CA1138169A - Method and apparatus for molding indicia in roto-molding plastic containers - Google Patents

Abstract

METHOD AND APPARATUS FOR MOLDING
INDICIA IN ROTO-MOLDED PLASTIC CONTAINERS

ABSTRACT OF THE DISCLOSURE
Disclosed is a method and apparatus for forming indicia on the outside of a roto-molded plastic container during the roto-molding process wherein a template bearing mirror image of the indicia is removably affixed to the interior wall surface of a mold.

Description

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.' ' Bacl;~round of ~h~ In~/ent;on l The prcsent invention relates to the roto-molding f ¦-¦plastic containcrs and morc specifically to a method and ¦apparatus for roto-moldin~ whcrcin a messagc or other indicia ican be molded directly into thc outer surface of the plastic ¦¦cont~iner.
Roto-molding is a wcll known process for manufacturing ~¦plastic containers. Brie~]y, in roto-molding a charge oE
I,powdered ther~oforming resin, such as polyeChyl2ne, polypropylcne l or tlle li~e is placed into a hollow mold. The mold is then hcated¦
¦to tcmperatures which may range up to 700F dependin~ upon the !resin material. The mo]d is simultaneously rotatcd about two ¦axcs while it is being hcated. This distributes the powdered jres n over the entire interior surfacc of the Illold and fuses the resin into a seamless homogeneous structure with a uniform wall thickness. The llold ls then cooled, opencd and the plastic ¦ container stripped froil the intcrior of the ~old.
I lleretoforc~ when it was desircd to forrn any indicia, such ¦1 as liquid level lines, coclpany namcs, or othcr appropriate li ~essages into th2 side oE the roto-molded pl~stic containcr, lt was nccessary to pl~ysicolly engravc a mirror ima~c of thc indicia into thc wall of thc mold. Thus, consl(lerablc time, effort and exl)cnse was rcquircd to cllan~c thc mold in this ¦ fasl:ioll and consc-luclltl~, thc roto-~oldin~ of containers with '' I; ' . ' , I ~

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indicin or otl-cr rlnrkings moldcd dircctly into thc cxtcrior of thc contnlner ls scldol~, lf cvcr, undcrtnken.
' Thc use of removablc te~pl;ltcs temporarily fi~cd to thc interior ~.urfacc of the roto-mold llkewise h.l3 not llerctoforc provcn successful. Thls is due in lnrge measure to tlle problcm of prcvcntln~ ~he tcmplnte from bccomin~ detached fro~ the side wall of thc mold ul)on exposuro for rclatively lon~ durations to thc hlgh temperaturcs lnvolved ln roto-T~olding, so~etimcs approachin~ 700~ for pcr.i~ds of 10-15 mlnutes or more. Furthcr, the nlr present in thc spacc betueen the tcmplate and the side wall of the mold during the heatin~ cycle expands and causcs deEects in the surfnce of the molded article on cooling.
Thc present lnvention, however, docs provide a mcthod and apparatus for modifyin~ a roto-mold so as to permit the noldln~
of indlcla into thc slde of the roto-rnolded plastic contniner.
In this l^espect, the invention :involves In one nspect, the use of n metal tcmplate whlch has en~raved ou its surace a nlrror imar,e of the lndicia to be molded into the sidc of the plast:lc ~contalner. Thls metal template is mechanically fnstened to the ,intcrior surace of the roto-mold by screws, bolts or the li~e ~ith a RTV (Room Tcmp~.rnturc VulcaniYln~) silicone rubber nsed ~as a caulklng materlal to prevent the for~ation oE bubbles in the plastic contalncr due to air entcring from around the templnte.
In another el:lbodlment~ the tcmplate itself, is cast in a sllicone RTV rubbcr,- thc tcmplate then belng attaclled to thc r~old using additlollal RTV s:;liconc rubbcr as tlle adhesive agen~:.
Thls lat'cr uethod i.s prcferred ln that it docs not destroy the intc~rity of thc T:lOld as co-npnrcd to tllc mechanical tcch~ le 'dcfined above, rt is thcrcore thc pr~ nry obJect oE thc invcntloll to providc a mctl-od nnd npl>aTatu3 for rGto-r.lolding n~cssnges lnto ~.' , , .
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, ~L~1 3~16''3 ~he outer surface of a plastic container without the need for engraving the indicia into the wall of the mold.
In summary of the above, therefore, the present inven-tion may be broadly seen as providing a method of roto-molding articles having indi~ia formed in the exterior wall of the article, comprising the steps of: a) providing a hollow mold having walls of heat conductive material, the interior mold surface formed by said walls being smooth, continuous and unbroken; b) forming a flexible template from a curable silicone rubber, said template having a smooth base surface and an upper surface formed with a mirror image of indicia to be molded into the exterior wall of the article; c~ affixing the template to the smooth interior mold surface with a heat resistant, strippable silicone compound adhesive applied between the template base surface and the interior mold surface;
d) introducing a charge of thermoforming resin into the mold;
e) heating the mold externally while rotating the mold to distribute and fuse the resin over the smooth, interior mold surface and the template thereby forming an article wherein the exterior wall of the article has the mirror image of the template upper surface molded directly therein; and thereafter f) removing the article from the mold while retaining the template adhered to the mold interior surface.
The above method may be carried out by way of apparatus for roto-molding comprising: a) a hollow mold of the type for receiving a charge of thermoformlng resin to be roto-molded to form an article, the hollow mold having heat conducting walls with a smooth, unbroken, unadorned interior surface; b) a flexible template made of a curable silicone rubher, the ~, ~

sd~ 3 : ' ' ' 1~1.3~1~9 ~ mplate having a smooth base surface and a top surface formed with a mirror image of indicia to be molded in an exterior wall of the article; and c) means for removably affixing the template to the mold interior surface, the means including a heat resistant, strippable silicone com-pound adhesive applied between the mold surface and the template base surface.
The invention will be better understood and additional objects thereof will become more apparent from 10 the accompanying specification and claims considered together with the accompanying drawing.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a roto~mold showing a template attached to the wall thereof in accordance with the present invention;
Figure 2 is a sectional view on an enlarged scale of a portion of the roto-mold wall having mechanically fixed thereto a metal template in accordance with the present invention;
Figure 3 is a sectional view of another embodiment of the invention wherein the RTV Silicone Rubber template is cast in situ on the interior surface of the mold wall; and Figure 4 is a view similar to Figure 2 showing a silicone rubber template fixed to the wall of the roto-mold by an adhesive.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, Figure 1 shows a roto-mold generally indicated at 10. The process of roto-molding plastic , sd/ -3A-1 3~6~

containers is well-known in the art. Briefly, the roto-mold consists of a metal shell 12, usually cylindrical having a smooth interior surface 14. The mold has an opening 16 at one end which can be sealed with a metal cover (not shown). In roto-molding a charge of thermoforming resin powder is placed into the mold and the mold is heated and rotated about two axes.
This causes the powder to spread over the entire interior surface of the mold and fuse into a seamless homogeneous structure with a uniform wall thickness. After cooling the fused plastic article is stripped from the mold and another resin charge added.

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In ordcr to nold inclicia inl:o thc plastic art.iclcJ tl1c srlooth r1old intcrior s1lrfacc l~ ~3u3t 1.c providcd wit;~ a 1nirror imar,e of thc lndicia. 11crctoforc, onc ~/ay of accor1l lishing this was to enr~ravc the indic:(a dircctly into the surracc oL the rlold. This, ho~/evcr, was not nn acccptable practicc as it required 'considerablc ti1nc, cfEort and' expense in ordcr to chan~ c the indicia oncc it has bccn cngraved dircctly into tlle 1nold surface.
i~The prcscnt invcnt:ion, 11owcvcr, elirlinatcs thc nec:d Eor cnE~raving the L1old surfncc by providing for thc nttacllloene oE a rcmovable -~ tcr,1plate 18 to thc interior surface of thc r.lold.
.l 11eretofore, rcmov.-blc templates have not becn widely used i in roto-nolding becausc of thc problems involvcd in maintaining, the template affixcd to the mold during extended periods of j~success~ e uses of the mold. This is duc to the relatively - ,high teMperatures to wl1ich the rnold is e~.posed for extended pcriods of time tllat may run, for exanple> up to 700F for lO to 15 rlinu,es. -I' Thc present invcntion, however, provides embodiment5 whcrcin a tcmplate 18 Eixed to tl1e smooth interior surface l4 of the - ' mold can easily be removed after molding.
In one embodir.1ent as shown in Pigure 2, tuo or more holes 20 are drilled ~hrour,11 a section of the mold uall 12. Attachcl to this section of thc wall is a template 22 rnade of a metal, prefcrably alllMinun~ and cngraved with thc n1irror i1nage of the indicia to be mol(1cd :lnto the wall of thc plastic container.
Ueldcd to thc rcar s:tdc of tcml)late 22 are two or rnore studq

2~ co-;.csponding to the drillcd holes 20. Thus, thc mctal tcmplat~
can bc attnc]lcd to thc mold wall siMply by inscrtinr, studs throllg llolcs 20 and thleadil1g on nuts 26. A layer 28 of RTV-c;ilico11e G0 Rubbcr (l~crd. T~l Or Ccncral 1'lectric Co.) i.: thcn placcd bctwe n ' 1net<ll tcl;lplatc 22 and 1nold wall 12 alon~ tl1c per:ipl1cry oE tcmplate¦

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22. This layi:l 28 placcd in thc li-luid stntc will cure to form n rcsilient mass ~-hich scnia coTnplctcly tl-c ilrc.~ bcncat11 the tcrlplate 22. Tl-is seal prcvents nny air transfcr, whlch mig11t othcrwisc occur durin~ the heating and cooting cycles from nir ¦poclccts which may be trapped bcneat)l thc tcmplate. If the air transfcr occurs it is li~cly to ca~sc bubbles or other dcfccts ln the rnoldcd product adjaccnt to the area of thc template, In another embodin:ent of the lnvention, as shown in Figure

3, the tcmplate mi1terial is cnst in placc directly on the smooth nterior surface l~ oE the mold wall 12. In tllis cmbodlment a small enc1Osec1 dam 30 of ~odcling clay or tl1e like is bu;lt up on tl1e smooth surfi1ce l/~ of the rlold. ~ liquid 2TV silicone indicated at 32 is pourec1 into thc space surrounded by the dam.
An iMpression plate 34 is then pressed doun onto the liquid ~TV
Isllicone and kcpt there until thc silicone has solidifir.d. The ¦¦knpression plate 34 is then rcMoved, leavin~, a mirror image oE
jlthe i~pression on the surface of the silicone. The moldinp, clay 1Idam 30 is then reMovcd and the ed~es of the slliconc are trimmed.
¦!T21C cast 1~TV silicone rub1)er on curi--g adheres to the inside wall ¦lof the mold. After being used for Molding containers the formed ¦¦templatc 32 is then peeled EroM tl1e smooth surfnce 14 of the mold .
In still another em1)odimcnt of the invcntlon, as shown in 1~ ure ~, a tcmplate ~0 of either ~TV or heat cured silicone 'is made outsi(1e of thc roto-Tnolc1 and then att~ched to thc smooth i~inner surfacc l~ of thc roto-r~old UsinE a layer ~2 oE ~TV 60 ¦silicone rubbcr (1~egd. T;l of Gcneral ~lcctric Co.) as an ad11esive.
ing thc silicone tcr1platc ~0 froT~ an P~TV siliconc is a relatively simplc matter and involvcs only tho fabrication oE a suitablc matri~ board and thcn ~-o~rin~ thc liquid .'~TV siliconc into thc matri~ bonrd.- ~1hen thc siliconc i3 sct, it is si1n1)ly .' I

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pcelcd from the rnatrix board as a pad and ls a mirror irlagc of the ¦
surfacc of thc marrlx board. ~ erc the silico--c ~cr;plate ~0 is ~adc from a heat cured slllcone, a slightly more involvcd proccss is involvcd. In this respcct, thc hent curable sillcone is placcd in thc ~atrix board and subjccted to l~lgh pressure and tcmperature, as for exnmplc, pressure of l,000 lbs per square ~inch at 350~. Thcse condition-: are maintained for approximately ¦15 minutes to cure ~ho slliconc coinpound, Thereaftcr, tlle jltenplate is rcmovccl from the matrix board and subjectcd to ! a post cure heat treatrnent at ~25F for about 3 to ~I hours, If dcsircd, an additional post cure hcat treatllent can be nccom-plished by s~bjecting the tcmplate to about 550F for an additiona l to 4 hours. These post cure heat treatin~ cycles are carried out to insure that there is no shrinkage of the tcmplate during the actu 1 roto-molding process.
The heat cured silicone templatc 40 is then affixed to the smooth surface 14 of the rnold with an RTY silicone layer 4Z bc;ng used as thc adhesive. An empty cyclc is flrst performed on the roto-rnold whereill the nold is heated to approximately 525F for - , lO to 15 ~linutcs withollt resin. Thcrcaftcr, a conventional ,ro;:o-rnoldin~ process can be carried out wherein the powdercd I,resin is introduced directly into the mold as sct forth a~ove.
A sultable adhcsive used for the tcmplate should be hcat resistant, havc a low coefficient of cxpansion and be casily strippable from thc rnold wall~ so that aftcr moldin~ onc type of indicia on the contaillcrs thc adhesivc is strippcd and thc templatc easily removcd. Thc inncr wall of the r~old cnn then be clcancd and a diffcrcnt tcrllplate attnched. Thc rcmovcd tcrnplat may be prescrvcd to be u3cd aKain.
~ n cxamplc of such hcat rcsi~.tan-, strippal~lc and resilicnt on curin~ adllcsivc is a RTV slliconc rubbcr. Onc 3uch adhcsive li : , , "

~ 3~'3 i i6 Rrrv 60 (Rc~'cl Tll of (:cncr.-tl ~lcctric) silicone rubbcr whicl ¦,is a <tin~cthyl silo~anc cor,o-ount curcd with dibutyl tin ditaurnte or tin octoate. ¦ -Thc tcrlylatc itsclf could be Madc Ollt of ;t varicty of Mater-¦ials: r~ct;t], a photorcsist on a rtcta], .siliconc rubber (RTV or hcat c~lred), or f]uorocarbons~ Thesc are only a fct~ cxartple jof l~atcritls suita1)1c as; tc~nplates altllough thQ invcntion is not liGtited to tllcse alonc. In order for the matcria~ to be sultablc ~or tcMplatcs, it Iqust have tile following propertics.
It should be heat resisttnt a~ 750F. for at lcast 15 Minut~s, bc bond.lblc to thc inncr wall of the mol~t, bc resilicnt and non-stic~iny to the rcsin Materinl bcin~ roto-moldcd.
In thc prcfcrred embodiment, the tc~lplate is macte out of ja hcat cured siliconc rubbcr and affixcd as a pad to the inner wall of thc ciold with RTV 60 a silicone aclhesive (Re~istered T~I of Gcneral l.lcctric). This ~lethod is prcfcrre~l in that it ¦
docs not dcstroy the integrity of the ~old due to Mccllanical ,dariazQ.
¦I Cnred RTV 60 has thc followin~ typical propcrties.

sp. y,r. 1.47 I ttar(tness Shorc A 60 I Tensilc Stren~th psi 800 Elonzat:ion % 130 lcar Resistancc ~lie B lbjin. 40 ¦ linear Shrinl;aze .2-.6 Coefficicllt of ThcrMal Exl)ansioll 11.4 x 10-5 The physical proycrtics of heat curcd silicone temylate are ¦~as follows:

!i llar(!lless Shorc A 50 ! Tcnsilc: Stren~,ttl psi 1000 , Elon~atioll 7. 400 ' Tcar ResLstancc dic B lb/in 65 Collyressivc S~rcn,~t;l Q
i 2? hollrs 350 F. ?3%
¦ 70 ho-lrs 300 F. 21,'.

Thou~lt thc speciEication dcscribes moldin~ indicia on thc ¦outsid~ E a ro~o-lr.old~d co::ta nor ehc ~Irocos~ lo~ds itsclE

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to moldin~, indicia on thc outsidc Or ~ny roto^-~,olclcd articlc.
Thus~ it should bc a~prcciated eh.~t the invcntion clocs providc a metllod and a~l)ar~tus Eor roto-~oldin~ spccializcd messa~es or othcr indici~ ~ircctly into tllc walls of a roto-mol~lcd COntRinCr and that this is accoLnplished without matcrially alterin~, thc inte~rity oE the ~old itself.
Havin~ thns descril-ed thc invcntion in detail, what is clai~e~
as new is:

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of roto-molding articles having indicia formed in the exterior wall of the article, comprising the steps of:
a) providing a hollow mold having walls of heat conducting material, the interior mold surface formed by said walls being smooth, continuous and unbroken;
b) forming a flexible template from a curable silicone rubber, said template having a smooth base surface and an upper surface formed with a mirror image of indicia to be molded into the exterior wall of said article;
c) affixing said template to said smooth interior mold surface with a heat resistant, strippable silicone com-pound adhesive applied between said template base surface and said interior mold surface;
d) introducing a charge of thermoforming resin into said mold;
e) heating said mold externally while rotating said mold to distribute and fuse said resin over said smooth, interior mold surface and said template thereby forming an article wherein the exterior wall of said article has the mirror image of said template upper surface molded directly therein; and thereafter f) removing said article from said mold while retaining said template adhered to said mold interior surface.

2. A method of roto-molding as in claim 1 wherein the forming of said template is accomplished by first molding a heat curable silicone to the likeness of said template and then heat curing said likeness to form said template and thereafter the affixing of said template to said interior mold surface is accomplished by using an RTV silicone rubber to adhere said template to said interior mold surface.

3. A method of roto-molding as in claim 1 wherein the forming and affixing of said template is accomplished simultaneously by casting said template directly onto said interior mold surface by the steps of:
a) forming a dam on said interior surface enclosing the area to be provided with said template;
b) pouring a liquid RTV silicone into the area surrounded by said dam;
c) pressing an impression plate onto the surface of said liquid RTV silicone and maintaining the position of said impression plate until said RTV silicone has set; and thereafter d) removing said dam and impression plate to leave said template so formed adhering to said interior mold surface.

4. Apparatus for roto-molding comprising:
a) a hollow mold of the type for receiving a charge of thermoforming resin to be roto-molded to form an article, said hollow mold having heat conducting walls with a smooth, unbroken, unadorned interior surface;
b) a flexible template made of a curable silicone rubber, said template having a smooth base surface and a top surface formed with a mirror image of indicia to be molded in an exterior wall of said article; and c) means for removably affixing said template to said mold interior surface, said means including a heat resistant, strippable silicone compound adhesive applied between said mold surface and said template base surface.

5. Apparatus as in claim 4 wherein said template is made from a heat curable silicone and said adhesive is an RTV silicone.

6. Apparatus as in claim 4, wherein said template is cast in situ on said mold interior surface from a liquid RTV
silicone rubber compound.

7. In an apparatus for roto-molding containers includ-ing a hollow mold adapted to receive a charge of the thermo-forming resin to be molded, the mold having walls of a heat conducting material for conducting externally applied heat to the resin charge and the mold having at least a portion of the inner surface of a wall substantially smooth and unadorned, means for applying heat to the exterior of the mold and means supporting the mold for rotation about two axes during the heating thereof, the improvement comprising:
(a) a flexible template of silicone rubber having a substan-tially smooth base surface for attachment directly to said smooth portion of said mold inner surface, a top surface of said template being formed with a mirror image of indicia to be molded into a wall of the roto-molded container; and (b) a heat resistant strippable silicone compound adhesive between said template base and said mold wall for attaching said template to said mold.

8. Apparatus as in claim 7 wherein said adhesive is a room temperature vulcanizing silicone.

9. Apparatus as in either claim 7 or 8 wherein said template is a heat curable silicone.

10. A method of rotomolding articles having indicia formed in the exterior wall thereof, comprising the steps of:
(a) forming a dam on the interior surface of a rotomold which encloses an area corresponding to the area on the article to be provided with the indicia; (b) pouring a liquid room temperature vulcanized silicone into the area surround-ed by said dam; (c) pressing an impression plate onto the surface of said liquid RTV silicone and maintaining the position of said impression plate until the room temperature vulcanized silicone has set; (d) removing said dam and impression plate to leave a template which carries a mirror-image of the indicia to be molded adhering to the interior surface of the rotomold; (e) placing a charge of thermoforming resin into the rotomold and heating the mold externally while rotating said mold to distribute and fuse said resin over the interior mold surface including said template thereby forming an article in which the exterior wall of the article has the mirror-image of said template surface molded direct-ly therein; and thereafter; (f) removing the article from said mold while retaining said template adhered to the interior mold surface.