Plastic Injection Molds

8/11/2019 Plastic Injection Molds

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Plastic Injection Molds

Fundamental Manufacturing Processes Series Study Guide - 1 -

Training Objective

After watching the program and reviewing this printed material, the viewer will

become familiar with the variety, design, and productive use of plasticinjection molds.

Mold components are identified and explained

The various mold types are featured

Aspects of mold maintenance and storage are detailed

The Injection Molding Process

Injection molding is the most widely used method of producing parts out ofthermoplastic material. The molten thermoplastic material is injected into the

mold at high pressure. Once this material cools and solidifies, the mold opensand the part is ejected. During the injection molding cycle, the mold servesseveral purposes, including:

Determining the finished shape of the part

Venting trapped air or gas during injection

Acting as a heat exchanger to draw heat from the part to aid in

solidification

Providing a means of ejecting the part from the mold

Mold Components

All molds contain a number of common design features. These features include:

Mold base

Mold cavity Mold core

Sprue bushing

Runner system

Gates

Vents

Cooling system

Ejector system, plus other components

The mold base is an arrangement of steel blocks manufactured to specificdimensions. Mold bases may be purchased from commercial mold base manufacturers

or produced in-house by moldmakers.

The basic mold base consists of two halves. The 'A' half, which is also referred

to as the stationary half, or the injection half, and the 'B' half, which isalso referred to as the moving half, or the ejector half. The mold cavity which

creates the outer image or surface of the part is usually mounted on the 'A'half of the mold, while the mold core which reproduces the inner image of thepart is typically mounted on the 'B' half of the mold. Collectively the cavityand core halves are known as the 'cavity set'.

Mold bases, cavity and cores are commonly made from special mold steels or fromother materials such as beryllium copper, stainless steel, aluminum, brass,


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kirksite, and epoxy. The softer mold materials are generally used for prototypemolds and limited production runs.

All plastics have their own shrink factor, meaning they shrink at a certain rateas they cool and solidify. Depending on the type of material to be injection

molded, moldmakers must take it's shrink factor into consideration whenproducing the cavity set. For example, if a material shrinkage is calculated tobe one-hundredth of an inch for a part six inches in length, a total of six-

hundredths of an inch must be added to the mold design to compensate forshrinkage.

Additionally, draft angles or tapers are machined into the side walls of the

cavity set to facilitate part removal from the mold. These tapers typically

range from 1 to 2 per side. Once completed, cavity sets may be heat treated to

protect them from the harsh injection molding environment. Molds may also becoated or plated with wear resistant surfacing material, such as nickel and hardchrome.

The interfacing plane between mold halves is called the parting line. Depending

on the complexity of the part, there may be several such parting lines. Properalignment of the mold halves is accomplished by using leader pins and bushings.

The mold halves are mounted on platens which are components of the injection

machine. Most injection machines have three platens:

The stationary platen, which holds the 'A' half of the mold.

The movable platen, which holds the 'B' half of the mold and moves back

and forth on the injection machines four tie bars.

The rear stationary platen, which holds the other end of the tie bars,

thus anchoring the entire system.

A locating ring on the mold centers to a hole on the stationary platen. This

then allows the nozzle of the heating cylinder to seat firmly against the spruebushing on the 'A' half of the mold. The sprue bushing directs the moltenmaterial from the heating cylinder nozzle into the mold's runner system.

A mold's runner system is the channel or network of channels through which thematerial flows to reach the cavity set. Surface runners are the most common

runner design, and are half-round channels machined into the surfaces of each ofthe mold halves.

Once the molten thermoplastic flows through the runner system it reaches thecavity set through an interface called the gate. The mold gate restricts andcontrols the flow of plastic into the mold. Passage through the gate causes africtional rise in material temperature, extending the materials flow into the

mold. Common types of gates include:

The edge gate, which is usually located on the parting line, and is the

most common gate type.

The submarine gate, which brings material under the parting line to fill

the cavity from below.

The tab gate, which redirects material flow into the mold.

The ring gate, which is used in molding round or cylindrical parts.

The fan gate, which is used to spread material quickly over a large area.


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To remove trapped air and process gases during injection, a mold venting systemis needed. The number and size of the vents are determined by part geometry,material type, viscosity, and the rate of injection. These vents are ground on

the parting line of the mold.

The hot thermoplastic remains in the mold under pressure until it cools. Thiscooling is usually achieved by water circulating in channels machined into themold. Proper cooling contributes to controlled part shrinkage, part strength and

quality. Overall, the speed of the injection molding cycle is controlled by theefficiency of the cooling system.

Once the parts are sufficiently cooled and solidified, the mold opens and an

ejector system, usually in the form of knockout pins, is used to aid in part

ejection. Ejector systems are mounted on the ejection side of the mold and are

typically activated by pneumatic or hydraulic cylinders. In addition to knockoutpins, other ejector methods include stripper plates, stripper rings, and airpressure ejection. Sometimes a sprue puller is used to remove molded plasticfrom the sprue bushing as the part is ejected.

Injection Mold Types

Typical injection molds designed to meet specific production requirements

include:

The cold-runner two-plate mold, which consists of two plates with the

cavity and core mounted in them. The sprue, runners, and gates, along withthe part are molded simultaneously, and then separated after ejection.

The cold-runner three-plate mold, which includes a stripper plate thatautomatically separates the sprue, runners and gates from the parts during

ejection.

The hot-runner mold, which uses an electrically heated manifold that

maintains material temperature in the runners at the same level of the

material in the injection cylinder. The runner system is contained in aplate of its own and does not open during ejection of the part. This isalso known as 'runnerless' molding and can decrease cycle time by 25

percent or more.

The insulated-runner mold, which uses large diameter runners and no

heaters. During injection the outer layers of plastic in the runnersolidify and insulate the inner material, keeping it at molding

temperatures.

Mold Maintenance and Storage

Over time and repetitive use, mold components and surfaces will degrade. The use

of inserts and laminated construction for mold surfaces subjected to high wearis recommended. Rust is also a major factor. Cleaning and lubrication arecritical measures between manufacturing cycles and for short-term and long-termstorage.


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Review Questions

1. Injection mold bases, cavities, and cores are most commonly made from:

a. special aluminumsb. epoxies

c. beryllium copperd. special mold steels

2. The usual range of mold draft angles is:a. 5 to 10 per sideb. 1 to 4 per sidec. 1 to 2 per side

d. 0.5 to 1 per side

3. The sprue bushing is located on the:a. 'A' half of the moldb. 'B' half of the moldc. movable platen

d. rear stationary platen

4. The purpose of a fan gate is to:a. speed solidificationb. control injection speed

c. spread material over a large aread. redirect the flow of material

5. Edge gates are usually located on:a. the 'A' half of the mold

b. the spruec. the parting lined. below the parting line

6. Mold vents are found:a. around the whole cavity setb. on the parting linec. near the gatesd. within the 'B' half of the mold

7. The major factor in injection cycle speed is:

a. the type of thermoplasticb. the temperature of the plasticc. injection speed and pressured. efficiency of the cooling system

8. A cold-runner three-plate mold has:a. no sprue bushing

b. a stripper platec. integrated sprue pickers

d. a larger gate system

9. The mold design known as 'runnerless' is the:a. the hot-runner moldb. the two-plate cold-runner mold

c. the insulated-runner moldd. none of the above


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Answer Key

1. d

2. c3. a

4.

c5.

c6. b

7. d8. b9. a

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Plastic Injection Molds