Posted by Paula Hynes | 10 / 08 / 15 0 Comments

In our series on creating the perfect high-quality plastic part, we began by outlining four key factors that impact the process including part design, tool building, material selection and manufacturing. In this article, we will focus on the final stage, manufacturing.

A quick review

But before we begin, let’s recap the processes that have already taken place to get to this point.  First we came up with a concept and a part design. We did some fine tuning to make sure we could manufacture the part quickly and efficiently. We kept our eye on the key design elements including wall thickness, rib and boss design, corner transitions and weld lines.

Taking into consideration the end-use of the part, we began the process of assessing various resins. Will the part need to be food safe? Does the application require UV or chemical resistance? We discussed the difference between amorphous and semi-crystalline resins and the various grades that are available. The choice of resins will also have an impact on the design of the mold. Certain plastics tend to shrink more when cooled and this needs to be taken into consideration for the tool building phase of the project. 

Building the mold is the final step before we start manufacturing parts. The design engineer, using CAD software will design the cavitations to accommodate resin shrinkage. To ensure the mold cavities fill properly, gate and vent placement get special attention. The ejector pin locations and type are specified to avoid any defects. 

To recap; the part has been designed for manufacturability, the resin has been selected based on the requirements of the part, and the mold has been built to maximize speed without forsaking quality.

Time to get the presses rolling?  Not quite yet.

Before a production mold is ramped-up and running, a sample mold is created to test the cavitation and the resin. This vital step can help avoid costly reworks of an injection mold. If any problems surface, they can be addressed before the production mold goes into service.

Choosing the right injection machine for the job plastic injection molding press

Injection machines (presses) come in all shapes, sizes and configurations. The selection of the machine will depend on several variables including the size of the mold, the number of cavities and the selected resin.

Most plastic injection companies will provide a molding equipment list on their website.  It may look something like this:

3- 68 Ton Injection Molding Presses

5- 123 Ton Injection Molding Presses

5- 154 Ton Injection Molding Presses

5- 202 Ton Injection Molding Presses

5- 233 Ton Injection Molding Presses

4- 400 Ton Injection Molding Presses


So, what does this mean?

Plastic injection molding machines are classified or rated based on tonnage, or more specifically the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 4000. The higher the press ton rating, the larger the machine.

A machine rated for 68 tons can deliver 68 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues.  Too much or too little pressure can also cause flashing, where the excess material appears on the part edge. Pressure also impacts the viscosity of the plastic being used in the project. Melt Flow Index or MFI is a measure of the ease of flow of the melt of a thermoplastic polymer. Plastic compounds react differently to pressure based on their MFI. The higher the MFI, the higher the pressure needed.

How much clamping force or pressure is required?

There are many factors that are taken into consideration when determining the size of the press.  The size of the part, the polymer being used and something called the safety factor.  The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor.  As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part.  Many calculations include the platen size as well as the mold and part size; however, to get an estimate of the press size your project will need, we have simplified it even further.

Many plastic injection professionals use a general rule of thumb of 2.5 times the surface square inches of the part to be produced.  So, if you have a part with 42 square inches then you would need a press size with 105 tons of pressure.  If you add 10% for a safety factor, you will need to use a press with a minimum of 115 tons of clamping force.  A press size of 120 tons would be able to accommodate your plastic injection molded product.  

Roll the presses

When the production mold is finalized, and the machine has been prepped, manufacturing begins.  An initial run will be thoroughly examined for any part defects.  If none are found, full production can begin.  Depending on the part and the agreement with the molder, quality checks can take place on a regular basis.  Most reputable molders will do their own quality checks.  They will check for strength, color correctness, and any common defects that can occur like flash or warping.

Automation = Efficiency

Now it comes down to inventory.  You need to be sure you have the right quantities in the right place at the right time.  At Rodon, we keep a safety stock for our clients to ensure we can meet any unseen demands.  We also monitor the quantities on hand, in real-time, so we can help our clients estimate their future needs.  This saves everyone some heartache.

MRP systemWe utilize an MRP system (Material Requirement Planning system).This tool allows the manufacturing team at Rodon to monitor jobs from the minute they are initiated to the time they are delivered including all of the downstream impacts.  According to Plant Manager, Tom Moore “The team manages to our customers requirements.  By monitoring production runs throughout the day, we can insure the needs of the customers are met.  We manage the efficiency of each job.  These measures insure we always have the materials and products required.”

Our demand-driven planning allows us to meet the JIT manufacturing needs of our discerning clients.  Rodon’s manufacturing team’s experience with cycle times, resin properties, and part demand has resulted in a 99.8% satisfaction rating.  When you are looking for an injection molder, ask them how they monitor and manage workflows and processes.  Professional companies should have manufacturing systems in place that help improve efficiencies and lower costs.

Got questions?  We have answers.

We get a lot of questions at Rodon about the plastic injection process and how we can meet the needs of our clients.  Below we have listed some of the most frequently asked inquiries.  If you would like to get additional information, please visit our Contact Us page and we’ll try our best to get you the answers you’re looking for.

What are the minimum quantities you can order?

Rodon is a high-volume plastic injection molder.  Our molds are built to exact specifications and can withstand years of continuous use.  Given the resources we invest in making the highest quality molds, we begin our production runs with a minimum order of 1,000,000 parts or more.

Do you make prototype tools?

We make prototypes based on the needs of the customer, the application and the size of the part.  We do not offer "job shop" molding and prototyping.  When we make a tool, we invest a great deal of time ensuring the design is precise while being cost effective. We offer financing terms and other payment arrangements to help defer the cost of the mold.

Can you provide a quote on making a plastic injection mold?

Yes, we provide the tool making cost when we supply a quote to produce a part.  To accurately quote a part we need to get answers to a few questions.  You can fill out our quick quote form to start the process.

Here are a few questions to consider:

  • Is the product no larger than 10" x 10" x 10"?
  • Will the annual volume exceed 500,000 parts?
  • Are material specs available?
  • Is there an existing tool?
  • Can you describe the part, supply samples, drawings or prototypes?

Does your company sign Non-Disclosure Agreements (NDAs)?

Yes, as an American manufacturer, we are all too familiar with the problems some of our customers have had with foreign producers, who may or may not honor NDAs.  We review the NDAs with in-house counsel to ensure the intellectual property of our customers is fully protected.

How long does it take to produce a part?

The length of time needed to get your parts delivered is determined by several factors. First, the tool needs to be made.  Depending on the complexity, this stage will take approximately three months.  Once the tool is made, we do a short production run to ensure the customer is satisfied with the final part.  If adjustments are required, additional time may be needed.  With the tool and sample parts finalized, the production can begin.  Average turn-around time on a production run is approximately 2-3 weeks.

Do you offer extended services such as assembly, packaging or decorating?

We offer some packaging at the press but do not offer custom packaging, decorating or assembly at the press. Additional secondary operations can be performed by approved 3rd party vendors with whom we work.

You say you beat China pricing, how do you do that?

The Rodon Group has invested a great deal in new equipment and robotic automation.  We pride ourselves in producing the highest-quality tools available ensuring hassle-free production for years to come.  These factors allow us to provide low-cost per piece pricing right here in America.  Globally competitive prices and elimination of offshore risks add up to a delivered part price that makes Rodon the envy of our industry.  See our Cheaper that China comparison chart to see how "Rodon Beats China Pricing."

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Topics: Plastic Injection Molding


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