塑料注射成型 中英文翻译

没有给我们规定热的方向。

如果能量能自发的从冰

块流向周围的水中，

这 可能和能量的守恒完全一致，

但这一过程决不发生。

这一概念是 热力

学第二定律的实质。

很明显，冷冻机是一种物理系统，用于 厨房的电冰箱、

冷场库和空调装

置，它不仅必须遵从第一定律（ 能量守恒）也必须遵从第二定律。

为了弄清冷冻机为什么没有违背第 二定律，

必须对这一定律加以说明，

热力学第二定律

换句话说，

热之所以能从较冷的物体流向较热的物体，

是外界力量做功的结果，

现在我

们弄清了某一日常的 自然过程。

如水和冰之间的热流动和冷冻机热从里面向外面流动之间的

< br>区别。

在水、

冰 系统中，能量的交换是自发产生的，

因而热的流动是水流向冰。水放出了能量

另一方面， 在冷冻机中，能量交换不是自发产生的，而需要改变热的流动方向，并通

过进一步加热较 暖的周围环境而使冷冻机内部变冷，就必须依靠外力做功。

Injection Molding

Many different processes are used to transform plastic granules, powders, and liquids into

product. The plastic material is in moldable form, and is adaptable to various forming methods. In

most cases thermosetting materials require other methods of forming. This is recognized by the

fact

that

thermoplastics

are

usually

heated

to

a

soft

state

and

then

reshaped

before

cooling.

Thermoses, on the other hand have not yet been polymerized before processing, and the chemical

reaction takes place during the process, usually through heat, a catalyst, or pressure. It is important

to remember this concept while studying the plastics manufacturing processes and polymers used.

Injection

molding

is

by

far

the

most

widely

used

process

of

forming

thermoplastic

materials. It is also one of the oldest. Currently injection molding accounts for 30% of all plastics

resin consumption. Since raw material can be converted by a single procedure, injection molding

is suitable for mass production of plastics articles and automated one-step production of complex

geometries.

In

most cases, finishing is not necessary. Typical products include toys, automotive

parts, household articles, and consumer electronics goods.

Since

injection

molding

has

a

number

of

interdependent

variables,

it

is

a

process

of

considerable complexity. The success of the injection molding operation is dependent not only in

the proper setup of the machine hydraulics, barrel temperature variations, and changes in material

viscosity.

Increasing

shot-to-shot

repeatability

of

machine

variables

helps

produce

parts

with

tighter

tolerance,

lowers

the

level

of

rejects,

and

increases

product

quality

(i.e.,

appearance

and

serviceability).

The

principal

objective

of

any

molding

operation

is

the

manufacture

of

products:

to

a

specific quality level, in the shortest time, and using repeatable and fully automatic cycle. Molders

strive to reduce or eliminate rejected parts in molding production. For injection molding of high

precision

optical

parts,

or

parts

with

a

high

added

value

such

as

appliance

cases,

the

payoff

of

reduced rejects is high.

A typical injection molding cycle or sequence consists of five phases;

1. Injection or mold filling

2. Packing or compression

3. Holding

4. Cooling

5. Part ejection

Plastic granules are fed into the hopper and through an in the injection cylinder where they

are carried forward by the rotating screw. The rotation of the screw forces the granules under high

pressure against the heated walls of the cylinder causing them to melt. As the pressure building up,

the rotating screw is forced backward until enough plastic has accumulated to make the shot. The

injection

ram

(or

screw)

forces

molten

plastic

from

the

barrel,

through

the

nozzle,

sprue

and

runner

system,

and

finally

into

the

mold

cavities.

During

injection,

the

mold

cavity

is

filled