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Two Ways to Avoid Weld Lines With Hot RunnersTwo Ways to Avoid Weld Lines With Hot Runners

April 1, 1997

8 Min Read
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A high percentage of today's larger moulded parts are appearance parts:housings, cabinets, panels, bezels, and so forth. To the product's potentialbuyers, appearance is the most obvious sign of quality, or its lack. Naturally,surface quality requirements for these products are high and continuallyrising. However, as quality levels have been increasing, wall thicknessesof large parts have been decreasing. A typical wall today is 2 to 3 mmthick; a few years ago it was 4 to 6 mm.

As wall sections become thinner, weld and flow lines become more visibleÑlinesthat are a problem in a high-quality appearance part. Multiple gating isone solution to the problem, but frequently it affects the part's mechanicaland optical properties. It can also impair paint adhesion in the finishingcommonly done with large parts. Fortunately, weld and flow line problemscan also be solved by actively influencing mould filling to gain exactcontrol of flow-front formation.

Sequential and Cascade


Two special-purpose injection moulding techniques based on hot runnertechnology can, in almost all cases, put the weld and flow lines whereyou want them. In the first, sequential injection moulding, the lines aredisplaced to noncritical areas of the moulding. The second technique, cascadeinjection moulding, can produce parts free from weld lines. Wolfgang Homes,head of the applications department of Eurotool, a manufacturer of hotrunner systems, clarified the two methods for IMI, noting that choosingbetween them depends greatly on the geometry of the moulded part.

Sequential injection moulding is suitable for large two-dimensionalparts. The needle valve nozzles of a hot runner system (Figure 1) are openedand closed to displace weld lines into noncritical areas of the part. Cascadeinjection moulding is suitable for oblong mouldings where the nozzles canbe arranged in series. Injection starts at the central nozzle. The adjacentnozzles are opened stepwise in series as the melt front arrives.

Mould Filling Simulation

Designing hot runners and moulds for both cascade and sequential mouldingrequires very thorough knowledge of the flow-front profile. The knowledgegained from the mould filling simulations done with special-purpose computersoftware is indispensable for both these techniques. Mould filling analysishelps determine gating locations, as well as optimum process parametersfor volume flow, melt temperature, mould surface temperature, etc.Ñwhichalso form the basis for designing the hot runner. Simulation results indicatemould geometry including runner and gate diameters, locations of the nozzles,etc.

TABLE 1. Comparing Three Ways To Control Nozzles in Sequential InjectionMoulding

A D V A N T A G E S

D I S A D V A N T A G E S

Time-dependendant via core puller control.

Low costs, mostly integrated in the injection moulding machine.

Inaccurately defined starting time, only single opening and closingpossible, limited number of core pullers, reproducibility consuming muchtime, inflexible, setting limited to one machine.

Stroke-dependent via induction switches.

Precise starting point, easily reproducible results, independent frommaterial-specific data.

Additional control unit necessary, higher costs, only single openingand closing possible, hardly flexible, transfer to other machines possibleonly with additional expenditure.

Separate control unit linked to the machine.

Starting signal for exact starting time of the machine, multiple openingand closing of the nozzles possible, easily reproducible results. Flexiblesince not fixed on a machine.

Cost-intensive.


Sequential Moulding Shifts Weld Lines

Weld lines are unavoidable when moulding parts with large surface areaswith several gating points, and air entrapment can occur under unfavourableconditions. This results from the confluence of melt fronts from severalgating points (Figure 2) and the melt splitting at cores or openings.

Sequential injection moulding uses pneumatically or hydraulically controlledneedle valve nozzles to influence weld line locations. By controlling theopening and closing of the nozzles, it is possible to displace the flowfronts, that is, move the weld lines to invisible areas and/or to pointswith lower mechanical load. An effective venting system should be providedin these weld line areas when the mould is built. Regardless of this, however,the higher the melt temperatures of the meeting flow fronts, the betterthe weld line strength and surface quality will be.

In terms of process technology, there are three approaches to controllingneedle valve nozzles. The time-dependent approach is accomplished usingthe core puller control of the machine. The stroke-dependent method isactuated via induction switches on the screw advance stroke. The thirdapproach uses a separate control unit that receives the start signal fromthe machine nozzle. Advantages and disadvantages of these methods are summarizedin Table 1. Tests are currently being conducted on pressure-dependent controlof the needle valve nozzles.

The hot runner system with 10 needle valve nozzles shown in Figure1 is controlled by induction switches as a function of stroke. Machinesetup begins with the basic settings determined in the simulation calculation.Using the flow pattern thus established, the operator optimizes openingand closing times for the needle valve nozzles until the weld lines havethe desired locations and the machine settings are proven stable.

Cascade Moulding Avoids Weld Lines

You can avoid having any weld lines with cascade injection moulding.This is achieved by using the needle valve nozzles to control mould fillingso that only one flow front exists at a time. Each needle valve nozzleopens only when the melt front has moved across its gating point. The techniqueis particularly suitable for long mouldingsÑprotective and sidestrips, or bumpersÑwhere the hot runner nozzles can be arrangedin series.

Nozzle 1. Nozzle 2. Nozzle 3.

Step 1.Opening of the central nozzle.

Step 2.Opening of the Two Outer Nozzles, closing of the centralnozzle2 (transfer of the melt).

Step 3.Filling of the rest of the article and phase of follow-uppressure (all nozzles open).

Step 4.Closing of all nozzles.

FIGURE 3.Cascade control of needle valvenozzles yields a uniform melt front without weld lines in the moulded part(here the central nozzle also has a needle valve).



The key points of this technique are as follows (Figure 3):

  • Only the central nozzle is opened at the beginning of the injectionprocess. When injecting with constant volume flow, the injection pressurerises as the flow path extends.

  • The next nozzle does not open until the melt front has moved acrossthe gate. Melt from that nozzle flows into the already present melt andcontinues the mould filling operation. The existing melt front is retained.

  • Preceding nozzles can be left open or closed as required.

  • When the mould cavity has been filled through the last pair of nozzlesby stepwise repetition of this process, all nozzles are opened for follow-uppressure, then closed simultaneously. Cascade moulding permits multiplyingthe flow length obtainable with one gate without provoking the formationof weld lines.

The number of required gating points, as well as the geometry of thehot runner system, is determined from mould filling simulation and analysis.In the cascade moulding of auto bumpers from elastomer-modified PP, a wallthickness of 3 mm required a hot runner with five gates (Figure 4). Typically,runner and gate diameters are 16 to 20 mm and 4 to 6 mm, respectively.

Various types of cascade moulding are presently in use. For example,if the nozzles being opened stepwise for mould filling are left open untilthe end of follow-up pressure, the central nozzle can be an open designwith the others being needle valve nozzles. Though less costly, this solutioninvolves the risk of the part's central region being overfilled becausethis nozzle is always open and pushing melt into the cavity.

If it is desirable to close a nozzle as soon as the next one has begunto supply the melt, all nozzles must have needle valve design with separatecontrol. The improved control possible with this solution ensures a moreprecise monitoring of mould filling. After volumetric filling of the mouldcavity, all nozzles must remain open during follow-up pressure.

Two variants of this solution are possible by gating through the hotrunner nozzles either directly or through a secondary runner. Changeoverpoints for the nozzles can be determined easily with direct gating, andsince the moulded part has no residual sprue, no postmoulding treatmentis required. However, careful design is needed for good thermal separationof the hot runner and mould, particularly in the gate area.

Though they are far less complex to design, secondary runners can reducethe thermal influence of the hot runner on the mould. However, sprue removalis required. Also, determination of changeover points requires more timebecause with the nozzle still closed, arriving melt can enter the coldrunner. This can be avoided by timing the opening of the needle valve tocoincide exactly with the arriving melt stream, allowing mould fillingto continue without interruption. In this solution, secondary runners mustbe as short as possible and the residual sprue should be from 10 to a maximumof 20 mm.

Back moulding, or hinterspritzen, is an important application of cascademoulding increasingly used in making automotive interior panels. A blankof cloth or film is placed into the mould and back-coated with melt byinjection moulding. A moulded part with a textile face is thus obtainedwithout the time and expense of separate lamination, and the bond betweenpolymer and textile can be much stronger than with lamination. The decisiveadvantage of cascade moulding in this case is that the cloth or film isevenly pressed onto the mould wall by the uniformly spreading melt front.

Contact Information:
Eurotool Hot Runner Systems
Mr. Wolfgang Homes
P.O. Box 3123
D-64615 Bensheim
Germany
Phone: (49) 6251 933232
Fax: (49) 6251 76969


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