EN-82-07

2, °1

INJECTION MOLDING OFRUBBER OBTURATOR PADS

FRANK TESTROET

-4.

AUGUST 1982

TECHNICAL REPORTENGDISTRIBUTION STATEMENT

I .ELECT 5:

:::, ENGINEERING DIRECTORATE

cm ROCK ISLAND ARSENALROCK ISLAND, ILLINOIS 61299

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EN-82-07 I iS(L _ _ _ _ _ _

4. TITLE (and Susbtil~) S. TYPE OF REPORT IPERIOD COVERED

INJECTION MOLDING OF RUBBER OBTURATOR PADS Technical ReportS. PERFORMING ORG. REPORT NUMBER

I. AUTHORt(a) S. CONTRACT OR GRANT NUMUER4'o)

Frank B. Testroet

9. PERFOR1MING ORGANIZATION NAMS AND ADDRESS 10. PROGRAM ELEMENT PROJECT. TASKCDR AREA A WORK UNIT NUMBERS

RokIsland Arsenal AMlS Code 3297.06.7710Rock Island, IL 61299______________

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATS

CDR, Rock Island Arsenal August 1982ATTN: SMCRI-ENM 13. NUMBER OF PAGES

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IL. SUPPL.EMENTARY NOTES

It. Key WORDS (Casflsua ma roveres. oftl Of necessay and IdentIfy 6Y bOck n&NNIher)

1. injection Molding

2. Obturator Pad, 155mm

3. Neoprene RubberIM ASSTAC? (odm m paorso o N nineoinv a fdontify by Weoek asloet)

he purpose of this effort was to reduce production costs for fabricating 155mmrubber obturator pads by using the injection molding process. A neoprenerubber compound specified for molding 155mm rubber obturator pads was testedto determine Its processabillty using the injection molding technIque. Thiscompound was shown to be compatible with the Injection molding process and anInjection mold was fabricated for molding 155mm obturator pads. The l55nwnrubber obturator pa can be vulcanized In 8 minutes by injection molding as

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UNCLASSI FIED* ~SCUmRY CLAISPICATION OF THIS PAGE(Dhan DO&ate naqg

20. ABSTRACT (cont)

compared to 2 hours for compression molded pads. Inject ion molded pads met* the dimensional requirements of Drawing No 11578862 and were successfully test* fired in the M109 Self Propelled Howitzer and the t4198 Howitzer.'$- -

ii UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE(Whow Data Entated)

FOREWORD

This work was authorized as part of the Manufacturing Methods andTechnology Program of the US Army Materiel Development and Readiness

* Command and was administered by the US Army Industrial Base EngineeringActivity.

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TABLE OF CONTENTS

PageDD Form 1473. ..... ................ .. .. .. ....

DD Form 11473.................... . . . ..... . . ... .. .. .. .

Foreword. ..... ................. .. .. .. . . ..

Table of Contents. ............. ............IV

Introduction .. ............ ................

Procedure. ............ .............. ...

Results and Discussion. ...... .............. .. 3

Conclusions ....... .............. ....... 13

Recoummendations ....... .............. ..... 13

INTRODUCTION

The purpose of this effort was to reduce production costs forfabricating rubber obturator pads for 155mm weapon systems by use of theinjection molding technique. This technique is a continuous high tempera-ture process that is capable of increasing production output and loweringmanufacturing costs because of reduced stock preparation time, no physicalhandling of molds, shorter cure times and lower finishing costs.

Rock Island Arsenal (RIA) has been manufacturing rubber obturator padsby conventional compression molding for over 20 years. Compression moldingof obturator pads involves milling the mixed stock to slab form, extrudingthe rubber, accurately weighing the rubber, and fcrming the rubber into.preforms in the form of a donut. The preform is then loaded into a hotmold and manually placed onto a steam heated platen of a hydraulic press.Pressure is applied to close the mold for final forming and excess rubberis forced out of the mold as flash during the curing process. For thickitems such as obturator pads, lengthy cure times are required because ofthe low thermal conductivity of rubber. Compression molding time of 2 hoursis specified for 155mm obturator pads.

When using the compression molding process, only 3 molds can be usedsimultaneously at RIA; consequently, production is limited to 12 pads per8-hour shift.

Conversely, the injection molding process requires no weighing or pre-form preparation. In this process, extruded rubber Is fed directly intothe heated Injection cylinder where it Is preheated and then forced throughrunners and gates where additional heat is generated by friction prior toentry into a preheated closed mold. A much shorter cure time is requiredbecause all of the rubber is at an elevated temperature when it enters themold. In addition, virtually no flash is produced. Some items, dependingon their volume and configuration, can be cured in three minutes or less.

The ram-type injection molding technique has been used at RIA for theproduction of barrier bags for 152mm caseless ammunition, various shapedbellows, grommets, retaining straps, gaskets and numerous other items. How-ever, none of these items have the bulk or the rigid dimensional requirementsof the 155mm obturator pad. The configuration and dimensional requirementsof the obturator pad are shown on Drawing No 11578862, "Pad", in Figure 1.

PROCEDURE

A neoprene compound for all 155mm obturator pads, as specified byDrawing No 11578862, "Pad", in Figure 1 was used throughout this study.This compound was developed specifically for fabricating obturator padsand could not be altered by the addition of processing aids or other com-pounding ingredients that would enhance flow of the rubber during injectionmolding. A Banbury internal mixer, size No. 1, was used to mix the neoprenecompound.

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Molding was performed with the Lewis ram-type vertical injection moldingmachine, shown in Figure 2. This machine is a Model 200V-Ram that has a fillingcapacity of 48 ounces, a 200 ton clamping capacity, a 2.5" diameter ram, andan effective plunger displacement of 65.4 cubic inches per shot. The machineis controlled with limit switches, timing relays and hydraulic valves. Valves,pressure and temperature regulators were operated manually to adjust injectiontemperature, pressure and speed. Mold temperature and clamping times were alsovaried to find the best conditions for injecting and curing the rubber.

Initial trials were conducted using an on-hand T142 track pad mold anda filler ring mold, PN 8427067, to observe whether the rubber would flow wellenough to be successfully injection molded in thick and circular sections.

A three-plate injection mold 15.245" x 15.245" with a stack height of 4.375"was fabricated with a two-runner system with two gates and two vents to produceobturator pads within dimensional tolerances cited on the aforementioned drawing.Initially, gates and vents measured 0.125" in diameter and operating parameterssuch as cylinder temperature, mold temperature, dwell time, injection time andpressure were varied in an attempt to produce physical properties which meetdrawing requirements and equal or exceed the performance of pads that are com-pression molded 2 hours at 290°F.

Tsst pads measuring 6" x 6" x 0.075" were compression molded 30 minutesat 307 F for purposes of quality control.

The injection mold was later modified from a two-runner to a four-runnersystem to allow for a more even distribution of the rubber during injectionof the rubber. The gates and vents of the two-runner system measured 0.125"in diameter and were alternated equidistantly on the flat surface of the mold.Final retooling of the mold reduced the vent size to 0.0625" in diameter.

Obturator pads were checked for dimensional accuracy on an optical com-parator using instructions for optical chart 11585963 which is used for qualityassurance inspection of compression molded obturator pads. Physical propertiesof the injection molded obturator pads were measured from slices taken from theperiphery of the pads.

Additional pads were sent to Watervliet Arsenal for similar opticalinspection. These pads were then sent to Aberdeen Proving Grounds (APG) fortest firing.

RESULTS AND DISCUSSION

Track pads, T142, were injection molded with a mold temperature of 3500Fand injection pressure of 1000 psi while cylinder temperature, injection timeand dwell time shown in Table I varied.

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FIGURE 2. VERTICAL RAM-TYPEINJECTION MOLDING MACHINE

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TABLE I

TIME AND TEMPERATURE PARAMETERSFOR INJECTION MOLDING TRACK PADS

Pad No. Cylinder Temp.,0 F Injection Time, Sec. Dwell Time, Min.

1 150 48 5

2 150 48 10

3 150 48 15

4 180 60 10

5 200 60 IV

The rubber flowed well for all of the pads but started to prec t at acylinder temperature of 2000 F. Test specimens were cut from the t . padsnumbered two and four, and the physical properties shown in Table veremeasured.

TABLE II

PHYSICAL PROPERTIES OF INJECTION MOLDED TRACK PADS

Drawing No. 11578862Property Measured Pad No. 2 Pad No. 4 Requirement

Tensile Strength, psi 2280 2360 1500 Min.

Modulus @ 1OOE, psi 770 710 none

Elongation, % 250 260 200 Min.

Hardness, Shore A 69 67 70 + 5

These stress strain properties indicate that the rubber was properly

cured throughout the 1.5" thickness of the pads within 10 minutes.

The filler rings, PN 8427067, which measure 5.754" in O.D. and 0.430"thick were injection molded with a cylinder temperature of 140°F, injectionpressure of 1000 psi, injection time of A0 seconds, dwell times of 2.5 and 3minutes and with mold temperature of 350 F. Physical properti8 s of aninjection molded filler ring that was cured 2.5 minutes at 350 F are given

*in Table III.

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TABLE III

i PHYSICAL PROPERTIES OF INJECTION MOLDED FILLER

Property Measured Result

Tensile Strength, psi 2270

Elongation, % 250

Hardness, Shore A 69

Conformance to drawing tolerances was confirmed by measurement with amicrometer and caliper. Thus, the feasibility of injection molding of thicksectioned and round items using the specified compound was demonstrated.Based on these results, the two-runner system injection mold was subsequentlyfabricated.

The injection mold was then placed in the Lewis injection press and* obturator pads were molded to test the mold. Injection cylinder temperature

was set at 150 F, as it waa shown earlier that the rub.er stock was precur-ing in the cylinder at 200 F. Mo!d temperature of 320 F with injection timeranging from 15 to 36 seconds and injection pressure of 1500 psi with a cure

* time of 3, 5 and 7 minutes were selected for initial test. Injection timesof 15 and 20 seconds were not long enough to fill the mold, but 24, 30 or36 seconds were sufficient. Rubber was observed to flow well during theinjection cycle; however, the obturator pads were out of round. The padswere determined to be undersize on the O.D. dimension in the area of thevents. Increasing injection temperature to 175 F and injection pressure to2000 psi failed to alleviate the problem. The 0.125" diat.ater vents allowedexcessive pressure drop and uneven filling of the mold between gates and

• . vents during the cure time which resulted in pads that were undersize et thevents.

Two additional runners were added to the mold producing a four-runner*" system with four gates and four vents, each measuring 0.125" in diameter,

alternating equidistantly around the mold.

Preliminary attempts to injection mold obturator pads were made using24 seconds injection time with cylinder temperature at 130 0 F, injectionpressure of 1700 psi and mold time of 8 minutes at 330UF. Again it was shownthat the O.D. dimensions in the area of the vents were undersize in the ventarea, producing pads that were not truly round. Dirr-nsions of the pads were

:" measured on an optical comparator at the 7.0263" datum line of Drawing11578862.

The four-runner system allowed good loading and retention of the rubberin the area of the gates, but pressure drop-off at the vents permitted therubber to shrink back from the vents during curing of the rubber, causingthe undersize radius dimensions in that area. Therefore, the vents were

*reduced to 0.0625" and injection molding parameters were examined in an

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effort to mold dimensionally stable obturator pads. The final pararetersselected for injection molding obturator pads were an injection cylindertemperature of 160 F, an injection time of 30 seconds at 1800 psi injectionpressure, and a cure time of 8 minutes at 320 F. These conditions proved tobe the optimum ones for producing obturator pads that displayed the requireddimensions. Typical measurements at the four gates and four vents are pro-

F' vided in Table IV. Variance of the radius is indicated by these measurements.

TABLE IV

TYPICAL MEASURED VARIANCE OF 7.0263 INCHES DATUM DIAMETERINJECTION MOLDED OBTURATOR PAD

Molded Using Molded Using

Preliminary Parameters Final ParametersPosition Gate/Vent Specified Measured Variance Measured Variance

Datum Datum of Radius Datum of RadiusDiameter Diameter Dimension Diameter Dimension

__ _ _ _(Inches) (Inches) (inches) (Inches) (Inches)

#1 Gate 7.0263 7.0263 0.000 7.0263 0.000

#2 Vent 7.0263 7.0133 -0.013 7.0263 0.000

#3 Gate 7.0263 7.0263 0.000 7.0263 0.000

#4 Vent 7.0263 7.0153 -0.011 7.0263 0.000

#5 Gate 7.0263 7.0263 0.000 7.0263 0.000

#6 Vent 7.0263 7.0173 -0.009 7.0263 0.000

#7 Gate 7.0263 7.0263 0.000 7.0263 0.000

#8 Vent 7.0263 7.0163 -0.010 7.0263 0.000

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The physical properties given in Table V were obtained with specimenscut from injection and compression molded obturator pads and a standard

* ASTM compression molded test pad for comparison with drawing requirements.

TABLE V

PHYSICAL PROPE, 'ES OF

INJECTION AND COMPRESSION MOLDED PADS

Injection Compression CompressionMolded Molded Molded Requirements of

Property Measured Obturator Pad Obturator Pad Test Pad Dwg.No.11578863

Tensile strength, psi 2250 2100 2200 1500 Min

Modulus @ lO%E, psi 530 480 630 -

Elongation, % 250 210 220 200 Min

Hardness, Shore A 69 67 68 70 + 5

Brittleness at low 5 specimens shallTemperature Pass Pass Pass pass @-50 F or

colder

To illustrate the sequence of operations for producing injection moldedobturator pads, the injection mold in the open position is shown in Figure 3.Figure 4 shows the closed mold during curing of the rubber and Figure 5 showsthe cured obturator pad being removed from the mold.

A series of twelve obturator pads were subsequently fabricated, using the_ same conditions, i.e., l6oF injection cylinder temperature, 30 secondg injection

time with 1800 psi injection pressure and an 8 minute cure time at 320 F. Threeof these twelve pads were inspected by the RIA Quality Assurance Directorate,SMCRI-QAO-P, using optical inspection procedures per Drawing No 11578862. Allthree pads met the drawing requirements. Two of these three pads marked 7A and12B were sent to Watervliet Arsenal for inspection. Both pads conformed to

" specifications as listed in the Watervliet Arsenal Inspection Activity Sheetshown in Table VI. These pads were then forwarded to Aberdeen Proving Groundfor live firing tests.

Obturator pad 12B was installed in a M109 Self Propelled Howitzer.Sixty-five rounds were fired using zone 8 charges without any problems. Thispad was reported to be in "fine" condition after 65 rounds had been fired.

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Obturator pad 7A was installed in a M198 Howitzer. Subsequently, 430 roundswere test fired using zone 8 charges. The pad was found to be in normal conditionas compared to a compression molded pad. Additional rounds could have been firedusing either pad since they were removed from the Howitzers only because ofculmination of other testing.

CONCLUSIONS

The neoprene compound specified by Drawing No 11578862 can be successfullyinjection molded to form thick sectioned, multifaced rubber items with verytight dimensional tolerances.

Stress-Strain properties are equivalent to or better than those of com-pression molded specimens.

Injection molded 155mm obturator pads can function satisfactorily as indicated"* by the firing test at APG.

Molding time of obturator pads was substantially reduced using the injectioni molding technique. Forty-eight obturator pads can be injection molded as compared

to 12 compression molded pads during an eight-hour shift.

j RECOMMENDATION

It is recommended that:

a. 155mm obturator pads be injection molded.

b. The injection molding process be considered for other thicksectioned rubber items, especially for high volume orders.

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