Acrylic Plastic Injection Molding D.O.T. Case Study

D.O.T. Reflectivity Regulations in Bicycle Application Achieved through Engineering Physics

Manufacturing companies from all different industries usually come to PMC because their existing plastic injection molder is having problems producing their parts.

Some of the common problems we see are:

However, in this particular case it was a lack of applying engineering physics when providing acrylic plastic injection molding.

Find out below how Plastic Molded Concepts used engineering physics to provide a solution that allowed these acrylic parts to meet industry regulation requirements.


A manufacturer of bicycle reflectors contacted us because their plastic injection molding company was unable to meet D.O.T. reflectivity regulations when they introduced a second machine with an 8-cavity mold.

The plastic injection molding company never had an issue meeting D.O.T. reflectivity requirements with their 4-cavity machine. However, when a large increase in demand presented itself, they were forced to add a second molding machine & an 8-cavity mold in order to increase their monthly production requirements.

The project engineer reached out to us & explained that he couldn’t get the new machine & 8-cavity mold to produce the part reflectivity that was required by D.O.T. regulations.

We knew we needed to observe conditions of the first machine with the 4-cavity mold first, so we could duplicate those same conditions in the second machine with the 8 cavity mold and determine where the issues were occurring.

To measure the cavity pressure in the 4-cavity mold, we needed the project engineer to add a transducer. The project engineer received approval to add the transducer & 3 weeks later they told us they were ready for us to perform our research.

What Went Wrong

During our observation of the acrylic plastic injection molding process in the 1st machine with the 4-cavity mold, we noticed:

The mold was actually being opened by excessive plastic pressure – just enough to cause the cavity pressure to rise immediately after the injection process stopped & the screw began to refill for the next shot.

The rise of cavity pressure at the end of injection was because the clamp pressure was exceeding the injection pressure. This was occurring because the machine was actually too small for the runner and the geometric area of all the parts.

Although, the tonnage of the molding machine was inadequate for the projected area, the machine was still able to produce acceptable parts according to D.O.T. reflectivity standards.

After observing the molding process in the first machine, we replicated the same conditions in the second machine. This allowed us to analyze the problematic contributing factors.

Our Findings

Since the first machine was half the size of the second and had half as many cavities, it produced enough clamping pressure to open the mold slightly so that it would re-clamp onto itself.

This amount of clamping pressure allowed the production of a coined-in diamond pattern that was achieving the desired reflectivity.

When they bought the second, larger machine, which was twice the size & had 2x as many cavities, the clamping pressure was actually a little higher than they needed.

This higher clamping pressure was preventing the injection pressure from opening the mold that slight bit which, in turn, prevented the mold from re-closing on itself & coining-in the appropriate diamond pattern surface finish that was required in order to meet D.O.T. standards.

The Solution

Since there was not a transducer in the second machine, we asked the engineer to begin turning down the clamping pressure 50 PSI at a time. After each reduction we would take shots of the reflectivity chamber and test them.

With the clamping force reduced appropriately, the second machine was able to allow the clamp to open slightly during injection. Now, after injection stopped, the clamp could re-close and coin-in the diamond pattern’s desired reflective surface.

After several clamping PSI reductions, the machine began producing acrylic parts that met the D.O.T. reflectivity requirements.

We ensured that both machines were producing parts with equal reflectivity measurements.

The Follow-Up

We identified our findings and solutions to the engineer. We explained the issues in machine size capabilities & clamping pressure.

We also explained our process for identifying the cause & solutions, which included adding a transducer to monitor cavity pressure in the first machine; using a 2 channel chart recorder to monitor the conditions; replicating the conditions in the second machine; and applying the laws of engineering physics to provide the necessary solution.

As many companies do, this one spent 3 weeks of downtime trying to solve their problem before contacting us for assistance. After arriving on-site & beginning our process, we were able to solve the issue in 3 hours (most of which was spent getting the new machine up-and-running from a cold start to making parts).

Domestic Plastic Injection Molding Company Solving Complex Conditions

In the plastic injection molding industry, there’s little left that’s still considered to be “black magic”. Although everything in our industry is a scientific provable process, many molders do not have the adept skill, knowledge or capabilities to identify the proper solution(s).

At PMC, we have in-house Master Molders who are formally trained in providing molding analysis, troubleshooting and end-solutions. Our Master Molders are part of an elite few (less than 1,000 in the entire world!), who have this level of skill and training.

Plastic Molded Concepts is a domestic plastic injection molding company located in the U.S. We specialize in solving complex molding conditions for a wide variety of industries. Request a quote today!

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