
5. Weld line When multiple molten flow fronts coincide with each other, a deep weld mark appears on the surface of a plastic part. This most commonly occurs at the confluence of multiple molten flow fronts.
6, Burning it is not a flat surface. It is usually dark or black spots; usually, they are found in sections where it is difficult to fill and trap the gas easily.
7, Black spots black impurities are evident on the surface of the plastic parts, mainly caused by the use of mixed materials.
8. Discoloration The slight difference in color between the actual plastic part and its required color is very apparent, and it is generally because the pigment is incorrect, the mix ratio is incorrect, or the mold is set at the wrong temperature.
9. Wrinkles The plastic parts show wavy lines on the surface caused by the cooling of the resin flowing.
10. Deformation The plastic parts have distortions, undulations, curves, and this is particularly common in the bosses, ribs, and round-shaped injection parts. These are especially common to PP injection molding.
11. Wrong materials In contrast to the specified materials, the one here can be identified by checking out the packaging label, the density, the burning continuity, the color of the flame, the color of the smoke, and the length of flames.
12. Stick to the mold Incomplete plastic part staying in the cavity, in contrast to short-shot, or due to the lack of designing the mold ejector system, the plastic part is hard to be ejected from the cavity, generally in thin-walled areas, bosses, and snaps.
13. Scratch The surface of the plastic parts rub against the surface of the cavity during the ejection so that a lot of them have scratches on the surface.
14, Over-flow It is caused generally by cavity damage and occurs on the active section, bosses, ejector pins, and parting surface.