Everything You Need To Know About Plastic Molding

When designing your plastic part, it’s crucial to consider what the part’s fit and function will be. 

First and foremost, the function of the product will affect the fit and form of the plastic part in almost every case. So as you draw out your design, make sure you’re cognizant of its fundamental usage.

The Design & Discovery Process

What special properties need to be included to make your plastic part stand out as more functional than potentially competing products?

Ask yourself these function-related questions:

1. What is the minimum required strength required for your product to perform successfully?
2. What is your product’s intended longevity?
3. Does your product need to be created with impact resistance in mind?

These, among many other product-specific questions, should be answered before you worry about the style of your custom plastic product.

Answering these questions provides you with a better understanding of what type of resin should be used to manufacture your part.

Determining Your Product’s Fit

Now that you understand exactly what your part needs to do, you’ll need to figure out what it will look like, how it will feel, and how it will interact with other elements within the product’s environment.The goal is to strive for optimal performance.

In this context, “Fit” refers to how a plastic part mates to another part or occupies a specified amount of cubic inches within a box or on a shelf. *

For Example:

• How a container lid mates to its container
• How the plastic housing envelopes a motor
• How the product occupies shelf space at Walmart

Some Tips for Perfecting Your Product’s Fit

The thickness of the part’s walls should stay consistent whenever possible.

Inconsistencies in the wall thickness can cause the plastic to warp. When the plastic cools, it cools from the outside inward which can cause the outside walls to be pulled inward (called sinks), internal stress, or internal voids.

Make sure the walls of the part aren’t exactly perpendicular

This wall feature is called a draft angle. It allows the part to come out of the mold smoothly. We recommend a draft angle of between 2 and 5 degrees.

Consider Rounded corners

Rounded corners allow material to flow through the part more efficiently. This also reduces stress on the material during the cooling process, in turn reducing its tendency to crack, bow, warp, or have fragile corners.

Make note of parting lines

A parting line is a line where the two halves of the mold meet. This can create a thin blemish around the part depending on how it was designed. If you need a sharp edge on your part, you can reduce blemishes incorporating that edge in the parting line.

Consider hole depth to diameter ratio

Most plastics companies recommend the hole depth-to-diameter ratio to stay under two. There are two types of holes:– Through Holes– Blind Holes

Unlike through holes, blind holes don’t protrude through the plastic part. The pins that are used to make these blind holes shouldn’t be too long, because the heat and pressure can potentially warp the insertion area.

Eliminate undercuts if possible

To save money, it would be wise to avoid using undercuts. Undercuts will almost always result in a more expensive part, due to a more complex mold design and typically more process time is needed to create them. 

An undercut is any protrusion or indentation that houses any non-standard mating part of the plastic. (For example: A T-shaped connector)

Consider utilizing ribbed features

If you want to strengthen the molded part without adding additional wall thickness, ribbing is a great way to accomplish this. Taller ribs can lead to issues like warping and bending. But if your piece has subtle and simple ribbing, it should serve the function well.

How to Design Your Plastic Part For the Shelves of Walmart and Other Retailers

It’s all about the cubic inches.

Let’s say you own an ice cream company, and you’re hoping to modernize the ice cream container so that it increases user-friendliness and fits perfectly in the frozen section at Walmart.

You’ll need to design for a specific amount of shelf space. 

It’s important to know that this shelf space allotted varies for each product type. For example, Walmart won’t accept a 10-gallon ice cream tub.

Your contact for any of these big box retailers will be able to provide you with the exact amount of cubic inches your product can occupy. Be careful, because if you don’t abide, they don’t have to stock your product. 

Design Hint: You’ll need to understand the Supplier Standards, obtain a Universal Product Code (UPC), and you must carry Product Liability Insurance to sell your product in big box stores.

Also, you’ll need to notify your plastic part manufacturer that the retail market is your end goal! 

On this page:

What is biodegradable plastic and how does it differ from other types of plastic?

The term “biodegradable” when used for marketing purposes includes a time component regarding the length of time it takes for the plastic to fully degrade. According to the Federal Trade Commission’s “Green Guides”: “It is deceptive to make an unqualified degradable claim for items entering the solid waste stream if the items do not completely decompose within one year after customary disposal. Unqualified degradable claims for items that are customarily disposed in landfills, incinerators, and recycling facilities are deceptive because these locations do not present conditions in which complete decomposition will occur within one year."

Plastics are derived from organic products. The materials used in the production of plastics are natural products such as cellulose, coal, natural gas, salt and, of course, crude oil. Crude oil is a complex mixture of thousands of compounds. To become useful, it must be processed.

The production of plastic begins with a distillation process in an oil refinery involving the separation of heavy crude oil into lighter groups called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules. One of these fractions, naphtha, is the crucial element for the production of plastics.1

Most petroleum-based plastic is not readily biodegradable; ie. it is not consumed by microorganisms and returned to compounds found in nature. What this means is that unless the petroleum-based plastic has been specifically designed to biodegrade, and although it may partially degrade, the plastic may last in the environment for tens to potentially hundreds of years, depending on the type of plastic and its disposal environment. The two major processes used to produce plastics are called polymerisation and polycondensation, and they both require specific catalysts. In a polymerisation reactor, monomers like ethylene and propylene are linked together to form long polymers chains. Each polymer has its own properties, structure and size depending on the various types of basic monomers used and that influence properties such as moldability and rigidity.

What is the difference between biodegradable plastic and compostable plastic?

Plastic that is compostable is biodegradable, but not every plastic that is biodegradable is compostable. Whereas biodegradable plastic may be engineered to biodegrade in soil or water, compostable plastic refers to biodegradation into soil conditioning material (i.e., compost) under a certain set of conditions. In order for a plastic to be labeled as commercially “compostable” it must able to be broken down by biological treatment at a commercial or industrial composting facility. Composting utilizes microorganisms, heat and humidity to yield carbon dioxide, water, inorganic compounds, and biomass that is similar in characteristic to the rest of the finished compost product. Decomposition of the plastic must occur at a rate similar to the other elements of the material being composted (within 6 months) and leave no toxic residue that would adversely impact the ability of the finished compost to support plant growth. ASTM Standards D6400 and D6868 outline the specifications that must be met in order to label a plastic as commercially “compostable”. There are currently no ASTM standard test methods in place for evaluating the ability of a plastic to compost in a home environment.

What are biobased plastics?

Biobased plastics are manufactured from plant materials instead of being made from oil or natural gas. Because they are plant based, there is a tendency to assume that this type of plastic must be biodegradable. However, biobased plastics can be designed to be structurally identical to petroleum based plastics, and if designed in this way, they can last in the environment for the same period of time as petroleum based plastic. Just as with petroleum-based plastics, biobased plastic can be engineered to be biodegradable or to be compostable.

The ability of biobased plastics to be recycled varies. Some forms of biobased plastic cannot be recycled together with petroleum-based plastics due to chemical structure incompatibility, while other biobased plastics may have compatible chemical structures that allow for recycling together with petroleum-based plastics. In order to determine what waste disposal options are available for a biobased plastic item, it is necessary to read the product’s label as to its compostability and recyclability.

Which organizations are responsible for creating standards and enforcing correct labeling of biodegradable and compostable plastics?

The American Society for Testing and Materials (ASTM International) sets definitions and standards, while the Federal Trade Commission (FTC) is responsible for enforcement against false or deceptive product labeling.

If a plastic product is labeled “compostable,” can I add it to my home compost pile?

No. Unless the label indicates that the product is okay for home composting, you should not try to compost it at home. Plastic that is labeled as compostable is generally intended to be sent to an industrial or commercial composting facility which has higher temperatures and different breakdown conditions than those found in a typical homeowner’s compost bin. If your community has a residential compost collection program, check with your local government or recycling company to find out if they will accept compostable plastic under this program.

What if my community doesn’t have a composting recycling pick-up program. Should I dispose of compostable plastic in the plastic recycling pick-up bins?

No. Compostable plastics are not intended for recycling and can contaminate and disrupt the recycling stream if intermixed with petroleum-based plastics that are non-compostable. If your community does not have a composting recycling pick-up program that accepts compostable plastic, contact your garbage/recycling company or local government to find out if there are any drop-off locations for your compostable plastic items.

What do I do with plastic caps (on water bottles and other bottles)? Do I leave them on or take them off before disposing the bottle in the recycling bin?

Until recently, many consumers were advised to take the caps off and dispose of them in the garbage can before placing the bottle in the recycling bin. However, processing technology has improved to the extent that the Association of Postconsumer Plastic Recyclers now recommends that plastic lids be left on the containers as they are placed in the recycling bins.  At facilities with newer processing technology, the bottles (with caps on) will be ground into flakes before being washed and the cap flakes separated from the bottle flakes during a water bath float/sink process and then both types of plastic can then be captured and recycled. Note, however, that due to the fact that not all recyclers may have equipment that enables processing of the bottles with caps left on, that you should check with your local recycling facility to see what their policy is with regard to disposal of the bottle caps.

Should I be placing single-use, non-compostable plastic bags in the recycling bin?

Contact your local government or recycling company to find out whether this practice is allowed, as policies on this vary. Loose plastic bags are difficult to handle in the recycling stream and can clog equipment. Some recycling companies allow recycling of single use bags if they are bundled together in a tight, tied package. Many supermarkets and “big box” stores have recycling bins for the collection of single-use plastic bags.

What is the purpose of plastic bag bans?

The Pollution Prevention Act establishes a national objective for environmental protection: “[T]hat pollution should be prevented or reduced at the source whenever feasible.” Similarly, the Resource Conservation and Recovery Act sets the order of preference for managing materials as: source reduction, reuse, recycling, and disposal.

With these objectives in mind, a number of communities are initiating bans of plastic bags that are intended for single use, such as those commonly provided in grocery stores. The rationale behind the bag bans includes the following:

• They are typically made out of petroleum-based plastic and don’t biodegrade when they are disposed of or escape into the environment;
• when plastic bags are disposed of on land they may be blown into creeks, lakes, or oceans where they can entangle marine life or the animals may mistakenly eat the plastic bags thinking that they are food;
• the light-weight plastic is not easily recyclable; and
• the bags are often used only once before being thrown away.

The various bag bans differ but typically contain many of the same elements:

• Replacement bags typically must be compostable or recyclable. That means that if the replacement bags are made of plastic, the plastic must either be compostable and labeled as such, or the bag must be heavier and thicker such that it won’t interfere with the industrial recycling process, is washable, can carry more weight, and be used multiple times (as many as 125 uses).
• Replacement paper bags are typically required to be made of 40% recycled paper and be 100% recyclable.
• Many plastic bag bans require retailers to charge a fee per replacement bag which is kept by the retailer.
• The purpose of the fee is to both to encourage customers to bring their own reusable bags (minimizing the need to manufacture new bags) and to cover the cost to the retailer of providing bags that are other than single use.

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