Lormac Plastics Supports the Del Mar Electronics and Design Show

del mar

 

 

 

 

Lormac Plastics supported the 2015 Del Mar Electronics and Design Show with a supplier sponsorship . It was a 2 day event on May 20 and 21 at the Del Mar Fairgrounds. There was a fantastic turnout of over 4,000 attendees. There were a host of seminars, giveaways, and incredible products and services on display. It was a wonderful networking and learning event. Thanks to all who organized and participated in the show.

 

Thermoforming Design Guidelines

Vacuum and pressure thermoforming for the purpose of this discussion is to be considered a single sheet of “heavy gauge” (>.060” thickness) thermoplastic, which is held in a rectangular or square platen, heated in a oven to an optimum forming temperature, then formed over a single sided tool.

As thermoforming is a done with a single sided tool, the critical surface of the part should always face the tool as this is the “controlled” surface with the best tolerances and surface treatment. Features formed on the controlled side of the part will telegraph through to the back side and vise versa.

Below are general thermoforming design guidelines.

1. Tooling
The single sided tool can be positive or negative in design. The common tooling materials are: medium density fibreboard (MDF), urethane, epoxy, and aluminum. The material is either fabricated, CNC cut, and/or cast to form the tool. Small vacuum holes are drilled in the tool. A sealed vacuum box and vacuum system are used to form the heated plastic to the tool. Pressure may be applied to the side opposite the tool, possibly along with a plug assist to enhance certain features for cosmetic, close tolerance parts.

2. Draft
Draft is the degree of taper of a vertical sidewall to facilitate removal of the part from the tool. On side walls greater than 2 inches (5cm), draft is most likely needed. Recommended degrees of draft vary with the geometry, plastics material, and tool material. Always provide the maximum draft possible for any given vertical feature, as this can help minimize de-molding problems and may help reduce part cost. 1.5-2º of draft is a minimum on a negative feature, and a deep texture will require more. For positive features, minimums are in the 3-5º range. The deeper the feature or rougher the surface texture, the more draft is required.

3. Radius
Radius is defined as the detail provided at the intersection of two planes or surfaces. These surfaces can be horizontal, vertical, or both. The greater the radii the better from the perspective of material distribution and part rejection (and therefore cost). Tight radii in the range of .015” can be attained in a negative pressure-formed part. Over a positive feature, the thickness of the material plus .030-.060” or more is desirable.

4. Draw Ratio
The draw ratio expresses the relationship between the beginning surface area of the unformed sheet which covers the opening of a feature, and the ending surface area of the interior of the feature once formed. A 3 to 1 ratio is generally a maximum draw ratio. Avoid multiple tall features too close to each other. Generally, the distance between two features, like ribs or cooling vents, needs to be no less than 2x the material thickness. A caution about corners: Avoid intersections of walls that are >90º, as it can be very difficult to get material to flow into that feature.

5. Under Cuts
Undercuts are features protruding from or into the tool surface, which would prevent removal of the part from the tool. Such features are possible with the use of movable “core-pulls” in the tool (note: this increases both tooling and production costs). These cores are used to form the feature, then, are retracted to allow the part to be removed. Under cuts are generally not to exceed 5/8” in depth.

6. Reference Points
Reference points should be designed into a part, which allows a measurement from a controlled (molded) surface or point to a critical feature of the part, such as drilled hole centers, cutouts, or other features.

7. Ribs and Bosses
Ribs may be formed into the part according to the parameters above. Ribs are used to support a flat surface. Ribs or other reinforcement may also be machined independently and glued in at added cost. Internal bosses may also be independently machined and glued in.

8. Texture
Texture may be incorporated into a given part either by using a textured raw material or by texturing the face of the tool. Texturing is of course an added expense and requires increased draft. More than one texture can be done on a part, as can areas of no texture.

9. Joining Lines
Two parts may be joined together such as a front and back case. The preferred joint is a lap joint, which would be formed as a rib or under-cut on the edge of one part. The joint should also require a witness line of about .060” to accommodate tolerance variation of the joint.

10. Trim, Holes, Cut-Outs, Vents
Trimming dimensions need to be referenced from the molded side of the part. Vents, or openings may be molded in by the use of a positive feature, and then machined off from the back to give a finished outside edge where hardware or electronics are mounted from the inside to show through to the exterior.

11. Hardware
All manner of hardware can be attached in secondary operations. And oftentimes hardware can be “insert-molded” into the part.

12. Tolerances
Tolerances are a variable, which ranges from the thickness and surface of the extruded stock, through tooling, trim and precision routing. Always provide the greatest tolerance possible to reduce costs and lead-time.
• Formed details are +/-.030” up to 12”, plus .002” (0.2%) per inch above 12 inches
• Drilled hole centers are +/-.015” up to 12”, plus .001” (0.1%) per inch above 12 inches
• Drilled hole Diameters are +/-.005” up to .375” dia., plus .001” (0.1%) per 1/16 above .375” dia.
• Trim dimensions are+/-.030” up to 12” plus .002” (0.2%) per inch above 12”

13. Materials
Almost any thermoplastic can be thermoformed. There are a great many resins and combinations of resins (alloys or capped sheet) available to meet almost any application’s requirements, and more are being developed every year. The table below lists the major categories of resins that Lormac Plastics uses on a regular basis. “Key properties” is a brief description of the more salient characteristics, but each resin type is quite diverse in the range of properties and properties that can be “tuned” to a purpose. The “Notes” section below indicate some of the range of modified properties available for the different resin families.

MATERIAL KEY PROPERTIES

materials

Lormac Plastics Inc. is an official Sponsor to the San Diego State University Aztec Formula SAE Team

 

Aztec Racing logo

 

Aztec Racing Formula SAE Team is a 501(c)3 non-profit student-run organization at San Diego State University. The purpose of the team is to promote collegiate education through the design, manufacturing and marketing of a small, open wheel racecar. The cars are built with a team effort in under one year and are taken to the annual competition for judging and comparison with approximately 80 other racecars from colleges and universities throughout the world. The end result is a great experience for young students in a meaningful engineering project as well as the opportunity of working in a dedicated team effort.

Lormac Plastics is an official sponsor of the program and is working with the student engineers on the design and production of custom vacuum formed plastic body panels. Lormac Plastics will donate design assistance, machine time, and materials to assist the students and program. “We are excited and proud to be working with such a talented group of engineering students” says Steve Klopf, President of Lormac Plastics. “Lormac strives to give back to the community in meaningful ways, especially when its students and race cars!”

The engineering students spend most of their time focused on the functionality of the car to ensure that it will perform without breaking down. With limited time and resource, the body panels and aesthetics of the car take a back seat. “Historically, body panels have been the ugly duck of our car in that there were never enough resources to put in the grueling time and effort in order to make our car look as good as it can. When I heard about the possibility of going plastic with our car, and the ease with which we can accomplish this goal, I was truly excited” said Rida Alvi, President of the Aztec Racing Team. “… having a sponsor as engaged and willing as Lormac Plastics provides the added dimension of a true partnership and keeps the team excited to pursue goals that were previously out of reach.”

About San Diego State University Aztec Formula SAE Team: San Diego State University’s SAE student chapter has a history of collegiate racing teams. It started with a number of successful showings in the Mini Baja Competition, with such achievements as 4th place in cost analysis, 3rd place in maneuverability, and a 7th place finish in the 1980’s and early 1990’s.

SDSU then moved on to the larger Formula SAE competition, with their first successful showing in 1994. The team struggled with a lack of funding and organization for the next few years. However, they were determined to succeed, and in 2004 began building a car that would reinvigorate the Aztec Racing tradition. The hard work paid off, as SDSU Formula Racing’s 2008 team was given the SAE Perseverance Award for being one of the top three rookie teams at the FSAE West Competition in Fontana, California. In 2012, the team presented a newly designed “AR-12” formula racing car that was very successful, and for the first time in Aztec Racing History, completed all racing events – a feat less than half of teams were able to complete.

About Lormac Plastics, Inc.: For over 45 years Lormac Plastics has specialized in providing high quality, custom vacuum and pressure formed products for a variety of modern design technology industries. Lormac Plastics occupies a modern, well-equipped, 10,000 square foot facility conveniently located between San Diego and Los Angeles in Escondido, California.

 

 

 

Vacuum Forming Techniques: Prestretching

Vacuum Forming techniques in Lormac’s San Diego facility.

Prestretching

When more uniformity or precise varying of wall thickness is desired, prestretching of the hot sheet before it touches the mold may be required. In using straight Drape Forming, as the hot sheet comes in contact with any portion of the mold, it cools that part of the sheet and decreases the sheet’s ability to flow. This causes most of the stretching to come from the section not yet in contact with the mold.

To reduce premature contact of the plastic sheet on specific mold surface areas that will cause stretching, a prestretching process is applied. There are several prestretching processes used by Lormac when vacuum forming in our San Diego Facility and we will discuss 2 of them in this post:

  1. Snap Back
  2. Plug Assist

Snap Back 

When prestretching is needed in the vacuum forming process, this method is automatically considered because the tooling is least expensive. A positive mold and a prestretch box box are easier and cheaper to make than a negative mold with plug assist. In this process, the prestretch box is pushed into the heated plastic sheet, causing a seal to form around the periphery. The hot plastic is pulled into the prestretch box by applying vacuum pressure or compressed air to form a bowl shape. To control consistency on the prestretch process, a photo electric eye may be used to automatically shut off the vacuum/compressed air when the stretch hits its mark. A rule of thumb is to prestretch the material approximately 2/3 the depth of the part.

When the plastic material hits the optimal prestretch height, the prestretch vacuum/compressed air is turned off and the plastic sheet is rapidly “snapped” to the detail of the mold surface using vacuum. After the plastic sheet is pushed completely against the mold, the prestretch box is moved away from the part to allow for optimal cooling.

The Snap Back process can be applied in both mold up and mold down formations. Mold Up formation is when the mold is mounted in the vacuum forming machine upside down and at the top of the machine. Better distribution of material is gained when using this formation as it allows you to take advantage of the natural sag of the heated plastic, however, for larger parts, the weight of the tool may prohibit mounting in this formation. Mold down formation is when the mold is mounted right side up at the bottom of the machine. the prestretch box has to work a little harder to billow the heated plastic sheet against its natural sag, but the process works well in keeping uniform wall thickness. At times, blowing carefully controlled compressed air between the sheet and the mold may help in billowing.

snapback vacuum formng

Snapback Vacuum Forming

 

 

 

 

 

Plug Assist

When using a negative mold and prestretching is required, plug assist is used. In this process, the mold is pushed into the heated plastic sheet creating a seal around the edges. At the same instance the seal is created, the heated plastic sheet is pushed into the mold using a plug assist. As the plug assist enters the the hot sheet, the air between the sheet and the mold is compressed, causing the sheet to billow around the plug. this action prevents the hot sheet from contacting the relatively cold mold as the sheet is stretched into the cavity. the plug stops within 10% of the bottom of the mold when the vacuum is rapidly applied transferring the sheet form the plug to the mold. Using a heated plug will avoid chilling the sheet during the prestretching process.

As with the Snap Back process, a mold up formation is optimal in plug assist for the same reasons, taking advantage of the natural sag and creating more air between the mold and the plastic sheet to increase billowing.

 

plug assist vacuum forming

plug assist vacuum forming

Pressure Forming – Vacuum Forming in San Diego

Pressure Forming and Vacuum Forming in San Diego

Lormac Plastics has been providing vacuum and pressure forming services in San Diego for over 40 years.

Thermoforming is a method of processing plastic resin into finished parts from sheet or film. The plastic sheet is heated to its particular thermoforming temperature and immediately shaped into the desired configuration. At processing temperatures, the sheet is very pliable, enabling it to be formed rapidly with great detail and minimum force. Pressure is maintained until the part has cooled.

Lormac plastics provides 2 thermoform processing methods in our San Diego facility and we will discuss the basic of both in this post:

vaccumforming

1. Vacuum Forming: With vacuum forming, a positive or negative mold of the desired part is created and mounted into vacuum forming machine. A sheet of plastic (a blank) is clamped into the clamp frame of the vacuum forming machine to ensure the sheet is held firmly in place during the vacuum forming process. The plastic blank is retracted into the heating oven where it heats to the required forming temperature. Once heated, the plastic blank is moved out of the oven and over the positive or negative mold. The plastic sheet is then draped over the mold and a vacuum is applied to quickly remove the air between them. Removing the air between the mold and the plastic blank allows atmospheric pressure to move (form) the pliable plastic to the mold. Cooling is applied once the plastic is formed to the shape. When the part has cooled, it can be removed form the mold. There are several techniques applied in the vacuum forming process. Each technique depends on the design of the part being formed. We discuss vacuum forming techniques in a separate blog post.

pressureform

2. Pressure Forming: To thermoform plastic parts that require more design detail such as sharp edges, close dimensional tolerances, and detailed surface shapes, pressure forming would be used over vacuum forming. Pressure forming applies high pressure to the plastic blank along with the vacuum to form the desired part. Common pressures are 35 to 60 PSI, however, higher pressures may be required for large parts of higher hot strength materials. The heated plastic blank is draped into a negative mold and a pressure plate is clamped on top of the negative mold. Vacuum is applied to remove the ambient air between the mold and the plastic sheet, and compressed air is used to pressurize the empty space in the mold cavity. The added pressure literally pushes (forces) the pliable plastic into the negative mold shape forcing sharp edges, undercuts, and other high tolerance design shapes. The vacuum and pressure are applied until the part cools and can be removed from the mold.

As with vacuum forming, there are several pressure forming techniques used depending on the design of the part being formed. We discuss these techniques in a separate post.

Feel free to contact Lormac Plastics with any questions or feedback and stay tuned for more thermoforming techniques.

Here are a few other resources on the vaccum forming and pressure forming processes:

Wikipedia vacuum Forming

Wikipedia Thermoforming