Design of Runner System

The sprue bushing receives the plasticated material from the injection nozzle, whichcloses off the barrel and is pressed firmly against the sprue bushing.. Figure 5.1 shows a runner syst

The sprue bushing receives the plasticated material from the injection nozzle, whichcloses off the barrel and is pressed firmly against the sprue bushing Frequently, a singlecavity mold has only a sprue; the part is then said to be sprue-gated (Section 6.1) Withmulti-cavity molds, the sprue bushing feeds the melt into the runners These areconnected to the cavities via the gates.

The gate is an area of narrow cross-section in which flow is restricted Its purposesare fourfold:

- to separate the molded part easily and cleanly from the runner system,

- to hold back the cooled skin that has formed on the cold walls of the runners (avoidingflash on the molded part),

- to heat the melt through shear before it enters the cavity,

- since the cross-section of the opening can be readily altered, the runner system can bebalanced in such way that the melt enters each cavity at the same time and in the samecondition

A runner system usually consists of several components This is particularly evident

in multi-cavity molds Figure 5.1 shows a runner system composed of

sprue

runners,

gate

Runner(primary)Secondaryrunner

Section A-B

Sprue bushingSprue

Land (of gate)

A

Sprue

Secondary runner

[Runner primary)B

Figure 5.1 Runner system [5.1]

5 2 C o n c e p t a n d D e f i n i t i o n o f V a r i o u s T y p e s

o f R u n n e r s

Depending on the temperature control, different types of runners may be distinguished:

- standard runner systems,

5.2.2 H o t - R u n n e r S y s t e m s

Hot runners may be viewed as extended injection nozzles in the form of a block Heatbarriers isolate it from the cold mold It contains the runner system consisting of centralsprue bush, runners and gates or nozzles The temperature of this block lies in themelting range of the thermoplastic melts Hot-runners offer the following advantages:

- no loss of melt and thus less energy and work input,

- easier fully automatic operation,

- superior quality because melt can be transferred into the cavity at the optimum sites.The disadvantages are:

- pressure consumption in cold runners is very high, a fact which makes the design moreexpensive,

- since the slightest temperature differences cause very large differences in viscosity, it

is practically impossible to fulfill the requirement of introducing "the material intoevery cavity at the same time in the same condition"

For these reasons, specialty types only have established themselves for rubber andelastomers; cold runner molds are not used at all for thermoset molding compounds.

5 3 D e m a n d s o n t h e R u n n e r S y s t e m

The dimensioning of a runner system is determined by a multitude of factors, which,

in essence, result from the configuration of the molded part and the plastic materialemployed (Figure 5.2) The demands on quality and economics are listed inFigure 5.3

Factors affecting runner design

Molding Geometry Volume Wall thickness

Quality requirements dimensional optical mechanical

Molding material Viscosity Chemical composition (amorphous, crystalline) Fillers

Freezing time Softening range Softening temperature Sensitivity to heat Shrinkage Molding machine

Type of clamping Injection pressure Injection rate

Injection mold Automatic demolding Manual demolding Temperature of runner system

Functions and demands

1 Cavity filling with a minimum of knit lines

2 Restrictions to flow as few

6 Length as short as technically feasible

to keep losses in pressure, temperature and material small

7 Cross section so large that freezing time equals or slightly exceeds a little that of the molding Only then can holding pressure remain effective until part is solid

8 Runner system should have little or no effect on cycle time

9 Place of gating at the thickest section

In addition, there are several special types that will be discussed along with the varioustypes of runner To provide a first overview, the types of runners listed in Figure 5.4 andtheir characteristic features are summarized in Figure 5.5.

5 5 T h e S p r u e

The melt enters the mold via a sprue which is generally machined in the sprue bushing.Together with the injection nozzle, which seals off the barrel, it must ensure a leakproofconnection between the barrel and the mold during the injection process, which entailshigh mechanical load and thus is characterized by wear The sprue bushing musttherefore be replaceable Except for hot manifolds, where planar contact surfaces (Figure5.6) are frequently required, the sprue bushing of a normal manifold matches that shown

in Figure 5.7 To be able to fulfill its functions, the following properties are required:

- wear resistance: therefore of made hardened steel,

- flexural fatigue strength: therefore a strong but not too large flange, and roundededges,

- since the sprue always leaves a mark on the molded part: as small a diameter aspossible,

- for a perfect seal, the orifices of the nozzles and bushing must be aligned The diameter

of the nozzle orifice (dN) must be 1.5 mm smaller than that of the sprue bushing (ds)

Figure 5.4 Gating systems

5 4 C l a s s i f i c a t i o n o f R u n n e r S y s t e m s

The design engineer can choose from a large number of runner systems to offer theoptimum quality and economics to the user These are:

I Runners which remain with the molded part and have to be cut off afterwards

II Runners which are automatically separated from the molded part and are demoldedseparately

III Runners which are automatically separated from the molded part during demoldingbut remain in the mold

This results in the classification shown in Figure 5.4

7 Pinpoint gate (with reversed sprue)

Application: for

temperature-sensi-tive and high-viscous materials, high-quality parts and those with heavy sections

Advantages: results in high quality

and exact dimensions

Disadvantages: postoperation for

sprue removal, visible gate mark

Application: for parts with large

areas such as plates and strips

Advantages: no knit lines, high

qual-ity, exact dimensions

Disadvantages: postoperation for

gate removal

Application: for axially symmetrical

parts with core mounted at one side only

Advantages: no knit lines and no

re-duction in strength

Disadvantages: postoperation for

gate removal

Application: for sleeve-like parts

with core mounted at both sides

Advantages: uniform wall thickness

around circumference

Disadvantages: slight knit line,

post-operation for gate removal

Application: primarily for smaller

parts in multi-cavity molds and for elastic materials

Advantages: automatic gate removal Disadvantages.for simple parts only

because of high pressure loss

Figure 5.5 Summary of gate types [5.2 to 5.6]

(continued on next page)

Sprue Molding Parting line

Molding Parting line Sprue^

Gate Runner

Sprue Parting line

Molding

Gate

Sprue Runner Parting line

Gate Molding

Sprue

Tunnel gate Molding

Parting line

Application: for multi-cavity molds

and center gating

Advantages: automatic gate removal

Disadvantages: large volume of

scrap, higher mold costs

Application:for parts with automatic

gate removal

Advantages: no postoperation Disadvantages: preferably for ther-

mally stable materials (PE, PS), ited use for others

lim-Application: for thin-walled parts

and rapid sequence of cycles

Advantages: no loss of material for

runner system

Disadvantages: mark on part from

nozzle

Application flat and light-weight

parts in multi-cavity molds

Advantages: better utilization of

ma-chine's plasticating rate

Disadvantages:large amount of

scrap from voluminous runner tem, higher mold costs

sys-Note: today generally used with hot

manifold, thus no scrap but more expensive

Application: for materials with a

large softening and melt ture range and rapid sequence of cy- cles

tempera-Advantages: automatic gate

separa-tion, material loss from runner only after shutdown

Disadvantages: Danger of cold

ma-terial getting into cavity after ruption

Figure 5.5 Summary of gate types (continued)

(continued on next page)

Sprue Runner

Parting line 2

Molding Gate

Parting line 1

Gate Molding

Sprue Parting line

Machine nozzle

Molding Parting line

Runner system Parting line I

The radius of the spherical indentation in the sprue bushing (Rs) into which the tip ofthe nozzle extends, must be 1 mm greater than that of the nozzle tip RN [5.7] (Figure5.7).

Application of the following rules to the dimensions of the sprue (Figure 5.9) will ensureperfect quality and reliable operation:

- The diameter at the foot of the orifice should be roughly 1 mm greater than the gatedmolded part at its thickest point or greater than the diameter of the connecting runner.(This ensures that it freezes last and that the orifice remains open for the holdingpressure.)

- The orifice must be tapered (> 1° and < 4°) and totally smooth, without furrows etc.,around its circumference in order that the sprue may be pulled out of the orifice whenthe mold is opened For this reason, it must not have any flash at its upper end(Figure 5.8)

- The lower orifice edge must be rounded to prevent the melt from pulling away fromthe wall to form a jet of material that would remain behind as a visible flaw on thesurface of the molded part

If these requirements are met, the sprue in single-cavity molds is pulled from the orificeand thus demolded by the molded part, which remains on the ejector-side of the moldhalf

Figure 5.6 Plane area of contact

between machine nozzle and sprue

bushing

Figure 5.7 Curved area of contact between machine

nozzle and sprue bushing [5.3]

Detail A

Figure 5.5 Summary of gate types (continued)

Characteristics

Applications: for high-quality,

tech-nical parts, independent of cycle time, also suitable for materials difficult to process

Advantages: no material loss from

runner system, automatic gate ration

sepa-Disadvantages: expensive molds

especially due to control equipment

Correct Undercut from flashprevents demolding Insufficient sealresults in flash Figure 5.8 Correct and incorrect

design of areas of contact

Figure 5.9 Guidelines for dimensioning sprues [5.8]

Figure 5.10 Design of sprue pullers

[5.9]

In multi-cavity molds, where the sprue serves only to feed the material to the runners,special demolding support is required A sprue puller is installed opposite the sprue, theprofiled tip of which acts as an undercut that grips the sprue (Figure 5.10) Duringejection, the undercut releases the sprue, which can then drop out This design also hasthe advantage of providing a cold-slug well.

Another, less common option for removing the sprue from the bushing is shown inFigure 5.11 The sprue bushing is spring-loaded After the mold has been filled and thenozzle is retracted from the bushing, springs push back the bushing and loosen the sprue

Figure 5.11 Spring loaded sprue bushing [5.7]

left side: Big spring = high force;

right side: Small spring = low force, therefore several

circumferential springs

5 6 D e s i g n o f R u n n e r s

Runners connect the sprue via the gate with the cavity They have to distribute thematerial in such a way that melt in the same condition and under the same pressure fillsall cavities at the same time

The plasticated material enters the runners of a cooled mold with high velocity Heat

is rapidly removed from the material close to the walls by heat transfer, which then forms

a skin This provides a heat-insulating layer for the material flowing in the center of thechannel A hot, fluid core is formed, through which the plastic flows to the cavity Thishot core must be maintained until the molded part is completely solid; then the holdingpressure can act fully to compensate for the volume contraction during solidification.This requirement on the one hand and the wish for minimal pressure loss andmaximum material savings on the other, determines the optimum geometry of the runner.The dimensions of the runner obviously depend on the maximum thickness of themolded part and the type of plastic being processed The thicker the walls of the moldedpart, the larger the cross-section of the runner must be As a rule, the cross-section must

be roughly 1 mm larger than the molded parts are thick A large cross-section promotesthe filling process of the mold because the resistance to flow is smaller than in thinrunners of the same length It pays therefore to dimension the runner according tohydraulic laws Section 5.9.7 explains how a runner system is optimized and balancedwith computer assistance

Figure 5.12 summarizes the factors affecting runner design The objectives of a runnerand the resulting demands can be taken from Figure 5.13 Figure 5.14 presents the mostcommon cross-sections of runners and evaluates their performance

Nomograms for a number of materials and their volumes or weights passing throughthe runner are presented in Figure 5.15 The data are empirical but the diameters of

runners are to be determined as a function of their lengths with an acceptable pressureloss of less than 30 MPa.

The surface finish of a runner depends on the plastic to be molded One can generallyassume that it is of advantage not to polish a runner, so that the solid skin is betterattached to the wall and not so easily swept along by the flowing material With someplastics, however, runners have to be highly polished or even chrome plated in order toavoid flaws in the molded part Critical plastics in this category are PVC, polycarbonateand polyacetal

The cardinal demand for all mold cavities to be filled simultaneously with melt in thesame condition is met very easily by making the flow paths identical However, as shown

in Figures 5.16-5.18, this can only be accomplished to a certain extent or at the expense

of other drawbacks This is why it has become standard practice to balance the bution system by means of different runner or gate cross-sections

distri-5 7 D e s i g n o f G a t e s

The gate connects the cavity (or molding) with the runner It is usually the thinnest point

of the whole system Size and location are decided by considering various requirements(see Figure 5.19):

- it should be as small as possible so that material is heated but not damaged by shear,

- it must be easy to demold,

- it must permit automatic separation of the runners from the molded part, withoutleaving blemishes behind on the part

Figure 5.13

Functions of runners [5.2]

1 Conveying melt rapidly and unrestricted into cavity in the shortest way and

with a minimum of heat and pressure loss.

2 Material must enter cavity (or cavities) at all gates at the same time under the

same pressure and with the same temperature.

3 For reasons of material savings, cross-sections should be kept small although

a larger cross-section may be more favorable for optimum cavity filling and

maintaining adequate holding pressure Larger cross-sections may increase

cooling time.

4 The surface-over-volume ratio should be kept as small as feasible.

Items affecting runners

Figure 5.12

Factors which affect design and size of runners [5.2]

Figure 5.14 Cross-sections for runners [5.2, 5.10-5.12]

The gate can be designed in various configurations Thus, one distinguishes between apinpoint and an edge gate A special form is the sprue gate, which is identical with thesprue itself, as described in detail in Section 6.1

In all gate types, except for the sprue gate, the gate is always the narrowest point inthe gating system

When flowing through narrow channels like a runner or gate, the material encounters

a considerable resistance to flow Part of the injection pressure is consumed and thetemperature of the melt is noticeably raised This is a desirable effect because

1 the melt entering the cavity becomes more fluid and reproduces the cavity better, and

2 the surrounding metal is heated up and the gate remains open longer for the holdingpressure

Cross-sections for runners

Advantages: Smallest surface relative to cross-section, slowest

cooling rate, low heat and frictional losses, center of channel freezes last therefore effective holding pressure

Disadvantages: Machining into both mold halves is difficult and

expensive

Advantages: Best approximation of circular cross-section, simpler

machining in one mold half only (usually movable side for reasons of ejection)

Disadvantages: More heat losses and scrap compared with circular

cross-section

Alternative to parabolic cross-section

Disadvantages: More heat losses and scrap than parabolic