When we receive technical drawings for custom hollow parts at our engineering center, the most frequent issue we encounter is specifications written with injection molding logic rather than blow molding physics. This mismatch often leads to delayed tooling launches and costly design revisions for our clients.
To define blow molding parameters effectively, you must specify the resin’s melt strength, define minimum wall thickness rather than uniform gauge, and ensure geometry follows the width-to-depth ratio (W>2D). These constraints determine the parison’s ability to stretch without webbing or rupture during production.
Let’s examine the critical technical details you need to include in your request for quotation.
How do I determine the best plastic material and resin grade for my product's function?
Our sourcing team in Singapore often sees requests specifying generic “plastic” or standard HDPE 1 standard HDPE without considering the specific chemical or thermal environment the part will endure. Ignoring these factors can lead to immediate stress cracking in the field.
Determine the best material by analyzing the product’s exposure to chemicals, UV radiation, and impact stress. HDPE offers superior chemical resistance for containers, while PP provides higher heat tolerance, and PETG delivers clarity for cosmetic applications requiring glass-like aesthetics and precision.
Understanding Melt Strength and Drawability
Selecting the right material for blow molding is fundamentally different from injection molding. In our daily operations, we do not just look at the final hardness of the plastic; we look at "melt strength." This is the ability melt strength 2 of the plastic parison (the hollow tube) to hang under its own weight without stretching out like taffy before the mold closes.
If you choose a resin with low melt strength for a large part, the bottom of your container will be paper-thin while the top remains thick. For purchasing novices, you must specify the Melt Flow Index (MFI). Unlike injection molding which prefers high flow (high MFI) to fill complex cavities, blow molding requires low MFI (typically fractional melt, less than 1.0 g/10min) to maintain stable parison geometry.
Matching Resin to Environmental Stress
We always ask our clients: "What goes inside this container?" or "Where will this panel sit?" If your part holds detergents or oils, standard HDPE might fail due to Environmental Stress Cracking (ESC) 3 Environmental Stress Cracking (ESC). In these cases, we recommend a Copolymer HDPE or High-Molecular-Weight HDPE (HMW-HDPE) which has long polymer chains that tie the material together against chemical attack.
Below is a guide we use to help clients select the right polymer family based on function:
| Material Type | Key Characteristic | Best Application | Common Purchasing Specification |
|---|---|---|---|
| HDPE (High-Density Polyethylene) | High chemical resistance, excellent impact strength. | Industrial containers, fuel tanks, outdoor panels. | Specify "HMW-HDPE" for large parts to ensure wall uniformity. |
| PP (Polypropylene) | High heat resistance, stiffer than HDPE, autoclavable. | Medical bottles, hot-fill food containers. | Request "Clarified PP" if visual transparency is required. |
| PETG (Polyethylene Terephthalate Glycol) | Glass-like clarity, good barrier properties. | Cosmetic bottles, medical devices. | Ensure mold polish specification is high to utilize clarity. |
| PVC (Polyvinyl Chloride) | Rigid, chemical resistant, clear or colored. | Construction bellows, chemical bottles. | Check local regulations; some markets restrict PVC use. |
The Regrind Factor
Sustainability is a major purchasing Sustainability 4 metric today. Blow molding produces "flash" (scrap material) at the tail and neck, often 20-30% of the shot weight. You must define the allowable percentage of regrind (recycled flash) 5 regrind (recycled flash) in your specs. A mix of 10% to 20% regrind is standard and safe for most non-structural parts. However, for high-stress parts like pressure vessels, we strictly specify 100% virgin material to prevent contamination or polymer degradation.
What are the critical wall thickness and weight tolerances I need to specify in the drawings?
We consistently remind our US clients that expecting injection-molding precision on a blown part leads to unnecessary rejection disputes and frustrated quality managers. Blow molding involves stretching, not precision filling.
Specify a minimum wall thickness for critical areas rather than a nominal uniform thickness, as corners naturally thin during expansion. Set weight tolerances between ±3% to ±5% and dimensional tolerances around ±0.015 inches to account for material shrinkage and elasticity.
The "Minimum Wall" Concept
One of the hardest habits to break for new buyers is asking for "3mm wall thickness everywhere." In blow molding, this is physically impossible. As the parison expands, material stretches thin in corners and deep areas, much like a balloon gets lighter in color as you blow it up.
Instead of a uniform dimension, your drawing should identify Critical Control Points. Mark the corners or the deepest draw sections and specify a "Minimum Wall Thickness" (e.g., Min 1.5mm). This tells us how thick the starting parison must be. If you set this too high, you waste material on the flat sections just to keep the corners thick.
Parison Programming Capabilities
To optimize costs, you should ask suppliers if their machines have Parison Programming. This technology allows the machine to vary the thickness of the plastic tube vertically before it is blown.
- Without Programming: The tube is one thickness. To pass QC on the corners, the straight walls become excessively heavy.
- With Programming: We can program the machine to make the tube thicker only where the corners will be formed.
When you define the "Part Weight," you are essentially defining the cost. A tolerance of ±5% is standard for general industrial parts. If you demand ±1% weight tolerance, we have to run the machines slower and reject more parts, which increases your unit price.
Comparing Tolerances
To give you a realistic baseline for your drawings, here is how blow molding tolerances compare to the injection molding tolerances you might be used to:
| Feature | Injection Molding Tolerance (Typical) | Blow Molding Tolerance (Typical) | Why the difference? |
|---|---|---|---|
| General Dimensions | ±0.005 inches | ±0.015 to ±0.020 inches | High shrinkage rates and lack of internal mold pressure. |
| Wall Thickness | ±0.002 inches | -0.010 / +0.030 inches | Stretching relies on air pressure, not a rigid core. |
| Part Weight | ±1% | ±3% to ±5% | Parison swell varies slightly with temperature changes. |
| Neck Finish (ID) | ±0.002 inches | ±0.005 inches (Calibrated) | Calibrated necks are accurate; uncalibrated necks vary more. |
Shrinkage and Volume
Always specify the Overflow Capacity (OFC) if you are buying bottles. Plastic shrinks for 24 to 48 hours after molding. A bottle that holds 500ml right off the line might only hold 495ml the next day. We account for this in the mold design, but your specification must state whether the volume requirement is "at mold" or "after 24-hour conditioning."
How should I detail my requirements for surface finish and mold parting lines?
During our Design for Manufacturing (DFM) 6 Design for Manufacturing (DFM) reviews, we often find parting lines placed in areas that complicate flash trimming or weaken the bottle’s structural integrity. This is a common oversight that affects both aesthetics and cycle time.
Detail your requirements by marking parting lines in non-critical aesthetic zones to minimize visible flash. Specify pinch-off width and acceptable flash projection limits in your drawings to ensure the mold cutting edges remain sharp and the final part seals correctly.
Strategic Parting Line Placement
The Parting Line (PL) is where the two halves of the mold meet. In blow molding, this is also where the excess material (flash) is pinched off. Unlike injection molding where the PL is a subtle witness line, in blow molding, it is a structural weld.
If your design has a handle, the parting line must run through the center of that handle. You cannot hide it on the side. We advise clients to design a "step" or a cosmetic rib to disguise the parting line if aesthetics are paramount.
Your drawing should explicitly state: "Maximum Flash Projection: 0.2mm." This forces the supplier to keep their pinch-off knives sharp. If this is not defined, you may receive parts with sharp, jagged edges that are dangerous to handle.
Pinch-Off Geometry and Strength
The area where the plastic is welded together is the weakest point of a blow-molded part. This is called the Pinch-off.
- Bad Spec: Ignoring the pinch-off design.
- Good Spec: Requiring a "compression molding" style pinch-off for heavy parts.
For purchasing, you simply need to define the Drop Test requirement (discussed in the next section). If the pinch-off is poor, the bottle will split at the seam upon impact.
Surface Texture and Venting
Air must escape the mold as the balloon expands. If the mold is perfectly airtight, the air gets trapped between the plastic and the metal, causing surface dents or an "orange peel" look.
To prevent this, we often sandblast the mold cavity to create micro-vents. This means a high-gloss "Class A" finish is very difficult to achieve in standard Extrusion Blow Molding (EBM) 7 Extrusion Blow Molding (EBM).
- Recommendation: Specify a textured finish (like Mold-Tech MT-11000 series) for the main body to hide scratches and flow lines.
- Gloss Requirements: If you need high gloss, specify Polished Mold Surface with Venting Inserts. Be aware this increases mold maintenance costs.
Draft Angles
Because the plastic shrinks onto the mold core (or into the cavity), you need draft angles to get the part out. draft angles 8
- Standard Draft: 3° is standard.
- Texture Draft: Add 1° of draft for every 0.001" depth of texture.
If your design has vertical walls with 0° draft, the part will drag against the mold upon opening, creating "drag marks" or scuffing. We always flag this during the quoting phase.
What quality control parameters must I establish to ensure the parts pass performance testing?
Before any container leaves our partner factories in Asia, it must prove it can survive the journey to your warehouse. Visual checks are never enough because invisible thin spots can cause catastrophic failure under load.
Establish quality control parameters that include leak testing for hermetic seals, top-load testing for stacking strength, and drop tests for impact durability. Define acceptable regrind percentages to maintain structural integrity while optimizing material costs.
Functional Performance Testing
You are not buying a piece of plastic; you are buying a function (holding liquid, protecting a component, etc.). Your Purchase Order or Quality Agreement 9 Quality Agreement should list "Pass/Fail" criteria based on performance, not just dimensions.
1. Leak Testing
Every reputable blow molding line should have an inline leak tester 10 inline leak tester. This machine pressurizes every single bottle to check for holes.
- Spec: "100% Inline Leak Decay Test at 3 PSI for 2 seconds."
- Why: This catches pinholes in the pinch-off area that the human eye cannot see.
2. Drop Testing
This verifies the strength of the weld line (pinch-off) and the material distribution.
- Spec: "Fill container with water/product. Drop from 1.2 meters (4 feet) onto concrete. Bottom-down, Side-down, and 45-degree angle. No leakage allowed."
- Adjustment: If you are shipping dangerous goods, you must reference UN packaging standards (e.g., UN3H1).
3. Top Load (Stacking) Strength
Blow molded parts are mostly air. Shipping them is expensive ("shipping air"). To save money, you want to stack them high in the truck.
- Spec: "Part must withstand 50kg top load without permanent deformation."
- Context: If your warehouse stacks pallets 3-high, the bottle at the bottom of the bottom pallet must support the weight of everything above it. We need this number to design the structural ribs into the part.
Visual Defect Criteria
To avoid subjective arguments ("This looks ugly" vs "This is normal"), create a Limit Sample Board. However, before you have physical samples, use a defect table in your contract.
| Defect Type | Description | Acceptance Standard |
|---|---|---|
| Die Lines | Vertical lines on the surface caused by the die head. | Acceptable if not palpable (cannot feel with fingernail). |
| Black Specks | Carbonized burnt plastic. | Max 0.5mm diameter; no more than 2 per part. |
| Flash | Excess material at parting line. | Max projection 0.2mm; must not be sharp. |
| Rocker Bottom | Bulging bottom preventing the bottle from standing flat. | Not Allowed. (Indicates insufficient cooling time). |
Cooling and Warpage
Cycle time drives cost. If a supplier runs the machine too fast, the plastic is still hot when it ejects. This causes the bottom to bulge ("rocker bottom") or the side walls to warp inward ("paneling").
- Purchase Spec: Include a "Flatness Tolerance" for any mating surfaces.
- Tip: If you see "paneling" (sucked-in walls) on your samples, it usually means the part was capped before it cooled down, or the cooling cycle is too short.
Conclusion
Defining requirements for blow molding requires a shift in mindset from "precision geometric replication" to "functional flow management." By specifying melt strength appropriate for your environment, respecting the physical limits of wall thinning, detailing pinch-off expectations, and mandating functional leak and drop tests, you protect your company from receiving unusable parts. At DEWIN, we help clients navigate these technical waters every day, ensuring that the transition from a CAD drawing to a physical product is seamless and cost-effective.
Footnotes
1. Technical product page for High Density Polyethylene from a major global resin manufacturer. ↩︎
2. Educational resource explaining polymer viscosity and melt properties in processing. ↩︎
3. Official guide on stress cracking mechanisms from the National Physical Laboratory. ↩︎
4. Reference to the UN Sustainable Development Goals regarding responsible consumption and production. ↩︎
5. US EPA guidance on sustainable materials management and recycling hierarchies. ↩︎
6. Industry article from the Society of Manufacturing Engineers on DFM principles. ↩︎
7. Process overview from a leading manufacturer of blow molding machinery. ↩︎
8. General engineering definition of draft angles required for molded parts. ↩︎
9. Best practices for defining quality expectations from the American Society for Quality. ↩︎
10. Manufacturer information on leak testing equipment specifically for empty plastic containers. ↩︎
