Blow molding materials

Materials:

Theoretically any thermoplastic material can be used for a blow molding operation. but practically only those material which show high hot strength and good stretch property at extrusion temperature are suitable for blow molding. The most common materials which show these properties are HDPE, LDPE, PP, medium impact PS, PVC, and PC.  Acetal, nylon, ionomers and acrylonitrile styrene are other materials which may be used. But because of their cost and/or sharp melting point or other properties, they have not been proved to be as suitable as the materials mentioned earlier.

HDPE:

Advantage:

1.       It has greater rigidity of the material which permits the use of thinner bottle walls and thus provide light weight, low cost packaging.

2.       It has lower water vapor permeability

3.       It has ability to withstand higher working temperature.

4.       HDPE pack can be decorated with high speed silk screen printing method.

Disadvantage:

Considerable knowhow is required for the correct design of HDPE bottle. On an average a bottle requires approximately 2years development period and 30% of its production costs are usually accounted for R&D.

LDPE:

It is used for more flexible items.

PP:

It is used for high gloss, heat resistance, stiffness and good strength.

PS:

It exhibits good color, high glass like clarity, decorative finish pack and it will not damage sinks, glass shelves and other delicate bathroom fittings and it will not rust. Also it has a moderate cost.

PVC:

With the advances in the understanding of PVC compound chemistry, new blow molding equipments are developed; which can manufacture bottles and packs of very light weight and at a high production rate. These bottles are rigid, strong and of comparatively low permeability. It can possess glass like clarity. Thin walled PVC products have extremely competitive material cost. Main problem with PVC is its toxicity when used for the packaging of edible and potable substances. To overcome this thicker walls are necessary. This results in the increased cost.

PC:

 It is used where transparency and strength are required. But this is availed at somewhat higher cost.

Other thermoplastics materials which are now coming into use for blow molding and which increase their application as blow molding material are:

Phenoxy material is  used for eye wash bottles.

Methyl pentene polymer used for experimental bottles.

Cellulose acetate butyrate used for Sun Tan oil bottles.

ABS for pressure containers used for spray.

This material though expensive is used due to technical consideration like freedom of corrosion, light weight etc.

DECORATING

Any design that improves the part function should be considered.

LETTERING WITH PLASTIC PRODUCTS:

Letterings in plastic product are required to include permanent detail or designs. These letterings may be raised or depressed on the plastic part.

Raised letters:

 This can be included by indenting the mirror image of desired letters/ designs in the mold. The mold surface should be highly finished to prevent any mold sticking. Also the letters should be checked periodically for any imperfections and some mold releasing agents should be applied.

Depressed letters:

This can be included by incorporating raised letters in the mold or post molding indentation. The raised letters can be incorporated in the mold by spark erosion or by assembling the required design on the mold surface. Both this methods involve higher cost for the part production due to the following reasons:

·         Increased mould fabrication cost.

·         Increased difficulties in part ejection.

·         Depressed letters either remove the material (reduce wall thickness and strength of part) or reduce the volume of the container.

TEXTURING:

Texturing in plastics means to display (create) effects on the plastic surface which simulates the fibrous structure of meat. Mould in or formed in textured surfaces on plastic can be obtained by variation in integral coloring methods.

Some decorative filler are added to the base resin at the compounding levels so as to produce such visual effects of wood, metallic finish etc.

TOLERANCES

·         The tolerances specified for any molded or machined plastic part should be as loose as possible because extremely tight tolerances require more expensive molds, special controls during the molding operation and possibly the use of shrink fixtures during cooling. All these result in a part which is more expensive.

·         In many plastic, the dimensional changes due to temperature variation from –55⁰ C to 120⁰ C is considerably more than the allowable tolerances.

·         For thermoplastics injection molding, compression molding generally results in parts with best dimensional tolerances. The dimensional tolerances for thermoplastic parts made by any method vary from part to part and material to material.

·         The tolerances for thermoset plastic parts depend on the type of resin, filler, molding condition and the type of mold used.

·         Parts made with thermoset plastic can be designed with a tighter tolerances than corresponding thermoplastic parts because thermoset plastic are more stable under thermal condition than the unfilled thermoplastic, when used for the same part.

·         Filled thermoplastic depending on the amount and type of filler can approach or be equal to the dimensional stability of thermoset plastic.

·         Tolerances vary depending on the location of the parting line and the depth of the flash. The molder should be informed by the critical dimension.

·         The flash can affect the dimension in one plane and not in the other.

·         Tolerances suggested by the resin manufacturer are given in inches/inch at the ambient temperature and 50%RH.

·         Shrinkage of parts made by thermoforming, blow molding, rotational molding etc. (except compression molding and transfer molding) will show higher variation in tolerance than injection molding. This variation is determined empirically.

·         Glass reinforced thermoplastic have considerably tighter tolerances than unreinforced material.

·         The molder can make slight changes in the finished dimension of a molded part without the cost and trouble of re-matching, re-heat-treating, and re-plating the mold.

·         The molder can increase or decrease the dimension or prevent the warpage.

INSERTS:

Definition:

An integral part of a plastic molding consisting of metal or other material which may be molded into position or may be pressed into the part after the molding is completed.

Types:

There are basically two types of inserts:

1)      Molded in inserts: They have medium or coarse knurl to provide adequate gripping power. Some of the molded in inserts are :

·         Blind hole female insert with internal threads.

·         Open end female insert with internal threads.

·         Blind hole female insert with internal threads and counter bore.

·         Male stud with internal thread.

·         Eyelet projecting.

·         Eyelet both end projecting.

·         Projecting rivet.

·         Double projecting insert with external threads.

·         Blind hole female insert with internal threads and double sealing shoulder.

·         Metal stamped insert.

·         Female insert with undercut.

·         Drawn eyelet.

·         Rod or pin type inserts.

2)      Post molded inserts: They may be threaded or may be installed by heat or ultrasonically and may also be coarse knurled.

Selection criteria:

The insert should have a shoulder fitting into the mold to prevent entry of the molten plastic into the threads.

Design criteria:

·         If the orientation not perpendicular to the parting line is to be used, it will be difficult and expensive to mold in the inserts. Hence a post molded installation should be considered.

·         Location of an insert relative to the edge of the part or an adjacent wall is very important.

·         Sometimes a thermal stress is imposed on the plastic, in addition to the mechanical stress, due to its higher coefficient of thermal expansion i.e. when the molded part is cooled the plastic tend to shrink more than the metal, causing stresses in the plastic which can result in cracks around the insert during operation. Using a sufficient amount of plastic around the insert, either in the form of a Boss or an increased distance from walls, edges or other insert will prevent consequences of the thermal stresses.

·         Inserts with sharp corners should be avoided to minimize cracking.

·         Inserts should always be used sparingly because they are placed in mold or part by hand, unless large quantity production justifies automation. In either case their use is expensive.

Significance:

Metal inserts are used in plastic parts:

·         To take wear and tear if frequent disassembly is required.

·         To carry mechanical stresses above the limits of the plastic.

·         To act as electrical contact.

·         For decoration.

THREADS

·         Threads in the molded plastic parts are commonly made either by molding in or by tapping.

·         Molded threads can be held to better tolerances than tapped threads of same size. Because tapped threads tend to have a minute chips and cracks on the surface.

·         Molded threads must start and stop abruptly.

·         Feather edges should never be specified because they are the weak areas in the part or mold.

·         External threads can be molded using a split mold, however, the excessive flash may be present which will interfere with the subsequent assembly. A chase operation can prevent this, but it is an extra and costly step.

UNDERCUTS

Definition:

Undercuts are indentation or projection on the walls of a plastic part which interfere with simple ejection from a two part mold; hence they must be avoided wherever possible.

·         Undercuts unduly complicates the mold design and increases the cost considerably because special split molds are required, retracting mechanisms may be required etc.

·         Internal undercuts require more expensive tooling than the parts with external undercuts.

·          Post mold machined undercuts are often cheaper than those molded in, especially external undercuts and circular undercuts.