Blow molding is the collective term for the production of hollow article in thermoplastics material by various methods whose common characteristic is the inflation of preform,or parison,inside a shape-giving blow mold .it has forerunner in the glass industry.
ONE DIFFERENTIES BETWEEN:
#Blow molding in a flowable state within the thermoplastic temperature stage.
#Streth blow molding in the thermoplastic temperature range.#Blow molding in a flowable state within the thermoplastic temperature stage.
In principle one can expert a greater blow up ratio as well as greater fredom in design of the blow molded article an blow mold with material in the termoplastic temperature range,whereas one obtains considerably more strength in the thermoplastic range.
The processing base classification of blow molding techniques resuts in two essentially different metode:
#Production of preform (parison) and subsequent transfer in molddable condition into the blow mold(singgle-stage process).
#Heating of the separately produced parison an sub sequent transfer in molddable condition into the blow mold(two-stage process).
*Extrusion Blow Molding.
*Injection Blow Molding.
*Dip Blow Molding.
The extrusion blow molding process has attained the greatest importance by far.The pressent state of the technology spans an extrusion blow molding range from 1 ml to 10 m3.The injection blow molding and dip blow molding processes are limited to producing articles of 10 ml to 101 or 11 content respectively and to the production of holowware of relatively simple shape.
EXTRUSION BLOW MOLDINGBasic Design Guidelines
The essential design charateristcs of the blow mold relate to the shape of the hollow article to be produced and the most faroable method of introducing the blowing medium.Etrusion with parison lenght variation or timer controlled variation of the die gap accommodate longitudinal differences in wall thickness of the blow molded article and the sectionwise of the die gap to provide crosswise wall thickness variations are part of today's tecnhology.
The blowing pressure is usually in the region of 2 to 20 bar (30-300psi ),preferably 5 to 10 bar ( 75-150 psi ).This requires a clamping force of at least 100kN ( approximately 10 tons ) per unit pressure and square meter of the projected area on the mold mounting platens.
These data are of particular importance for the design of design of blow molds when the weight of the mold is of prime consideration.
MOLD MATERIAL
It is essential to classify the mold materials according to the size and the number of article required.Here the following subdivisions are useful :
+ Blow mold for continous production and large batches
+ Blow mold for small batches and prototype production
Only metal molds can be considered for continous production and long runs,whereas for small batches and prototypes,casting resins in water permeable backed shell contruction are also suitable.
With metals it is necessary to further subdivide into tool steels,aluminium,beryllium and zinc alloys.Fastenings and guide components and,if possible also the areas of hight edge inserts should in addition be hardened.
Tool steels should be considered mainly for long production runs in the volume range of up to 10 liters.The blow mold is produced by machining.Any of the steel types generally used in plastic processing can be chosen.If blow molding plastic contain corrosivee volatiles,it is necessary to employ corrosion or acid resistant steels.Apart from the exeptions alreadymentioned,hardening is not necessary and crome plating is not customary.For large volume articles with a content of three cubic meters and up welded machined plate contruction can also be considered.Cast steel is of no value for mold base.
Given the right care,the life span of steel blow molds isgreater thanthe longest production run achieved up to now ( 10'7th parts per blow mold ).Inserts subject to wear and tear either have to be refurbished or exchanged at intervals of 10'5th or 10'6th cycles.
The advantages of blow molds produced in aluminum,berylium and zinc alloys include excellent thermal conductivity,low weight and economical manufacture by precision casting.These material should be considered mainly for blow molding in the range from 10 liter volume and up,but they can also be used for mass production of smaller hollow articles if they are equipped with steel inserts.Plating may be considered where processing of plastics materials containing corrosive volatiles is a problem,in which case even gold plating has been suggested.Due to the reduced strength and hardness,damage to edge and surfaces can easily be cause.With proper care,life spans of 10'5th to 10'6th cycles be expected.
MOLD PARTING LINE and INSERTs
The blow mold should be parted in technologically most suitable plane for the hollow article.As a rule one obtains two equal or mirror image halves,equally divided.With asymmetrical and awkwardly spaced angled blow molded articles the location of the parting line is the determined by the most farorable blowing position and the avoidance of ejection difficulties caused by undercuts.That type of blow mold tool may also require several parts or inserts e.g.bottom inserts that may rotaye or be able to slide against each other.These may be actuated by compressed air or hydraulic cyliders or by means of synchronous cams.Loase insert in dovetail guides that are ejected with the blown molded article for rause may also be considered.
Cylindrical articles are parted along the axis of rotation and elliptical shape along the large diameter.Cube shape articles can be parted along the diagonals or parallel to the side faces.Diagonal parting may permit a smaller,more favorable blow ratio,but it results in an excessive stretching in the corners opposite the parting plane with a corresponding thinning out in these places on the article.
Assembled mold halves and inserts should be designed so that the joints donot show on the molded part.It is well known that even perfectly flush assembly can cause visible lines on the molded part.The joins runs along the area of the standing rim and at the top along the portion of the neck covered by the closure.
Of the multitude of mold inserts,only the bottom and neck inserts, the edge and knife inserts for welding,cutting and trimming and the replaceable plate and punches containing engravings and other markings will be mentioned.All inserts subjected to high edge loading during the clamping of the parison should be hardened to 60-62 RC.
Cylindrical articles are parted along the axis of rotation and elliptical shape along the large diameter.Cube shape articles can be parted along the diagonals or parallel to the side faces.Diagonal parting may permit a smaller,more favorable blow ratio,but it results in an excessive stretching in the corners opposite the parting plane with a corresponding thinning out in these places on the article.
Assembled mold halves and inserts should be designed so that the joints donot show on the molded part.It is well known that even perfectly flush assembly can cause visible lines on the molded part.The joins runs along the area of the standing rim and at the top along the portion of the neck covered by the closure.
Of the multitude of mold inserts,only the bottom and neck inserts, the edge and knife inserts for welding,cutting and trimming and the replaceable plate and punches containing engravings and other markings will be mentioned.All inserts subjected to high edge loading during the clamping of the parison should be hardened to 60-62 RC.
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