The importance of material selection for the injection molding of plastics

HTI Plastics – plastic resin containers

Choosing the right plastic material for injection molding protects you from costly tool adjustments after production has started.

LINCOLN, NE, U.S. October 11, 2021 / – HTI Plastics
For example, suppose you are new to making an injection molded product (using plastic or other materials). In this case, it is important to know how materials influence the development and manufacture of a product.

Case in point: Recently a company that sells outdoor / sports equipment contacted us to ask if we could make out of plastic an aluminum part that is used in one of their products. The company’s goal was to reduce the cost of the existing part.

Our engineering department uses finite element analysis software to develop solutions to these types of challenges. This software allows us to tackle very complicated problems by breaking them down into small elements that can be solved in relation to each other. In this special case, FEA (Finite Element Analysis) enabled our engineer to optimize the design of the molded part with the greatest possible rigidity for each material used. The analysis presented a comparison of the part design with the aluminum part, which made it possible to choose the right material and show how the strength of the part would theoretically compare to the existing one.

In other cases we could help a customer save money. For example, not so long ago a customer asked us to use a high-end material for a product – 50% carbon fiber-filled nylon with Teflon® – but we had concerns about how the part was filled, how to achieve a uniform finish, and the Produce costs. Our engineers, working with our material suppliers, were able to suggest a 30% glass fiber reinforced nylon that cost about 20% of the original high-end material we were supposed to use at the beginning. In addition, the material we recommended was suitable for better cosmetics and even surpassed the high-end material in the customer’s drop test.

Why is the choice of material important?
Choosing the right material for injection molding protects you from costly tool adjustments after production has started. Since injection molds are built for a specific amount of shrinkage, choosing the right material is critical to a successful injection molding project. There are several factors to consider when choosing the right materials. We’ll mention these later.

If the mold is not designed with the right choice of materials, it can become costly to the customer later when changes are required. Even if experienced engineers do not adequately advise the customer, this could be a missed opportunity to use improved material that could help optimize the quality and function of the product.

Plus, by choosing the right material in advance, you can save time so that your product is ready for the market faster.

In addition to the aforementioned finite element analysis, our engineers at HTI Plastics also use the Moldflow simulation software from Solidworks Plastics. Solidworks helps us determine if we can make a particular part design in a particular material.

There are cases when the material chosen is not suitable for part design and it is ideal to find out early in the process. It also helps us to point out potential concerns based on the material selected – some materials have poor weld line strength or are more prone to warping. The right choice of material allows us to consider these possibilities in advance.

Conversion of products into plastic parts.
To convert from one material to another, we use finite element analysis. If we have two designs and known material properties, we can directly compare the properties to see if the alternative material works satisfactorily.

Which factors have to be considered?

There are many factors to consider when selecting materials for an injection molded product to ensure optimal performance.
– Part size, thickness, aesthetics, color, living hinge
– Costs
– Mechanical properties: rigidity, tensile strength, impact strength, fatigue resistance
– Electrical insulation
– Thermal properties
– Welding / gluing
– Chemical resistance
– FDA conformity
– Parts in large numbers
– transparency of the material
– UV exposure
– sterilization requirements
– Lifespan
– overmolding
– surface finish
– UL certification

HTI engineers – material selection process.
Our engineers first take into account all of the aforementioned factors. After checking all the factors, they look for the lowest cost materials that meet the requirement, but we also need to consider the logistics like availability of the material, order quantities, lead time, etc. We also have access to great technical resources from our material suppliers, so we check with them often to ensure we have the most current material suggestions and information as the industry continues to evolve.

HTI Plastics uses the automated material handling system MOTAN.

The most popular injection molding materials and why.
At HTI Plastics, the most popular materials are polyethylene, polypropylene and nylon. With materials (as with most things), the better properties come at a higher cost. Polyethylene is on the low end in terms of cost and properties, but offers a good mix of mechanical properties, chemical resistance, barrier properties and processability that is very attractive for single-use products as well as some long-lasting products. Polypropylene is similar to polyethylene, but a little more performing – we’re gaining stiffness, it works well with glass filler for even greater stiffness, and it’s great for living hinges. Polyethylene and polypropylene are considered bulk resins because they are inexpensive and manufactured in large quantities. As you step up to higher performance, you get engineering resin grades like nylon. Nylon further increases stiffness (especially with glass filler) and high temperature performance, making it suitable for use in places where PE or PP are near their melting points. There are hundreds if not thousands of grades of each of these materials so there are many different options to suit each one to a customer’s needs.

Selection of medicinal resins.
Food grade and over the counter pharmaceutical products must generally meet FDA CFR Title 21 regulations for food contact surfaces. Many of our products are made with FDA materials using GMP procedures to provide parts that are safe for food contact and storage or dispensing of pharmaceutical products.

Ryan Lund
HTI plastics
+1 402-474-8577
email us here

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October 11, 2021, 2:40 PM GMT

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