Knowledge of plastic hanger mold polishing

Knowledge of plastic hanger mold polishing:

1.0 Surface finish of hook mold

   When making a mold, it must be noted that in different parts of the mold, the required surface finish also changes. Customers generally have higher requirements on the surface of the mold that the plastic touches during injection molding, especially for transparent plastic products. The glossy surface has a positive impact on the appearance and value of the product.

1.1 American Standard:

SPI divides mold finish into 12 different standards. The treatment materials used for each finish are different. The SPI standard is made of H13 steel 3/8in thick and 1in in diameter, and then machined on a circular plane with the surface treatment materials listed in the table below.

Surface finish standard and processing material used for processing (SPI AR-106) Unit: inch

Finish code

Surface treatment material used

Roughness average Ra μ–in

A-1

Grade 3 Diamond Paste

0-1

A-2 Grade 6 Diamond Paste

1-2

A-3

Level 15 Diamond Paste

2-3

B-1

600 grit sandpaper

2-3

B-2

400 grit sandpaper

4-5

B-3

320 grit sandpaper

9-10

C-1

No. 600 Whetstone

10-12

C-2

No. 400 Whetstone

25-28

C-3

No. 320 Whetstone

38-42

D-1

Spray No. 11 glass beads

10-12

D-2

Spray No. 240 oxide sand

26-32

D-3

Spray No. 24 oxide sand

190-230

1.2 Japanese Standard:

The JIS standard divides the mold finish into 6 grades. The treatment materials used in each stage have specific specifications. The JIS standard is to use steel with a diameter of 18mm and a hardness between HRC56 and 60, and then process it on a circular plane with the surface treatment materials listed in the table below.

Surface finish standard and processing material used for processing (JIS K7140) Unit: mm

Finish code

MR-1

MR-2

MR-3

MR-4

MR-5

MR-6

Surface material used

No. 8000 (1 to 5μ) diamond powder

1200 size (8 to 20μ) diamond powder

360 steel paper

No. 150 Whetstone

No. 120 sandblasting under the air pressure of 5kgf/c㎡

Spray No. 46 under the air pressure of 5kgf/c㎡

Surface Roughness (uRz)

minimum

-

0.06

0.24

1.2

4.8

15

maximum

0.03

0.12

0.48

1.7

6.6

19

2.0 Factors Affecting Polishing:

2.1 Steel material for moulding:

Deviations in hardness or other properties of localized areas of the mold surface can cause problems during polishing, such as various types of slag and sand holes.

Vacuum deoxidation reduces large slag and oxygen embrittlement, and produces more homogeneous materials.

Electrofusion slag removal (ESR) treatment can greatly improve the polishing performance of materials, and better results can be achieved by vacuum degassing. ESR treatment can reduce the content of slag in the steel.

2.2 Heat treatment:

Heat treatment can affect polishing in many ways, and a piece of steel that has been hardened seems to have some characteristics that are not suitable for polishing, mainly due to the presence of small oxide particles on the surface of the steel.

In addition, the higher the hardness, the more difficult it is to grind the mold, but after polishing, a high surface finish can be obtained. The higher the hardness of the mold, the longer the polishing time is required to achieve a higher surface finish, but as the hardness increases, the problem of over-polishing will be reduced accordingly. If the heat treatment process is improper, the steel will produce too much retained austenite or bainite, and the polishing effect will be poor.

2.3 Polishing technology:

Generally speaking, the polishing technology is the most important factor affecting polishing. If the polishing technology used is appropriate, and the heat treatment is appropriate, and the steel material is good, satisfactory results can be achieved. However, if the polishing technology used is Improperly, even the best textures can be ruined.

2.4 Surface condition before polishing:

Grinding EDM steel surfaces is more difficult than other conventionally machined and heat treated surfaces. Due to EDM (ie, electric discharge) processing, a thin layer of hardened layer is easily formed on the surface of the mold. The hardness of this layer is higher than that of the parent material, and the harder surface layer must be removed, which is more difficult to polish than the original parent material. Tempering after EDM can also improve polishing performance.

3.0 Typical Polishing Procedure:

machining

milling

Turning

Grinding

EDM processing

rough grinding

Whetstone particle size number

thick

80#

120#

150#

180#

220#

320#

Fine 400#

polished

Sandpaper grit number

thick

320#

400#

600#

800#

1000#

Fine 1200#

polishing

Diamond paste particle size

thick

45μm

25μm

15μm

10μm

6μm

3μm

fine 1μm

3.1 Under normal circumstances, after the mold is processed by different machines, the following process should be used:

◆ After milling, rough grinding, fine grinding and polishing;

◆ After grinding, rough grinding, fine grinding and polishing;

◆ After EDM machining, fine grinding and polishing;

3.2 Grinding:

Grinding is the basis for the success of subsequent polishing operations. By grinding, the scratches left by the machine are removed to make the surface smoother. When grinding, pay attention to:

a. The grinding process should not generate a lot of heat and pressure, which will affect the structure and hardness of the material, and there should be sufficient coolant.

b. During each texture size change, hands and workpieces should be cleaned to prevent coarse abrasive particles or dust from being carried into the next working stage to affect finer textures.

c. Every time the texture size is changed, the grinding direction should be changed. Grind toward a 45° or 90° angle to the previous sanding direction until the grain is finer and more uniform than the previous sanding.

3.3 Polishing:

Diamond paste is a commonly used polishing compound and the best results are obtained when used properly.

Polishing tools, from different types of fibers to sticky pads, are suitable for different hardness materials. Such as nylon, paper, velvet, cotton, etc.

When polishing, pay attention to:

a. Polishing should be carried out in a dust-free and ventilated place, because hard dust particles are easy to adhere to the diamond paste and will damage the surface that is about to be ground;

b. Each polishing tool should be used for only one type of diamond paste, and pay attention to dust:

c. Hands and workpieces should be washed every time the diamond paste is replaced;

d. Adjust the polishing pressure to suit the hardness of the polishing tool and the particle size of the diamond paste. For the finest diamond paste, the pressure is only as large as the weight of the polishing tool.

e. Polishing should start from the corners, edges, pieces or other parts of the mold.

Problem solving in 4.0 polishing:

  The main problem with polishing is over-polishing. The so-called over-polishing means that the longer the polishing time, the worse the surface condition. Excessive surface polishing usually refers to the following two phenomena: "orange peel" and "pinhole". It should be noted that there is usually a greater chance when machine polishing is used.

4.1 Orange peel peeling layer:

  After polishing, orange peel-like irregular peeling occurs on the surface of the workpiece. The main reason is that the material is soft and the polishing pressure is too large. The hard material can withstand higher polishing pressure and is not prone to excessive polishing.

  When you see the deterioration of the polished surface, increase the polishing pressure and continue polishing, which will inevitably increase the polishing of the surface.

  To restore the surface of the material after deterioration, the following methods can be used:

Method 1: Before polishing, use fine grinding to remove the deteriorated layer on the surface, and then start polishing after the final grinding. A lower polishing pressure should be used when polishing.

Method 2: Release the stress when the temperature is 25°C (45F) lower than the last tempering temperature, and repeat the grinding before polishing until the surface condition is satisfactory. Start polishing again, but at a lower polishing pressure than the previous polishing pressure.

   If the result is still unsatisfactory, the hardness of the material should be increased. There are the following methods to increase the hardness:

◆ Nitriding or carburizing heat treatment methods can be used to increase the surface hardness of the steel.

4.2 Pinhole:

Very fine pinholes that typically occur during polishing are caused by a layer of slag that has been ground away during polishing, in the form of hard oxides. Important factors that are common in this process are:

◆Polishing time and pressure;

◆The purity of the steel, especially the related slag;

◆Polishing tools;

◆Abrasive

One of the reasons for pinholes is that the hardness of the parent material and slag is different. During the polishing process, the speed of grinding away the parent body is higher than that of grinding the slag, and the polishing can gradually grind out the lower layer of slag particles until the surface of other materials is removed by further polishing, so that pinholes remain. The problem is more likely to occur when the abrasive rule is below 10 μm and the polishing tool is too soft.

One way to minimize the occurrence of pinholes is to select high-purity tool steel, which is mainly determined by the vacuum exhaust and electronic slag refining (ESR) in the production process. In addition, the following methods can be used:

◆ Before polishing, use a soft, free-cutting grindstone for grinding, followed by polishing.

◆ Avoid using the softest polishing tools when the abrasive rule is 10μm or less.

◆ The polishing time should be as short as possible, and the use pressure should be as small as possible.

SPE standard and SPI standard correspondence table

#1 = A-1

#2 = A-3

#3 = B-3

#4 = C-3

#5 = D-2

#6 = D-3

Information comes from plastic hanger accessories network