40% of injection molding defects are caused by water entering mold.

Time:2024-02-01 19:18:46 / Popularity: / Source:

Pure nylon injection molded sockets are prone to defects such as bubbles, dents, seams, loose tissue, and surface flow marks during injection molding due to uneven glue thickness, large ratio of thick glue area to thin glue area, and large shrinkage rate.
injection molding defects 
In order to meet usage requirements, ensure product quality, and improve product qualification rate, with exhaust system and cooling system designed reasonably, mold was improved from glue feeding method and gate position, from direct glue feeding to submerged glue feeding.
After many improvements, above-mentioned bad defects of product have been solved. Improved injection mold has greatly improved qualification rate of socket.
Sockets used as structural parts in electronic systems have increasingly higher requirements for surface quality and dimensional accuracy. In order to meet its reliable physical and mechanical properties, product is required to have dense structure, small surface shrinkage, firm welding, high strength, small internal stress, no bubbles and no flow marks.
However, during molding process of plastic parts, after adjusting injection process parameters, problems such as insufficient filling, bubbles (see Figure 1) and surface flow marks often occur in product. Analysis shows that main reason for above defects is unreasonable design of mold's gating system.
In order to meet usage requirements, ensure product quality, and improve product qualification rate, this article improves mold in terms of glue feeding method and gate position.

●Glue feeding method●

Glue feeding method is method for molten material to enter mold cavity, which can be divided into two categories: glue feeding through a small nozzle and glue feeding through a large nozzle.

Thin nozzle glue feeding method

Fine nozzle glue feeding method is a glue feeding method often used in plastic molds. It is filled evenly during injection. After injection is completed, it is actively disconnected from product. Disconnection position is not obvious and surface quality is high. However, mold structure is complex, processing is difficult, and cost is high.

Large nozzle glue feeding method

Large nozzle glue feeding method is the most commonly used glue feeding method in plastic molds. There are two commonly used glue feeding methods:

Direct glue feeding method

Direct glue feeding, also known as side glue feeding, is the most commonly used glue feeding method. Its advantages are simple structure, convenient processing, fast injection speed, high production efficiency, easy mold adjustment and mold modification, etc.; disadvantages are that it is easy to produce flow marks and weld marks; material head cannot be automatically cut with product and needs to be trimmed and trimmed again. Afterwards, a small scar will be left on the surface of product, which reduces surface quality of product.

Submersible glue method

Gate position selection of submersible glue method is very flexible. When surface quality of product is very high, gate position can be designed on inner surface of product, otherwise it can be designed on outer surface of product; and it can effectively reduce product welding marks; During injection molding, material head will be automatically cut with product. Incision is small and there is no need to trim it. In mass production, efficiency is extremely high.

●Gate location●

Principle of gate location selection: Gate location is mainly determined according to product shape and requirements. Selection of gate location has a direct impact on product quality.
Following aspects should be mainly considered when determining location of gate:
In order to reduce direction change and energy loss of molten material when flowing, flow of each part of molten material when filling cavity should be the shortest.
Gate position should be set at maximum wall thickness.
Gate location should be conducive to cavity exhaust.
Gate position should be conducive to reducing occurrence of weld marks, warping deformation, etc.
When surface quality requirements of product are high, gate should not be set on the surface.
Problem analysis
If gate location is chosen unreasonably, following problems will occur:

1. Bubbles

During injection molding, gas formed in product mainly comes from gas existing in cavity and flow channel, water vapor formed by evaporation of water contained in plastic raw material at high temperature, and gas produced by decomposition of plastic at high temperature.
Mold is generally designed with an exhaust groove. During injection, most of gas will be discharged from exhaust groove out of cavity. However, there is still a small amount of gas due to improper selection of gate position, which causes melt to form air pockets, that is, bubbles, during filling during injection. When this kind of bubble is compressed during injection, it will generate a large back pressure, which will increase internal stress of product and cause serious quality problems.

2. Weld marks

Product adopts direct glue feeding method, and gate is not in optimal feeding position of product. Molten material does not flow smoothly in flow channel, resulting in flow marks on the surface of workpiece.
During injection, when several molten materials come together in cavity, they are not completely fused together at their intersections, and cannot fuse with each other to form a fusion mark, that is, a fusion mark.

3. Surface shrinkage

During injection molding process, large surface shrinkage of product is the most frequent undesirable phenomenon. Plastic of injection mold shrinks in volume when cooled, and surface of early cooling part hardens first. Due to uneven cooling, surface easily shrinks (see Figure 2).
injection molding defects 

4. Insufficient filling

Gate position of product is in thin glue area. When filling into thick glue area during injection, time for plastic flow will increase, and span of plastic flow will also be larger.
Molten material filled first will cool down first, and cooled molten material will affect continued filling of uncooled molten material, resulting in insufficient filling.

●Improvement of gate position●

Selection of socket gate location is unreasonable because socket gate position is in thin glue area, which is a direct glue feeding method and is not at maximum wall thickness. Moreover, during filling, melt flow and cooling are uneven, resulting in serious surface shrinkage and unsmooth exhaust. Bubbles, ripples, insufficient filling, etc. often occur, which cannot meet requirements.
For phenomenon of shrinkage, bubbles, surface ripples and insufficient filling, the first improvement method adopted is: re-determine position of gate, continue to use direct glue feeding method, and try to fill thick glue area of socket with melt from thick glue area to thin glue area, increase diameter of runner, and shorten process (see Figure 3).
injection molding defects 
Improved mold was tested again. Injection molded product was slightly optimized compared to before, but bubbles and shrinkage still existed (see Figure 4), could not meet requirements.
injection molding defects 
After further analysis, it was concluded that gate location was still not in the best position. Considering surface quality of product, gate was chosen at sharp end of thick glue area of product. However, during filling process, air pockets are formed under gate position when molten material flows, resulting in bubbles and shrinkage.
After analyzing reasons, gate position was adjusted again and glue feeding method was improved. Original direct glue feeding method was changed to submerged glue feeding method. Glue feeding position was at the most uniform place in thick glue area (see Figure 5).
molding process of plastic parts 
Improved mold was tested again. Surface shrinkage of molded socket was greatly reduced, and there were no problems such as bubbles, surface ripples, or insufficient filling (see Figures 6 and 7), meeting usage requirements.
molding process of plastic parts 
molding process of plastic parts 
During injection molding process of plastic parts, many problems such as bubbles, weld marks, surface shrinkage and insufficient filling often occur. Under condition that exhaust system and cooling system are reasonably designed, this article improves mold from aspects such as glue feeding method and gate position. It changed from direct glue feeding to submerged glue feeding. After many improvements, it finally solved defects such as bubbles, weld marks, surface shrinkage and insufficient filling in the product.
Gate position and glue feeding method are crucial to quality of product. A reasonable pouring system can improve product production efficiency and qualification rate. After improvement, qualification rate of socket has been greatly improved.
This method of glue feeding has been gradually applied to production of other products, which can meet needs of mass production.

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