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Injection Mold Design Process for Custom Water Bottle Lid

This article will discuss the injection mold designing process for the custom plastic lid of a water bottle made up of vacuum insulated stainless steel. The cap is injection molded by infusing high-temperature molten plastic into the mold. It is then pressurized, cooled, hardened, and removed from the mold to materialize a plastic cup lid

We will also explore the prerequisites and crucial points of designing the injection mold for the insulated water bottle lid. It incorporates plastic product properties, the mold, and the molding machine’s structure and composition, and manipulating some significant systems. Consequently, we acquire the design of the complete mold

Process Analysis of the Product

Plastic selection is an essential process in producing plastic cup lids because of the material’s diverse characteristics. Characteristics like fluidity and shrinkage vary from one plastic material to another. During the design process, the mold structure is considered to facilitate the overall manufacture of the mold. During the process, the product structure must be simple to make uncomplicated molds, and the thickness of product walls should also be kept low and uniform.

It is because of shrinkage that occurs in plastic products. The difference in wall thickness can lead to product deformation, and cup design must also be easy to use. All this is done to balance a product’s economics to produce it at the lowest cost possible

Quality Required in Bottle Lid Production

In the case of the thermos bottle lid, we have to follow strict requirements. If there is a significant error in lid size, it will lead to a less tight head, destroying the insulation measures. In general, the dimensional tolerance of the outer diameter of the cup is kept below two

An electric discharge machining process is used to fulfill these high accuracy requirements of the thermos bottle lid, which guarantees as much accuracy as possible. For demolding easily, the wall thickness of the cup lid is kept as low as possible. 3mm is the standard wall thickness of the product

Surface Roughness

Surface roughness is one of the critical factors in determining the lid quality. Lid surface roughness is directly related to the roughness of the mold surface. In normal circumstances, the mold surface roughness level is 1-2 lower than that of the product surface. In industries where mold is continuously used, surface roughness increases with time

So, after a specified time, it needs to polish the mold for the desired product. There are different roughness requirements for different cup lids, both on male and female parts of the mold. For transparent covers, both parts’ mold roughness must be the same

Lid Material

Polypropylene is the main constituent of the cup lid. Polypropylene is semi-crystalline material that comes in excellent resin variety. When colorless, it is a wax-like un-harmful compound with translucent nature and no smell. It has a high shrinkage rate that lies between 1% to 2.5%. This property leads to the deformation and shrinkage during the production of polyethylene lids

Selecting Injection Molding Machine

Most industries use one mold with two or four cavities to produce bottle lids. One mold with one cavity is rarely used or generally utilized for the production of large products. In this article, we are discussing the production of small particles with one mold and four cavities

Typically, the approach for one mold and four cavities is utilized to manufacture lids on a mass level. Usually, this is the most suitable and feasible technique for manufacturing a considerable quantity of caps of the same size.

Technically, the machine’s performance while working on manufacturing lids on a mass level shows a specific value of pressure, temperature, injection stroke, the diameter of the screw, and others

On the other hand, the values of factors mentioned earlier vary when manufacturing lids of different sizes in different quantities

Method of Injection

Meanwhile, the method of injection utilized in this process is the screw-type Ejection method. In the screw-type ejection method, there are no ejector rods on any of the machine’s sides. As far as the ejector is concerned, it is of mechanical type.

Along with this, the method of clamping associated with this process is mechanical-hydraulic clamping

Checking the Injection Volume

Product mass can be determined quickly, i.e., 30 grams. The Cup lid is manufactured using one mold and two cavities, so the mold system’s pouring volume is 15.72 cm³

Product V = m ／ p

By putting suitable values in the formula, we can determine whether the machine is capable of molding this particular product or not. In bottle lids, we can use the Injection molding machine because this formula is established

Checking the Injection Pressure

It is inferred from the machine’s above performance that the injection molding machine’s injection pressure is 120MPa. Moreover, for producing cup cover, it is 80MPa. We take 80MPa here because the injection molding machine’s injection pressure lies between 120 to 80 to meet the requirement.

Checking the Clamping Force

Following is the formula for checking the clamping force for the injection molding machine

p (nA + A1) ≤F

In this formula, F is the rated clamping force, and A represents the projected area of a single plastic part present on the parting molding surface. The P is the pressure exerted by plastic melt on the cavity. A1 is the projected area of the casting system existing on the mold parting surface

If the sizes of the plastic parts are:

A1 = 1300 mm2, A = 2826 mm2

P (nA + A1) = 80 * (2 * 2826 + 1300) = 556160N

It is less than the injection molding machine’s rated clamping force, i.e., 900KN, to meet the injection molding’s suitable requirements

Above, we calculated the mold’s clamping force, an essential parameter in designing injection mold

Checking the Opening Stroke of the Mold

The formula for mold opening stroke is

s≥H + H1 + (5-10) mm

In the above formula, H represents launch distance which can also be used as punch height. The S is the maximum stroke that a moving plate of injection molding machine can achieve. H1 represents the plastic parts, including the pouring system

H + H1 + (5-10) = 20 + 75 + 10 = 105mm

From the above injection molding machine’s performance, we can infer that the maximum value of opening mold stroke is between 300mm and 105mm

As inferred from the above calculations, it is suggested that the mold stroke is suitable for the production of our product, i.e., plastic cup lid

Therefore, the Injection molding machine is fulfilling the requirements

Designing the Parting Surface

The surface separates the mold into two parts, i.e., core and cavity. This surface can be parallel as well as perpendicular to the clamping direction of the mold. In some cases, it is also inclined to the mold clamping direction. The parting surface of the mold is designed to touch the product’s outermost diameter

Since our concern is to produce a two-cavity mold, the mold will be a double parting mold. Following parameters are considered while designing the mold parting surfaces.

1. The dimensional accuracy of the product is considered.
2. Surface requirements of the products are looked upon.

Following principles will get followed in the selection of parting surfaces for our product.

1. The product must remain attached to the male part of the mold to make mold opening easy. Otherwise, the product will remain stuck in the female mold. However, it must get done carefully without damaging the design of the product.
2. Making processing intensity easy
3. Making room for exhaust

Taking measures to restrict overflow

Designing the Pouring Syste

Here, the pouring system alludes to paths through which molten plastic flows from the injection nozzle to the mold cavity. The main runner consists of main channels, gate, runner, and cold feed well. The main channel starts from the nozzle and extends to the end of the runner. There comes a runner, ranging from the end of the channel to the end of the gate.

Another part of the main runner is sprue brushing. The beginning part of the main runner is made detachable, which can be replaced with sprue bushing. The way traveled by the molten plastic from the runner end to the product end is called the gate. Cold feed, which is also pronounced as cold feed pockets, is the mold where cold feed heads generated during the injection process are stored.

Designing the Main Channel

The main channel is of conical shape, having a taper of 2 degrees and a single side of 1 degree. The main channels are short length because long length channels lead to solidifying plastic, deteriorating product quality. Here, the main channel is 50mm long, tapered for the plastic’s smooth flow and removing the plastic if sticks

Shunt Design

The mold of our concern is two-cavity mold, so designing the shunt is imperative. The shunt is developed as shortest as possible, and the numbers of bends are minimum to minimize heat and pressure losses.

This approach is mainly used in recently manufactured injection molding machines. The primary purpose is to avoid excessive loss of heat and pressure.

Sprue Bushing

Sprue bushing can easily get damaged because it frequently collides with an injection nozzle. So, the main channel extends to the sprue sleeve. Processing of the sprue sleeve is usually done with suitable materials. It proves beneficial in detaching but also facilitates the repair or replacement of the sprue bushing.

Gate Design

The gate experiences side pouring, paint pouring, overlapping pouring, latent pouring, fan-shaped pouring, tab pouring, and horn pouring, depending on the product that is being manufactured. In manufacturing our product, we will use point-in -place pouring. It is preferred in the manufacturing of such products because it facilitates the shear rate-sensitive plastic formations. Though small in size, the gate plays a crucial role in molding. The gate is closed carefully because its shape and size influence the product quality.

• Make it as short as possible
• Usually rests in the thick product wall
• Must consider the molecular effects
• Avoid melt rupture
• Welding marks should get cleared for a good finish

For cup lid production, we follow the point-in-water pouring technique to achieve automatic film release

Design of Cold Cavity

The cold cavity length is generally taken 1.5 to 2 times the diameter of the main road. It enhances the product quality by the smooth flow of molten plastic into the cavity, hindering any cold material from entering. A pull pin can get used at the cold cavity location to remove the main channel’s cold material

Designing the Temperature Regulation System

Following is the table of resin molding temperature and mold temperature

We are using polypropylene as the material of our product. Its molding temperature and mold temperature are shown as 200-270 and 20-60, respectively. Mold temperature in the production of this product is generally kept at 50 C. Mold temperature here means the surface temperature of the male mold part and female mold part.

This mold temperature directly affects the quality of lids produced. Before the production process, mold is preheated to a suitable temperature to ensure its quality. Filling difficulties are reduced in this way. 

The temperature control system, also named the cooling system, affects the plastic mold product’s quality and relates to the mold production efficiency.

• Mold Waterway: Cooling through a molded waterway is used to ensure the quality of the product. It sometimes controls the temperature of the mold cavity. It makes the molten plastic in the mold freeze resulting in the rapid formation of the product. 
• Cooling Checkpoints: The water temperature entering the mold and leaving the mold after cooling is measured. The outgoing water temperature must not exceed 5 degrees centigrade from incoming water. 
• Waterway Configuration: Most cooling process happens near the gate. The waterway must be equidistant from the product throughout the mold in calculation and theoretical estimates, but this is quite a romantic process impossible to achieve in practice

Calculating Cooling Water Channel Size

First, we will calculate the water-cooling surface area. The formula for calculating the cooling water surface is as follows

A = Mq / 3600a (θm－θw)

In this formula, A is the total surface area of the cooling water channel. M represents the mass of the resin injected per minute. The Q is the heat released per unit mass of the resin in the mold. The A refers to the surface heat transfer coefficient of the cooling water. Furthermore, Θm represents the molding surface temperature, and Θw is the cooling water’s average temperature.

Secondly, let us calculate the cooling water length

L = 1000A / 3.14d

L refers to the cooling length of the cooling water circuit. A is the cooling water’s total surface area, and D represents the cooling water hole’s diameter. 

Designing the Clamping Guide Mechanism

The mechanism that ensures the male part’s correct positioning with the female part of the mold is pronounced as the clamping guide mechanism, which performs the primary task of positioning, guiding, and withstanding lateral pressure. In this respect, the column guide mechanism is the most common clamping guide mechanism. 

This mechanism consists of a guide column, guide sleeve, and a guide hole

Designing the Guidepost

Generally, in a set injection mold, there are eight guideposts. Oil grooves on the guidepost are greased to lubricate the closing process. Typically, the guide part of the guide post is made 8 to 12 mm higher than a punch to avoid premature entry of the male mold in the female mold and minimize an accident’s chances. The head of the guide column is hemisphere-shaped for easy guidance of it to the guide hole.

The Head size of the guide column is not very strict. Grinding the rough surface on the grinder is necessary. The guide column is made up of material that is wear-resistant and not easily breakable. Guideposts are usually placed at the four corners of the mold. Therefore, there are four guideposts for the male and female parts of the mold. 

Guide column diameter depends on its template position. The diameter of the guide post is generally taken 1 to 1.5 times of the template. For example, the mold guidepost’s diameter is 25 mm, and it is 30 mm long and 40 mm wide from the edge of the mold

Designing the Guide Bush

The guide sleeve’s front end is chamfered for the guide post’s perfect entry into the sleeve. A hard material is used in sleeve making, but that material must not be more rigid than a guidepost. The guide sleeve may remain tight in the template in this process, but the guidepost brings out the guide sleeve to put it in the mold opening. The guide sleeve is fastened with the template by drilling a small hole inside the template

Designing of Guiding Hole

Guide holes can be classified as with guide sleeves and without guide sleeves. The process is not very much accurate, but the smooth passage of the guide post is ensured. Nevertheless, when too large, it cannot be processed to be used as a guide. The guide sleeve can also be rendered unusable

Designing the Exhaust System

The exhaust system’s primary function is to vent out the gases in the cavity during the molding process. One of the most critical issues in designing the mold is designing an exhaust system. An exhausted groove is generally made on the sub’s parting surface or disassembly to vent out the cavity’s air. 

The depth of the exhaust hole depends on the mold. It must now exceed two. The depth hole is generally made through electric discharge machining with a lower current at a time. Discharge machining is done to achieve maximum accuracy. This depth hole must not be huge; otherwise, we will end up producing burr products. 

The inadequate exhaust of air leads to a series of quality problems in the product. Let us relate some of the issues that occur. In this case, the mold cavity is not filled with molten plastic. The product has bubbles on its surface. Air gets trapped inside the mold with noticeable welding lines.

Apart from this, the following problems are caused by poor exhaustion of air

• In case of the inadequate exhaust of the mold gas, pressure is generated inside the mold. When it reaches a certain level of compression, it penetrates the product, destroying the product quality. 
• It usually happens that when molten plastic enters the cavity, it pulls the gas out of it, and gas is vented out through the exhaust channel. When the exhaust system is problematic, molten plastic way to fill the mold cavity will get hindered, and the mold cavity will remain unfilled to some extent. 
• The gas in the mold cavity is highly compressed. Due to its high pressure, the temperature inside the mold cavity rises sharply, which results in the melting and burning of surrounding dispensations.
• If gas is trapped inside the mold cavity, the molten plastic will enter at a different speed. So, it is subjected to the formation of the flow marks and fusion marks on its surface. It leads to the reduction in the desired mechanical properties of the plastic parts to be produced.
• Lastly, the presence of air in the mold cavity affects the speed of molten plastic. When molten plastic enters the cavity at such a reduced speed, it drastically affects the process’s efficiency.

Mold Structure and Calculation of Size

The Cavity of the Structure

The cavity is inlaid the mother template of the mold, which is easy to process. The inlaid kernel is fastened with the mother template. Because of their small size, these inlaid mold kernels are very easy to maintain. They avoid the scrapping of the whole mold template in case of a damaged mold core.

Calculating the Radial Dimension of the Cavity

The following formula is used for the calculation

Dm=(D+DQ-3/4△

Dm is the radial dimension of the cavity, and B refers to the manufacturing error of the mold parts. D is the product’s maximum size, while Q represents the shrinkage rate of product material. The △ is product tolerance

Structure of the Core

The core-like cavity is also inlaid on the standard form. This inlaid male mold cavity is again easy to process. The male mold in the case of the manufacturing cup lid is used to shape the internal structure. While designing, the male mold shrinkage rate of the product is considered. Here we will look after the tolerance of the cup lid

Cavity Depth and Core Height Dimension Calculation

Let us move towards calculating the dimensional depth of the mold cavity. Here we will use the following formula

Hm=（h+hQ-2/3△)

The above formula is the numerical way of calculating cavity dimensions. In this formula, the term Hm represents the depth of the cavity. The term h here represents the maximum size of the product that can get achieved.

Now, let us move towards the calculation of dimensions of the core. Here we will use a different formula for this purpose. 

hm=（H+HQ+2/3△)

By putting values, we can determine the core dimensions directly. Here H is the minimum depth size of the product. 

The above two parameters play a crucial role in the design of the injection mold. Here one thing that is imperative to consider is tolerance. Our product, i.e., cup lid shows shrinkage, and we have to pay keen attention. Otherwise, we will end up confronting product deformation and quality degradation

Drawing of Overall Mold Assembly

Here in the above figure:

• Number 1 is the positioning ring.
• Number 2 is the tie rod.
• Number 3 is the upper fixing plate.
• Number 4 extends to the removing plate.
• Number 5 is the male template of the mold.
• Number 6 is the female template.
• Number 7 represents the thimble.
• Number 8 is die-foot.
• Number 9 is the upper thimble plate.
• Number 10 is the lower thimble plate.
• Number 11 refers to the garbage nail.
• Number 12 is the lower fixing plate.
• Number 13 is the guiding post.
• Number 14 is core.
• Number 15 is again a guiding post.
• Number 16 is KO hole

Henceforth, in this article, we explained the whole process of designing the water bottle’s lids. We hope this article helped a lot. You are more than welcome to share it.