Friendly reminder: Items 1-6 are relatively simple and clear. See summary of Part 7 at end of article for a detailed interpretation.

1. Definition

Recess (Sink Mark) - when material cools and shrinks on a thick wall or in places of ribs and recesses, surface dents appear, also called shrinkage pits, shrinkage, pits, shrinkage traces wait.

Manifestations of shrinkage

2. Influence of different polymeric materials on shrinkage marks

Pressure-volume-temperature relationship of polymers

As you can see from figure above, specific gravity of amorphous polymers is less affected by temperature changes, so it has less effect on shrinkage marks.

Recommended reading: What is crystalline plastic? and effect of crystallization on properties and manufacturability

3. Examples of Product Design Mistakes

Parts tend to shrink if wall thickness is different

The picture above shows that:

Original design: • Excessive wall thickness, • Uneven wall thickness, • Sharp corners inside and out

Improved design:• Reduced wall thickness, • More uniform wall thickness, • Chamfered inside and outside

Details are prone to countersinking

You can see from figure above that:

Original design.• This design may have drawstrings and air pockets.

4. Rib design highlights

The structural strength of ribs indicates a

Constructive points of a strong edge b

•Part wall thickness T

•The draft slope (d) is 0.5°~1.5°

• Rib height (H) less than 5T, typically 2.5-3T

• Distance between ribs is greater than 2T–3T

• Chamfer radius (R) is 0.25-0.40T

The thickness of fin (W) is 0.4-0.8T

5. Key points of counterbore design

Key points for countersink design

Key points of counterbore design b: To ensure strength, outer diameter of counterbore is usually twice inner diameter

Design point with counterbore c: The above two methods can strengthen counterbore structure

6. A brief description of solution for removing sinkholes

6.1, molding process

a. Increase injection pressure b. Increase pressure after holding c. Decrease injection speed according to tightening d. Reduce mold temperature e. Reduce injection temperature

6.2, mold design

a. Enlarge sprue b. Shorten sprue c. Reposition sprue d. Widen sprue e. Decrease flow resistance f. Make sure nozzle and mold are well aligned

6.3, product design

a. Make ribs and countersunk holes thinner b. Add chamfers to ribs and countersunk holes c. Decrease wall thickness d. Create a template surface to hide sink marks

7. Solutions at a Glance for Eliminating Drawstrings on Injection Molded Parts

Shrinkage marks in injection molded products, also known as product shrinkage, are local depressions that form on surface of product, some of them are pits and some are ravine-shaped. After internal cooling, a layer of condensate forms on surface of product. thin, which is not enough to resist shrinkage of material under internal stress caused by internal shrinkage stress causing stretching and deformation of surface of product. Traces of shrinkage mainly appear on back side of ribs, columns and buckles, and can also appear in corners where local thickness of flesh varies greatly.

7.1, ribs too thick

When position of fin is thicker, joint between fin and main wall thickness is also thicker, where plastic is concentrated, and surrounding area of ​​the fin and main wall thickness is first solidified by cooling, and center of joint between fin and main wall thickness remains in molten state.Later cured plastic shrinks on first cured plastic and exerts a suction force on surrounding plastic. If coagulated layer is weak in any one place (usually only on surface of mold opposite rib), surface at that place may fall into depression. As a rule, appearance of sinks is determined by properties of material, holding pressure and distance from sprue. Sometimes thickness of fin is designed to be 50% of wall thickness of main body or even less. Estimating human experience is sometimes inaccurate. It is recommended to analyze it with a CAE mold flow analysis. . The risk of sink marks can also be predicted using above method for product columns and corners.

7.2. The temperature of rib and relative surface of mold is too high

The surface temperature of mold corresponding to rib is too high, surface condensate layer is thin, and rigidity is insufficient. As molten adhesive at center of joint solidifies, residual stress is likely to pull thinner layer of condensation into recess and form shrinkage marks. Then, it is necessary to strengthen cooling of mold surface against ribs, lower temperature of mold to ensure that condensation layer is formed quickly, and when condensation layer is thicker, surface hardness is relatively large and not easily formed by surface depressions. The mold temperature can be set according to recommended value in material properties table and gradually adjusted in 5 degree increments, producing 5 to 10 pieces each time. make further adjustments according to result. Here, you can use mold temperature measuring instrument to accurately measure actual mold surface temperature, reducing waste of materials for machine setting. This adjustment can also be analyzed by CAE mold flow analysis, and risk can be predicted according to mold flow analysis sink index, and influence of different mold temperatures on sinks can be understood.

7.3. Gutter and sprue size affect product shrinkage

The design of runner and gate is too small, and material flow resistance is relatively large. If injection pressure is not enough, mold cavity will not be filled tightly, and density of melted adhesive will be small, which will easily cause surface depressions and shrinkage marks. Some products have uneven local adhesive flow or flow channels that are too small, which will also cause high resistance to material flow and insufficient local pressure, which also increases chance of draw-downs. I suggest to use mold material flow analysis CAE software to analyze material flow and pressure before mold design and fill process simulation CAE software will provide reasonable data to help us design runners and gate sizes intelligently and analyze obstacles in product structure. flow, modifying product based on analysis results can properly prevent sink marks on surface of product.

7.4. Unreasonable design of number and location of mold sprues will affect product shrinkage marks

Unreasonable design of location and number of sprues will also result in product shrinkage. Here is a parameter called stream length ratio that can be shared with colleagues. The material flow length ratio is ratio of material flow distance to wall. Thicknessthe main body of product., is an important reference for us to design gate number and gate position, please pay special attention to mold colleagues. This reference data value can be found in regular material property sheet, which is used to evaluate fluidity of a material. If material flow length ratio is too large due to unreasonable number of gates and gate positions, it will cause too much resistance to flow. If injection pressure is not enough, mold cavity cannot be densely filled, and melt density is small, there is a high probability of shrinkage depression. Many of our older moulders often design location and number of sprues based on experience. I don't think it's recommended to rely entirely on experience to judge. The results of CAE computer simulation analysis can give us mold designers very accurate reference data. analysis More accurate results can be obtained by comprehensive analysis of material properties, product structure, mold temperature, water cooling, etc. Mold designers should learn how to use scientific analysis tools to reduce mold design errors.

7.5. Too high temperature of injection molding machine will affect shrinkage of product

If temperature of injection molding machine cylinder is too high, density of melted adhesive will decrease. When product cools down, melted adhesive in contact with mold surface will first solidify into a coagulated layer. With gradual solidification of hot melt adhesive, a cavity is formed in center of cavity, and a shrinkage stress is formed on inner wall of cavity. If surface condensation layer is not rigid enough, surface of product will sag inward, forming shrinkage traces. The improvement method is to set reasonable temperature of material tube within range allowed by material, trying not to use limit value as much as possible, which can effectively reduce chance of shrinkage of product. CAE mold flow analysis can also analyze reasonable reference values ​​to prevent sink marks on product in advance. May I ask how many molding machine technicians saw mold flow analysis parameters before setting up machine? I'm afraid there aren't many, right? I believe that scientific data analysis is an absolute trend in technology development.

7.6. Product cooling time is too short, resulting in product shrinkage

If cooling time of product is not enough, condensate layer on surface of product will be too thin, and there will not be enough rigidity to withstand stress caused by shrinkage of internal melt, and it is easy to form shrinkage pits on surface of product. How to set cooling time, you can refer to recommendedthe values ​​given in CAE mold flow analysis table and material physical properties table. May I ask how many machine setup technicians have reviewed material property sheet requirements before setting up machine? It is recommended to set cooling time based on analysis and suggested value when setting up machine, to shorten cooling time as much as possible within allowable range of surface quality of product, and to shorten molding cycle.

7.7. Insufficient injection pressure and insufficient damping cause shrink spots on product

Insufficient injection pressure and holding time, low plastic pressure in cavity resulting in insufficient filler, low product filling density, low product density and easy formation of streaks on product surface. When injection molding screw buffer is zero, screw will no longer move forward and pressure cannot be increased. Insufficient holding pressure can easily cause product to shrink. Mold flow analysis CAE must analyze reference values ​​of holding pressure, buffer time and position, and fine-tune injection molding machine according to analysis results. Did you read mold flow analysis report before technicians tried mold?

7.8. The screw check valve of injection molding machine is damaged and leaves no marks on product

Injection molding experts know that check valve of an injection molding machine is designed to prevent molten plastic from flowing out in front of screw in material tube during injection and to maintain pressure. If check valve of injection molding machine is worn and broken during long-term use, and damage is serious, plastic will flow back through gap, buffer and holding pressure will decrease or disappear, and product will easily cause breakdown and depression. Therefore, frequent inspection, maintenance and cleaning is required. In addition, plastic pellets should be kept clean and free of debris, especially recycled materials should not be mixed with hard objects such as blades and iron shavings. If a serious defect is found, it is recommended to replace it immediately.

There are often many factors that cause product defects. We must comprehensively consider product design, mold design and injection molding process, accurately analyze and find out root cause of problem, and comprehensively consider cost, efficiency and quality of product. Sometimes requirements cannot be fully met, it is necessary to comprehensively consider accounting and make an informed choice. My principle is: priority of improving structure of product is highest priority. If this is not possible, it can be improved by mold design. Even if mold cost increases, do not leave problem of injection molding adjustment, because mold cost is a one-time increase. Sexually, cost of injection molding is long-term and large-scale, and injection molding only requires a reasonable and small adjustment. Therefore, when considering problems, it is necessary to systematically analyze, carefully calculate and make a reasonable choice in order to be a good press specialist.c-forms.

7.9. Shrinkage caused by excessive shrinkage of material

Strength brother's note: It is generally not appropriate to select high shrinkage polymer materials for producing products with more ribs or thicker products. From point 2 above, generally amorphous polymers Material shrinkage rate small, and filler, especially aspect ratio filler, can quickly reduce shrinkage rate of plastics such as fiberglass.

Recommended Reading: Factors Affecting Plastic Injection Molding Shrinkage and Overall Plastic Shrinkage

Source: Internet, first author unknown, thanks a lot! The Brother Resilience team is shutting down

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