1. Definition and composition of plastics,

Plastic refers to a material which is mainly composed of polymer synthetic resin, has plasticity and fluidity under certain temperature and pressure, can be molded into a certain shape, and keeps the shape unchanged under certain conditions.

Composition: polymer synthetic resin (40 ~ 100%)

Accessories: plasticizer, filler, stabilizer, lubricant, colorant, foaming agent and reinforcing material.

The role of auxiliary materials: improve the usability and processability of materials and save resin materials (expensive).

(2) Classification of plastics:

More than 300 varieties, of which more than 40 are commonly used.

The name is the general name of all synthetic resins: polyethylene, polypropylene, polyvinyl chloride, phenolic resin, oxygen resin, commonly known as bakelite (phenolic resin), plexiglass (polymethylmethacrylate), glass fiber reinforced plastic (glass fiber reinforced thermosetting resin); English name: nylon (polyamide) PA polyethylene PE

Classification: Thermosetting plastics and Thermoplastics (according to the molecular structure of plastics)

1, thermoplastic

Scaffolds with linear molecular chains become soft when heated, but irreversible when cured.

2, thermosetting plastics:

It has a network molecular chain structure, softens when heated and is irreversible after curing.

General plastic: refers to large output and wide use. A cheap plastic. For example, polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic plastics and aminoplastics account for 60% of the plastic output.

Engineering plastics: refers to a kind of engineering materials with high mechanical properties, which can replace metals, such as nylon, polyphosphate, polyoxymethylene, ABS and so on.

Special plastic: interstitial oxygen resin

(3) Characteristics of plastics

1, light weight, density 0.9 ~ 0.23g/cm foam 0.189g/cm.

2. High specific strength:110 of the strength of metal materials. FRP has higher strength.

3, good chemical stability

4. Excellent electrical insulation performance

5, good thermal insulation performance

6, easy to shape and process, easier than metal.

7. Disadvantages: strength and rigidity are not as good as metal, and it is not heat-resistant. The thermal expansion coefficient below 100C is large, and it is easy to creep and age.

Moldability of thermoplastics;

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< 2 > plastic state:

1, glassy state: the general plastic state TG is higher than room temperature.

2. High elastic state: When the temperature is higher than TG, the polymer becomes as soft and elastic as rubber.

3. Viscous flow: When the polymer is at the fluidization temperature, the plastic flow and viscous liquid flow shift successively, and the plastic molding process is introduced into the viscous flow of the material.

(3) Liquidity:

Under the action of a certain temperature and pressure, the plastic can fully fill all parts of the mold cavity, which is called fluidity.

The fluidity is poor, and injection molding needs a lot of pressure; Too good fluidity, easy to drool, resulting in product overflow.

〈 4 〉 Rheology: The property that a polymer produces fluidity and deformation under the action of external force is called rheology.

Newtonian fluid and non-Newtonian fluid.

Newtonian fluid: mainly depends on shear stress, shear rate and absolute viscosity. The liquid or solution fluid of low molecular compound belongs to Newtonian fluid.

Most polymer melts behave as non-Newtonian fluids during the molding process.

〈 5 〉 Crystallinity: whether it can crystallize when condensed.

Amorphous plastics and crystalline plastics.

Crystallization: nylon, polypropylene, polyethylene, amorphous plastic: ABS.

< heat sensitivity and water sensitivity.

[7] Compatibility: In the molten state, no matter whether the two plastics can be fused together, they will peel off in layers.

(8) Stress cracking and melt cracking.

(9) Thermal performance and cooling rate.

(10) molecular orientation (orientation).

(1 1) contraction.

(12) Toxicity, irritation and corrosiveness.

Design principles of thermoplastic products

I. Size, accuracy and surface roughness

<1> size

The size mainly meets the use requirements and installation requirements, and at the same time, the processing and manufacturing of molds, the performance of equipment and the fluidity of plastics should be considered.

< 2 > accuracy

There are many influencing factors, such as mold manufacturing accuracy, plastic composition and process conditions.

(3) Surface roughness

It is determined by the roughness of the mold surface, so the roughness of the mold surface is generally lower than that of the product, and the mold surface should be polished. Through the product, it is required that the surface smoothness of mold cavity and core should meet Ra < 0.2 um.

If there is no tolerance requirement on the plastic ring, the size is still determined. Generally, Grade 8 in the standard is adopted. The size of the hole can be marked with positive tolerance, while the size of the shaft can be marked with negative deviation. The dimension of center distance can be brown plus or minus tolerance, and the dimension of mating part is higher than that of non-mating part.

Second, the demoulding slope

Due to the cooling shrinkage of plastic parts in the mold cavity, the plastic parts cling to the core and the protruding parts in the mold cavity, which makes it difficult to take out the plastic parts. Forced removal will lead to friction and wear on the surface of plastic parts. In order to facilitate demoulding, the inner and outer surfaces parallel to the demoulding (and shaft core) direction must be considered when designing plastic parts, and sufficient demoulding inclination should be designed, which is generally 1- 1.

Generally, the inclination of core is greater than that of cavity, and the inclination will not decrease with the increase of core length and cavity depth.

Third, the wall thickness

According to the use requirements (strength and stiffness) of plastic parts, the structural characteristics of products and the requirements of mold forming process.

The wall thickness is too small, the strength and stiffness are insufficient, and plastic filling is difficult.

If the wall thickness is too large, it will increase the cooling time, reduce the productivity, and produce bubbles and shrinkage cavities.

It is required that the wall thickness should be as uniform as possible, otherwise, internal stress will easily occur due to different cooling and curing speeds, resulting in deformation and cracking of plastic parts.

Fourth, reinforcing ribs.

Design principle:

< 1 > the middle stiffening rib should be lower than the outer wall by more than 0.5 mm, so that the bearing surface is easy to be straight.

(2) Local accumulation of plastics should be avoided or reduced.

(3) The arrangement of ribs should follow the flow direction in the cavity.

Verb (abbreviation of verb) bearing surface

Plastic parts generally do not use the whole plane as the bearing surface, but use the frame and feet as the bearing surface.

Six, rounded corners

All corners of plastic parts are required to be rounded (arc) transition, because sharp corners are easy to concentrate stress.

Plastic parts have rounded corners, which is beneficial to the flow filling and ejection of plastic parts. The appearance of plastic parts is good, which is beneficial to the strength and life of the mold.

Seven, hole (slot)

Three processing methods of holes in plastic parts;

(1) model directly molding.

(2) Moulding blind holes and drilling them through.

(3) drilling holes after the plastic parts are formed. Make a shallow hole first. All right.

1. The aperture ratio (length and aperture ratio) of the molded through hole should be smaller. aperture