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Exterior Surface (Mold Cavity) Design

December 31, 2021

First we talked about Interior Surface (Mold Core) Design, link to the blog : https://cmppin.com/blog/interior-surface-mold-core-design/

The inner and outer walls of the part are formed simultaneously and integrally, but interior and exterior designs are essentially independent so we review them separately. As the design develops, the designer should begin thinking about the interaction of the plastic and the mould that will produce the part. The visual exterior of many products is formed in one half of the mould called a cavity. Following are some of the features of mould cavities the designer will want to consider.

Cavity – Cavity Blow Ratio = W>D

A bottle is a typical example of a blow moulded part formed using 2 cavity mould halves. A round bottle has a blow ratio that is comprised of a width=diameter and a depth=radius (2:1). The result is excellent material distribution in a round bottle. But, not all parts will be round. As designers start to push the limits of drawing down into cavities, how far should they go? The answer depends on the elongation elasticity of the material and how thin a wall you are willing to accept. But as a rule of thumb, the material won’t stretch much further down into a cavity (Depth=D) than the width of material available to fit into the cavity (Width=W). So, try not to design your cavity-cavity part to be deeper than the width.

Cavity – Core Blow Ratio = W>2D 

Many industrial parts are formed using a combination of cavity and core mould elements where the core forms interior shapes. The core changes the blow ratio parameters.

The diameter of the cylindrical parison that forms a double wall part must allow enough material to enter the mould to adequately form each half of the part. Half of the cylindrical parison is used to form the exterior half (cavity) of the part and the other half of the cylindrical parison forms the interior half (core) of the part. Since there is no flow of material along the mould walls (only stretching), it follows that the depth of the cavity (D) should be no more than one-half the length or width of the cavity(W). A part design utilizing cavity depths that exceed this relationship will be subject to severe thinning or blow-out. So like the relationship between diameter and radius, cavity-core parts should have overall blow ratios of W>2D.

With multiple or divided cavities each cavity should meet this W>2D requirement.

The design of certain complex parts will require changes in the parting-line location to stay within this relationship. These steps in the parting line must include clearance for repeated opening and closing of the mould halves and be positioned so they do not shear the parison during mould-close. A parting line angle of 10° draft or greater is generally designed into mould parting line steps. When a 10° draft is not possible, options like angling the mould in the machine so that the parting lines form a positive draft relative to one another or moving mould sections can be used.

Sidewalls & Draft

The plastic parison sticks and begins to solidify as soon as it hits the mould. The material then stretches to fill the cavity as blowing progress. There is no flow of material along the mould walls. There are three aspects of thinning to consider.

  1. The thinning caused by stretching results in weakness. Any thin, weak location is susceptible to further thinning because it has become thin & weak. The thinning progresses rapidly in these locations.
  2. The apparent rigidity (or strength) of any area on the part varies proportionately with the square of the wall thickness.
  3. Variation in wall thickness can result in warpage.

The thinning along sidewalls and in corners is the reason that parts should have outside draft angles. The exterior draft is not critical to part removal from cavities since the plastic shrinks away from the outer mould walls as it cools. The draft is recommended when exterior walls are to be textured.

Plastic contouring of heavy parison sections to match these critical areas can improve the condition but not eliminate it. Because of this, cavity design must avoid features that contribute to thinning. Designs that utilize sharp 90° corners will result in parts with extremely thin, weak corners. There are a variety of corner configurations that improve or alleviate this problem. The most common approach is angling the sidewall and putting a radius or a chamfer-angle at the corner.

Part removal may be a problem with back-draft sections. Back-drafted areas can lock the part in the mould. If possible, apart from the backdraft on one side should have an equal positive draft on the opposite side. Thus, apart from a 15° backdraft on one side and a 15° positive draft on the other side can be removed like a part with no draft. Otherwise, moulds may need moving sections to remove the back drafted feature.

Shrinkage & Warpage

Shrinkage varies by material, the rate of temperature change and the thickness of the material. For PP and PE materials, the material thickness is the best predictor. Thin wall parts may shrink as little as 1% and thick parts over 10%. A .060” thick part will shrink approximately 1.65% as it cools and a .125” thick part will shrink about 1.85%. The shrinkage expectation must be taken into consideration when setting the mould size.

Designs that allow wall thinning variation to occur in the part may result in warped parts. The thin areas will shrink less before cooling than the thick areas. The variation in shrinkage rates and distances can cause the part to warp. Some wall thickness & shrinkage variation occurs in every moulded product because the cooling rate of the plastic will vary. The skin of the material against the mould metal will cool and take a set before the material does not touch the mould metal. The result is a tendency for outer walls to warp inward and is offset by the tendency of the inner wall to warp outward. The use of structural ribs, welds between walls, arcs or steps can create a structure that helps reduce warpage.

Cooling

To control dimensions, surface appearance and warpage, it is important to have as much control over the cooling of the part as possible. Flow rate is a major factor in heat removal and cycle time. By creating turbulent flow, heat extraction and cycle times can be improved. To control warpage in many designs, it is essential that mould cooling be targeted to provide extra heat extraction in the heavier wall portions of the part. The overall flow pattern also affects the part quality. The water warms as it flows through the mould.

The mould should contain several inlets and outlets in an oscillating cooling pattern.

A cold mould surface can also cause problems in reproducing surface details such as texture. Tooling engineers can target water lines near each critical section of the mould to provide the dimensional control and appearance you need.

Venting

When the mould closes the parison is captured at the pinch-off. A certain amount of air is trapped between the outside of the parison and the mould cavity. When air is blown to expand the parison, the trapped air becomes compressed by the expanding parison until an interior – exterior pressure equilibrium is reached. When this occurs, the parison will not completely touch the mould wall. The results are visible surface abnormalities, loss of texture & engraving detail, the appearance of creases and drag-lines, and longer cycles from poor mould cooling.

Venting can be easily located at the edge of an insert in the cavity. Slotted vent inserts or porous metals can be purchased and fit into nearly any location. Some venting methods will produce visible markings on the finished part. Texture, inserts and other techniques can be employed to mask the markings made at the vent location.

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DESIGN GUIDELINES FOR BLOW MOULDING MOULD

Guide pins and guide bushings from common standard parts suppliers are usually used as blow mold guides. The guide length should not exceed 1 * D. This is particularly important for mold movement machines to avoid damaging the parallel guidance of the clamping unit and the blow mold guide. The bore for the bushing must be drilled towards the rear of the mold or be cleared laterally to be able to easily remove pressed in material and ensure a clean closing of the blow mold.

HOW TO SELECT TOOLING IN BLOW MOULDING MACHINE :

With a basic understanding of the differences in the types of head tooling
available, we can now look at calculations that will help make our tooling
choices. It is important to keep these points in mind:
Parison diameter is dependent on head tool size, resin characteristics,
regrind content, temperature, and punch-out speed.
Head tooling diameter is determined by part shape and size.

ROLE OF AIR IN BLOW MOULDING

In a blow molding machine, air has multiple purposes: pre-blow and inflation air expands the parison until it contacts the mold wall to reproduce the shape of the blow mold. The inflation air presses the material against the mold wall to reproduce surface details such as lettering, surface textures, etc. Furthermore, the inflation pressure promotes good and permanent contact between the mold wall and the polymer, since the polymer starts to shrink during the cooling period. In addition to these functions required directly with the actual blow molding process, air is required for pneumatically activated functions in the machine, such as the insertion of a blow needle to move molded parts (core pulls) or doors in the machine’s safety guard.

PLC in EBM 

Until the late 1970s to early 1980s blow molding machines were primarily delivered with contractor control. This type of control offered the advantage that the electrical components could be checked by appropriately trained maintenance personnel and easily replaced in the event of a fault. Changes in the machine control system could be made with the help of simple auxiliary devices. However, the disadvantages were the susceptibility to faults and short service life due to wear of mechanically moving parts, the large number of components (up to 150 contactors, 12 temperature controllers, etc.), the size of the components, and how these elements were connected. This type of control was called “hard-wired programmed logic controller”.

CLAMPING UNITS IN EXTRUSION BLOW MOULDING

In all extrusion blow molding processes, a blow mold consisting of two halves closes around the parison once it has reached its full length. These two halves of the mold are mounted on the so-called mold mounting platens, which from part of the clamping unit (or closing unit).

RADIAL WALL THICKNESS DISTRIBUTION IN BLOW MOULDING

In blow moulding, the wall thickness distribution of the finished article is essentially determined by the wall thickness distribution of the parison, its position in relation to the blow mould cavities, and the local stretching paths.

SWELLING BEHAVIOUR OF THE PARISON

The visco – elastic behavior of the thermoplastics (here the memory effects) in conjunction with the flow channel geometry in the head causes the diameter of the parison to change after leaving the die. The wall thickness also changes and thus the length of the parison. This phenomenon is called ‘; die swell’’.

MANDREL HEADS IN EBM

In mandrel heads (also referred to as ‘’heart – shaped – curve heads’’, or cardio did heads) the solid core part is massively integrated into the basic structure of the head. These heads are also called side-fed heads.

SPIDER HEADS IN EBM

The main task of parison heads (blow heads or even just heads) is to divert the compact melt flow as supplied by the extruder into a vertical downwards flow movement and to form melt tube with a uniform circumferential wall thickness distribution.

EXTRUDER WITH GROOVED FEED ZONE

Blow moulding machines with grooved barrel extruders are universal machines for all types of polyolefin; the grooved barrel system is,however, indispensable for high molecular weight, high viscosity raw materials. In these extruders, the cylinder wall in the feed section features tapered longitudinal grooves.

SMOOTH BORE EXTRUDER

Blow molding machines with smooth – bore extruders are more or less limited to the processing of low to medium molecular weight polyolefin types. Smooth bore extruders are less expensive than those with a grooved feed zone and show less screw wear.

Is a Blow Molding Machine Used to Make Anything Other than Plastic Bottles?

For manufacturing plastic bottles, experts rely on a state-of-the-art blow molding machine. With this particular process, a manufacturer produces a broad range of finished products in varying sizes and shapes. Because of that, multiple industries depend on a professional company to make a specific type of plastic bottle needed.

What Are the Future Trends of Plastic Molding Machines?

Although the plastic molding industry took a hit this year due to the instability within the automotive industry, it’s still on target for breaking records. After all, the plastic molding machine is something that a broad range of industries rely on for both internal operations and the making of consumer goods. As demand increases, you can expect to see some exciting changes for plastic molding machines.

Can Extrusion Blow Molding Produce Complicated Shapes

When it comes to extrusion blow molding capability, most people think of standard plastic bottles, tight-head containers, jars, and larger water containers. However, the extrusion blow molding process can do much more. In fact, when using a technologically advanced extrusion blow molding machine, it can produce finished products with complicated shapes, and in varying sizes.

Rotational Molding: Advantages and Disadvantages

Rotational moulding offers a number of benefits, but it’s not the best production process for every part. So how do you decide if it’s a fit for you? Understanding the advantages and disadvantages of the process is the first step toward making a decision with confidence.

HOW TO START A TOY-MAKING BUSINESS WITH EBM IN INDIA?

Before starting the toys making business, it is important to do market research. It is important to collect as much information as possible about this business so that you get the knowledge of exactly what kind of toys are in demand in the market.

WHY INDIAN GOVT. ENCOURAGES TOY MAKING BUSINESSES MAKING BUSINESS

Indian government motivates and urges the Indian startups in the toy sector to maximize manufacturing and increase the reach of Indian made toys to the world. After holding a meeting with senior officials and ministers, the Prime Minister make a statement and urge the start-ups and young blood to innovate in the toy sector. Also, PM has given the suggestion to the educational institutions to arrange competitions in toy technology and design.

Biodegradable vs. Recyclable Plastics

Biodegradable plastics undergo digestion by microbes to form natural bi-products and what is known as humus (a sludge that makes an excellent plant fertiliser). It also produces gas which is officially known as biogas. Biozone’s landfill-biodegradable plastics have proprietary additives that help them biodegrade naturally in landfills. Once the biodegradable packing ends up in a landfill, it attracts the necessary microbes that break down the plastic into its natural components.

The Future of the Extrusion Blow Molding Machine

Everywhere you look, competition among different industries has exploded. Thanks to a thriving market, companies that once struggled now see incredible success. Just as businesses need ways to stay ahead of the competition, so do extrusion blow moulding machine manufacturers. Because of that, they continually push to come up with more innovative solutions.

Why CMP Machines are the BEST?

Innovation is at the heart of any successful company. Innovation keeps an organization at the top of their game, allowing them to flex their competitive muscles and take pride in the work they do.

What is Multilayer Extrusion Blow Moulding Machine?

Nowadays literally all humankind use plastic products 24/7 all over the world. It is super easy to get access to all kinds of plastic products. You might have come across bottles that have a shiny layer on the outer side, or different colours on the outer and a different one in the inner. So, what it is? It is a multilayer extrusion product. Well, let me explain to you how what it is exactly.

Interior Surface (Mold Core) Design

The inner and outer walls of the part are formed simultaneously and integrally, but interior and exterior designs are essentially independent so we review them separately. As the design develops, the designer should begin thinking about the interaction of the plastic and the mould that will produce the part. The interior surface of double-wall blow moulded parts is normally formed by a mould core. Since the mould core must fit inside the cavity, there should be no question it meets the same core blow ratio = W>2D overall size requirement as the cavity.

About Parison and Parison Control

Parison blow moulding of intricate plastic bottles and other plastic parts requires air pressure control that is precise, repeatable and rapid responding. Proportion-Air offers electronic closed-loop pressure and flow control used to improve processes in the plastics blow moulding industry.

Why CMP Blow Moulding Machine?

Multiple industries and outside manufacturing companies depend on advanced extrusion blow moulding machinery. Without the right machine, there would be no way to produce superior quality products and we, at CMP, provide quality over quantity. Let us go through the advantages you will get for using CMP’s machine.

Information about the series of Extrusion Blow Moulding Machines in CMP

Have you ever seen a person at the fair or the mall during the holidays, blowing glass to create beautiful, artistic designs? It’s mesmerizing how a random piece of coloured glass can be manipulated by precisely placed hot air and become such a masterpiece. This art form inspired the current method of manipulating plastics to form many of the items we use every day, from water bottles to toys and so many things in between. This artful method is called Extrusion Blow Molding and it’s a common and inventive way of mass-producing hollow, lightweight products that are also durable and waterproof

How to select a Blow Moulding Machine?

The reputation of the blow moulding machine manufacturer you do business with has a lot to do with the quality of the equipment it sells. The right company takes great pride in offering its customers top-of-the-line products at a competitive price. Buying a blow moulding machine can be a nerve-racking experience for fresh entrepreneurs.

Types of products manufactured in Extrusion Blow Moulding Machine

Blow moulding is a manufacturing process for forming and joining together hollow plastic parts. In general, there are three main types of blow moulding: extrusion blow moulding, injection blow moulding (one stage), and injection stretch blow moulding (two stages). In the extrusion blow moulding process (EBM), plastic is melted and extruded into a hollow tube (a plastic parison). This plastic parison is then captured by closing it into a cooled metal mould. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container, or part. After the plastic has cooled sufficiently, the mould is opened and the part is ejected. It is designed to manufacture high volume hollow plastic products.

Types of material used in Extrusion Blow Moulding Machine

When choosing a material for blow moulding; cost, density, flexibility, strength, and other factors should be considered into what resin is best for your part. If the elastomeric properties of the material are exceeded, a hole will tear through the material resulting in a defective part. Other relevant properties are largely based on your application and might include mechanical, physical, chemical resistance, heat, electrical, flammability or UV resistance.

Extrusion Blow Moulding Machine VS Injection Blow Moulding Machine

If you operate a business that requires specially formed plastic bottles or containers, such as juice and water bottles or liquid soaps and shampoos, it’s to your benefit to understand the various manufacturing processes for making those bottles and containers.

What are the 2 types of Extrusion Blow Moulding Machines?

Blow moulding is a manufacturing process used for the production of plastic bottles, containers, and custom shapes. At Central Machinery and Plastic Products, we use extrusion blow moulding (EBM). EBM bottles are easily identifiable by their pinch line across the base of the bottle. This line is created as the mould cavity closes on the parison and the tail is trimmed off.

The process of Extrusion Blow Moulding Machine

Extrusion Blow Molding is the simplest type of blow moulding. A hot tube of plastic material, called a parison, is dropped from an extruder and captured in a water-cooled mould. Once the moulds are closed, the air blown through the top of the neck of the container. When the hot plastic material is blown up and touches the walls of the mould, the material freezes, the container now maintains its rigid shape.

What is Extrusion Blow Moulding Machine?

Blow moulding is an umbrella term for forming hollow plastic parts by inflating a molten plastic tube or parison until it fills a mould and forms the desired shape. Think of it as inflating a balloon inside of a water bottle. The blow moulding process begins with melting down the plastic and forming it into a parison. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass. Water channels are carved into the mould to assist in cooling.

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